M55 Rocket Assessment

M55 Rocket A

Summary

U.S. Army Program MDemilita

Aberdeen Proving

July

July 2002

ssessment

Report

anager for Chemical rization Ground, Maryland 2002

Executive Summary In the early to mid-1960s, the United States produced over 400,000 M55 chemical rockets for military use. None of the rockets were ever used in combat, and in 1981 they were declared obsolete and of no military value. The chemical rockets are currently in storage and awaiting destruction at five U.S. Army stockpile sites: Anniston Chemical Activity in Alabama; Blue Grass Chemical Activity in Kentucky; Pine Bluff Chemical Activity in Arkansas; Deseret Chemical Depot in Utah; and Umatilla Chemical Depot in Oregon. The M55 rockets that were once stored on Johnston Island in the Pacific have been safely destroyed. Destruction of M55 rockets is currently under way at the Tooele Chemical Agent Disposal Facility in Utah. The rockets weigh nearly 60 pounds and are almost seven feet long. Each rocket has a warhead and motor section. The warhead contains approximately 10 pounds of chemical nerve agent (GB or VX) along with explosives that disperse the agent when the rocket impacts the ground. The M28 propellant in the rocket motor provides thrust when the rocket is launched. The rockets are stored inside fiberglass shipping and firing tubes, which are then placed in pallets and stacked inside earthen bunkers (or igloos). Over time, the M28 propellant naturally degrades through a series of chemical reactions that generate heat. To control this chemical reaction, chemical compounds called stabilizers were added to the propellant when it was manufactured. The stabilizer is consumed over time as the propellant ages, and as the stabilizer depletes, the propellant generates additional heat. If the propellant gets hot enough, it could ignite on its own—a phenomenon known as autoignition. Some of the rockets have begun to leak agent while in storage. Agent leakage occurs much more frequently for rockets containing GB agent

because the GB becomes more acidic as it ages. The Army has instituted special surveillance procedures for M55 GB rockets in order to find the leaks before liquid agent breaches the shipping and firing tube. When a leak is discovered, site workers overpack the leaking rocket in a specially-designed steel container. The steel container prevents leaked agent from escaping to the air inside the igloo. Concerns have been raised in the past regarding both the stability of the rocket propellant and leakage of the GB rockets. In response to these concerns, the Army has conducted extensive laboratory testing and analytical modeling. This work has been completed and the results are summarized in this report. Statistical analyses have been completed to evaluate the effectiveness of the Army’s GB rocket surveillance procedures in minimizing worker exposure to agent and agent release to the environment. These studies show that the surveillance procedures are effective at finding leaks before agent breaches the shipping and firing tube. Since agent is confined inside the shipping and firing tube, the potential for worker exposure to agent is small and agent release to the environment is minimal. Tests have shown that the stabilizer in nonleaking M55 rockets has depleted only slightly since the rockets were manufactured. The projected safe storage life of these rockets is greater than 100 years and the likelihood of an autoignition of a nonleaking rocket is essentially zero. Tests with chemicals similar to nerve agent showed that agent-contamination could greatly accelerate depletion of the stabilizer in the propellant. This has since been confirmed in comprehensive laboratory tests to investigate the effects of agent on propellant. These tests showed that stabilizer depletion is much faster following agent contamination and that the stabilizer could become fully depleted approximately 20 years after the propellant becomes contaminated. The tests also showed that a substantial amount of heat could be

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DRAFT

generated in the propellant once the stabilizer became fully depleted. To confirm predictions based on the laboratory tests, propellant samples were taken from 120 leaking M55 GB rockets stored at two stockpile sites. These rockets had been stored for 11 to 30 years after being overpacked. Chemical analysis of the propellant samples showed that samples with substantial contamination had significantly depleted stabilizer levels, and, in some cases, significant heat generation. However, in every case, agent contamination of the propellant was very localized, with typically less than 10 percent of the propellant grain being affected. Army scientists conducted tests to assess the ability of stored rockets to transmit heat to their surroundings. These tests showed that heat generated in the propellant is effectively dispersed across the surface of the rocket so that heat losses from the rocket are higher than expected. Based on the test results, detailed computer models were developed to evaluate the

likelihood that a leaking rocket auto-ignites while in storage. The computer models show that autoignition of a leaking rocket is very unlikely. In addition, the probability of autoignition is much smaller than the probability of other accidental rocket ignition events, such as ignition due to lightning striking a storage igloo. While these studies have demonstrated that the storage of the M55 rocket presents no immediate threat of autoignition, the rockets still present a significant risk of ignition due to other events such as lightning strikes and earthquakes. The Army is currently evaluating ways to reduce the risk due to storing the rockets; however, the only way to ensure complete safety of the surrounding public and the environment is to destroy these munitions in a safe and efficient manner. Detailed information in the form of technical reports, and a videotape that summarize the M55 test program can be obtained from the Program Manager for Chemical Demilitarization (PMCD) Public Outreach and Information Office.

M55 Rocket Assessment ii

Introduction The United States stockpile of chemical weapons initially included over 400,000 M55 chemical rockets that each contained approximately 10 pounds of nerve agent, either GB or VX. The M55 rockets were developed in the 1950s and were manufactured in the early to mid-1960s. None of the rockets were ever used in combat, and in 1981 they were declared obsolete and of no military value. The current stockpile of M55 rockets is stored at five chemical stockpile sites in the continental United States (figure 1) and is awaiting demilitarization. The M55 rockets that were once stored on Johnston Island in the Pacific have been safely destroyed, and destruction of the rockets is currently under way at the Tooele Chemical Agent Disposal Facility (TOCDF) in Utah. All of the M55 GB rockets previously stored at the Deseret Chemical Depot have been destroyed at the TOCDF. As of January 2002, there are approximately 350,000 rockets remaining in the stockpile. Over the years, concerns have been raised regarding the safety of long-term storage of the rockets. The initial concern was leakage of the M55 GB rockets, which have been more prone to leakage than any other chemical munition in the stockpile. More recently, concerns have been raised over the stability of the propellant in

the rockets and the potential for the rockets to auto-ignite. To address the leakage and propellant stability concerns, the Army undertook extensive testing and monitoring efforts to better understand the significance of these issues and to ensure that the public, workers, and the environment are adequately protected while the rockets are stored. This report describes the Army’s rocket studies and presents the conclusions drawn based on this work. Background In order to understand the Army’s M55 rocket studies, it is important to understand the factors that influence agent leakage and rocket stability. The following discussion provides an overview of these issues and a historical perspective on the work that has been done to address them. First, a brief description of the M55 rocket is provided.

Description of the M55 Rocket The M55 chemical rocket, shown schematically in figure 2, is over 6 feet in length and weighs approximately 60 pounds. It has two main sections: the warhead and the motor assembly. The warhead is a thin-wall aluminum shell that contains approximately 10 pounds of chemical agent (GB or VX) surrounding two explosive bursters. The bursters are initiated by a fuze threaded to the warhead. The motor assembly is a steel housing that contains the M28 propellant

JI

BGCA

ANCA

PBCA

DCD

UMCD

Figure 1. M55 rockets are currently stored at five locations

awaiting destruction. All rockets previously stored on Johnston Island in the Pacific have been safely destroyed.

1

FUZE

BURSTERS

CHEMICAL AGENTCAVITY

THIN-WALLALUMINUM

M28 PROPELLANT GRAIN

FINS ROCKETMOTOR

Figure 2. Schematic diagram of the M55 rocket.

July 2002

Figure 4. Earth-covered magazines called igloos are used for M55 rocket and other muition storage at the

chemical depots.

grain and an igniter assembly. The propellant burns rapidly and provides the thrust when the rocket is fired. The M28 propellant is a solid, double-base propellant grain containing two energetic materials, nitroglycerin (NG) and nitrocellulose (NC). The grain is manufactured with a tri-lobe core (figure 3) to provide a large surface for burning and a path for exhaust gases to exit the propellant and pass though the nozzles at the aft end of the rocket. The grain is approximately 4 inches in diameter, 31 inches in length and weighs about 19 pounds. Each grain has a nominal composition shown in table 1.

Figure 3. Section of an M28 propellant grain showing the tri-lobe core.

Table 1. Nominal composition of M28 Propellant Component Weight

percent Purpose

Nitrocellulose 60.1% Energetic Nitroglycerin 23.8% Energetic Triacetin 9.9% Plasticizer Dimethyl Phthalate 2.6% Plasticizer 2-nitrodiphenylamine (2-NDPA)

1.7% Stabilizer

Lead stearate 1.9% Ballistic modifier

The rockets are stored inside fiberglass shipping and firing tubes (SFTs). During storage, the rockets are placed in wooden pallets that contain 15 rockets each. The rocket pallets are stacked inside earth-covered magazines called igloos (figure 4) and are oriented such that all of the rockets point toward the rear of the igloo.

