Aging and condition monitoring studies of composite insulation cables used in nuclear power plants

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 1; February 2007

1070-9878/07/$25.00 © 2007 IEEE

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Aging and Condition Monitoring Studies of Composite Insulation Cables Used in Nuclear Power Plants

K. Anandakumaran

Kinectrics Inc. 800 Kipling Ave.

Toronto, Ontario M8Z 6C4, Canada

ABSTRACT Five composite ethylene propylene rubber/chlorosulfonated polyethylene (EPR/CSPE) insulated cables, a single composite cable with EPR/Neoprene, and a single Kerite insulated cable used in US nuclear power plants (NPP’s) were evaluated. All of the cables had a CSPE outer jacket. To study the rate of degradation these cables were thermally aged at 110 and 120 °C until embrittlement of the composite insulation and /or outer jacket occurred. The degradation of the composite insulation and outer jackets was assessed by measuring elongation at break and indenter modulus. Both the indenter modulus and elongation values of the composite insulation and outer jacket exhibited excellent superposition when shifted using the time-temperature superposition principle. Regardless of whether or not the EPR insulation was bonded or non-bonded to the CSPE or Neoprene composite jacket thermal induced hardening and fusion of the composite jacket with EPR insulation was the cause for loss of flexibility of the composite wires. The aging rate of the composite insulated wires were not affected by the aging mode, namely, whether the composite wires were aged as a complete cable or with the jacket removed. However, the aging rate of the composite insulation and the outer jacket varied between different cables. Threshold indenter modulus values for successful Loss of Coolant Accident (LOCA) performance for each of the composite insulation and outer jacket was correlated with the thermal aging periods of the studies available in the literature. Close correlation was found between the thermal aging periods used for satisfactory LOCA performance and the time to reach 50% elongation of the composite insulation. Indenter modulus values of the composite insulation and outer jacket when the composite insulation decreases to 50% elongation is also provided.

Index Terms — Hypalon, composite insulation, condition monitoring, cable indenter, elongation, thermal aging, LOCA performance.

1 INTRODUCTION

THE most common insulation materials for low voltage cables in US nuclear power plants consist of fire retardant ethylene propylene rubber (FREPR) or fire retardant cross-linked polyethylene (FRXLPE) with a CSPE (CSM as per ASTM) jacket. To obtain fire retardancy, additives such as brominated and/or chlorinated compounds with antimony oxide are incorporated in the insulation formulations. In the past some manufacturers supplied EPR insulation without fire retardant additives and achieved the desired cable fire retardancy by extruding a thin layer of Hypalon (DuPont’s trade name for CSPE) or neoprene. These cables were known as composite insulation system and will be referred to as such in the report. This study provides aging and condition monitoring results for the composite insulation systems,

which. can be of the bonded or non-bonded types, i.e., in the non- bonded design the jacket can be peeled off the EPR insulation. Previous aging studies [1] of composite insulation showed that the Hypalon component aged at a faster rate than the underlying EPR layer. The Hypalon tended to crack and the crack propagated through the EPR layer causing electrical failure during LOCA simulation tests. In addition, tensile testing of composite insulation provided evidence that degradation of the Hypalon jacket would result in low elongation values in the composite insulation. Hypalon jacket materials have been shown to possess resistance to radiation induced aging [2]. During normal service life in US NPP cables are exposed to a relatively low radiation dose in the 10 to 20 Mrad range [3, 4]. Hence in composite insulated wires, the thermal aging behavior of the Hypalon will be the limiting factor in determining the life of these cables.

Manuscript received on 19 April 2006, in final form 4 August 2006.

K. Anandakumaran: Aging and Condition Monitoring Studies of Composite Insulation Cables Used in Nuclear Power Plants 228

Traditionally, insulation degradation is assessed by the use of elongation measurements and the acceptance criterion [5, 6] for installed cables is 50% absolute elongation-at-break. It is generally assumed that 50% absolute elongation-at-break values will provide sufficient margin to ensure that the XLPE and EPR based insulation maintains the mechanical and electrical properties during a design basis event [7, 8]. This acceptance criterion has also been found to be relevant for composite insulated wires [8, 9]. For qualification testing of composite insulated wires, cable manufacture’s used the EPR insulated wires (i.e. without Hypalon). To determine the thermal aging periods for the simulation of normal service life, in the Arrhenius calculations the activation energy of the EPR insulation (ranging from 1.18 eV to 1.44 eV) [10] was used. High activation energy values in the Arrhenius calculations result in over estimation of the service life. For example, to simulate 40 yr of service at a service temperature of 60 °C with an activation energy of 1.18 eV requires 602 h of aging at 121 °C. This is only equivalent to 23.3 years of service life if the activation energy of 1.08 eV for Hypalon is used. Hence, the selection of the proper activation energy is critical when subjecting cables to accelerated aging. Even with the use of high activation energy values (i.e. with over estimated service life), the performance of some of the composite insulated wires was not satisfactory during the EQ testing. Therefore, if it can be demonstrated by non destructive means that the cables are in relatively good condition, replacement or additional EQ testing will not be required and only periodic monitoring will be sufficient for cable life extension. Indenter modulus values obtained using the Electrical Power Research Institute’s (EPRI) cable indenter have been shown to be ideal for monitoring the condition of Hypalon insulated cables [11]. In this study, five commonly used composite EPR/Hypalon insulated and Hypalon jacketed cables installed in US nuclear power plants were evaluated. As part of the study two cables, one with EPR/Neoprene composite insulation and the other with Kerite insulation were included. These cables were jacketed with Hypalon. The relationship between the condition of the composite insulation and outer jacket in terms of elongation-at-break and indenter modulus was derived, which led to derivation of threshold modulus values for successful LOCA performance.

