A DDITIONAL I NSIGHTS FROM THE N UCLEAR I NDUSTRY

The nuclear industry has decades of operational experience on fielded cable performance.

Data obtained from the nuclear power utilities provides a comprehensive perspective and overview of the cable systems found throughout the US. The Nuclear Energy Institute (NEI) is a board of nuclear professionals that provides a collective opinion on policy decisions, helps resolve technical initiatives, and responds to information queries. In 2005, NEI conducted a survey to obtain data on underground medium voltage (MV) cables.²⁷ Specific cable information (such as rated and applied voltages, manufacturer, insulation type, years in service, cable conductor shield attributes) was requested from the nuclear fleet. Based on the work, NEI explicitly states that, “[w]etting of energized medium voltage cable does accelerate the effect of aging.”²⁷

The effort received 81 unit responses which represented 51 plants. Data obtained from NEI’s survey revealed that 65 units reported underground conduits, 76 reported underground ducts, 23 reported direct buried circuits, and 21 reported enclosed trenches with supported cables.²⁷ A summary of MV cables found in US nuclear power plants was reproduced from the document and presented in Table 3. It should be noted that red EPR is synonymous with pink EPR and both terms may be used interchangeably throughout the text.

Table 3: Summary of data collected from the NEI survey

Insulation

EPDM: ethylene propylene diene monomer, PILC: paper insulated lead jacketed cable

Data presented in Figure 2 shows the predominant MV cable insulations used at nuclear power facilities are EPR and XLPE. When combining the different insulation types for all MV cables, EPR and XLPE represent approximately 95% of the survey data as shown in Figure 2.

Although not all plants responded to the survey request, the information coincides with numerous sources on the distribution of insulation materials at nuclear power facilities. The primary focus of the subsequent sections of this review will be on EPR and XLPE given the predominance of these materials in the nuclear fleet.

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Figure 2: Percentage of MV insulation materials from NEI survey data

As a part of the survey, NEI requested failure information for MV cables in underground, wet environments. Of the 74 units which reported data, 21 units experienced a total of 50 circuit failures as of the date of the white paper. The cables were designed to operate throughout the original plant lifetime, but prematurely failed due to degradation in submerged environments.

Figure 3 illustrates the total number of failures distributed over years of service.

Data from the NEI white paper separated by insulation type is presented in Figure 4. Filled XLPE failures were distinguished separately because they all occurred at one plant and for a specific type of filled XLPE. For the red EPR, the failure occurring during the fifth year of service was caused by a manufacturing defect. The three red EPR failures occurring during the sample’s tenth year of service were all located at the same plant; however, additional information was not provided. There were no failures of brown EPR reported in the survey.

Authors of the white paper indicated that the four butyl rubber failures appeared to be low, but noted that only four reporting units used that particular insulation material. Although the NEI survey requested and presented cable manufacturers, there was no attempt to correlate cable manufacturer data to electrical failures.

Data presented in the NEI white paper provides a reasonable evaluation of cables found at nuclear power plants, although half the US facilities did not participate. The paper offers recommendations to help mitigate submergence related failures by 1) improving methods to keep the cable systems dry, 2) anticipating cable failures, and 3) sharing failure resolutions.

EPRI supplemented the data obtained from the NEI survey which helped provide a framework for an aging management program.¹⁵ Supplemental details to the NEI data, as presented by EPRI, may be found in Table 4. The recommendations for the aging management program may

Butyl Rubber

2% EPDM

0%

Black EPR 34%

Brown EPR 14%

Red EPR 31%

XLPE 16%

PILC 2%

Unknown 1%

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be found in Table 5. It should be noted that pink EPR is listed separately by manufacturers.

Given the data from the white paper, the recommendations may be reasonable.

Figure 3: Total number of submerged cable failures for the 21 reported units versus cable years in service at time of failure

Figure 4: All cable failures separated by insulation material 0

1 2 3 4 5 6 7

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Number of Failures

Years of Service

0 1 2 3 4 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Number of Failures

Years of Service Filled XLPE

XLPE Black EPR Red EPR Butyl Rubber

Table 4: Cable susceptibility under wet conditions (reproduced from EPRI 30020000557)

1975 - 1980 Moderate 34 years 10 - 12 years Water degradation failures have been observed in the nuclear industry starting at 24 years of service

10 - 12 years Failures were observed starting at 10 years of service, with many failures between 10 and 25 years Butyl

Rubber

GE, Collyer, Okonite

1967 - 1972 Small 42 years 20 - 25 years Water degradation failures have been observed in the nuclear industry starting at 25 years of service date in the nuclear industry with 20 - 30 years of service

Brown EPR Kerite Bulk 1972 - 1985, some 1990 - 2003

Moderate 37 years 20 - 25 years No water-related failures have been observed to date in the nuclear industry

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Table 4: Cable susceptibility under wet conditions (reproduced from EPRI 30020000557)¹⁵ Material Manufacturers

31 years 20 - 25 years No water-related failures have been observed to date in the nuclear industry, one

31 years 20 - 25 years Some early failures with water combined with manufacturing

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Table 5: Aging management recommendations based on insulation material type (reproduced from EPRI 30020000557)¹⁵ Material Manufacturers

1975 - 1980 Implement aging management program

1967 - 1972 Implement aging management program

TR-XLPE Not known 2004 Implement aging

management program for cables with more than 30 years of service.

In 2009, a position document created by EPRI on the suitability of EPR MV cables for wet and submerged environments provided perspective from industry on the intended application.¹⁰ The paper reviewed the content of NRC violations and the regulatory concerns and issues which provides a comparison between the construction of submarine and underground cables through standards and manufacturers as well as case studies and long-term aging tests of EPR cable

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samples. From these bases, the paper contends that medium voltage EPR cables were appropriately designed for wet and submerged exposures; however, the current state of installed cables was intentionally not addressed by the paper.

In an ongoing effort to better understand the aging and condition assessment of MV insulation materials in submerged environments, EPRI has maintained an analytical experimental program for degraded and failed cables from nuclear power plants.28,29,30,31 Insights developed from the research are intended to help define acceptance criteria differentiating good and severely degraded cables. The cable analyses of field returned samples were mostly done by Cable Technology Laboratories (CTL). The work is classified as EPRI Proprietary Licensed Material and only a limited amount of material was presented in Table 6.

Table 6: Summary of cables removed from service or failed in service and subsequently evaluated by CTL generally coincides with the NEI survey which complements the aging management program recommendations put forth by EPRI. Exceptions include the Anaconda pink EPR and Kerite brown EPR; however, these cables did not fail and were removed for other reasons.