GB Rocket Leakage About 10 years after the M55 GB rockets were manufactured, they began to leak. During this time, leakage was usually detected only when liquid agent appeared on the outside of the rocket SFT. When leakage was detected, the rocket (still in its SFT) was overpacked in a steel container. Once a leaking rocket was overpacked, it was removed from its normal storage location and moved to an igloo designated for overpacked items (figure 5). The overpacking operation introduces workers to increased risk of agent exposure. The potential for agent exposure is greatest when liquid agent has breached the SFT. In order to reduce the potential for worker exposure to agent and to minimize the potential for agent release to the environment, the Army has implemented a surveillance program for the M55 GB rockets. In this surveillance program, which is referred to as the Storage Monitoring Inspection (SMI) program, the interior of the SFT is monitored using extremely sensitive agent detectors. The objective of the SMI program is to catch the leakage at an early stage; in other words, before agent has breached the SFT. Most of the leaking rockets found during the early 1970s were liquid leakers, in other words, leakers that were initially detected with a substantial amount of liquid agent in the SFT

M55 Rocket Assessment 2

and, in some cases, with liquid agent outside the SFT. Since the SMI program was instituted, most leaking rockets have been detected after only trace amounts of agent leaked from the warhead. In nearly every case, the leaked agent is confined to the inside of the SFT. Leakers of this type are referred to as vapor leakers. In recent years, questions have been raised regarding the effectiveness of the Army’s surveillance programs in minimizing worker exposure to agent and protecting the environment. In addition, concerns have been expressed that recent leakage experience may indicate accelerating degradation of the rockets and increasing public health risk due to agent leakage. To address these concerns, the Army has reviewed the existing surveillance program and performed statistical analyses of historical leakage data. The results and conclusions from these studies are discussed later in this report.

Propellant Stability The NG and NC in the propellant degrade slowly under normal storage conditions. This degradation process generates heat and releases nitrogen oxide (NOx) gases that subsequently react with moisture in the propellant to form acids. Both heat and acids accelerate the degradation process.

Chemical compounds referred to as stabilizers are added to the propellant to prevent autoignition by absorbing the NOx gas species as they are released. The stabilizer used in the M55 propellant is 2-nitrodiphenylamine (2-NDPA). A single molecule of 2-NDPA can absorb as many as six gas molecules through a series of reactions that produce reaction products (called daughter products). Each subsequent reaction yields a daughter product that is a less effective stabilizer (in other words, the daughter products absorb NOx molecules more slowly than the original stabilizer). Therefore, as the stabilizer degrades over time, the propellant becomes less stable and the rate of heat generation in the propellant increases. If the heat generated by the propellant exceeds the rocket’s ability to transmit heat to its surroundings, the temperature of the propellant could increase sufficiently that it auto-ignites.

Figure 5. Overpacked leaking rockets are stacked in

pallets inside storage igloos.

Accidental ignition of the rockets is a significant concern because ignition of one rocket can cause a fire that eventually spreads to the other rockets in the igloo. The resulting series of explosions could cause the igloo structure to fail and allow the release of a substantial quantity of chemical agent vapor to the atmosphere. In 1994, the National Research Council (NRC) and the General Accounting Office (GAO) expressed concerns regarding the stability of the M28 propellant (NRC, 1994; GAO, 1994). Both organizations acknowledged the work that the Army had done in assessing propellant degradation, but both were concerned over the lack of understanding of the long-term behavior of the rockets in storage and the uncertainty in how long the rockets could safely be stored. Until more detailed studies of propellant stability could be completed, Congress tasked the Army with preparation of contingency plans for actions that would be taken to prevent the M55 rockets from reaching a potentially dangerous condition. The resulting study culminated in the publication of the Master Action Plan for M55 Rockets (Cain, et al., 1996). The Master Action Plan discussed actions that would be taken if the stabilizer concentration reached specific “trigger” levels.

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For example, at a 2-NDPA concentration below 0.2 weight percent, the rocket was thought to be unsafe for continued storage and would have to be destroyed within 60 days. In order to address the uncertainty in the stability of the M55 rocket stockpile, the Army began an extensive research program to investigate propellant stability and the potential for rocket autoignition. Initially, the focus of this program was the stability of uncontaminated propellant and the effectiveness of the propellant stabilizer. However, a panel of propellant experts convened by the Army to guide the program suggested that agent-contaminated propellant was a more significant concern and recommended that the Army investigate the effects of chemical agent on the propellant. The panel’s recommendation was based on the results from tests of propellant contaminated with GB and VX agents (Fitzgerald, et al., 1996) and results from tests of propellant taken from leaking rockets stored on Johnston Island (Battelle, 1996). These tests showed that agent contamination could significantly accelerate depletion of the stabilizer, but were not sufficient to allow predictions of how leaking rockets would behave under actual storage conditions. Based on the experts’ recommendations, the Army undertook a comprehensive program to study the effects of chemical agent, particularly GB nerve agent, on propellant. The Army’s program involved extensive laboratory testing coupled with computer modeling. The computer models were used to extrapolate the results from the test program to field storage conditions and to estimate the likelihood of autoignition for both leaking and nonleaking rockets. Predictions from the computer model were then checked against the results from laboratory analyses of propellant samples taken from actual leaking rockets stored at two different stockpile locations. The results and conclusions from the laboratory tests and computer models are discussed later in this report.

Agent Leakage Studies A small fraction (approximately 0.5 percent) of the GB M55 rockets have warheads that have begun to leak chemical agent. Some leakage has also occurred with the VX rockets, but the fraction of VX rockets that has leaked is considerably less (less than 0.01 percent). Inspection of the leaking GB rockets has indicated that the GB corrodes the aluminum warhead and causes pinhole leaks to form. Leakage is more significant for GB rockets because contaminants present in GB have, over time, increased the acidity of the agent. The increased acidity is believed to be the cause of the observed warhead corrosion and leakage (AMSAA, 1985). During the period that GB was manufactured for use in the M55 rockets, the manufacturing process and the purity requirements changed. Because of this, some manufacturing lots contained more contaminants and have become more acidic with time. Rocket lots containing this agent are much more prone to leakage. Stockpile sites with a larger percentage of rockets from these lots have had a higher incidence of leakage. The Army has instituted a surveillance program in order to detect agent leakage at an early stage. The SMI program includes periodic sampling of the gas volume inside the shipping and firing tubes of a number of randomly selected M55 GB rockets from each munition lot at each stockpile storage site (DA, 1990). A rocket is overpacked if GB in either liquid or vapor form is detected. The SMI program also includes quarterly visual inspection of the M55 VX rockets at each stockpile site. There are two different types of containers used to overpack M55 rockets: modified M1 gas identification set containers (referred to as PIGs) and single round containers (SRCs). Both are made from steel and are designed to be tightly sealed in order to confine any leaked chemical agent. Overpacked munitions are taken to

M55 Rocket Assessment 4

separate storage igloos that contain only overpacked items. To ensure that the leaking agent is effectively contained within the overpack containers, these igloos are monitored for agent at least once each week.

Storage Monitoring and Inspection Monitoring and inspection of the rockets was initiated in 1971. The primary objective of this early surveillance was to ensure the serviceability of the rockets should they be needed during a battlefield situation. The monitoring and inspection program included visual inspection of the GB and VX rockets (figure 6) and intrusive monitoring of a selected number of GB rockets. During intrusive monitoring, the sampling port plugs on the ends of the shipping and firing tube were removed and the interior of the tube was monitored using an agent detector. The agent detectors available at the time were capable of detecting GB at concentrations as low as 170 parts per billion (ppb). To ensure the safety of personnel performing the inspection, the interior air in the igloo was monitored for agent prior to personnel entry and also during the inspection. After the M55 rockets were declared obsolete in 1981, the objective of the monitoring program changed from ensuring that the rockets could be used in a battlefield situation to ensuring that they could be stored safely. The current SMI

program has evolved from a program established in 1984 in response to a U.S. Army Development and Readiness Command (DARCOM) [now, U.S. Army Materiel Command (AMC)] directive. The DARCOM directive made the following statement regarding the objective of the SMI program:

These minimum essential procedures are established to assure the earliest possible detection of leakers; provide maximum protection to DARCOM workers, the public, and the environment; and permit proper and timely containment of leaking munitions. In addition, the procedures are necessary to provide required information to properly characterize the status of the M55 GB rockets and to predict deterioration trends.

To achieve these objectives, the SMI program included visual inspection of all shipping and firing tubes, interior monitoring of the storage igloos, and intrusive monitoring of a representative sample from each GB rocket lot in the stockpile. The agent detectors used in this inspection were capable of detecting agent at concentrations below 17 parts per trillion (ppt). The SMI program has continued to evolve over the years to take best advantage of improving monitoring technologies and the growing understanding of leakage trends. In the current SMI program, a sample of GB rockets is intrusively monitored each quarter using agent detectors. The detectors currently used in the SMI program have the same sensitivity as those used when the program was initiated in 1984. The number of samples monitored from each manufacturing lot depends on the total number of rockets in the lot and the leakage history with that lot. More samples are taken from larger lots and from lots that have had a higher percentage of leakers. The number of rockets intrusively monitored at each stockpile site each year ranges from about 2,500 to 7,100 depending on the site.