2 EXPERIMENTAL 2.1 CABLE SAMPLES

The cables used in this study, with the exception of US#23, were obtained from a US cable distributor who purchased these cables from the surplus inventories of US nuclear power plants. The cables were manufactured by BIW (Bostrad 7E), Samuel Moore (Dekorad type 1952), Okonite, Anaconda (Durasheath), AIW, and Kerite. Cable descriptions and the year of manufacture, as marked on the cable jackets, are provided below.

US#1: 3/C, #14AWG, bonded EP-CSPE insulation, 0.75 mm insulation + 0.39 mm composite jacket, 1.9 mm CSPE outer jacket, 600V (1990). US#2: 7/C, #14AWG, bonded EPR-CSPE insulation, overall semicon tape, 0.82 mm insulation + 0.34 mm composite jacket, 1.67 mm CSPE outer jacket, 600V (1982). US#3: 2/C, #16AWG, bonded EPR-CSPE insulation, copper coated Mylar shield, 0.82 mm insulation+ composite jacket, 1.25 mm CSPE outer jacket, 300V. US#8: 9/C, #14AWG, Kerite insulation, 0.97 mm insulation, 2.3 mm CSPE outer jacket, 600V (1980). US#10:4/C, #14AWG, non-bonded EPR-CSPE insulation, 0.83 mm insulation + 0.40 mm composite jacket, 1.25 mm CSPE outer jacket, 600V. US#15:9/C, #14AWG, non-bonded EPR-CSPE insulation, 0.80 mm insulation + 0.53 mm composite jacket, 1.55 mm CSPE outer jacket, 600V (1983). US#23:7/C, #12AWG, non-bonded EPR-Neoprene, 0.83 mm insulation + 0.42 mm composite jacket, 1.50 mm CSPE outer jacket (1973). This cable was provided by a US utility as part of assessing the condition of installed cable in 2004.

2.2 SAMPLE PREPARATION For each of the seven cable design, ten 200 mm long complete cable sections (with outer jacket intact) and 10 single conductor composite insulated wires were prepared. The ends of these specimens were not sealed. In addition, 30 tubular composite insulated wires with the copper conductors removed were prepared for elongation measurements. The ends of the tubular specimens were sealed with RTV silicone to minimize air access to the inside of the tube during aging.

2.3 THERMAL AGING

The samples were aged in ASTM type II forced air circulating ovens with 100-120 air exchanges per hour (Despatch Model LAC1-38A-4) at temperatures of 110 and 120 °C for periods lasting from 4 to 8 months. Aging oven temperatures were continuously recorded using an HP Datalogger. Periodically, all three types of samples were removed from the ovens for tensile testing and indenter measurements. Indenter measurements were initially conducted on the 200 mm long specimens of the complete cable samples. Following the indenter measurements, the outer jackets of the samples were cut open to obtain jacket specimens for elongation measurements and composite insulated wires for indenter measurements. For cable US#23, elongation measurements were also obtained for composite insulated wires aged with the outer jacket.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 1; February 2007

2.4 TENSILE TESTING

Tensile and elongation tests were performed with a Lloyd tensile testing machine at a crosshead speed of 50 mm/min. Typically, three 100 mm long tubular composite insulation specimens and three Die cut outer jacket tensile specimens (ASTM D638 V) were tested and the data averaged to obtain the results.

2.5 INDENTER MODULUS The EPRI indenter uses an anvil which is driven against the cable jacket or insulation at a constant speed during which both force and deformation depth are measured to yield Indenter Modulus. At a specific force, the anvil is retracted to preclude damage to the cable. The indenter is a portable, self contained device which is battery powered and robust enough to be used in the field. The clamping assembly can be changed to accommodate large size multi conductor cables as well as individual small size wires. For each cable, a minimum of 5 measurements was taken. If the standard deviation variations exceeded 10%, ten or more measurements were obtained. Indenter modulus measurements were made on outer jackets and the composite insulated wires aged inside the complete cable and single conductor composite wires that were directly heat exposed (Note: Composite insulated wires aged inside the complete cable were labelled as jacketed aged in the indenter plots). The software parameters controlling the operation of the indenter were set as follows: Desired Velocity: 5.08 mm/min Maximum Deformation: 0.1 to 0.7 mm Maximum Force Allowed: 9 Newtons Modulus Force Range, for outer Jacket: 4.5 to 8.5 N Modulus Force Range, for composite insulation: 1 to 3 N Ambient Temperature: 23 -25 °C

3 RESULTS AND DISCUSSION

3.1 SIMILARITY BETWEEN COMPOSITE JACKET1 AND OUTER JACKET

Formulations of composite jacket and outer jacket were characterized by measuring filler, fire retardant type and content, melting temperature, and by Fourier Transform Infrared (FTIR) analysis. CSPE composite jacket and outer jacket formulations were found to be similar for the same manufacturer, except for cable US#2. The composite jacket of cable US#2 contained 40% more filler than its outer jacket. The outer jackets of cables US# 2 and US#3 were slightly different from the other cables in that the antimony oxide fire retardant additive was present. Cables US#1 and US#23 were made by the same manufacturer, but at different manufacturing periods. The formulation analysis confirmed that the outer jackets were similar.

