End of an Era for the Los Alamos Critical Experiments Facility: History of critical assemblies and experiments (1946–2004)

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Annals of Nuclear Energy 33 (2006) 1339–1359

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NUCLEAR ENERGY

End of an Era for the Los Alamos Critical Experiments Facility:History of critical assemblies and experiments (1946–2004)

David Loaiza *, Daniel Gehman

Los Alamos National Laboratory, P.O. Box 1663, J562, Los Alamos, NM 87545, United States

Received 20 September 2006; accepted 29 September 2006Available online 4 December 2006

Abstract

The Los Alamos Critical Experiments Facility (LACEF) was the last operational, general-purpose, critical-mass laboratory in theUnited States. The long history of remote operations and large-scale critical-mass experiments at LACEF began in 1948, and it effectivelyended in July 8th, 2004, when the last critical experiment was performed on the Planet critical assembly. The experimental activities at thePajarito Site began in April 1946 as a way to obtain subcritical measurements for weapons safety guidance. A year later, the first Kiva(a concrete-reinforced building) was constructed, and 18 months afterward the first remote critical operation was reported with the Topsycritical assembly. In the early years, the Pajarito Site primarily supported the weapons program; later, for almost 17 years, the neutronicsof the Rover nuclear-propulsion program dominated activities at Pajarito Site. More recently, Pajarito Site added some new dimensionsto its operations in order to support emergency response, the Nuclear Criticality Safety Program, and radiation-detection development.The long history of critical-assembly measurements and operations is documented in hundreds of peer-reviewed technical papers, lab-oratory reports, personal files, and video sessions with some of the pioneers. It is the intent of this paper to capture, in one single doc-ument, a summary and the highlights of the glorious days of this facility. In essence, this paper is a summary of the programs conductedin the last 58 years and of the numerous critical assemblies and reactors that operated at LACEF. It also provides a list of references tothe reader who might want to learn more about this facility’s rich history.Published by Elsevier Ltd.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13402. Assemblies and programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1341

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2.1. Topsy (1948–1955) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13412.2. Elsie (1950s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13412.3. Little Eva (1950s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13422.4. Comet (1950s–2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13422.5. Planet (1950s–2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13432.6. Lady Godiva (1951–1957). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13442.7. Heavy-Water Solution Assembly (1952–1956) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13442.8. Jezebel (1954–1977) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13452.9. Hydro (1957–1959) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345

2.10. Rover rocket-propulsion program (1955–1972) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13462.11. Honeycomb (1956–1990) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13462.12. Supercomet (late 1950s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1346

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rresponding author. Tel.: +1 505 667 4936; fax: +1 505 665 3657.ail address: [email protected] (D. Loaiza).

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1340 D. Loaiza, D. Gehman / Annals of Nuclear Energy 33 (2006) 1339–1359

2.13. Godiva II (late 1950s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13472.14. Flattop (1958–2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13472.15. Mars (late 1950s–1988). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13482.16. Modular high-temperature gas-cooled reactor (1960s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13492.17. Venus (late 1950s–1962) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13492.18. Kiwi (1959–1968). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13492.19. Dumbo (early 1960s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13512.20. Moly-Godiva (late 1950s–1964) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13512.21. Phoebus (1965–1968) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13512.22. Pewee (1968–1971) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13522.23. Nuclear Fuel Furnace (1972) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13532.24. Godiva III (late 1950s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13532.25. Godiva IV (early 1960s–2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13532.26. Big Ten (early 1960s–1994) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13532.27. Los Alamos Molten Plutonium Reactor Experiment (1961–1963) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13542.28. Plasma Cavity (1972–1979) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13552.29. Parka (1972–1979) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13552.30. Kinglet (1973–1974) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13562.31. SHEBA (1980–2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13562.32. SKUA (1980s–1996). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1357

3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1357Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1358References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1358

1. Introduction

Through mid 1945, the Los Alamos Scientific Labora-tory pursued a strategy of overlapping approaches in itsprograms to test and refine critical mass and performother calculations affecting bomb design and deployment.These activities included work in subcritical and criticalassemblies, nuclear constants, and other measurements.Aside from a full-scale test, there was no means to deter-mine precisely the extent of supercriticality achieved in agun or implosion-assembly device or to measure otherchain-reaction properties such as the neutron populationgrowth rate. However, model experiments with subcritical(keff 6 0.95), near-critical (0.95 < keff < 1), and critical(keff P 1) components could confirm theoretical estimates.Aqueous solutions, hydride, metal pseudospheres, hemi-spheres, and building blocks were used for these experi-ments. Thus, critical experiments at Los AlamosNational Laboratory (LANL) began with the constructionof the first water-boiler reactor at the Omega Site. Thewater-boiler reactor attained criticality in May 1944,beginning an era of criticality studies (Paxton, 1981). In1946, following a fatal criticality accident at the OmegaSite involving Harry Daghlian, criticality experimentswere moved to Pajarito Site, also known as TechnicalArea 18 (TA-18). Thus Pajarito Site became the home ofthe Los Alamos Critical Experiments Facility (LACEF).Pajarito Site was originally used by the RadioactivityGroup because of its outlying location, in order to avoidradiation background arising from other research activi-ties. It had also been used as an explosives firing site for

diagnosing implosion methods before becoming a criti-cal-mass laboratory (Paxton, 1981).

The site began hands-on subcritical experiments in April1946. Unfortunately, one of these hands-on subcriticalexperiments inadvertently went critical and caused thedeath of Louis Slotin. As a result of the accident, allhands-on operations were terminated, and future near-crit-ical and critical experiments were remotely operated. Theconstruction of the first Kiva, a concrete-reinforced build-ing constructed to house the experimental assemblies, wascompleted in 1947. Ironically, the last critical-mass experi-ment at Pajarito Site was also performed in this same Kiva,56 years later. Two additional experimental bays (Kivas 2and 3) were constructed in the 1950s. The critical assem-blies housed in the Kivas were operated remotely fromthree distinct, but grouped, control rooms located a quar-ter of a mile away from Kivas 1 and 2 and somewhat lessdistant from Kiva 3. In the 1990s, the names for the exper-imental bays were changed from Kivas to CASAs (CriticalAssembly and Storage Areas) to promote a more politicallycorrect environment.

The Kivas were constructed with reinforced concreteand masonry blocks. Each had a traveling crane in themain assembly room. The Kivas also had vault-type struc-tures to store active materials. In accordance with operat-ing procedures, a system of radiation-level detectorspermanently installed in each Kiva served the roles ofscram actuation and operational instrumentation. EachKiva was surrounded by an exclusion area that had to bevacated before remote operations could be performedand by blinking beacons that forewarned the beginning

D. Loaiza, D. Gehman / Annals of Nuclear Energy 33 (2006) 1339–1359 1341

of a critical operation. When the gate to the exclusion areawas open, unintentional operation was prevented by inter-locks and key-actuated switches that required the same‘‘captive’’ key for supplying power to the assemblies andfor opening the gate.

The Los Alamos Critical Experiments Facility was bornwith a myriad of documentary requirements, imposed as anatural reaction to the two fatal hand-assembly accidents.The scope of operations and general procedures weredefined by operating regulations from which no departurewas permitted. Supplementary procedures that were spe-cific to a particular experiment were covered by anapproved Experimental Plan. As experience accumulatedand demands for critical experiments broadened, proce-dures evolved to become less rigid but more effective. Even-tually, as a result of this evolution, the procedures weregenerated by the principal investigator, concurred with bythe criticality safety group, and approved by higher-levelmanagement. In general, content changes in the procedureswere consistent with the development of a safety culture inthe laboratory, and the most important function of theprocedures was to protect the people who handled the fis-sile material. Safety was considered to be one of the goalsof design and operation, not something superposed.