Figure 6. Workers perform visual inspection of M55 GB rockets during the Storage Monitoring Inspection

Program

Effectiveness of the SMI Program The SMI program is very costly and labor-intensive for the stockpile sites to implement. Consequently, it is important to understand

5 July 2002

whether the program is achieving the objectives outlined in the original DARCOM directive. Statistical assessments have been completed to examine the effectiveness of the SMI program in protecting workers, the public, and the environment and in providing the necessary information to characterize degradation of the M55 GB rocket stockpile. In 1997, a statistical assessment of the SMI program was completed (PMCD, 1997b). The two objectives of this assessment were (1) to determine whether the SMI program was meeting its objectives, and (2) to determine whether the M55 GB rocket stockpile was becoming more prone to leakage. The 1997 assessment reviewed the leakage history of the M55 GB rocket stockpile and determined the number of liquid leakers and vapor leakers found each year. Figure 7 shows the leakage history since 1971. The figure shows that prior to the start of the SMI program in 1984, few leakers were discovered each year and most that were discovered were liquid leakers. Since the start of the SMI program, more leakers are being found because many more rockets are being intrusively monitored; however, there have been very few liquid leakers. This trend indicates that the SMI program has been effective in reducing the potential for worker exposure to liquid agent during

overpacking operations. In addition, because most leaks are found before agent breaches the shipping and firing tube, the SMI program has been effective at reducing the potential for agent release to the environment. Therefore, although discovery of leaking rockets at a stockpile location often receives substantial press attention and raises concerns among the public, the health threat from this leakage is negligible. The 1997 statistical assessment also considered leakage trends to determine whether the existing leakage data were sufficient to characterize stockpile degradation and, if so, to determine whether the data indicated a trend toward an increased leakage rate. The study concluded that the leakage data generated by the SMI program is sufficient to identify trends in the status of the GB rocket stockpile and to evaluate risks due to operations involving leaking rockets. Statistical analysis of the data showed that the GB rocket stockpile does not show signs of an increased leakage rate, although there are some individual rocket lots that may be degrading and should be monitored more closely. One rocket lot at the Anniston site, lot 1033-45-181, was identified as a specific concern. The report also noted the relatively high number of cases in which multiple leakers were identified during a single SMI operation involving lot 1033-45-181. The potential for cross-contamination of vapor samples taken from the monitored rockets was theorized as a possible reason for this. If true, then the higher leakage rate with this lot would be an artifact of the monitoring procedures, rather than an indication that the rockets in the lot are degrading.

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1971 1976 1981 1986 1991 1996

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Figure 7. The number of liquid leakers found each year has

decreased since the SMI program was initiated in 1984.

Another statistical assessment has recently been completed that focused specifically on Anniston lot 1033-45-181 (SAIC, 2002). This study found that the increased leakage trend noted in the years prior to publication of the 1997 report has not been sustained in the years since then. Although the data may indicate a slight increase in the leakage rate since 1991 for this rocket lot, the trend is probably not sufficient to indicate

M55 Rocket Assessment 6

that the rockets in lot 1033-45-181 are deteriorating at an accelerated rate. The study noted that Anniston lot 1033-45-181 has an unusually high leakage rate compared to other lots containing GB manufactured to the same specifications, but noted that “problem” lots such as this have occurred at other stockpile sites in the past. For that reason, it was concluded that the leakage experience with lot 1033-45-181 is not indicative of cross-contamination or other problems with the implementation of monitoring procedures at Anniston. Testing of Uncontaminated Propellant For most propellants, the Army continually monitors the stockpile from time of production to time of disposal. The status and safe storage life predictions for the propellants are normally based on two test programs: a master sample program and accelerated aging tests. The first extensive testing of the propellant in M55 rockets was associated with the 1985 M55 Rocket Stockpile Assessment, which was undertaken to characterize the condition of the rockets (AMSAA, 1985). As part of this program, the Armament Research, Development and Engineering Center (ARDEC) at Picatinny Arsenal, New Jersey was tasked with analyzing the level of 2-NDPA stabilizer in the propellant. ARDEC was provided with propellant samples from each M28 propellant lot from each of the stockpile sites. Samples were taken from the front, center, and aft of the propellant grain. Results of this initial analysis indicated that the M28 propellant had exhibited minimal stabilizer loss after 20 to 25 years in storage and that the rockets were safe for continued storage (Wachter, 1986). The propellant samples collected from the various rocket storage locations became the M28 propellant Master Field Samples. The Master Field Samples are stored at Picatinny Arsenal,

and were reanalyzed each year until 1993. The program was discontinued in 1994 due to funding cuts and because the results of the analysis indicated essentially no change in stabilizer concentration from year to year. In lieu of annual testing of the propellant samples, the Army instituted Safe Interval Prediction (SIP) testing in 1995. The SIP tests use accelerated aging of the propellant at elevated temperatures to determine a safe testing interval for the propellant. The safe interval is an allowable time period between reanalysis of the stabilizer level in the samples. Based on the SIP test results, the safe retest interval was calculated to range from 15 to 31 years (Geo-Centers, 1996); in other words, the propellant samples would not need to be reanalyzed for at least another 15 years to ensure adequate verification of stabilizer levels. According to the current schedule for destruction of the M55 rockets, all of the rockets will have been destroyed by the time reanalysis of the Master Field Samples would be needed. Testing of Contaminated Propellant In the late 1980s, researchers at ARDEC studied propellant degradation following exposure to diisopropyl methylphosphonate (DIMP), a chemical similar to agent (Robertson, 1988). These tests showed that the DIMP greatly accelerated depletion of the stabilizer in the propellant. It was predicted that agent exposure could have a similar effect. Concern over the potential effect of agent on the M28 propellant led to two additional studies that were completed in 1996: (1) laboratory analysis of propellant exposed to GB and VX agent under laboratory conditions, and (2) laboratory analysis of propellant samples taken from leaking rockets stored at one of the stockpile sites. In 1995, researchers at the U.S. Army’s Edgewood Research Development and

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Engineering Center (ERDEC) [now the Edgewood Chemical Biological Center (ECBC)] exposed M28 propellant to the nerve agents GB and VX. These tests showed that agent contamination greatly accelerates stabilizer depletion (Fitzgerald, et al., 1996). Subsequently, propellant samples were taken from 21 overpacked leaking rockets stored on Johnston Island and sent for analysis at ERDEC. The ERDEC analysis showed that the propellant from one rocket had substantial agent contamination and significantly depleted stabilizer and NG in a portion of the propellant grain (Battelle, 1996). The ARDEC and ERDEC tests provided an indication of the effect of agent on M28 propellant but were not sufficient to allow predictions of how the rockets would behave under actual storage conditions. For that reason, additional testing was needed.

Laboratory Test Program A comprehensive agent-propellant testing program was conducted at the Midwest Research Institute (MRI) in Kansas City, Missouri (PMCD, 2000). The objective of this test program was to characterize the thermal, chemical, and physical behavior of M28 propellant following exposure to GB nerve agent. Since comprehensive laboratory testing had not been performed previously with agent-contaminated propellant, a significant amount of analytical methods development had to be conducted. Procedures used at ARDEC for testing propellant were used as a basis for development of procedures for analyzing the agent-exposed samples. Thirty years of normal storage could only be simulated if tests were performed at high temperatures, a process known as accelerated aging. All sample exposures were performed in laboratory ovens under controlled temperature conditions.

The propellant samples were exposed to various amounts of GB vapor under controlled laboratory conditions at temperatures ranging from ambient (room temperature) to 70°C (figure 8). Nearly all agent exposures and most of the subsequent tests were conducted at elevated temperatures (usually at 65.5°C) in order to accelerate the degradation reactions in the propellant. Results at elevated temperatures could then be extrapolated to ambient temperature based on the kinetics of the controlling reactions. All M28 propellant employed for this study came from the inventory of Master Field Samples stored at ARDEC. The propellant was sent to Alliant TechSystems in Rocket Center, West Virginia, where it was cut into the appropriate geometries for testing. Munitions grade GB (nominal purity 94 percent) was obtained from ERDEC. A variety of chemical analyses were performed on the agent-contaminated propellant to determine the time-dependent degradation of the nitrate esters (NG and NC), stabilizers (2-NDPA

C a p

M e t a l H o l d e r

P r o p e l l a n t C u p

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0 . 0 5 in. D i a meter W i r e

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A g e n t V i a l

A g e n t

Figure 8. Propellant samples were exposed to GB agent vapor under controlled laboratory conditions to simulate

exposure to agent under storage conditions.

M55 Rocket Assessment 8

and its stabilizing daughter products), and the chemical agent. Stabilizer and NG concentrations were measured using high performance liquid chromatography (HPLC). Early in the testing program, NC content in GB-exposed propellant was measured by size exclusion chromatography (SEC). These tests were subsequently discontinued because the NG concentration measurements were much easier to perform and were thought to provide sufficient information on nitrate ester depletion in the propellant. Degradation of the GB was determined by subjecting extracts of the exposed propellant samples to gas chromatography with flame photometric detection (GC-FPD) and ion chromatography (IC). Tests were conducted to determine the following aspects of propellant behavior following agent contamination:

• Absorption of GB vapor by the propellant

• Stabilizer and nitrate ester depletion as a function of time

• Degradation of GB in the propellant • Heat evolution as a function of time

following agent exposure • Impact sensitivity of the propellant

following agent exposure • Rate of diffusion of agent into the

propellant grain • Thermophysical properties of both

contaminated and uncontaminated propellant.