3.2 VISUAL INSPECTION 3.2.1 OUTER JACKET

The cable specimens were visually inspected for any change in physical appearance such as discoloration, change in sheen, surface deposits or cracking as a function of aging period. The outer jackets of six of the seven cables evaluated were black. The outer surface of the jackets of US#23 was painted blue. Upon aging a white dust covering was found on the outer jackets of cables US#2 and US#3, which was identified as antimony chloride or by-product of antimony oxide [12]. On cable US#2 a white dust appeared after 2 months at 110 °C followed by the formation of a thick white coating upon further aging. For cable US#3, only a whitish dust was formed after 4 months at 110 °C. The blue colored outer jacket of cable US#23 darkened upon aging, after 3500 h at 110 °C. The black colored outer jacket of cable US#15 turned to brownish-grey after 6 months at 110 °C. The outer jackets of cables US#8 and US#10 did not exhibit any visible differences even after aging for 5000 h at 110 °C.

3.2.2 COMPOSITE JACKET Five composite jackets were black, US#3 was beige colored and cable US#15 had white colored painted surfaces. Only cables US#3 and US#15 showed visible changes upon aging. The composite jacket of cable US#3 discolored within a month at 110 °C and after 3 months the composite insulation became brownish in color. However, the white colored (painted surface) composite jacket of cable US#15 showed only a slight discoloration after 6 months at 110 °C.

3.3 MANUAL MANIPULATION None of the composite insulated wires or outer jackets exhibited evidence of cracking within the thermal aging period of this study. However, flexing the composite wires resulted in cracking of either only the composite jacket or both the insulation and composite jacket depending on the exposure period. During the early stages of aging only the composite jacket cracked, i.e. the conductor was not exposed. On further aging, both the composite jacket and insulation cracked simultaneously, exposing the conductors, regardless of whether the cable had bonded or non-bonded composite insulation. It was noticed that during aging the Hypalon layer fuses with the EPR insulation.

The composite EPR/Neoprene insulated wire of cable US#23 also exhibited similar behavior to the EPR/Hypalon insulated wires, i.e. cracking of the aged composite insulated wires upon flexure. For the EPR/Neoprene insulated wire, it was noticed that upon application of gentle pressure with pliers on the composite insulation, the Neoprene layer crumbled and could be removed, exposing the flexible EPR insulation. It was also noticed that the edges of severely aged Neoprene pieces were sharp and could cut into the EPR insulation when flexed. This could result in mechanical and 1: Also referred as inner jacket, bonded jacket, or insulation jacket.

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electrical failure during LOCA even though the EPR insulation was flexible.

3.4 TIME- TEMPERATURE SUPERPOSITION The elongation and indenter modulus values of the composite insulation and the outer jacket were plotted as a function of aging time for the seven cables evaluated. Through the utilization of the time -temperature superposition principle the elongation values obtained at the two aging temperatures were superposed to the lowest temperature of aging [13]. The time–temperature superposition principle involves shifting the time-dependent data at each temperature to a common reference temperature by multiplying the aging times by empirical multiplicative constants, At, determined from the Arrhenius relationship, given by At= exp Ea/R Tr

-1 – Tf-1

where, Tr and Tf are the aging and reference temperatures respectively, Ea is the activation energy for the degradation process and R is the gas constant. When the aging parameter versus the shifted aging times are superposed to a single curve, it implies that the aging parameters follow an Arrhenius aging behavior and thus allows calculation of the activation energy of the degradation process. To verify the Arrhenius relationship aging data is required from at least three aging temperatures. The Arrhenius relationship was already established for similar CSPE materials [13]. The activation energy of each of the cables was determined using this previously established Arrhenius relationship with the shift factor determined with data from two aging temperatures.