A large number of remote critical experiments have beenperformed over the last 56 years at the LACEF. The exper-iments used numerous fissile materials, assemblies, andreactors. The experimental programs and operations couldbe categorized in any of several ways—such as by chrono-logical order, by their technical characteristics and objec-tives, or by their program. We have chosen to presentthis summary of the critical assemblies and reactors inchronological order to give a perspective of the evolutionof the programs. A short summary is provided for eachassembly, reactor, or program. For a more detaileddescription of the Pajarito Site programs in the early years,the reader is directed to Paxton (1981, 1983). This docu-ment is only meant to provide a brief discussion of the crit-ical assemblies, reactors, and program highlights for thelast 56 years.

2. Assemblies and programs

The earliest experiments performed at Pajarito Site sup-ported the weapons development programs that took placebetween 1946 and 1955. The first remotely controlled crit-ical assembly (a bomb mockup) was designed to be similarin size to Fat Man, the weapon dropped on Nagasaki. Thismockup was built as a vertical-lift machine. The experi-ment was devised to demonstrate the safety of implosion-weapon components, confirm the intended reactivity ofproduction cores, and provide guidance for new implosiondesigns (Paxton, 1983). Later, the site transitioned to sup-port programs related to basic measurements of nuclearparameters, kinetic behavior of chain-reacting systems,and nuclear-propulsion programs. In recent years, LACEFhas also supported the testing of radiation detectors,

nuclear criticality training, neutron multiplication studies,and emergency response training, in addition to supportingthe Nuclear Criticality Safety Program.

2.1. Topsy (1948–1955)

Topsy reached the critical state in the fall of 1948, about18 months after Kiva 1 became operational. Topsy wasdesigned to provide the first information of its kind onfast-neutron fission chains in a readily computable system(to check detailed weapon calculations). Super-prompt-crit-ical pulses that had previously been performed were toofleeting for neutronic experiments, and the early criticalmockups of a mercury-cooled fast reactor at another labo-ratory site were too complex for reliable calculations. Thus,Topsy became the first of a series of assemblies to providefast-neutron data for checking the powerful and simpli-fied computational techniques that relied upon high-speedmachines being developed at the time. The core was a pseud-osphere (1/2-in. steps on the surface) made of 94 wt% U-235reflected by 9 in. of natural uranium. In 1955, Topsy opera-tions were terminated, and the machine was replaced by theFlattop machine (Paxton, 1983).

Topsy was designed such that the carriage that held thecore portion of the assembly contained a hydraulic lift thatmoved between a safety-test section (a water tank) and thecritical-assembly section. Before a criticality experimentwas performed, the core unit was positioned within thewater tank. In order to achieve criticality, the core wasraised into the reflector cavity. Other smaller cavities withinthe reflector accommodated control rods. A radial ‘‘gloryhole’’ provided access for internal measurements, such asfor foils for neutron activation and fission chambers forRossi-a measurements (Paxton, 1983). Some of the experi-ments performed on Topsy were a parallelepiped coreshape, a uranium core with reduced average density or con-centration, and a spherical core of plutonium (White,1953). Fig. 1 shows an archival photograph of Topsy witha natural uranium reflector.

2.2. Elsie (1950s)

The Elsie critical-assembly machine was utilized forexperiments in the 1950s. The applications for Elsieincluded the testing of gun-type weapons that were lessmassive than Little Boy. The core was a split cylinderof 93 wt% U-235, the upper portion secured in a verticalreflector cylinder with a cap. The reflector consisted ofvarious materials of interest to weapons programs. Thecore mass of Elsie was adjusted to reach the critical statewhen both parts of the core came into contact. Horizon-tal control rods entered the reflector immediately abovethe upper core. Elsie contained a vertical glory hole,through the reflector cap and into the core, for internalmeasurements (Paxton, 1983). Fig. 2 shows an archivalpicture of Elsie.

Fig. 1. Topsy critical assembly with natural uranium reflector.

Fig. 2. Elsie critical assembly.

Fig. 3. Little Eva critical assembly.


2.3. Little Eva (1950s)

Similar to Topsy, Little Eva contained a pseudospheri-cal core of approximately 22 kg of 94 wt% enriched ura-nium, but it had a thinner natural uranium reflector(Paxton, 1983). The system was mounted on a drill-pressframe. The assembly consisted of 1/2-in. U (94 wt%) cubessurrounded by a U-238 reflector. The outside dimensionsof this reflector were approximately the size of an 8.5-in.cube (Sayeg et al., 1960).

Unlike the other critical-assembly machines, Little Evawas housed in a trailer for portability. Little Eva was usedfor demonstrating neutron-distribution measurements byphotographic emulsions. The assembly was moved to theNuclear Rocket Development Site at the Nevada Test Sitein 1958 to calibrate foils for reactor diagnostics (White,1953). One of the experiments performed on Little Evaconsisted of measuring the leakage neutron flux and spec-trum, and the neutron and gamma dose versus distancerelations for the assembly. The fast-neutron flux was mea-sured using Np-237, U-238, and S-32 threshold detectors,the thermal flux was measured using bare and cadmium-covered gold foils, and the gamma dose was measured withtetrachloroethylene chemical dosimeters. The measure-ments showed that the assembly had the lowest gamma-to-neutron dose ratio of any assembly at the time (Sayeget al., 1960). Little Eva is depicted in Fig. 3.

2.4. Comet (1950s–2004)

The Comet assembly was a general-purpose assemblymachine that was used for many critical experiments,nuclear safety studies, and criticality safety training. Mea-surements were made by bringing two parts of a configura-tion (the stationary platform and movable platen) together

Fig. 4. The Zeus assembly mounted on the Comet machine.


and measuring the resulting neutron count rate. Typicalconfigurations included the Flattop cores, the Thor core,Jemima plates, and uranium foils. In addition to the vastnumber of critical experiments performed on this machine,class demonstrations for criticality safety training were alsoperformed on Comet. These demonstrations were used toprovide students with simple, but effective, involvementin criticality safety principles through the use of hand-stacking and approach-to-critical experiments. Both theComet and Planet (discussed later) assembly machinestaught students how to extrapolate a critical configurationusing highly enriched uranium (HEU) foils interleaved witha moderating material. For example, a common demon-stration on Comet used Lucite plates as the moderatormaterial. Students were also introduced to hand-stack prin-ciples followed by the remote operation of the assemblymachine (Group Q-14 Quarterly Progress Report, 1980).

The Thor core was brought critical in the Comet assem-bly. The spherical core was composed of 9.8 kg Pu-239,reflected by thorium. Other experiments included weaponssafety tests in storage or immersion, and additional criticalsafety measurements. The lowest critical mass ever mea-sured was determined with Comet. This configuration con-sisted of a polyethylene–uranium core with a thickberyllium reflector. Less than 300 grams of U-235 wereneeded to reach the critical state.