The tests showed that GB vapor is rapidly absorbed by the M28 propellant. In tests with substantial excess agent available, the propellant absorbed more than its own weight of GB and the propellant grain lost its structural integrity, becoming a viscous, gelled mass with an appearance similar to tar (figure 9). When the agent source was more limited, the propellant grain maintained its solid structure. The researchers at MRI determined that the quantity of GB that would cause the propellant to gel was approximately 200 microliters (approximately 220 milligrams) of GB per 300

milligrams of propellant. (This is the quantity of liquid agent supplied in the exposure vial and not the quantity of agent actually absorbed by the propellant.) More importantly, it was determined that exposure to 50 microliters of GB (55 milligrams) per 300 milligrams of propellant was sufficient to greatly accelerate degradation of the propellant. This agent exposure level, which corresponds to an agent concentration of approximately 6 to 8 percent by weight, did not cause the propellant to lose it solid structure.

Figure 9. M28 propellant turned into a tar-like mass following exposure to large quantities of chemical

agent GB (left photo), but remained solid when exposed to a limited agent source (right photo).

When the propellant was exposed to agent at or above 50 microliters per 300 milligrams and maintained at 65.5°C, complete depletion of the 2-NDPA and its di-nitrated daughter products occurred within 28 days, while depletion of the tri-nitrated daughters was complete within 50 days. Below an exposure level of 25 microliters per 300 milligrams of propellant, depletion was much slower, but was still faster than that for unexposed propellant. (For comparison, previous testing has shown that depletion of the original 2-NDPA in unexposed propellant requires on the order of 400 days at 65.5°C.) The NG and NC in the propellant deplete along with the stabilizer. As the nitrate esters and stabilizers deplete, the heat generation in the propellant increases to a peak value and then decreases as the nitrate ester concentration falls further (figure 10). The time of the peak heat generation rate is the critical time for propellant autoignition since that is the time at which the propellant would be heating the fastest.

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The time-dependent heat generation rate in the propellant was determined by microcalorimetry for a wide range of test conditions, including a range of agent exposure levels and a range of temperatures from 40° to 70°C. For most samples, the microcalorimetric cells were tightly sealed; however, for some samples, the tops of the cells were loosened to allow released gases to escape. Measurements were taken periodically for each of the exposed propellant samples in order to characterize the time-dependent heat generation rate as the stabilizer degrades. From these data, the peak heat generation rate and the time of the peak were determined. The magnitude and time of the peak heat generation rate varied significantly among the different test conditions. The highest peak heat generation rates were measured for samples that were exposed to agent and immediately sealed in the microcalorimeter cells. Samples in unsealed microcalorimetric cells had much lower peak heat generation rates than samples in sealed cells. These results are important because the overpack containers used to confine leaking M55 rockets may allow gas pressure to build, resulting in a higher peak heat generation than in leaking rockets that have not been detected and overpacked. The temperature-dependent heat generation data were found to accurately fit an Arrhenius equation, with a calculated activation energy on the order of 20 to 24 kcal/mole. This was an

important finding from the test program because it indicated that the propellant degradation mechanisms do not change as the temperature is reduced. As a result, Arrhenius-type equations can be used to predict the heat evolution rate and stabilizer depletion rate at ambient temperature based on the results at elevated temperatures.

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Figure 10. As the effective stabilizer concentration

decreases, the heat generation rate reaches a peak value.

Based on a comparison of the time-dependent heat evolution curves and the stabilizer depletion curves, it was determined that the heat evolution rate begins to increase when the di-nitrated stabilizer daughter products deplete and reaches a peak some time after the tri-nitrated daughter products have been depleted. This result indicates that the tri-nitrated daughter products are effective stabilizers. Previously, only the 2-NDPA and its di-nitrated daughter products were thought to be effective in stabilizing the propellant and preventing accelerated heat production. Chemical analyses were performed to measure the time-dependent concentrations of GB and GB degradation products in the propellant. These analyses showed that GB degrades in the propellant. The three primary degradation products are isopropyl methylphosphonic acid (IMPA), methylphosphonic acid (MPA), and DIMP. All three of these compounds are known to be hydrolysis products of GB and DIMP is also a common impurity in GB. In order to ensure the safety of personnel performing manual movement and transportation of rockets during storage or disposal operations, the sensitivity of agent-contaminated propellant to impact (for example, as a result of dropping the rocket) was determined using a Bureau of Explosives dropped weight test apparatus. Comparisons were then made to the sensitivity of uncontaminated propellant. These tests showed that the sensitivity of the agent-contaminated propellant is essentially the same as that of uncontaminated propellant.

Tests were conducted to investigate GB vapor diffusion in the propellant at ambient temperature. The diffusion coefficient for agent

M55 Rocket Assessment 10

vapor was determined based on the agent concentration gradient measured in two test configurations: propellant cylinders with agent vapor on the outside and thin propellant disks with agent vapor maintained on one side. The calculated diffusion coefficient was approximately 2.4 × 10-8 cm2/s for both configurations. It was observed that agent is readily absorbed at the surface of the propellant and then diffuses inward.

Figure 11. M55 rocket segments were retrieved from

the TOCDF deactivation furnace feed chute for sampling of M28 propellant.

Additional tests were done to collect data needed for computer models of propellant behavior. Tests were done to determine the thermal conductivity, heat capacity, and heat of combustion for both contaminated and uncontaminated propellant. The laboratory test program provided a wealth of new information on the behavior of M28 propellant following agent contamination. However, there were still important gaps in our understanding of what happens under actual field storage conditions. For that reason, it was important to analyze additional samples of propellant from leaking rockets that had been stored in the field for several years.

Analysis of Leaker Rocket Samples To provide further validation of the laboratory results, propellant samples were taken from 99 M55 GB leaker rockets stored at the Deseret Chemical Depot (DCD) in November 1999. Most of the rockets selected for analysis were detected and overpacked during the early 1970s, but a few were overpacked as late as 1989. Propellant sampling was performed during normal demilitarization of the leaker rockets in the TOCDF. During the demilitarization process, the rockets were sheared such that there were as many as five segments containing propellant. Prior to shearing, the rocket shipping and firing tube had been marked so that the axial position of the propellant segments could be identified. After the segments were cut, staff from the Army’s Technical Escort Unit (TEU) retrieved segments from the deactivation furnace feed gate and took them to a sampling station within the toxic area where samples of the propellant were cut (figures 11 and 12).

Figure 12. Army Technical Escort Unit staff in the toxic area of the TOCDF sampled M28 propellant

collected from leaker rockets.

Propellant samples from each rocket were collected from three locations: one near the aft end of the rocket (near the nozzles), one near the middle of the propellant grain, and one near the forward section of the propellant grain. If a propellant segment had an area that had gelled or softened due to agent contamination, a sample was taken from the gelled or softened area and from the adjacent solid portion of the segment. The fraction of the propellant segment that visually appeared to be gelled or softened was recorded. Each propellant sample was identified with respect to the rocket from which it was taken, the axial location in the rocket, and whether the sample was gelled or solid. The

11 July 2002

propellant samples were then shipped to MRI for chemical and thermal analysis. At MRI, propellant samples were subdivided and the resulting smaller samples were subjected to either chemical or thermal analysis using procedures that were essentially the same as those used in the earlier laboratory test program (PMCD, 2002b). The primary objectives of these tests were to verify model predictions of the propellant behavior following agent contamination and to determine the fraction of the propellant grain affected by agent. The propellant samples were visually inspected upon receipt at MRI. The majority of the samples were similar to uncontaminated M28 propellant. Structural integrity was intact and individual grains could be observed. Other samples were soft, with little structural integrity. These samples, which are referred to as gelled samples, often had liquid present in the plastic sample container with the propellant. These visual observations are consistent with the observations made during the laboratory test program. In the laboratory test program, exposure of the propellant to GB under controlled laboratory conditions produced changes that ranged from little effect on structure (low level of GB exposure) to a complete loss of structural integrity, reducing the propellant to a viscous mass (high level of GB exposure). Tests were conducted to determine the concentrations of stabilizers and NG in the samples, the concentrations of GB and GB degradation products in the samples, and the rate of heat evolution of selected samples at ambient and elevated temperature (65.5°C). The tests indicated significant stabilizer and nitrate ester depletion in at least one propellant sample taken from 16 rockets. In every case, these samples had high levels of agent contamination and, in most cases, the samples had lost physical integrity and were completely gelled. Chemical analysis of these rockets showed that substantial degradation of the absorbed GB had occurred and nearly all of the