4 ELONGATION AT BREAK AND INDENTER

MODULUS

4.1 CABLE US#1 The superposed elongation at break values of the composite insulation and the outer jacket of cable US#1 are plotted in Figure 1. For both the composite insulation and outer jacket a shift factor of 2.3 resulted in a good overlap of the elongation values obtained at 120 °C and 110 °C. The activation energy corresponding to this shift was 1.08 eV. The trend of elongation values in Figure 1 suggests that composite insulation and outer jacket deteriorated progressively at the same rate for up to 1700 h. On further aging, the composite insulation deteriorated at a faster rate than the outer jacket. The most likely explanation for this phenomenon is that the Hypalon composite jacket fused with the EPR insulation, resulting in the premature failure of the composite insulation during tensile testing. It is known that for fused composite materials the embrittled material controls the elongation, as cracks in the brittle material tend to propagate through the flexible material. The indenter modulus values of the composite insulation and outer jacket are correlated with the elongation values as

shown in Figure 2. The indenter modulus values of the unaged composite insulation and outer jacket were 4.4 N/mm and 10.3 N/mm, respectively. The difference in modulus can be explained by the underlying composition of the samples. The completed cable does not have the underlying base as does the single wire which consequently results in higher indenter modulus values. It was also observed that regardless of whether the composite insulation was directly exposed or aged inside the complete cable, the indenter modulus remained the same. Good correlation was found between literature [14] data of naturally aged cable outer jackets and the accelerated aging data. The naturally aged cables were removed from Detroit Edison’s Fermi 2 NPP in 1992 after service for approximately 7 years at estimated ambient temperatures ranging from 49 to 82 °C. The data demonstrates that the indenter modulus values obtained from accelerated aging studies, shown in Figure 2, are representative of the US#1 cables aging in field environments.

4.2 CABLE US#2 Superposed elongation and indenter modulus values of the composite insulation and the outer jacket of cable US#2 are plotted as a function of aging time in Figures 3 and 4. A shift factor of 2.3, which was used earlier for cable US#1 resulted in a good overlap between the data at 120 and 110 °C. Therefore, the activation energy of degradation of the composite insulation and the outer jacket was the same.

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Figure 1. Time-temperature superposed elongation results for CSPE outer jacket and EPR/CSPE composite insulation of cable US#1 at a reference temperature of 110 °C.

The trend of elongation values in Figure 3 shows that the outer jacket was aging at a significantly faster rate than the composite insulation. For example, the elongation value of the outer jacket reached 50% absolute elongation after 1000 h while the composite insulation reached the same absolute elongation after 2000 h of aging at 110 °C. The indenter modulus data in Figure 4 reveal that for up to 1000 h (until jacket reached 50% absolute elongation), the modulus values

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 1; February 2007 of the composite insulation and outer jacket were similar and both increased at the same rate.

On further aging the composite insulation modulus continued to increase at the same rate but the modulus of the outer jacket increased at a much higher rate. Significant differences in the aging behavior of the composite insulation and the outer jacket are in agreement with the differences in their formulation and the observed differences during aging.

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Most importantly, the differences in the aging trends between the outer jacket and composite insulation suggest that embrittlement of the outer jacket does not necessarily correspond to deterioration of the insulation material of US# 2 cables.

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A comparison between the outer jacket elongation data in the Sandia study [13] and this study indicates that the outer

jacket used in this evaluation aged almost at twice the rate. This was due to differences in aging environments. In the Sandia study samples were aged inside ventilated cans to restrict the air flow rate. The samples from this present study were directly exposed to forced air environments as in the EPRI evaluation [8]. The elongation and indenter values of the composite insulation measured in this study are in agreement with the data reported in the EPRI report.

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Figure 4. Time-temperature superposed indenter modulus results for CSPE outer jacket and EPR/CSPE composite insulation of cable US#2 at a reference temperature of 110 °C.

4.3 CABLE US#3 The elongation and indenter modulus values of cable US#3 are illustrated in Figures 5 and 6. A shift factor of 2.3 provided a good overlap between the data obtained at 110 and 120 °C for both the composite insulation and the outer jacket. The 50% absolute elongation value was reached after approximately 790 hours of aging at 110 °C for the composite insulation, and after 1820 h for the outer jacket. The elongation data show that the composite insulation was aging at a very fast rate compared with the outer jacket. The aging rate of the composite insulation of cable US#3 was the most rapid among the bonded or non-bonded EPR/CSPE composite insulated wires. However, the aging rate of the CSPE outer jacket was similar to all the other outer jackets except that of cable US#2. Compared to other composite EPR/CSPE insulated wires evaluated it was noticed that the CSPE composite jacket of cable US#3 was bonded (fused) with the EPR insulation during the manufacturing process. This bonding seems to have been the cause of faster embrittlement and the high initial indenter modulus (11 N/mm) for cable US#3 in comparison to the other EPR/CSPE insulated wires (4 to 8 N/mm).

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4.4 CABLE US#10 Figures 7 and 8 apply to cable US#10. The construction of cable US#10 was based on a non-bonded EPR/CSPE composite insulation and a CSPE outer jacket. The superposition of the elongation and the indenter data obtained at 110 and 120 °C was achieved using a time shift factor of 2.4. The activation energy of this shift corresponded to 1.135 eV. A good agreement was found between the elongation values of the composite insulation and the outer jacket as shown in Figure 7, showing that the rate of degradation of the composite insulation and the outer jacket was the same. Both the composite insulation and outer jacket reached the 50% elongation value after approximately 4287 h of aging at 110 °C. This was the best thermal life exhibited by any of the bonded or non-bonded EPR/CSPE insulated wires evaluated. The common shift factor and the overlapping elongation values of the composite insulation and the outer jacket again validated that the degradation rate of the composite insulation

would be driven by the CSPE composite jacket regardless of whether or not the composite insulations consist of bonded or non-bonded EPR/CSPE insulations.