The Zeus series of experiments was also performed onComet between 1999 and 2003 (Mosteller, 2005). Two setsof moderated and reflected-by-copper experiments wereperformed on the Comet critical assembly. The first set ofexperiments (four experiments) was designed to test theadequacy of U-235 cross-sections in the intermediate-energy range. The Jemima plates were used for this set ofexperiments (approximately 93 wt% enriched uraniumcylindrical plates). They were moderated by graphite andreflected by copper. The final critical configurations werecomposed of C/U-235 ratios ranging between 13 and 51,and the critical mass ranged between 106.6 kg and125.6 kg (Mosteller et al., 2004). The second set of experi-ments (two experiments) was supposed to test the adequacyof iron cross-sections in the intermediate-energy range, butthe cross-sections fell into the fast-energy range. Instead ofusing a graphite moderator, as was used in the first set ofexperiments, iron was used as the moderator. The criticalconfiguration for both experiments was an Fe/U-235 ratioof 14.9 and a critical mass of 198.24 kg (Hayes, 2004).Fig. 4 shows the Comet critical assembly loaded with aZeus experiment.

2.5. Planet (1950s–2004)

Planet was a vertical-lift critical assembly initiallydesigned for safety tests of gun-type weapons. Experimentson Planet involved placing the lower portion of the config-uration on the platen, which rests upon the hydraulic lift.The upper portion of the configuration was placed on thetop of the stationary support plate. The critical state was

attained when the two portions were placed in contact withone another. Upon a scram (a rapid shutdown) of theassembly, the lower portion of the assembly fell back bygravity to its original starting position.

Several series of experiments were performed on Planet.Some of the more recent experiments included the Westing-house Idaho Nuclear Company (WINCO) slab tanks,waste matrix experiments, a Np-237 sphere surroundedby HEU, and experiments in support of the Jupiter IcyMoons Orbiter. The WINCO slab tank experimentsinvolved measuring subcritical configurations using thesource-jerk technique. The experiment consisted of two‘‘pancake-shaped’’ tanks filled with highly enriched ura-nyl-nitrate solution. One of the pancake tanks was placedon the stationary platform of Planet, and the other wasplaced on the movable platform. The reactivity of the sys-tem was measured by varying the distance between the twotanks (Spriggs et al., 1989). A series of waste matrix exper-iments was conducted in support of the Yucca Mountainproject and sponsored by the Nuclear Criticality Safetyprogram. These experiments interlaced HEU foils withwaste matrix materials (i.e., SiO2, Fe, MgO, Al, Gd, con-crete, etc.). These experiments were moderated andreflected by polyethylene to simulate a flooding scenario(Gehman, 2005; Loaiza, 2002a,b, 2004a,b).

To reduce the uncertainty on the critical mass of Np-237, an experimental series consisting of a central neptu-nium sphere (6.02 kg) surrounded by hemispherical shellsof HEU was performed on Planet between 2002 and2004. In order to change the neutron spectrum of the com-posite core (Np/HEU) and increase the number of fissionsin the Np sphere, a polyethylene reflector and a low-carbonsteel reflector were used (Loaiza, 2004a,b; Loaiza, 2005).


These experiments supported nonproliferation programs.Fig. 5 shows the Planet critical assembly loaded with thefirst Np/HEU experiment.

The last experiment performed on the Planet assemblywas in support of the Jupiter Icy Moons Orbiter, whichlater became the Prometheus Project. The original intentof this project was to design a space-reactor power systemto explore Europa’s ocean, in hopes of discovering simplelife. A series of critical-mass experiments was designed toaddress the design, development, and performance of thespace nuclear reactor. These experiments consisted of inter-lacing refractory materials rhenium, molybdenum, tanta-lum 2.5 wt% tungsten (Ta–2.5W), and niobium 1 wt%zirconium (Nb–1Zr) with moderating materials (graphiteor polyethylene) fueled by HEU plates. These experimentswere designed to assess the adequacy of and uncertainty inrefractory materials’ neutron cross-sections for use in thePrometheus nuclear-reactor design work. Unfortunately,only the Nb–1Zr experiment was taken to its critical statebecause of time constraints, and only hand-stacking opera-tions were achieved for all other experiments (Loaiza et al.,2005).

2.6. Lady Godiva (1951–1957)

Lady Godiva was in operation from 1951 to 1957 (Pet-erson, 1953). Its main purpose was to supplement Topsy inchecking weapons calculations and to perform fast-reactorresearch. Lady Godiva was a bare sphere of HEU, simplerthan the two-component Topsy core, and was used to pro-vide data and to benchmark for predicted super-prompt-

Fig. 5. Np sphere, with highly enriched uranium shells, mounted on thePlanet critical assembly.

critical behavior. Later, the assembly was used as a radia-tion source for biological studies and produced approxi-mately 1000 prompt-radiation bursts. Each pulse wasterminated by thermal expansion of the machine. The max-imum burst for Lady Godiva was 2 · 1016 fissions with ahalf-width of 35 ls, which resulted in a peak power of10 GW (Malenfant, 1981). The retirement of the LadyGodiva assembly occurred because of an accident thatcaused fuel warping and damage to the support frame(Paxton, 1981).

Lady Godiva’s core was spherical consisting of 52.6 kgof unreflected 93.5 wt% enriched uranium (Peterson,1953). The critical size of the core was determined by anear-critical pseudosphere experiment on the originalComet assembly machine. The core was subdivided intothree sections: a stationary central section and upper andlower retractable caps. The central core pieces containeda radial glory hole, with channels for two HEU controlrods (Paxton, 1983). Fig. 6 shows a picture of LadyGodiva.

2.7. Heavy-Water Solution Assembly (1952–1956)

The Heavy-Water Solution Assembly commenced oper-ation in 1952. Two sets of critical experiments with heavy-water solutions of UF6 (93 wt% U-235) were designed tosupport research in the weapons program. The results of

Fig. 6. The Lady Godiva critical assembly of highly enriched uranium.


the two experimental series were needed to provide EdwardTeller with information about thermonuclear weapons.Later, this information became valuable for reactor appli-cations. Fig. 7 shows the Heavy-Water Solution Assembly.

In the first set of experiments, the solution was in 25- or30-in.-diameter tanks without a surrounding reflector. Thesolution heights were adjusted to attain criticality, withD2O solutions at D/U ratios of 230 and 419 in the smallertank. In the large tank, the D/U ratios were found to be856 and 2081. In the second experiment, solution sphereswere centered in a 35-in.-diameter spherical container.D2O was pumped into the container from a reservoir atthe base. Criticality was obtained in six solution spheresranging from 13.5 to 18.5 in. in diameter. D/U-235 ratiosranged from 24 to 431 (Paxton, 1983).

2.8. Jezebel (1954–1977)

The Jezebel critical assembly began operation in 1954within the confines of Kiva 2. Like Lady Godiva, Jezebelaided Topsy in checking weapons calculations and fast-reactor research. In addition, Jezebel provided neutronicinformation about plutonium analogous to the informa-tion found for enriched uranium on Lady Godiva. Theassembly was mounted on a light, portable, tubular frame.The core was an unreflected spherical assembly of delta-phase plutonium. Its mass was 9.46 kg and it had a radiusof 6.66 cm (O’Dell, 1999). The core was composed of four

Fig. 7. The Heavy-Water Solution Critical Assembly.

parts: a two-part split central section that was held togetherby thin wires, and upper and lower caps—all nickel plated(0.13-mm-thick cladding)—also held by wires to minimizeextraneous reflection and provide alignment (Malenfant,1981). The Jezebel assembly was also used to investigateneutronic information about U-233. The U-233 compo-nents used on the assembly were retired because of intensegamma radiation from U-232 impurities (Paxton, 1983).Jezebel continued to be operated with its plutonium corethrough May 1977. Shortly after that, the parts were placedin storage because they needed replating. Fig. 8 presents anarchival photograph of Jezebel.