organophosporus contamination was from the three GB degradation products—DIMP, IMPA, and MPA. The tests showed that the extent of NG and stabilizer depletion in the propellant depends on the total organophosphorus contamination (GB, DIMP, IMPA, and MPA). The higher the level of contamination, the more pronounced the degradation of the NG and stabilizer in the propellant. It was observed that organo-phosphorus contamination levels of greater than about 10 weight percent are required to show significant reductions in NG and stabilizer. Loss of physical integrity and gelling of propellant occurred when the organophosphorus contamination level was greater than about 10 to 15 weight percent. The average organo-phosphorus content in the gelled samples was 29 weight percent. Non-gelled samples collected in close proximity to these gelled samples were found to have very low organophosphorus concentrations, indicating that agent contam-ination was very localized within the propellant grain. Not surprisingly, depletion of the nitrate esters and stabilizers in the propellant was also very localized. Samples with significantly depleted stabilizers and nitrate esters were found very near to samples with almost no stabilizer or nitrate ester depletion. Microcalorimetric analyses of the contaminated propellant samples were performed to determine heat generation by the propellant. The majority of the analyses were done with the samples maintained at 65.5°C to be consistent with the majority of the results from the laboratory test program. Some samples showed high heat generation rates compared to the uncontaminated control samples. This was particularly true for the gelled samples, some of which showed very high heat generation rates. The measured heat generation rates were comparable to the values measured during the earlier laboratory testing program. The microcalorimetric analysis also showed that the effects of agent were very localized, since samples taken in close proximity

M55 Rocket Assessment 12

to one another showed drastically different heat evolution rates. The results from the leaker sample analyses confirmed the earlier results from the laboratory test program. As in the laboratory test program, agent contamination greatly accelerated propellant degradation and caused an increase in heat evolution by the propellant. A key conclusion from the leaker sample analysis is that the effects of agent are very localized so that only a small fraction of the propellant grain undergoes accelerated degradation. Another important finding was that only a small fraction of the leaking rockets had agent-contaminated propellant. Apparently, the amount of agent leakage was either too small or the nozzle plugs in the aft end of the rocket effectively prevented migration of agent liquid or vapor into the motor section of the rocket. Heat Transfer Testing and Analysis The agent-propellant test program provided a great deal of information that could be used to assess the stability of the M55 rocket. However, in order to assess the probability of rocket autoignition during storage, it was necessary to know the magnitude of heat losses from the rocket under realistic storage conditions. Heat transfer tests were conducted to determine the rocket’s ability to transmit heat to its surroundings. A computer model was also developed to simulate heating of the rocket once stabilizer has become depleted and heat generation has reached a peak.

Heat Transfer Test Program Literature values for free convective heat transfer are available only for simple configurations and a limited range of conditions. Therefore, it was necessary to perform experiments to simulate heat transfer from the rockets under realistic storage conditions.

Heat transfer tests were performed at the Army Research Laboratory (ARL) at Aberdeen Proving Ground, Maryland (Bundy, 1998). The ARL tests used M60 rockets, which are inert surrogates of the M55 chemical rockets. Heat generation in the rockets was simulated using internal heaters, and the rockets and overpack containers were instrumented with thermocouples to obtain the thermal response of the rocket to a range of internal heat generation rates (figure 13). The internal heating rates used in the tests were selected based on the results from the laboratory test program at MRI. Both overpacked and nonoverpacked rockets were tested in a variety of storage configurations (figure 14) and at two different temperatures. Heat loss from the simulated rocket propellant was determined based on the heating rate and

Cross-sectionalView

Side View

SRC or PIG

SFT

M60 HeaterMgO Filler

TCs

Figure 13. The ARL tests were performed on instrumented M60 simulant rockets with internal heaters to simulate

propellant heating.

Rocket-Filled PIGs

Pallet of Rocket-Filled SRCs

B

A

4 ft C

E D

F

Rocket-Filled SRCs

Empty Overpack-Like Shells

Empty SFTs

7 ft

10 ft

Figure 14. Instrumented rockets (indicated by letters A-F)

were arranged to simulate a range of field storage configurations.

13 July 2002

the measured temperature difference between the simulated propellant casing and the ambient temperature. The measured heat loss from the rockets was higher than had been expected based on literature correlations for free convection. This is believed to be due to heat conduction within the propellant casing and within the walls of the steel overpack. In effect, the relatively low internal heating rate allows the heat from the propellant to be conducted to a large portion of the rocket and overpack, increasing the effective surface area for heat transfer to the surrounding air. As expected, the measured heat loss for overpacked rockets was slightly less than for nonoverpacked rockets. This is due primarily to the additional thermal resistance provided by the air gap between the rocket SFT and the overpack container. Also, the heat loss for rockets on the interior of a stack was slightly smaller than for rockets on the periphery of the stack. This is due to the more restricted airflow around rockets on the interior compared to rockets at the periphery.

Thermal Model of Propellant A computer model of the propellant was developed to allow results from the laboratory test program and heat transfer test program to be extrapolated to field storage conditions. The computer model treated internal heat generation in the propellant grain and heat losses from the propellant to the surroundings. The model predictions were used to identify the conditions under which autoignition of the propellant might occur. Calculations using the thermal model showed that, because of the relatively small heat generation rates, heat generated locally in the propellant is readily conducted to other parts of the propellant grain. As a result, it is the total heat generation in the propellant rather than the distribution of heat generation that is the critical parameter in the autoignition analysis. The calculations also showed that autoignition of the propellant is only possible when high heat

generation in the propellant is combined with low heat loss from the propellant surface. The likelihood that these conditions occur simultaneously is the objective of the rocket stability assessment discussed in the next section. Rocket Stability Assessment Stability of the M28 propellant in the M55 rockets has been a concern since the mid-1990s. Initial attempts to assess propellant stability were focused on determining how long the rockets could safely be stored. These studies suffered from a limited understanding of propellant degradation phenomena and the ability of the rocket to transmit heat to its surroundings. The testing and modeling efforts just described have provided a much better foundation for assessing rocket stability. Recent assessments have taken advantage of this new information and have utilized probabilistic modeling techniques to estimate the likelihood of autoignition for both nonleaking and leaking M55 rockets. The following discussion summarizes the previous attempts to determine the safe storage life of the rockets as well as the more recent assessments of the probability of autoignition. To provide perspective on the risk significance of rocket autoignition, the calculated autoignition probability is compared to the probability of other accidental rocket ignition events, such as due to lightning striking a storage igloo.

Previous Safe Storage Life Estimates Data collected during the chemical analysis of Master Field Samples have been analyzed on several occasions to estimate the safe storage life of the M55 rockets. The term safe storage life is used in the management of the weapons stockpile to define the maximum safe storage period for a munition. The munition must be removed from service and retired once the safe storage life is reached. The safe storage life is

M55 Rocket Assessment 14

calculated using conservative assumptions to ensure that the munition is removed from service well before the time at which it would become hazardous in storage. Thus, the safe storage life does not define the time at which autoignition would occur. There was considerable variation among the early safe storage life assessments for the M55 rocket (table 2). These variations were due to a lack of empirical data on M28 propellant and limitations and uncertainties in the safe storage life models. Because of these uncertainties, there was a large variability in the assessments and it was not possible to reliably determine if M55 rocket autoignition was a near-term concern. This uncertainty led to criticism of the Army by outside review organizations such as the NRC and GAO. To remedy this situation, the NRC recommended that the Army undertake a new and definitive study of the M55 rocket propellant. As an initial step to address the NRC and GAO concerns, in 1994 the Army prepared the M55 Rocket Storage Life Evaluation (USACDRA, 1994). This study used data from

the 1993 analysis of the propellant samples stored at ARDEC and included a statistical treatment of the uncertainties in the data. The 1994 evaluation was the most comprehensive study to date, but it suffered from some of the same uncertainties as in the previous analyses. Thus, autoignition was predicted to occur over a range of possible dates from 2013 to 2064.

Expert Elicitation Process Two weaknesses of the storage life evaluations shown in table 2 were the lack of a rigorous treatment of uncertainties and limited consideration of information from outside experts. These weaknesses have been addressed in recent evaluations using a process known as expert elicitation. Expert elicitations were first used extensively in the U.S. Nuclear Regulatory Commission (USNRC) assessment of nuclear power plant risks (USNRC, 1992). Since that time expert elicitations have been used successfully in a wide variety of applications.

SSA

LaBa

Pe

US

Table 2. Summary of Storage Life Evaluations Conducted on Uncontaminated M28 Propellant

tudy Reference Results Evaluation Approach and Limitations IC, 1985 Zero stabilizer in 27

years This was the first assessment of stabilizer life. The predicted storage life was based on ARDEC accelerated aging tests using the limited reaction kinetics data available at the time.

ndrum and czuk, 1990

100 years if stored at 25°C

The authors conducted extensive analysis of reaction kinetics, and developed a model and data for predicting M28 storage life. The model gave credit to stabilizing daughter products.

rry, 1994 0.5% remaining stabilizer by 2008 or 2007

Storage life predictions were presented by MITRE during a meeting involving propellant experts. Two methods were used to estimate when autoignition would be a concern. The first used a log-linear relationship for the time-dependent stabilizer concentration. The second assumed a constant depletion rated based on accelerated aging data (Robertson, 1988) and kinetic data developed by Landrum and Baczuk. Both models assumed autoignition would occur when stabilizer concentration reached 0.5%.