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4.5 CABLE US#15 The elongation and indenter modulus data for Cable US#15 are illustrated in Figures 9 and 10. This cable was of similar construction as cable US#10. The construction is based on non-bonded EPR/CSPE composite insulation and CSPE outer jacket. The elongation data of Figure 9 shows that a good agreement of the 110 and 120 °C data exists with a shift factor

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 1; February 2007 2.3 which was also used for the five of the seven cables evaluated. The elongation data also reveal that the degradation rates of the composite insulation and outer jacket slightly differed during the early stages of aging but overlapped upon reaching the 50% elongation value after aging for 3250 h at 110 °C.

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4.6 CABLE US#8 The elongation and indenter modulus data of cable US#8 are illustrated in Figures 11 and 12 as a function of aging time. The cable had a CSPE outer jacket and the insulation consisted of a single layer. The Kerite insulation formulation contained either CSPE or a similar material. Figure 11 shows that the aging trend of the insulation and jacket measured as elongation values at 120 °C was essentially the same. A reasonable superposition was possible between the insulation and jacket elongation values measured at 120 °C and the jacket elongation values measured at 110 °C using a shift factor of 2.4. It may be noted that insulation elongation data was not collected at 110 °C. The progressive decrease of the elongation values of the insulation is in agreement with its CSPE-like base polymer structure. The insulation and jacket exhibited a fairly good thermal life. As shown in Figure 11, the insulation and jacket required in excess of 5350 h at 110 °C to reach the 50% elongation value. The cable manufacturer reported that 1950 h of aging at 120 °C was required to reach the 50% elongation threshold [10], which compares favorably with the values in this study (2016 h at 120 °C).

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Figure 11. Time-temperature superposed elongation results for CSPE jacket and Kerite insulation of cable US#8 at a reference temperature of 110 °C.

4.7 CABLE US#23 The cable US#23 consisted of a non-bonded EPR/Neoprene composite insulation and a CSPE outer jacket. The elongation and indenter data of the composite EPR/Neoprene insulated wires of cable US#23 behaved similarly to that of EPR/CSPE insulated wires discussed in the earlier sections. As found with the EPR/CSPE composite wires, the presence of the CSPE outer jacket did not provide any extra protection for the EPR/Neoprene composite insulation. The aging rate was essentially the same whether or not the composite wire was exposed directly to the heat environment or aged inside the outer jacket (See Figure 13). Due to insufficient test cable length, data was not collected at temperatures differing by least 10 °C to determine the shift factors. The shift factor for the CSPE outer jacket was taken as that of cable US#1 and that for the Neoprene composite jacket was estimated from the literature activation energy

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value [13]. It is evident in Figure 13 that the shifted indenter modulus value of the longest aged composite insulation inside jacket does not overlap well with the trend of directly aged composite insulations. As shown in the figure, when the indenter modulus values sharply increase significant uncertainty was evident. When the deviation values are included, however, the common trend of the directly aged and aged inside jacket is followed.

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Figure 12. Time-temperature superposed indenter modulus results for CSPE jacket and Kerite insulation of cable US#8 at a reference temperature of 110 °C.

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Figure 13. Time-temperature superposed indenter modulus results for CSPE outer jacket and EPR/Neoprene composite insulation of cable US#23 at a reference temperature of 110 °C. The Neoprene composite jacket aged for 1085 h at 110 °C had elongation value in the 25% range. At this level of aging the Neoprene composite jacket exhibited cracking and separation from the EPR insulation during installation of the composite specimens into the tensile machine. However, the EPR insulation of the composite wire continued to elongate, in excess of 300%. On further aging, the composite EPR/Neoprene specimens were extremely hard and showed less than 10% absolute elongation so that cracking of these specimens occurred when flexed. It was previously explained that when the Neoprene layer of these specimens was removed with pliers, the elongation of the EPR underneath was in

excess of 50% absolute. At the same time, it was postulated that the sharp edges of the aged Neoprene jacket layer could induce micro cracks in the EPR insulation. The elongation data shows that when the Neoprene composite jacket elongation reaches approximately 25%, the Neoprene composite jacket will not damage the EPR insulation when flexed or bent. However, on further aging the indenter modulus values of the composite insulation will increase drastically. Therefore, the indenter modulus value when Neoprene composite jacket reaches 25% elongation can be taken as the threshold value for continued service performance of this type of cable. The corresponding indenter modulus value for the Neoprene composite jacket and the CSPE outer jacket was 24 N/mm as shown in Figure 13.

5 CORRELATION WITH LOCA PERFORMANCE

One of the objectives of this evaluation is to determine a indenter modulus range for the composite insulation and outer jacket, which will allow for the prediction of the performance of installed cables during normal service including an accident event. Performance of the cables under accident conditions was evaluated by Sandia [15] and Brookhaven National Laboratories (BNL) [9]. Cable samples in single conductor and complete cable configurations were subjected to a range of normal service radiation and thermal exposures and then followed by accident irradiation dose and a simulated LOCA environment. The cable performance was assessed via measurement of the insulation resistance values (IR) during the steam test or via a dielectric withstand test upon completion of the steam test.