2.9. Hydro (1957–1959)

The Hydro critical assembly was designed in 1957 as a10-kW neutron source for uranium exponential columns(ORAU Team, 2004). It consisted of a cylindrical core with93 wt% U-235 and copper inserts for conducting heat to ajacket of circulating water. There was no upper reflector.Hydro was operated outside Kiva 2 to avoid the effect ofroom-return neutrons, and it was also housed outside Kiva2 in a concrete-shielded underground enclosure. For oper-ation, the enclosure cover was retracted, and the assemblywas elevated to the base of an exponential column (Steinke,1968).

The exponential column was 21-in. in diameter and con-sisted of 0.12-in.-thick plates of 93 wt% U-235 interleavedwith various thicknesses of natural uranium. The averageenrichments for the critical exponential columns were

Fig. 8. The Jezebel unreflected plutonium assembly.

Fig. 9. The Hydro assembly on top of an exponential column outsideKiva 2.


4.9 wt%, 6.53 wt%, and 9.12 wt% of U-235. A series of sub-critical experiments averaging 10.9 wt%, 12.32 wt%,14.11 wt%, and 16.01 wt% of U-235 were also performedin this assembly. These results led to an infinite heightenrichment of 10 wt% U-235 for a 21-in.-diameter column(Paxton, 1983). Fig. 9 shows an archival picture of theHydro assembly elevated to the base of an exponentialcolumn.

2.10. Rover rocket-propulsion program (1955–1972)

The Rover rocket-propulsion program was the mainfocus of activities at the Pajarito Site between 1955 and1972. Most critical experiments during this time centeredon graphite-moderated propulsion systems such as theKiwi reactors, but other reactor concepts, such as usingrefractory metals to accommodate high temperatures andmaintaining the critical state of gaseous uranium within athick reflector of beryllium or heavy water, were also inves-tigated. During the Rover period, the development of pro-pulsion reactors went through an extensive design phase.First, parametric surveys were investigated to provide guid-ance for the designers. When the dimensions became fixed,details of the core, controls, and internal structures wereestablished via a mockup with good geometry. Finally,the reactor was checked and adjusted to specifications atthe Pajarito Site before heading to Nevada for testing. Crit-

ical assemblies such as Supercomet and Moly-G were alsodesigned in this 17-year duration. These assemblies werecreated for neutronic studies that were unrelated to theRover program.

As a result of the Rover program, computational capa-bilities to interpret and supplement critical experimentswere expanded at the Pajarito Site. This developmentincluded the conversion of room-temperature critical datato high-temperature data typically appropriate for reactorpropulsion systems (Paxton, 1983).

2.11. Honeycomb (1956–1990)

Honeycomb was a versatile machine used for mockingup relatively large critical assemblies. Honeycomb beganoperation in 1956 as part of the Rover rocket-propulsionprogram, but it was also used for many other programs.It was designed as a flexible system to accommodate studiesfor critical parameter investigations. Honeycomb was ahorizontal split-table machine with a matrix of aluminumtubes into which reactor materials could be inserted. Alu-minum tubes were selected as support components becauseof the aluminum’s low absorption and scattering cross-sec-tions with fast or thermal neutrons. Honeycomb used con-trol and safety rods placed in the core or reflector sections.The movable section of the table provided for disassembly.Fig. 10 shows an archival photograph of Honeycomb in itssafe (shutdown) configuration.

Honeycomb’s horizontal split-table arrangement con-tains a 1.83 · 1.83 · 1.83-m matrix of 7.62 · 7.62-cmsquare aluminum tubes. The initial criticality studies con-sisted of a survey of graphite-moderated 93 wt% U-235cores with beryllium reflectors. The fuel used in Honey-comb consisted of 330 kg of 0.005-in.-thick, 93 wt%U-235 foils. The length and width of the foils were barelyless than those of the inside dimensions of the aluminummatrix tubes. The U-235 foils were interlaced with graphiteplates. This configuration helped designers size the overalldimensions of the proposed graphite-moderated Roverreactors. The overall C/U-235 atomic ratio in this configu-ration was 180 (Paxton, 1976).

2.12. Supercomet (late 1950s)

Supercomet was a general-purpose assembly machine,similar to Comet but with a longer hydraulic-lift stroke.Supercomet was aimed at the studies of critical masses,but it was primarily used as a mockup of a proposed fuelwith plasma thermoelectric cells containing U-235, eachof which would establish an electromotive force due to fis-sion (Paxton, 1983). The purpose of these studies was topower a low-thrust ionic propulsion system for spaceexploration. A water tank could also be mounted in Super-comet for flooding tests. Utilizing the water tank, Superco-met was used for the safety testing of immersed weaponcomponents, poisoned arrays of Rover fuel elements, andthe prompt-burst assembly Godiva II. A salient experiment

Fig. 10. Honeycomb assembly loaded with U-235 foils and interlaced withgraphite.

Fig. 11. Supercomet with water tank on hydraulic lift and fuel structuremounted above.


on Supercomet precisely established the critical mass of analpha-phase plutonium sphere in water (Paxton, 1976).Fig. 11 shows an archival photograph of Supercomet witha mockup of a thermocouple reactor.

2.13. Godiva II (late 1950s)

In 1957, Godiva II succeeded Lady Godiva. Godiva IIwas specifically designed for super-prompt-critical opera-tion; therefore, the spherical geometry found in Lady God-iva was not preserved. Instead, a cylindrical system wasdesigned with emphasis placed on structural stability andreduced thermal shock. The fuel was 93.2 wt% enricheduranium, and the total uranium mass for Godiva II was57.7 kg.

Godiva II possessed several features that led to itsimproved performance as a burst irradiation assembly.Godiva II had rigid mechanical mounting and nickel clad-ding for reproducibility. Massive control rods were usedfor mass adjustments that were performed by remote con-trol. The machine was compact and portable allowing flex-ible operation. Electrical interlocks were needed to control

the burst-generation procedure. In addition to all these fea-tures, Godiva II was cooled by forced air coming from amanifold on the cage, a cooling process that minimizedthe wait time between bursts (Wimett et al., 1958).Fig. 12 shows an archival picture of Godiva II.

2.14. Flattop (1958–2004)

Flattop began operation in 1958 and was designed forfundamental reactor physics studies and to provide sam-ples for radiochemical research. Flattop was the successorto Topsy. It used spherical geometry instead of the moreflexible pseudospherical structure that was more appropri-ate when critical masses were not well established. Flattophad interchangeable spherical cores of 93 wt% U-235,98.1 wt% U-233, and 94.9 wt% Pu-239 surrounded by a19-in.-diameter natural uranium reflector. The reflectorwas subdivided into a stationary hemisphere and two

Fig. 13. Flattop with plutonium core and adjustment pieces.

Fig. 12. Godiva II critical assembly.


moving quadrants. Natural uranium control rods enteredthe fixed hemisphere from below. Upon a scram of thereactor, the two movable quadrants would rapidly retractto the ‘‘disassembled’’ position. Flattop first operated withan HEU core, and two years later, with a U-233 core. Incontrast to the core of the Jezebel assembly, the U-233 corewas smaller and easier to handle without significant gammaexposure (Paxton, 1981).

Like Jezebel, Comet, and Planet, Flattop was also usedfor criticality safety courses. The purpose was to allow stu-dents to operate this unique and inherently safe assemblyand expose them to the operating procedure. Studentshad the chance to see and handle the nickel-coated 5.9-kgplutonium-alloy core, load the assembly, and run it remo-tely at delayed critical and on positive reactor periods(Plassmann, 1980).