ACDRA, 1994 2013, 2043, or 2064 A panel of propellant chemistry experts recommended a model similar to

for 1 in 1 million chance of autoignition

that used by Landrum and Baczuk. The model was applied using the most recent propellant test data and included a quantitative analysis of the uncertainties. A stabilizing capacity factor was included to account for the effect of stabilizer daughter products. Three storage life estimates were developed based on three different capacity factors.

15 July 2002

The expert elicitations used for this assessment followed a structured process that is similar in many ways to the process developed during the USNRC study. This structured process is designed to utilize the available expertise in a manner that accurately reflects the collective uncertainty of the experts. It is useful for estimating parameter values for cases in which testing and analysis cannot provide a definitive value. Although the details of the elicitation process vary from one application to the next, every elicitation shares the following general steps: 1. Selection of experts to represent a cross-

section of relevant backgrounds and experience.

2. Detailed review of available technical information by the experts and open technical discussion of this information.

3. Training of the experts on techniques for developing probability distributions.

4. Development of a probabilistic framework for treating model uncertainty.

5. Elicitation of uncertainty distributions from the technical experts.

6. Application of the uncertainty distributions to the probabilistic model developed by the experts (in step 4).

Experts in the field of propellant chemistry and heat transfer were assembled for the expert elicitations in this study. Information packages were prepared and distributed to the experts 1 to 2 weeks prior to each of the elicitation meetings. These packages included a brief discussion of the issue to be addressed, a summary of how the results of the elicitation will be used, and a summary of the available information relevant to the propellant autoignition issue. This information provided a basis for the subsequent open discussion of data sources, models, and methods for analysis. At the beginning of each elicitation meeting, presentations of available information relevant to the subject of the elicitation were made. These presentations summarized the available test data and any model results. During the presentations, the experts were encouraged to

provide their opinions regarding the validity and accuracy of the data and models. The experts were also asked to identify strengths and weaknesses in the knowledge base, with special attention devoted to sources of uncertainty. The discussion was facilitated to ensure that all of the experts had adequate opportunity to express their views. Experts in various fields of science are usually not trained in probability theory and its application to risk analysis, and sometimes have difficulty thinking in probabilistic terms. For these reasons, probability training was provided at the start of each elicitation. During the training, the technical experts were exposed to various techniques for developing probability distributions and the problems that are often encountered when these techniques are applied to uncertain scientific issues. The technical experts were asked to list the most important factors that influence the probability of rocket autoignition. Potential modeling approaches were then proposed that address these factors. Subsequent discussion focused on developing a consensus approach upon which all experts could agree. A consensus model was developed, and the experts provided probability distributions for each uncertain parameter in the model. The experts were elicited to provide their individual probability distributions and supporting rationale. After the individual elicitations were completed, the experts’ individual distributions were combined into a single probability distribution. The resulting probability distributions were applied to the probabilistic model the experts had developed earlier in the elicitation process. The initial calculations with the model and the results from the analysis were provided to the experts for their review before the elicitation results were finalized. Separate expert elicitations were conducted to address autoignition of nonleaking rockets and leaking rockets. Experts were selected for their expertise in propellant chemistry or heat transfer. The following discussion summarizes

M55 Rocket Assessment 16

the results from the expert elicitations and outlines how this information was used to estimate the autoignition probability for nonleaking and leaking rockets.

Autoignition of Nonleaking Rockets An expert elicitation on autoignition of nonleaking rockets was held in September 1996 (PMCD, 1997a). During the elicitation, the expert panel developed a model for predicting autoignition of uncontaminated M28 propellant and then developed uncertainty distributions for the parameters of the model. The model was based on a model proposed earlier by Baczuk (Perry, 1994). The key factors considered by the experts in their analysis included: the stabilizer depletion rate, the effectiveness of the stabilizer daughter products, and the probability of autoignition as a function of the stabilizer concentration. Periodic chemical analyses of propellant samples taken from the field have shown that the concentration of 2-NDPA stabilizer in the propellant is depleting very slowly over time. Slow depletion of the 2-NDPA stabilizer has also been confirmed in accelerated aging tests of the propellant performed at elevated temperatures. The expert panel developed their uncertainty distributions for the stabilizer depletion rate based on results from the accelerated aging tests. The panel concluded that the di-nitrated stabilizer daughter products were likely to be effective stabilizers, but that more nitrated daughter products probably were not very effective. This elicitation was conducted before the results from the agent-propellant testing program became available. As already discussed, the agent-propellant testing program showed that the tri-nitrated stabilizer daughter products are also effective. The experts also concluded that the effective stabilizer concentration must drop well below 0.2 weight percent for autoignition to be likely. They also noted that autoignition may not occur even when the effective stabilizer is completely gone, since heat losses may be more than

adequate to compensate for heat generation within the propellant. (As discussed earlier, this assumption was later verified in the heat transfer tests conducted at ARL.) An analysis of the autoignition probability was completed using the expert panel’s uncertainty distributions. The results from this analysis indicated that the autoignition probability is negligible and the rockets could be safely stored for well over 100 years. This result is consistent with the analysis performed previously by Landrum and Baczuk (1990). It is also consistent with storage life estimates calculated based on the SIP test results (Geo-Centers, 1996), which indicated that the rockets could be stored for over 230 years.

Autoignition of Leaking Rockets An expert elicitation on autoignition of leaking M55 rockets was held in November 1998. This elicitation, which involved experts in propellant chemistry and chemical analysis, followed an approach similar to that used in the previous elicitation on autoignition of nonleaking rockets. All members of the expert panel had been involved in the previous elicitation and were familiar with the elicitation process. The experts identified the following key parameters governing autoignition of the propellant: the peak heat generation rate in the propellant, the time at which the peak occurs, the fraction of the propellant significantly affected by agent, and the magnitude of the heat loss from the rocket to the ambient air in the igloo. The propellant chemistry experts did not feel qualified to provide input on the magnitude of the heat loss from the stored rockets, so a second elicitation was held in December 1998 involving experts in heat transfer testing and analysis. The experts also did not feel that they had adequate information to provide their judgment on the fraction of propellant affected by agent. This lack of information was addressed by the subsequent collection and analysis of samples from leaking rockets at DCD (discussed earlier).

17 July 2002

During the November 1998 expert panel meeting, the experts were elicited on parameters related to the time of peak heat generation and the magnitude of the heat generation rate. They provided probability distributions for the time and magnitude of peak heat generation at 65.5°C (150°F), the temperature used in the majority of the agent-propellant tests. They also provided distributions on the activation energy associated with the key chemical reactions. The activation energy was used to extrapolate the elevated temperature results to ambient conditions. During the December 1998 elicitation, the heat transfer experts provided probability distributions for the effective heat transfer coefficient for heat loss from the rocket. The effective heat transfer coefficient characterizes heat loss from the rocket propellant to the air outside the rocket. It includes the effects of heat conduction through the rocket motor casing, fiberglass SFT, overpack container, and intervening air gaps, and natural convection to the air outside the SFT or overpack container. Both expert panels treated overpacked and nonoverpacked leaking rockets separately. The propellant chemistry experts believed that the overpack containers provide confinement of NOx, which could significantly increase the peak heat generation rate and the decrease the time before the peak occurs. The heat transfer experts thought that the overpack containers would reduce heat losses from the propellant, although they recognized that this effect was not likely to be very significant. The autoignition probability for M28 propellant depends on the magnitude of the peak heat generation rate, the magnitude of the corresponding heat losses from the propellant, and the time at which the peak heat generation occurs. Autoignition will not occur if heat losses from the rocket are sufficient to keep the propellant from being heated to the point that it ignites. A computer model was developed that calculates the probability of autoignition (PMCD, 2002a). The model predicts the time after agent exposure at which heat generation in the propellant

reaches a peak. The model then calculates the likelihood that the peak heat generation rate exceeds the rocket’s ability to transmit heat to its surroundings. For leaking rockets, the time of the peak heat generation in the propellant stored at ambient conditions was determined from the experts’ probability distributions. This time is the critical period for propellant autoignition. The mean time to peak heat generation for contaminated propellant was calculated to be approximately 15 to 22 years after agent contamination. This estimate is consistent with the chemical and thermal analysis of propellant taken from overpacked leaking rockets at TOCDF. Sixteen of the 99 rockets that were sampled at TOCDF had portions of propellant with significantly depleted levels of nitrate esters and stabilizers. These rockets had been overpacked for periods ranging from 11 to 32 years. Autoignition is assumed to occur if heat generation in the propellant exceeds heat losses from the propellant to the surroundings. Factors that influence the likelihood that this will occur include the magnitude of the peak heat generation rate, the fraction of the propellant that reaches this peak, and the magnitude of the heat loss from the rocket. The experts’ probability distributions for the magnitude of the peak heat generation rate and magnitude of the heat loss from the rocket were used in the computer model. A probability distribution for the fraction of the propellant reaching the peak was developed based on the analysis of propellant samples taken from leaking rockets at TOCDF. Analysis of the 99 leaking rockets at TOCDF showed that only 16 had agent contamination sufficient to produce accelerated degradation. Of these rockets, only a small portion of the propellant was significantly affected by agent. In most cases, only the samples that had gelled showed significantly depleted stabilizers and nitrate esters. Based on visual observations of the propellant recorded when the samples were taken, it was possible to estimate the fraction of the propellant