In this evaluation, the harshest normal service conditions used in the Sandia or Brookhaven study under which the cables satisfactorily functioned during the steam test are considered. The indenter modulus values are primarily affected by thermal aging [8], although, due to synergistic effects, a small increase in the indenter modulus can be attributed to radiation (Note: Elongation values are affected both by radiation and thermal exposure). As indicated earlier, during the normal service cables are exposed to irradiation levels below 10 to 20 Mrad. Ignoring the effect of radiation exposure, equivalent thermal aging periods used during the qualification tests were calculated using the appropriate activation energies of the Hypalon composite jackets. Using the Indenter plots constructed earlier, corresponding indenter modulus values for satisfactory LOCA performance are estimated. The indenter modulus values thus derived can be considered conservative. However, if the normal service radiation level for the qualification testing was reduced or excluded, cables could withstand additional thermal aging and hence the indenter modulus values will be higher.

5.1 CABLE US#1 In the Sandia study, a set of US#1 cables (single conductor and complete cable) were subjected to 40 Mrad of normal

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 1; February 2007 radiation exposure and thermal aging at 98 °C for 183 days followed by 110 Mrad of accident radiation. Both the single conductor and complete cables functioned satisfactorily during the LOCA steam test as demonstrated by insulation resistance measurements (1x105 Ω /100m). However, performance of the single conductor was unsatisfactory during the LOCA steam test for another set of US#1 cable subjected to 60 Mrad of normal radiation exposure and 274 days of thermal aging at 98 °C and then followed by 110 Mrad of accident radiation exposure.

Ignoring the effect of radiation exposure, aging periods equivalent to 183 days and 274 days at 98 °C are estimated to be 1552 h and 2282 h at 110°C respectively with an activation energy of 1.08 eV for Hypalon. Installed cables are only exposed to a radiation level of less than 20 Mrad and thus one can expect satisfactory performance of cables subjected to the upper limit of thermal aging, i.e. 2282 h at 110 °C. The indenter modulus range corresponding to 2282 h of aging at 110 °C for the composite insulation and outer jacket on Figure 2 is approximately 17 to 20 N/mm and 32 N/mm, respectively.

5.2 CABLE US#2 The Sandia study [15] indicates that cables similar to US#2 functioned satisfactorily during the steam test, after having been subjected to the harshest aging conditions of the study. This corresponded to 60 Mrad of normal radiation exposure and 274 days of thermal aging at 98 °C followed by 110 Mrad of accident radiation exposure. The IR values (2 to 7x103 Ω / 100m) measured at the peak temperatures of the steam test were low for instrumentation cable applications, but would be acceptable for power (600V) and control cable applications. The equivalent thermal aging period corresponding to 274 days at 98 °C was 2282 h at 110 °C for an activation energy of 1.08 eV. The indenter modulus values corresponding to 2282 h of aging for the composite insulation and outer jacket are approximately 32 N/mm and 200 N/mm respectively (See Figure 4). However, sudden rise in the indenter modulus values for the outer jacket suggests that some uncertainty may exist and values below 200 N/mm shall be appropriate for the outer jacket.

5.3 CABLE US#3 The LOCA performance of cable US#3 was variable. In a Sandia study [15] one of the multi conductor cables failed when subjected to 20 Mrad plus 91 days at 98 °C and an accident radiation dose of 110 Mrad followed by a LOCA steam test. However, in the Brookhaven study [9], similar cables performed acceptably after exposure to 121 °C for 252 h plus 77.3 Mrad of normal radiation dose and 156.6 Mrad of accident radiation dose. Equivalent thermal aging periods corresponding to the Sandia and Brookhaven studies using a CSPE activation energy of 1.08 eV are 761 hours and 629 h at 110 °C respectively. Assuming thermal exposure to be the primary cause of embrittlement, it can be expected that the cables would function with 50% remaining elongation. This

was attained at approximately 790 h of aging at 110 °C. Indenter modulus values corresponding to 790 h of aging in Figure 6 were 20 N/mm for the composite insulation and 15 N/mm for the outer jacket.

5.4 CABLE US#10 The cable US#10 was exposed to 169 h of thermal aging at 150 °C plus 53.6 Mrad of normal radiation dose and 154 Mrad of accident radiation dose in the Brookhaven study [9]. After the exposure to the LOCA steam test the cable passed the 2400 Vac dielectric withstand test. The thermal aging period of the study i.e., 169 h at 150 °C, translated to 4370 h of aging at 110 °C for an activation energy of 1.135 eV (See Figure 7). The indenter data provided in Figure 8 indicates that the threshold indenter modulus value of the composite insulation and the outer jacket at this level of thermal aging corresponds to approximately 20 N/mm and 40 N/mm, respectively.