One of the last experiments performed on Flattop was todetermine the reactivity worth of a neptunium sample. The

experiment was performed by placing a neptunium samplein the glory hole of Flattop. This neptunium sample wasthen replaced with an HEU sample in the same locationand, finally, the HEU sample was replaced with an emptycan. Measurements were reported as the difference in thereactivities of the samples. The mass of the neptunium sam-ple was 28.4 g, and the uranium sample was 29.9 g. Theexperimental worth of the neptunium sample with respectto the empty can was 18.96 cents, and it was 3.42 cents withrespect to the uranium sample (Brewer, 1997). Fig. 13shows an archival picture of Flattop with its plutoniumcore and the mass adjust buttons.

2.15. Mars (late 1950s–1988)

The Mars vertical-lift critical assembly was built early inthe Rover program and was used for neutronic and coreoptimization studies for the Kiwi, Phoebus, and NuclearFurnace reactors. The Mars assembly consisted of an ele-vated base plate for reflector support, an upper platformfor mounting the control drum actuators, and a personnelplatform. The core was mounted on the platen of a hydrau-lic cylinder and inserted into the reflector from below. Theintact core could be moved from beneath the machineusing a movable cart that could be rolled out on guide rails.Fig. 14 shows a recent photograph of the Mars assembly.

In the early 1980s, the National Science Foundationfunded a feasibility study led by the University of Florida

Fig. 14. Upper section of the Mars critical assembly.


to study pulsed, gaseous-core, nuclear-reactor systems. Theberyllium reflector for these experiments consisted of threeparts: a cylindrical section and upper and lower end plugs.The core was set up inside a cylindrical aluminum vesselwhich was 89 cm high by 46 cm in diameter. The fuel con-sisted of 112 HEU metal fuel elements and a central canis-ter containing gaseous U(93)F6. The worth of the HEU gasin the central canister was found to be 1.54 cents per gram.The worth of the HEU metal fuel elements was determinedto be 25 cents (Group Q-14 Quarterly Progress Report,1980, 1981a,b). Subcritical beryllium-reflected cavityassemblies such as the Plasma Cavity were also mountedon Mars.

The Compact Nuclear Power Source (CNPS) experi-ment was performed on the Mars assembly. CNPS was ajoint design effort between LANL and Atomic Energy ofCanada to power a short-range radar (20 kWe) station inthe North Warning System (a series of radar stations acrossArctic North America that provide surveillance of airspaceto prevent potential attacks coming from North America’spolar region).

The CNPS core was cylindrical with a 600-mm radiusand 1100-mm height. The core was 19.9 wt% U-235 metaland was moderated and reflected by graphite. There were492 fuel channels, 20 material-replacement channels, 12heat-pipe channels, and 5 control-rod channels. The fuelcompacts were 50 mm long with a density of 10.6 g/cc, aC/U-235 ratio of 0.274, and an O/U-235 ratio of 1.631.

The reflector consisted of bottom, top, and radial com-ponents. The radial reflector was 200 mm thick and wascomposed of twelve 30-degree segments (604-mm innerradius). The bottom reflector was 200 mm thick, had a600-mm radius, and was mounted to a 60-mm thick alumi-num support plate resting on a movable cart. The upperreflector component had an 804-mm radius and wasmounted on the radial reflector. The top and radial graph-ite reflector components, once mounted, rotated to alignheat-pipe and control-rod channels with those of the core(Hansen et al., 1988).

2.16. Modular high-temperature gas-cooled reactor (1960s)

The modular, high-temperature, gas-cooled reactor forthe Department of Energy’s (DOE’s) New ProductionReactor (NPR) program used most of the CNPS core withthe reflector assemblies already in place. The purpose of theprogram was to obtain neutronic data from the centralzone of the CNPS core and to use that data to benchmarkthe capability of the reactor physics design codes that wereto be used in the NPR design. Thus, this particular pro-gram was primarily designed for experimental benchmarks.The experiments were designed to provide insight into neu-tron spectra, reactivity worths of materials, and the spatialdistribution of reaction rates in the fuel-target block (Pater-noster et al., 1990).

2.17. Venus (late 1950s–1962)

The Venus machine was originally designed to accom-modate a neutronic mockup of a Kiwi-A series reactor. Itwas then adapted to mockups of Dumbo and an earlymember of the Kiwi-B series reactor. The Pewee Zepoexperiments were also loaded on Venus. Because of itsextensive flexible design, its limits extended well beyondthose of Comet and Supercomet. Venus was a vertical-liftmachine similar to the Mars machine; a portion of the con-figuration would be placed on the hydraulic lift and it couldbe retracted onto a movable cart. This movable cart madefueling and modifying the configuration very easy and con-venient. Upon a scram of the Venus machine, the lowersection of the configuration dropped to bring the assemblyto a far subcritical state, as in the Mars assembly (Paxton,1976). Fig. 15 shows an archival photograph of the Venusmachine mounted with a Pewee Zepo experiment.

The Venus machine was set up in Kiva 1. It was alsoused for the WINCO annular tank experiments and thepoison-tank experiments. The purpose of the WINCOexperiment was to generate benchmark data to validatecriticality safety calculations for a proposed reprocessingplant upgrade. In this experiment, the tank was an annularstainless-steel tank with a 30-in. outer diameter, 22.5-in.inner diameter, and 69-inch height. The fuel in the experi-ment was 93 wt% uranyl nitrate with a uranium concentra-tion of 300 grams U(93) per liter (Safety Analysis Reportfor Pajarito Site (TA-18) and the Los Alamos CriticalExperiments Facility (LACEF), 1992).

2.18. Kiwi (1959–1968)

The Kiwi series of reactors were part of the Roverrocket-propulsion program. The Kiwi experiments beganmid 1959 and ended in 1968. Each reactor was cylindrical,U(93)-carbide fueled and graphite moderated. The Kiwinonflying series of reactors was named after the earth-bound New Zealand bird.

The Kiwi-A reactor series included an axial D2O islandto conserve enriched uranium. It made proof-of-principle

Fig. 15. Venus machine loaded with the Pewee Zepo assembly.

Fig. 16. Kiwi-A is shown inside Kiva 1.

Fig. 17. Kiwi-TNT after planned destructive excursion.


tests possible at 100 MW. The reactors were graphiteloaded with uranium-carbide fuel elements distributed ina graphite matrix. The core was hydrogen cooled, andhigh-temperature gaseous hydrogen exited the core.Because severe erosion occurred in the first test, niobiumlining was used in the reactor coolant channels. Fig. 16shows an archival photograph of Kiwi-A.

The Kiwi-B1 reactor series was aimed at flyable systems.The elimination of the D2O and a higher uranium enrich-ment led to an increased power output of 900 MW. TheUC microspheres were still distributed in a graphite matrix.In these reactors, gaseous hydrogen was replaced by liquidhydrogen, a further step toward a flyable system. Liquidhydrogen remained the coolant/propellant in all succeedingtests (Paxton, 1983).

The Kiwi-B4 reactor series designs were too reactive tobe completely assembled; therefore, some of the fuel ele-ments were replaced by inert material to introduce some‘‘poison.’’ The reactor core consisted of 132-cm-long hex-agonal rods, 1.9 cm across, each containing 19 preciselylocated flow channels. Like the Kiwi-A series, niobium lin-

ing of the coolant channels was used for corrosion control.During the testing, it was discovered that the extra reactiv-ity was due to hydrolysis of the uranium-carbide distrib-uted as particles throughout the graphite fuel matrix. Thesource was moisture in the air. The hydrolysis was elimi-nated by distributing uranium carbide as beads with a pro-tective coating (Paxton, 1981).