M55 Rocket Assessment 18

grain represented by the samples that had accelerated degradation. A probability distribution for the fraction of the propellant grain affected by agent was developed. The median fraction of the propellant affected by agent was only 0.08 (8 percent) and the maximum fraction affected was less than 0.5 (50 percent). Because agent affects only a small fraction of the propellant grain, the total heat generated in the propellant when the stabilizer depletes is limited. The autoignition probability generally increases with time as more rockets leak, the stabilizer in these rockets depletes, and internal heat generation in the propellant increases. For that reason, it was necessary to calculate a time-dependent autoignition probability for each group of rockets that leak during a given year. The total autoignition probability for a given year was then determined by summing the contributions from rockets that leaked in prior years. Because the leakage experience with M55 GB rockets differs greatly from site to site, site-specific estimates of the autoignition probability were calculated. The median estimates of the autoignition probability for overpacked and

nonoverpacked leaking rockets are shown in table 3. The probabilities for other potential accidents involving rockets are also shown in the table. Table 3 shows that the calculated autoignition probabilities for overpacked rockets are small compared to the probabilities of other postulated accidental rocket ignition events such as due to a lightning strike or an earthquake. Lightning striking an igloo could cause an arc that ignites a rocket. An earthquake could cause an ignition of one or more M55 rockets if the pallet stacks in the igloo fall due to ground motion. For all stockpile sites, the autoignition probability for overpacked rockets was much less than 1 percent of the total probability of other accidental ignition events. The risk significance of an overpacked rocket ignition is further reduced by considering that ignition of one overpacked rocket is unlikely to create a fire that would cause adjacent overpacked rockets or other overpacked munitions to ignite. The lower probability of propagation for overpacked rockets is due primarily to the thermal protection provided by the overpack containers.

Table 3. Comparison of Autoignition Probabilities to the Probabilities for Other Accidental Ignition Events

Site

Overpacked Rocket

Autoignition Probability a

(Probability/Year)

Nonoverpacked Rocket

Autoignition Probability a

(Probability/Year)

Lightning Initiation

Probability b

(Probability/Year)

Earthquake Initiation

Probability b (Probability/Year)

Anniston 3 × 10-5 1 × 10-6 2 × 10-3 1 × 10-4

Umatilla 1 × 10-5 6 × 10-7 6 × 10-4 8 × 10-4

Blue Grass 1 × 10-5 5 × 10-7 2 × 10-3 2 × 10-4

Pine Bluff 1 × 10-5 2 × 10-7 5 × 10-3 3 × 10-4

Notes: a Autoignition probabilities change with time. The values shown were calculated for the year 2002. Although

the autoignition probability may be slightly higher than this in future years, it always remains well below the lightning or earthquake initiation probabilities.

b Lightning and earthquake initiation probabilities are from the following quantitative risk assessments (QRAs) for the Army’s chemical agent disposal facilities and storage sites: for Anniston (SAIC, 1997c), for Umatilla (SAIC, 1996), for Blue Grass (SAIC, 1997a), and for Pine Bluff (SAIC, 1997b). These numbers may change slightly as the QRAs for these sites are updated.

19 July 2002

Table 3 shows that the calculated autoignition probability for nonoverpacked leaking rockets is even smaller than that calculated for overpacked rockets. This is due to the lower expected internal heat generation rate in the propellant and the higher heat loss from the rocket to its surroundings. For all stockpile sites, the calculated autoignition probability for nonoverpacked leaking rockets was less than 0.1 percent of the total probability of other accidental ignition events. Although rocket autoignition is no longer considered to be a major concern, the M55 rockets remain the dominant contributor to the health risks due to continued storage of the chemical stockpile. For example, previous quantitative risk assessments have shown that the rockets make up 90 percent or more of the total risk due to storage (SAIC, 1996; 1997a; 1997b; and 1997c). Other Rocket Assessments In addition to the propellant, the M55 rocket contains other explosive components such as the two explosive bursters in the warhead section, and the detonator and booster in the fuze section. These components could degrade in storage or be affected by agent contamination. Studies have been done to examine the safety of long-term storage of these components.

The first such study to examine these other components was the independent evaluation of the M55 rocket completed in 1985 by the Army Materiel Systems Analysis Activity (AMSAA, 1985). For the AMSAA study, 94 rockets were randomly selected from the stockpile and disassembled onsite. The separated components were then sent to various Army laboratories for analysis. Each laboratory performed standard tests on the components, with the primary objective of determining the amount of degradation the component had undergone after 20 to 25 years in storage.

Figure 15. Studies have shown that accidental rocket ignition caused by lightning is a much greater concern

than autoignition. (Photograph courtesy of Dave Crowley, www.stormguy.com.)

The results from the AMSAA evaluation of agent leakage from the warhead were mentioned previously, as were the propellant stabilizer analyses conducted at ARDEC as part of the AMSAA assessment. In addition to these results, the AMSAA assessment found that the burster material in the rockets had not degraded to any measurable extent since the rockets were produced. It was theorized that agent contamination could cause production of highly sensitive metal-organic compounds; however, it was concluded that the quantity of such compounds that could be generated would be unlikely to create a hazardous condition. The AMSAA assessment also concluded the explosive components of the fuze assembly have not degraded during storage. In 1996, the Army completed a detailed study of the chemical reactions that could occur between agent and the explosive components of the M55 rocket (PMCD, 1996). This study began with a literature review that generated a list of chemical compounds that could form if agent were to contact the explosive components in the warhead or fuze. Seven compounds were identified that could potentially increase the sensitivity of the rocket to handling or processing accidents. The hazards due to formation of these compounds were assessed by estimating the likelihood that the compound would form and the severity of the potential hazard if it did. It was found that either the compound was very unlikely to form or that, if it formed, it would not be sensitive or energetic enough to cause the

M55 Rocket Assessment 20

rocket to explode. Based on this assessment, it was concluded that there are no significant hazards due to agent interactions with explosive components in the rocket Risk Reduction Plans The Army has implemented or is currently evaluating ways to reduce the risks associated with storing the M55 rockets. These efforts are focused on ways to reduce or eliminate risks due to lightning strikes or earthquakes. The following discussion outlines risk reduction measures that have already been implemented and others that are being considered. The metal reinforcement (rebar or wire mesh) used in the construction of the igloos could provide substantial protection for the stored rockets against the effects of lightning. To have this protective effect, the metal reinforcement must provide an electrically conductive path that effectively disperses the lightning’s electric charge and conducts the charge to ground. A structure that does this is sometimes referred to as a “Faraday cage.” For effective lightning protection, all metal in or attached to the igloo structure must be well bonded to all other metal in the structure and the metal structure must be electrically grounded. Initial inspection of the M55 rocket igloos indicated that the intrusion detection system (IDS) in each igloo was electrically bonded to the igloo’s lightning protection system but not to the metal reinforcement (steel rebar or wire mesh) in the igloo structure. Because the IDS is not electrically bonded to the entire igloo structure, arcing may occur inside the igloo due to the large voltage difference between the IDS conduit and metal reinforcement. Electrical arcing inside the igloo could cause an ignition of one or more of the stored rockets. To remedy this situation, the IDS conduit in each rocket igloo has been electrically bonded to the metal reinforcement. The igloos at the three eastern sites with rockets (Anniston, Blue Grass, and Pine Bluff) have

been tested to determine the lightning protection provided by each igloo. These tests have shown that some igloos provide poor lightning protection. In those igloos, arcing could occur that would ignite a rocket if the rocket is too close to the igloo wall. The required safe standoff distance has been determined for each rocket igloo and rocket pallets that violate this safe standoff distance have been identified. Two options are being considered to reduce the lightning risk associated with the rocket pallets that are too close to the igloo wall. One option is to move the rocket pallets away from the wall, while the other is to insert dielectric barriers between the rocket and the wall. The dielectric barriers help to shield the rockets against lightning. Neither of these options is a perfect solution. In order to gain access to the few rocket pallets that are too close to the wall, many other pallets must be moved. The risk associated with these pallet movements must be weighed against the risk reduction from moving the pallets away from the wall. Rocket pallet movement may also be needed in order to insert the dielectric barriers between the pallets and the wall. In addition, scratches or other defects in the dielectric barriers could significantly reduce the barrier’s effectiveness. The Army is currently evaluating these options. The risk associated with accidental rocket ignition during an earthquake can be reduced by stabilizing the stacks of rocket pallets, thus preventing pallets from falling. Options currently being evaluated by the Army include reducing the stack height to a maximum of four pallets (some stacks are five pallets high), banding the pallets together, or bracing the pallet stacks so that they cannot tip over. Whether or not a decision is made to implement risk reduction at a specific site will depend on the current schedule for destruction of the rockets at that site. Changes may not be made if near-term destruction of the rockets is planned.