5.5 CABLE US#15 The composite insulation of this cable was soft and very flexible. Indenter readings of significantly aged insulations varied as much as ±20% depending on the location of the measurement. The reported post-LOCA performance of a cable similar to cable US#15 was also different from other cables considered in the Brookhaven study [9]. A similar cable, though exposed to a minimal level of thermal aging, did not meet the post-LOCA electrical performance requirements. The outer jacket exhibited a number of cracks and pulled away from the mandrel in a number of sections due to extensive swelling of the jacket. The thermal aging period was 252 h at 121 °C and the radiation levels were 38.7 Mrad of normal radiation dose and 156.6 Mrad of accident radiation dose. An equivalent thermal aging time corresponding to 252 hours at 121 °C equates to only 629 h of aging at 110 °C for an activation energy of 1.08 eV. At this level of aging, only a minimal change in the elongation or indenter modulus values can be seen in Figures 9 and 10 in comparison to the as received values. No viable explanations can be given for the poor performance of this cable. For informational purpose, the indenter modulii of the composite insulation and outer jacket at the 50% elongation threshold extracted from Figure 10 were shown in Table 1.

5.6 CABLE US#8 According to the Sandia study [15], the measured IR readings were extremely low (350 to 1400 Ω /100m) during the LOCA test. The Sandia aging conditions were 60 Mrad of normal radiation exposure and 274 days of thermal aging at 98 °C followed by 110 Mrad of accident radiation exposure. Thermal aging for 274 days at 98 °C corresponded to 2170 h at 110 °C for an activation energy of 1.135 eV. The elongation value of the insulation at this level of aging (Figure 11) was 200%. Radiation exposure of 60 Mrad can reduce the elongation to 0% [12]. But, the most likely explanation for the low IR readings during the steam test is that the chemistry of Kerite insulation was altered by radiation or thermal aging

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as in PVC insulated cables or the insulation material had inherently poor electrical behavior when exposed to heat as in PVC [16]. The indenter modulii of the insulation and outer jacket after 2170 h of aging corresponded to 12 N/mm and 16 N/mm, respectively. The indenter modulus value at 50% elongation level was 30 N/mm for the insulation and 42 N/mm for the outer jacket.

6 SUMMARY OF THRESHOLD MODULUS VALUES FOR SATISFACTORY LOCA PERFORMANCE

The indenter modulus values of the composite insulation and the outer jacket of various cables are summarized in Table 1. Generally, as received modulus values for the composite insulation were in the 4 to 8 N/mm range and for the outer jacket in about 10 N/mm. Significant changes in the indenter modulus values can be seen as the composite insulated wires of each cable reached 50% elongation values. These observations agree with Gillen et al work that showed similar correlation for thermally aged materials between compressive modulus and 50% absolute elongation [13]. Most interestingly, close correlation was found between the thermal aging periods used for satisfactory LOCA performance and the time to reach 50% elongation of the composite insulation. Consequently, threshold indenter modulus values for satisfactory LOCA performance of the composite EPR/CSPE insulated cable and the indenter modulus values when the composite insulation reached 50% elongation values were in the same range. Table 1. Indenter Modulus (IM) values of composite EPR/CSPE insulated wires at various stages of aging

Cable ID As received

IM (N/mm)

IM When Composite Ins Reached 50%

Elongation (N/mm)

IM for Satisfactory LOCA Performance

of Composite Insulation (N/mm)

US#1, Ins1 4.4 15-20 17-20 Jacket2 10.3 28 32

US#2, Ins1 7.5 26 32 Jacket2 10.7 <200 <200

US#3, Ins1 10.7 20 20 Jacket2 10.2 15 15

US#10, Ins1 5.1 18 20 Jacket2 11.8 37 40

US#15, Ins1 8.1 35 Not Available Jacket2 10.6 22 Not Available

US#23, Ins3 4.4 11 Not Available Jacket2 10.3 22 Not Available

US#8, Ins4 6.5 30 12 Jacket2 10.3 42 16

1: EPR/CSPE Composite Insulation, 2: CSPE outer Jacket, 3: EPR/Neoprene Composite Insulation, 4: Kerite Insulation

7 CONCLUSIONS

Five composite EPR/CSPE insulated cables, a single composite cable with EPR/Neoprene, and a single Kerite insulation commonly used in US nuclear power plants were evaluated for thermal degradation. All cables had a outer CSPE jacket. The cables were thermally aged at 110 °C and

120 °C until embrittlement of the insulation and outer jacket occurred. The condition of the insulation and jackets was assessed by measuring elongation at break and indenter modulus. The elongation and indenter modulus values of the composite insulation and the outer jacket aged at 120 °C exhibited excellent superposition with values aged at 110 °C using the time-temperature superposition principle. The elongation and indenter modulus data of a field aged Hypalon outer jacket, similar to two of the cable outer jackets used in this study, exhibits very good correlation with the accelerated aging data and provides evidence that the indenter modulus values obtained in this study are representative of field aging. The cable degradation rates are different for different manufacturers and results obtained can be applied only to the unique formulation. When the weakest link of the composite insulation, namely CSPE or Neoprene, reached the embrittlement stage, it induced cracking of the EPR insulation when flexed regardless of whether or not the composite wires were bonded or non-bonded insulation types. The indenter modulus and elongation values of the composite insulation were not affected by the aging mode, i.e., whether the composite insulated wires were aged as a complete cable or with the outer jacket removed. There is a close correlation between the thermal aging periods used for satisfactory qualification of these cables and the time to reach 50% elongation for the composite insulation. Cables that exhibit indenter modulus in excess of the threshold values identified in this study will require removal of a sample for elongation or EQ testing to allow cable life extension.