Finally, Kiwi-TNT was a special reactor designed so itcould increase reactivity very rapidly. It was purposelydestroyed in Nevada to investigate a severe reactivity acci-dent. This served to check the required accident analyses,thereby increasing confidence in yield calculations.Fig. 17 shows an archival photograph of Kiwi-TNTremains after the planned destructive excursion. BeforeKiwi-TNT was sent to Nevada, it underwent a very uniqueexperiment. Kiwi-TNT was positioned in close proximityto the Parka reactor, as seen in Fig. 18. Both the assemblies

Fig. 18. Kiwi TNT and Parka reactors in close proximity.

Fig. 19. Mockup of proposed Dumbo propulsion reactor.


were operated simultaneously to measure their interaction.These experiments were performed to estimate the effect ofhaving several reactors clustered together to power a single,high-thrust rocket (Paxton, 1983).

2.19. Dumbo (early 1960s)

Dumbo was designed as an alternative to the Kiwi reac-tors. Dumbo was also part of the Rover rocket-propulsionprogram and used a hydrogenous moderator to reduce theoverall weight of the assembly. The concept was intendedto take advantage of the high-temperature characteristicsof refractory metals such as tungsten, tantalum, andmolybdenum. Within the reactor, annular tungsten–UO2

fuel columns were placed in the hydrogenous moderatorin a way that would increase the probability that neutronswould pass over capture resonances while slowing down.Liquid hydrogen in the moderator would be heated whilediffusing into the fuel column annuli and would beexhausted along the axes.

The initial Dumbo mockups consisted of a 25-in.-diam-eter polyethylene cylinder with seventy-seven 3-in.-diame-ter holes to accommodate various simulations of fuelcolumns. The assembly was known as Aunt Polly. Alumi-num and graphite structures supported tungsten andHEU foils within the 3-in.-diameter holes. Later configura-tions had beryllium reflectors up to 6-in. thick. In 1959,Dumbo was terminated because the hydrogen flow intothe fuel columns was nonuniform and the reactor failedto show any advantage over the Kiwi reactors (Paxton,1983). Fig. 19 shows a mockup of the Dumbo proposedpropulsion reactor.

2.20. Moly-Godiva (late 1950s–1964)

The Moly-Godiva assembly was developed during theRover program period at Pajarito Site. It was designedfor burst operations to perform neutronic studies. The coreconsisted of an unreflected, unmoderated, circular cylinderof uranium–molybdenum alloy. The assembly wasdesigned for pulsed operation with a maximum burst capa-bility of 2 · 1017 fissions and a minimum burst width of50 ls. The assembly was moved to the White Sands MissileRange in July of 1964, shortly after it was checked and cal-ibrated in Kiva 2. The Moly-Godiva is now known as theWhite Sands Missile Range Fast Burst Reactor (Paxton,1983; ORAU Team, 2004). Fig. 20 shows an archival pho-tograph of Moly-Godiva.

2.21. Phoebus (1965–1968)

The Phoebus reactor series was also part of the Roverrocket-propulsion program. These reactors were designed

Fig. 20. Moly-Godiva prior to shipment to White Sands Missile Range.

Fig. 21. Mockup of Phoebus-2 reactor with 4000 fuel element core.


as flyable systems. There were two generations of Phoebusreactors: Phoebus-1 and Phoebus-2. The Phoebus-1 reac-tors were about the same size and general design as theKiwi-B4 reactors. They had more fuel elements (1600),which led to an increased power density and duration ofoperation. The final test of this reactor demonstrated30 min operating time at a power output of 1500 MW witha propellant exhaust temperature of 2500 K. The Phoebus-2 reactors were aimed at increasing power and thrust. Thecore size of the Phoebus-2 reactor was increased to contain4000 fuel elements, but it retained the Phoebus-1 reactordesign. In Nevada, the Phoebus-2 reactor achieved a powergreater than 4000 MW (Paxton, 1981). Fig. 21 shows amockup of Phoebus-2 with its 4000-fuel-element core.

Fig. 22. Zero-power mockup of Pewee.
2.22. Pewee (1968–1971)
Pewee was one of the last reactors operated at Nevada insupport of the Rover program. It was designed for rela-tively inexpensive testing of new fuel element designs.The reactor contained 400 fuel elements, and the greatestpower density observed was in the Pewee-1 reactor:5 MW per liter of fuel. The project was cancelled in 1971due to funding restrictions. The overall program ended18 months later (Paxton, 1981). Fig. 22 shows of mockupof the zero-power Pewee reactor.

The zero-power Pewee assembly consisted of an 8-in.-thick beryllium reflector. The core was placed within agraphite barrel with a 21-in. outer diameter and 19.8-in.inner diameter. A typical core contained three hundred sev-enty-seven 30 wt% composite fuel elements and 163 moder-ator elements. The core composition was 57.7-kg uranium,182-kg carbon, 237-kg zirconium, and 1.7-kg hydrogen(Orndoff, 1970; Kirk, 1973).


2.23. Nuclear Fuel Furnace (1972)

The Nuclear Fuel Furnace was a small test reactor witha low fuel inventory designed to provide means of testingnuclear-rocket-reactor fuel elements. The Nuclear FuelFurnace was a heterogeneous, water-moderated, beryl-lium-reflected reactor containing 49 cells in which high-temperature fuel elements could be tested. The reactor coreconsisted of a cylindrical aluminum can that contained 49aluminum tubes. Each tube contained one fuel element.During operation, water flowed in a two-way pass systembetween tubes while hydrogen gas flowed through the fuelelements.

The principle objectives of the Nuclear Fuel Furnacetest series were to check out the operating characteristicsof the reactor and to operate the reactor at an averageexit-gas temperature of 2450 K for at least 90 min. Theseprimary objectives were achieved, and a wealth of informa-tion was obtained on the static and dynamic characteristicsof the reactor. The static operations were performed at TA-18, whereas the dynamic operations were conducted at theNevada Test Site. Fig. 23 shows an archival photograph ofthe Nuclear Fuel Furnace (Kirk, 1973).

2.24. Godiva III (late 1950s)

Godiva III was a replica of Godiva II, designed andbuilt by Los Alamos for operation at the Sandia NationalLaboratories Pulse Reactor Facility. The burst reactor was

Fig. 23. Nuclear Fuel Furnace with borated water shield.

called Godiva III by Los Alamos and was renamed SPR-1(Sandia Pulsed Reactor-1) by Sandia (Malenfant, 1981).

2.25. Godiva IV (early 1960s–2004)

Godiva IV, like its predecessors, was designed as asuper-prompt-critical pulse assembly machine. Godiva IVwas primarily an irradiation reactor, although its originalpurpose was to test design features, including materialselections that were expected to increase resistance to shockdamage. Godiva IV eliminated the reactivity change foundin Godiva II by absorbing the shock in a strong, heavilysprung retainer for the fuel rings that made up the core.The retainer was mounted at the centers of the shock suchthat stress was not transmitted to the support. The retainerconsisted of three massive C-clamps holding the reactorhead between two heavy rings. In addition to having theretainer for shock absorption, uranium 5 wt%/molybde-num-alloy rings were added for tensile strength and resistanceto deformation and cracking. The molybdenum–uraniumrings were added as a result of the lessons learned whileoperating Godiva II. With this design, pulses of 6 · 1016 fis-sions were produced regularly without damage to the93 wt% U-235/1.5 wt% molybdenum rings that constitutedthe reactor head. The damage threshold of the system wasapproximately 8 · 1016 fissions (Paxton, 1983). Peakpowers around 100 GW have been measured for GodivaIV.