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Conclusions The Army has completed an extensive program of testing and analysis to assess the safety of M55 rockets in storage. The Army program focused primarily on the stability of the M28 propellant in the rocket motor, but also addressed issues related to GB agent leakage from the warhead and the effects of leaked agent on the explosive components of the rocket. The Army studies showed that the propellant in nonleaking rockets is degrading very slowly and would be stable for well-over 100 years under normal storage conditions. The Army studies also showed that agent contamination greatly accelerates degradation of the propellant. After the propellant has become contaminated with agent, it could theoretically reach an unstable condition within 15 to 20 years. However, a number of factors cause this to be extremely unlikely. Most importantly, examination of propellant samples from leaking rockets has shown that only a small portion of the propellant grain becomes contaminated with agent. In addition, tests have shown that heat losses from the stored rockets are sufficient to balance the increase in heat generation that occurs as the rockets become unstable. Although autoignition of the leaking rockets is not impossible, it is much less likely than ignition due to other accidental causes, such as lightning striking a storage igloo. Studies have also shown that the M55 rocket stockpile is not becoming more prone to agent leakage as the rockets age. It was also found that the current Army surveillance program is effective at meeting its objectives to minimize both the potential for worker exposure to agent and agent release to the environment. Although propellant stability and agent leakage are not major concerns, the M55 rockets remain the dominant contributors to the health risks associated with continued storage of the chemical weapon stockpile. The Army is currently evaluating ways to reduce the risk

associated with storage of the rockets; however, timely destruction of these munitions remains the best way to ensure the safety of the public and the environment. Acronyms/Abbreviations 2-NDPA 2-nitrodiphenylamine AMC Army Materiel Command AMSAA Army Materiel Systems Analysis

Activity ANCA Anniston Chemical Activity ARDEC Armament Research,

Development and Engineering Center

ARL Army Research Laboratory BGCA Blue Grass Chemical Activity DA Department of the Army DARCOM Development and Readiness

Command DCD Deseret Chemical Depot DIMP diisopropyl methylphosphonate ECBC Edgewood Chemical Biological

Center ERDEC Edgewood Research,

Development and Engineering Center

GAO General Accounting Office GB nerve agent, Sarin GC gas chromatography GC-FPD gas chromatography-flame

photometric detection HPLC high performance liquid

chromatography IC ion chromatography IDS intrusion detection system IMPA isopropyl methylphosphonic acid JI Johnston Island MPA methylphosphonic acid MRI Midwest Research Institute

M55 Rocket Assessment 22

NC nitrocellulose NG nitroglycerin NOx nitrogen oxides NRC National Research Council PBCA Pine Bluff Chemical Activity PIG modified M1 gas identification set

container PMCD Program Manager for Chemical

Demilitarization ppb parts per billion ppt parts per trillion QRA quantitative risk assessment SAIC Science Applications International

Corporation SEC size exclusion chromatography SFT shipping and firing tube SIP safe interval prediction SMI Storage Monitoring Inspection SRC single round container TC thermocouple TEU Technical Escort Unit TOCDF Tooele Chemical Agent Disposal

Facility UMCD Umatilla Chemical Depot USACDRA U.S. Army Chemical

Demilitarization Remediation Activity

USNRC U.S. Nuclear Regulatory Commission

VX nerve agent References Army Materiel Systems Analysis Activity (AMSAA), Independent Evaluation/Assessment of Rocket, 115 mm: Chemical Agent (GB or VX), M55, Aberdeen Proving Ground, Maryland, October 1985. Battelle, M55 Rocket Propellant Analysis, Final Report, Contract No. SPO900-94-0002, Task 94, Columbus, Ohio, September 1996.

Bundy, M., Investigating Worst-Case Heat Transfer Conditions in the Storage of Unstable Solid Rocket Fuel, Army Research Laboratory, Aberdeen Proving Ground, Maryland, November 1998. Cain, T., et al., Master Action Plan for M55 Rockets, MP 95W0000121, Mitretek Systems, Inc., September 1996. Department of the Army (DA), Inspection of Supplies and Equipment Ammunition Surveillance Procedures, Supply Bulletin SB 742-1, Washington, DC, November 1990. Fitzgerald, G., et al., M55 Rocket Propellant Test Report, Phase II: Agent Study, ERDEC-TR-353, ERDEC, Aberdeen Proving Ground, Maryland, August 1996. General Accounting Office (GAO), Chemical Weapons: Stability of the U.S. Stockpile, GAO/NSIAD-95-67, Washington, DC, December 1994. Geo-Centers, Inc., Safe Interval Prediction Testing, GC-TR-2625-033, Newton Center, Massachusetts, August 1996. Landram, G., and R. Baczuk, Propellant Degradation Kinetics from Experimental Temperature, Time, and Concentration Data, ARDEC Contractor Report ARAED-CR-90005, Hercules Aerospace Corporation, Magna, Utah, June 1990. National Research Council (NRC), Recommendations for the Disposal of Chemical Agents and Munitions, National Academy of Sciences, Washington, DC, 1994. Perry, R., “Minutes for the M28 Propellant Stability Meeting of 12-13 July 1994,” Memorandum SFIL-CMS(50q), August 16, 1994. Program Manager for Chemical Demilitarization (PMCD), Evaluation of Potential Hazards of Chemical Agent-Contaminated M55 Rocket Explosive Components, Aberdeen Proving Ground, Maryland, January 1996.

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PMCD, Report of the Expert Elicitation on Autoignition of Nonleaking Rockets, Aberdeen Proving Ground, Maryland, May 1997a. PMCD, Assessment of the Storage Monitoring Inspection Program for M55 GB Rockets, Aberdeen Proving Ground, Maryland, May 1997b. PMCD, Preparation and Analysis of Laboratory Prepared GB-Contaminated M28 Samples – Final Test Report, Aberdeen Proving Ground, Maryland, October 2000. PMCD, Assessment of the Stability of M55 Rockets in Storage, Aberdeen Proving Ground, Maryland, March 2002a. PMCD, Preparation and Analysis of M55 Leaker Rocket Propellant Samples – Final Test Report, Aberdeen Proving Ground, Maryland, March 2002b. Robertson, D., T. Richter, and N. Slagg, M55 Assessment Program, ARAED-TR-88005, ARDEC, Picatinny Arsenal, New Jersey, May 1988. Science Applications International Corporation (SAIC), Probabilities of Selected Hazards in Disposition of M55 Rockets, SAIC-84/177QB, April 1985. SAIC, Umatilla Chemical Agent Disposal Facility Phase 1 Quantitative Risk Assessment, SAIC-96/2601, Abingdon, Maryland, September 1996. SAIC, Blue Grass Chemical Agent Disposal Facility Phase 1 Quantitative Risk Assessment, SAIC-96/1118, Abingdon, Maryland, January 1997a. SAIC, Pine Bluff Chemical Agent Disposal Facility Phase 1 Quantitative Risk Assessment, SAIC-96/1120, Abingdon, Maryland, March 1997b. SAIC, Anniston Chemical Agent Disposal Facility Phase 1 Quantitative Risk Assessment,

Rev. 1, SAIC-97/2620, Abingdon, Maryland, May 1997c. SAIC, Statistical Analysis of the Leakage History for Anniston M55 GB Rocket Lot 1033-45-181, Abingdon, Maryland, March 2002. U.S. Army Chemical Demilitarization and Remediation Activity (USACDRA), M55 Rocket Storage Life Evaluation, Aberdeen Proving Ground, Maryland, December 1994. U.S. Nuclear Regulatory Commission (USNRC), Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants, NUREG-1150, Washington, DC, 1992. Wachter, C., et al., M55 Rocket Stockpile Assessment: Evaluation of M28 Propellant Stabilizer Content, ARAED-TR-86008, ARDEC, Picatinny Arsenal, New Jersey, July 1986. Where to Find Out More

About the M55 Rockets The various technical elements of the M55 rocket stability studies have been documented in a series of technical reports, all of which are listed in the references. The Assessment of the Stability of M55 Rockets in Storage (PMCD, 2002a) is the technical report that summarizes and combines the technical information as part of the autoignition probability assessment. Other detailed technical reports contain documentation of the testing performed. These reports and a videotape that summarizes the Army’s M55 rocket stability studies can be obtained from the PMCD Public Outreach and Information Office.

About the Program General information concerning the Army’s chemical stockpile disposal program is also available from the PMCD Public Outreach and Information Office.

M55 Rocket Assessment 24

The mailing address and phone number for the PMCD Public Outreach and Information Office are:

Public Outreach and Information Office Program Manager for Chemical Demilitarization Attn: SFAE-CD-P Building E4585 Aberdeen Proving Ground, MD 21010-4005 (800) 488-0648

25 July 2002