ACKNOWLEDGEMENTS The author would like to thank O. Kiir for technical assistance and D.J. Stonkus, R. Lewak, and P.V. Castaldo for useful suggestions and comments.

REFERENCES [1] M.J. Jacobus, “Aging, Condition Monitoring, and Loss-of-Coolant

Accident Tests of Class 1E Electrical Cables: Ethylene Propylene Rubber Cables”, NUREG/CR-5772, SAND91-1766/2, Vol.2, 1992.

[2] K.T. Gillen and R.L. Clough, “Predictive Aging Results for Cable Materials in Nuclear Power Plants”, Sandia Report, SAND90-2009, 1990.

[3] J. Kingseed, W.M. Denny, and B.J. Grabusky, “Cable Aging Management Program for D.C. Cook Nuclear Plant Units 1 and 2”, EPRI Report TR-106687, 1996.

[4] G. Toman, “Nuclear plant evaluation via Visual/Tactile and Indenter Techniques”, International Atomic Energy Agency (IAEA) Meeting on “Enhancing Nuclear Power Plant Safety, Performance and Life Extension through Effective Aging Management. 2002.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 1; February 2007 [5] D.J. Stonkus, “Physical Degradation Assessment of Generator Station

Cables”, Proc. EPRI Power plant Condition Monitoring Workshop, San Francisco, February 16-18, 1988.

[6] W. Michel, “Prognosis on the Aging of Cables”, IAEA Specialists Meeting on Effectiveness of Methods for Detection and Monitoring of Age Related Degradation in Nuclear Power Plants, Bariloche, Argentina, 1995.

[7] K. Anandakumaran, W. Seidl, and P.V. Castaldo, “Condition Assessment of Cable Insulation Systems in Operating Nuclear Power Plants”, IEEE Trans. Dielectr. Electr. Insul., Vol. 6, pp. 376-384, 1999.

[8] G. Toman, EPRI Project Manager, “Initial Criteria Acceptance Concepts and Data for Assessing Longevity of Low-Voltage Cable Insulations and Jackets”, EPRI Technical Report 1008211, Final Report, 2005.

[9] U.S. Nuclear Regulatory Commission. Assessment of Environmental Qualification Practices and Condition Monitoring Techniques for Low-Voltage Electric Cables: LOCA test Results, Brookhaven National Laboratory. Nureg/CR-6704, Vol. 1, 2001.

[10] System 1000, Materials Aging and Radiation Effects Library, Revision 16.

[11] G. Toman, S. Hunsader, and D. Peters, “In-Plant Indenter Use at Commonwealth Edison Plants, Proceedings: EPRI Power Plant Cable Condition Monitoring Workshop, San Francisco, California, February 1993.

[12] M. Subudhi, “Literature Review of Environmental Qualification of Safety Related Electric Cables, BNL Report, Nureg/CR-6384, Vol. 1, 1996.

[13] K.T. Gillen, R.A. Assink, and R. Bernstein, “Nuclear Energy Plant Optimization (NEPO) Final Report on Aging and Condition Monitoring of Low Voltage Cable Materials”, Sandia Report, SAND2005-7331.

[14] L.R. Raisanen, “Fermi 2 Cable Aging and Surveillance Program”, EPRI Workshop on Cable Condition Monitoring, Rockville, Maryland, USA, 1996.

[15] M.J. Jacobus, “Loss-of-Coolant Accident (LOCA) testing of Aged Cables with application to nuclear plant life extension”, Nuclear Engineering and Design, Vol. 134, pp.267-275, 1992.

[16] K. Anandakumaran, S. Barreca, W. Seidl, and P.V. Castaldo, “Nuclear Qualification of Installed PVC Insulated Cables”, IEEE Trans. Dielectr. Electr. Insul., Vol. 8, pp.817-825, 2001.

K. (Anand). Anandakumaran has been with Ontario Hydro Research Division and its successor companies since 1988. He obtained the Ph.D. degree in polymer chemistry from City University of New York in 1983 and continued as a postdoctoral fellow at the Institute of Molecular biophysics at Florida State University and Pulp and Paper Research Institute at McGill University. He has extensive experience in determining the performance of safety related cables under the effects of radiation, thermal, and LOCA/MSLB steam environments. His experience includes condition monitoring, failure analysis and life assessment of PVC, PE, XLPE, EPR, Butyl, SBR, Hypalon, and Tefzel insulated cables and nitrile, Viton, silicone, and EPDM based seals, gaskets, O-rings and diaphragms. At present he is a project manager at Kinectrics Inc. for projects involving EQ testing, condition monitoring, and failure analysis of electrical cables used in various Nuclear Power Plants. He has published about 15 papers.

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Aging and condition monitoring studies of composite insulation cables used in nuclear power plants