The Godiva IV fuel components were all aluminum-ionplated and provided a total mass of 65.4 kg of HEU. ThreeC-shaped clamps fabricated from high-strength maragingsteel fastened the stack of six stationary fuel rings. The fuelrings consisted of six 1-in.-thick annular rings with a 7-in.outside diameter and 3.5-in. inside diameter. A safety blockfit inside the fuel rings. The supporting structure consistedof a three-legged stand with heavy aluminum shelf platesthat supported the assembly and housed the actuatingmachinery. The system used a pneumatically driven burstrod and a magnetic clutch on the safety-block drive thatsupplied the force for disassembly through a coil springand gravity. The safety block was inserted by using ahydraulic cylinder coupled through the clutch (Wimett,1973). Fig. 24 shows a recent photograph of Godiva IV.

A significant experiment with this reactor was the firstdemonstration of a laser pumped by uranium fission prod-ucts, with Godiva IV providing the neutron source. Its ini-tial success was accomplished with a helium–xenon laser inwhich the tube was lined with enriched uranium foil. Apolyethylene sleeve around the tube thermalized neutrons,and fission fragments excited the laser (Helmick et al.,1975).

2.26. Big Ten (early 1960s–1994)

Big Ten was a large, cylindrical, metal assembly with itsaxis mounted horizontally on top of a split-table machine.

Fig. 24. Godiva IV fast-burst assembly.

Fig. 25. Big Ten with the movable cart retracted.


The purpose of this assembly was to measure cross-sectiondata in a fast-neutron spectrum that extended to lowerenergies than those provided by small metal systems likeJezebel and Flattop (Paxton, 1976; Hansen and Paxton,1979). A portion of the assembly was fixed in a stationaryposition while the other portion was mounted on a mova-ble cart.

The Big Ten core was made of a 10-wt-%-averageenriched uranium and was reflected by natural uranium.The cylindrical core had a 21-in. diameter consisting ofinterleaved plates of 93 wt% U-235 and natural uraniumsuch that the average enrichment was 10 wt%. The corewas surrounded by an axial 6-in.-thick reflector ofdepleted uranium. Control rods with low-reactivityworths (depleted uranium) and safety rods of the samematerial were inserted into the reflector. A higher-worth10 wt% uranium control rod entered along the core axis.The total mass of U-235 in the system was approxi-mately 250 kg (Sapir, 1997). The major scram modewas achieved by retracting the movable part of the tableto its original position. Fig. 25 shows an archival photo-graph of Big Ten.

During Big Ten operational history, core length adjust-ments were made in order to measure different reactor con-figurations. For example, a core length of 22.6 in. was usedfor kinetic measurements (86 cents excess reactivity). Acore length of 21.4 in. was used for delayed critical mea-surements (18 cents excess reactivity) (Paxton, 1983; Pax-ton, 1980).

2.27. Los Alamos Molten Plutonium Reactor Experiment

(1961–1963)

The Los Alamos Molten Plutonium Reactor Experi-ment (LAMPRE I) was the first reactor built by Los Ala-mos during its continuing program to develop plutoniumfuels for fast-breeder-reactor applications. The work wasperformed at TA-35, two to three miles from Pajarito Site.Although this reactor was not operated within the confinesof Pajarito Site, several of its members participated as con-sultants and operators. LAMPRE I was designed as a radi-ation test reactor in order to obtain the necessary materialsexperience that would permit the design and fabrication offuture reactors. Because space and shielding were availablein the building that housed the reactor at TA-35, the spe-cific power of the reactor (approximately 40 W per gramof plutonium) was made high enough to provide initialexperience on the effects of fission product accumulationsin the fuel. Overall, the reactor operated at 1 MW.

The initial design of LAMPRE I called for plutoniumfuel to be contained in a single, connected region, cooledby sodium flowing through tubes welded to the top andbottom plates of a cylindrical container. Since knowledgeof the container and fuel materials was insufficient to thepoint that the design could not be completed within thedesired time frame, the design was changed so that LAM-PRE I could serve as a radiation test reactor in order toobtain necessary material experience that would permitthe design and fabrication of future reactors (LAMPRE IFinal, 1963). LAMPRE I first achieved criticality in early1961 and was terminated by mid 1963.

The reactor core was an array of tantalum capsules con-taining Pu–Fe alloy fuel. The fuel composition was 90 at.%plutonium and 10 at.% iron, and it was characterized by itslow melting point. The iron content was primarily pureiron to combat corrosion. There were 199 core locations,of which 140 centralized locations were filled with fuel cap-sules. The remaining locations were filled with stainless-steel reflector pins. Reactivity adjustment was performedby control elements (consisting of five removable reflector


sections). These control elements served as shutdownmechanisms (LAMPRE I Final, 1963). Fig. 26 shows anartist’s rendition of the Los Alamos Molten PlutoniumReactor Facility.

Fig. 27. Plasma Cavity with initial core retracted.

2.28. Plasma Cavity (1972–1979)

The Plasma Cavity assembly was created after the termi-nation of the Rover program. It was mounted on the Marsmachine and was intended to guide the design of a high-power plasma reactor of interest to NASA (Paxton,1976). The purpose of this design was to withstand veryhigh temperatures during operation. In the earlier versionsof this reactor type, the gas cores were simulated by thin,enriched uranium foils, either distributed on thin alumi-num shelves within a large cavity or lining it. Later versionsinvolved Rover fuel elements instead of the foil, and evenlater, a UF6 canister replaced the elements, but a completeUF6 flowing core was never obtained. Fig. 27 shows thePlasma Cavity with its massive containment core in theshutdown position.

The cavity of this assembly was 1 m diameter by 1 mlong. Surrounding the cavity was a 19-in.-thick berylliumreflector. The beryllium reflector was composed of beryl-lium parts from the Rover program (Phoebus-2, Kiwi B,Pewee, Nuclear Fuel Furnace, and Honeycomb) (Evanset al., 1978).

The reactor core consisted of ten 93 wt% U-235 foils dis-tributed throughout and gaseous UF6(93 wt% U-235). Thegaseous UF6 was first introduced into the system staticallybut was later added as flowing gas confined within anargon vortex. The maximum quantity of UF6 came to 50grams under flowing conditions. The reflector was takenfrom the Phoebus-2 in order to approximate the D2O cav-ity assembly with a beryllium reflector. In the Plasma Cav-ity, the reflector was an inch smaller than in the D2Oassembly (Paxton, 1983).

Fig. 26. The Los Alamos Molten Plutonium Reactor Facility.

2.29. Parka (1972–1979)

The Parka assembly was essentially a Phoebus-1 reactor(without a pressure vessel) that was used as a criticalassembly. The Parka core was composed of hexagonal,one-hole, fuel elements to mock up coolant-channel vol-ume fraction in the actual Phoebus-1 fuel elements (Sapirand Orndoff, 1976). The core was reflected by 50 mm ofgraphite and 114 mm of beryllium. The ends were essen-tially unreflected. The active core was 89 cm in diameterand was 132 cm high. The reflector surrounding the activecore was made of beryllium and contained 12 rotating,boron-loaded control drums (Evans et al., 1978). Sincethe drum-speed limit was 5 cents per second, the assemblywas protected from excess reactivity transients (McDon-ough, 1972).

The core readily accommodated special experiments.One experiment involved a central fast-reactor fuel zoneto investigate diagnostic techniques. Another experimentincluded a 10-kg sample of damp mixed oxide (60 wt%PuO2, 40 wt% depleted UO2). This experiment providedinformation to aid in the design of a proposed assemblyof the same material (Paxton, 1983). Fig. 28 shows theParka assembly that was used to investigate interactionsbetween reactors.

Fig. 28. Parka critical assembly.

Fig. 29. Kinglet critical assembly.


2.30. Kinglet (1973–1974)

The Kinglet assembly was operated in 1973 and 1974. Itwas intended to contribute to the Kinetic Intense NeutronGenerator Design. The assembly was placed in a sheetmetal building outside Kiva 1 (Paxton, 1976). The experi-ment was terminated after funding was discontinued.Fig. 29 shows an archival picture of Kinglet.

One of the attractive features of the assembly was toresult from a large volume of HEU solution circulatingthrough a small critical region within a beryllium reflector.The solution volume was 561 L and circulated in an 8-Lcritical volume at up to 2 MW of power. In this reactor,a large power density could be maintained with a moderatetemperature rise of the entire solution. Kinglet was meantto explore reactor (kinetic) stability and the effect of radiol-ysis (the shutdown mechanism with an absence of delayedneutrons). There was doubt over operational stabilitybecause nearly all delayed neutron precursors would decayoutside the critical volume (Paxton, 1983).

2.31. SHEBA (1980–2004)

SHEBA (Solution High-Energy Burst Assembly) wasdesigned in 1980 for proof-testing criticality-accident-detection systems and investigating solution reactorprompt bursts to better understand solution criticality acci-

dents (Cappiello et al., 1997). It is housed in the sheet-metalbuilding outside Kiva 1, which was vacated by Kinglet. Theassembly was principally used for calibration of criticalityaccident dosimeters for uranium reprocessing plants.SHEBA generated thermal neutrons like those emitted bysolution accidents. The solution vessel of the SHEBAassembly can be seen in Fig. 30.

SHEBA is an aqueous solution of UO2F2 (4.95 wt%U-235). Criticality was attained by adjusting the solutionheight in a cylindrical vessel with an inner diameter of54.6 cm. Shutdown occurred by draining the solution intostorage containers. A safety rod could be dropped into a6.35-cm diameter axial tube. With a solution density of1.04 kg(U)/L (H/U = 525), the assembly reached criticalat room temperature at a height of 36.5 cm (84.3-L volume)(Paxton, 1983).

Several experiments aimed at acquiring informationregarding different parameters of the assembly have beenperformed on SHEBA. Some of these parameters includefuel characteristics, delayed-critical solution height as afunction of solution temperature, initial reactor periodand reactivity as a function of solution height, flux profile,and the reactivity effect of voids in the fuel. An experimentof great interest for SHEBA analyzed the effect of void for-mation within the fuel solution caused by radiolytic gasproduction. By performing several ‘‘free-run’’ operations,a better understanding was obtained between the droppingof the fuel solution level due to gas production and the sub-sequent acceleration of the reactor period (Cappiello et al.,1997). Another series of experiments was designed to prove

Fig. 30. SHEBA inside a sheet-metal shed outside Kiva 1.

Fig. 31. The Skua fast-burst assembly.


tritium production capabilities using helium-3 targetsinserted in power reactors or DOE-owned test reactors(Cappiello, 1999).

2.32. SKUA (1980s–1996)

SKUA (Supercritical Uranium Assembly) was a large-sized fast-burst assembly. It was designed to producehigh-fluence pulses of thermal or fast-energy neutrons.SKUA had a 9.5-in. diameter cavity for irradiation of sam-ples with fast neutrons. For thermal neutron irradiation, amoderating insert made of polyethylene or zirconiumhydride could be inserted into the irradiation cavity.SKUA’s application was expected to include investigationof the influence of its intense radiation fields on air chem-istry and improved excitation of fission-pumped lasers.SKUA differed greatly from Godiva IV in size, and a cop-per reflector surrounded a stack of 93 wt% U-235 alloyrings. The reflector had three retractable safety block sec-tions. Three other portions rotated into the main body ascontrol drums (Paxton, 1983). Fig. 31 shows an archivalphotograph of the SKUA fast-burst assembly.

SKUA was designed as an approximate unit height-to-diameter cylinder of annular 93 wt% U-235 rings, moder-ated by ZrH1.6 and reflected by copper. ZrH1.6 was chosenas the moderator over polyethylene because of the mate-rial’s superior physical properties at high temperatures.The 175 kg of HEU in the system allowed for bursts of

2 · 1017 fissions with a mean system temperature rise ofapproximately 300 �C.

3. Conclusion

The Los Alamos Critical Experiments Facility started itsexperimental experience in April of 1946, primarily sup-porting the weapons safety guidance. Accidental criticalityhad to be avoided during the handling, storage, and trans-portation of weapon components and during the assemblyand manipulations of weapons. The experimental informa-tion it provided was essential to avoid impractical critical-ity-safety restrictions. As the weapons program expandedand weapons design became more sophisticated, theLACEF engaged in measurements to (a) aid in the designof nuclear explosive devices, (b) establish procedures andlimits for the safe, subcritical handling, storing, and trans-porting weapons, and (c) provide neutron physics parame-ters to validate calculations of weapons systems. Althoughthe weapons activities continued to be of primary concern,the activities gradually broadened to include active mate-rial processing and fabrication operations. In 1955, theRover nuclear-propulsion reactor program was added,and it became a dominant movement through 1972. Thisprogram started with parametric studies of reactor config-urations. Then as overall dimensions became fixed, detailsof cores, controls, and internal structures were establishedin a mockup with good geometry. Following the Roverprogram, fast-reactor safety studies were added to the crit-ical experiments activities. The work concentrated onassessing various reactor accidents. The activities alsoincluded development and evaluation of techniques tomeasure fuel behavior in destructive environments.


In more recent years, the activities expanded to supportother programs such as the Nuclear Criticality Safety Pro-gram (NCSP), the Emergency Response Program, andnonproliferation programs. The NCSP became the mainsponsor to the experimental activities that took place atthe Pajarito Site. The tasks supported under the NCSPinclude critical experiments, criticality safety training, andbenchmark projects. LACEF was the only facility capableof providing criticality safety demonstrations, hands-onexperience in handling special nuclear material, and char-acterization of multiplying systems.

The history of critical experiments at LACEF is vast anddiverse. We have attempted to summarize and present thevarious critical assemblies and reactors that have operatedat this site. Literally thousand of experiments have beenperformed during the last 58 years; it is impractical to pres-ent them all, but an effort has been made to summarize themore salient and significant activities. References have beenprovided for the reader who would like to learn more abouta particular program.

Acknowledgements

The authors would like to express their gratitude andadmiration to the operating staff of the LACEF. The workdescribed represents the cooperative combined efforts ofmany individuals working to enrich our understanding ofthe fundamental principles of criticality. One of the impe-tuses for forming a facility like the Pajarito Site was toemphasize the importance of experimental results, helpingtheorists become accustomed to tying their work closelyto observable phenomena. The application and contribu-tions made by the LACEF to technological advances inreactor design, criticality safety, and weapons-related workwere direct and massive. As the research community entersa hiatus in critical experimental activities, the work per-formed at this facility will bring new light and focus. Final-ly, the authors would also like to express our gratitude tothe many people who contributed ideas, discussions, re-views, and comments to this work. Any mistakes or omis-sions are the sole responsibility of the author.

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Documents

End of an Era for the Los Alamos Critical Experiments Facility: History of critical assemblies and experiments (1946–2004)