EPRI Research Update- Radiation and Temperature Monitoring and Submergence Qualifications

Transcription

1 EPRI Research Update- Radiation and Temperature Monitoring and Submergence Qualifications Andrew Mantey Principal Technical Leader EQ Technical Meeting November 2, 2018

2 Cable Technical Seminar Temperature and Radiation Study of Cables in Nuclear Power Plants EPRI Submergence Research Summary 2

3 Aging Stressors Low vs Medium/High Voltage Cables Low Voltage Cable Aging Stressors Major aging concerns Thermal if >50 C Radiation if > 200 kgy Medium/High Voltage Cable Aging Stressors Electrical stress induced degradation Water treeing/submergence Partial discharge in joints/splices or terminations Lesser aging concerns Chemical Polymer stability in water Lesser aging concerns Thermal Thermal/radiation Ohmic heating Chemical 3

4 Thermal and Radiation Aging Degradation of Cables Design Basis Accident Qualified Cable Radiation complicates things Ambient (typical design is for 40 C) Radiation dose accumulation Effect similar to thermal: insulation and jacket harden as they age Radiation can become the dominant factor in determining end of life at higher accumulated dose levels (> 200 kgy) and in some insulations dominate time to end of useful life Radiation Complicates Things 4

5 Cable Submergence Degradation Mode: Water Treeing Water Treeing, conditions needed: Voltage stress > 0.8 kv/mm Water or high humidity Polymer Medium voltage typically operates > 1 kv/mm Low voltage typically operates < 0.2 kv/mm Only Medium Voltage Cable Water Tree 5

6 Temperature and Radiation Study of Cables in Nuclear Power Plants 6

7 Temperature and Radiation Monitoring Study Determine whether electrical cables installed in nuclear power plants will be able to perform reliably if they are operated beyond the current licensing period. Inform the knowledge gap of understanding the expected degree and rate of degradation of cables involves the thermal conditions and radiation exposure that cables are subjected to in their operating environments. 7

8 Radiation and Temperature Assessment for Cables Phase 1: Existing radiation and temperature data collected from US NPPs Limited responses, mostly temperature data (14 plants) with limited radiation data While data indicated less severe conditions then qualification lifetime values, there was insufficient data to draw any specific conclusions Phase 2: 55 radiation and temperature monitors have been installed in containment and other high thermal/radiation areas Collect data radiation and temperature data for a fuel cycle Quantify cable operating environment Evaluate collected data to see if it can inform inverse temperature, synergistic effect research Provide methodology for future studies 8

9 Sensor Deployment Monitors were installed for 18 months 55 monitors installed 40 monitors were installed in Containment. 15 monitors were installed in the Auxiliary Building All monitors contained a thermal sensor and thermoluminescent dosimeter (range up to 1kGY) 12 monitors had an optical crystal in areas of expected higher dosage (range 1 10,000 kgy ) 9

10 Plant Monitor Installation Sequential alphanumeric numbers were added to the front and back of each monitor, as monitor serial numbers were recorded pre-installation to specific locations. 10

11 Results- Temperature Containment temperatures ranged from 26 C (80 F) to 56.0 C (132.8 F), mean 35.4 C (95.8 F ) only one (1) temperature monitor location exceeded that the plant design service temperature of 48.8 C (120.0 F) that monitor recorded 56.0 C (132.8 F) 11

12 Results- Temperature and Radiation Auxiliary Building temperatures all results were > 5.5 C (>10 F) less than the EQ zone normal temperature of 40 C (104 F) Mean temperature was 26 C (78.8 F) Main Steam area monitors results ranged from 32.8 C (91 F) to 43.6 C (104 F) Dose extrapolated to 80 years was < 120 kgy 12

13 Radiation and Temperature Results RCP Cable Run 13

14 Radiation and Temperature Studies Results Radiation levels were often two (2) or more orders of magnitude less than design values used for qualifications Temperatures were less than design values for the Westinghouse PWR monitored (one exception was 7 C (12 F) higher than design, the remaining 54 monitors were 2.8 C to 28 C (5 F to 50 F) less than design values) RCP cable route showed variances in operating conditions can be observed along cable runs Some older cable jackets and insulations like Neoprene, chlorosulfonated polyethylene, polyimide (Kapton) and Butyl rubber (from this EPRI study) may need replacement prior to the end of plant operation Reports issued ( , ) 14

15 Value for Performing Operating Environment Studies Determine how conservative design values for radiation and temperature are Use of actual operating environment temperature values can be used obtain longer qualified life At radiation levels obtained, dose related aging affects will be minimal Operating environment studies cost are a fraction of that required to extend qualified life by type testing 15

16 EPRI Submergence Research 16

17 Industry Actions for Submergence Cable aging management programs focus on wet/submerged cables: Manholes, vaults, embedded conduits inspected If possible keep cables dry, repair/replace structural degradation Frequency based on plant operating experience (at least annually) Condition monitoring to assess cables with degraded insulation Medium voltage, shielded cable- tan delta/withstand testing Low and medium voltage non-shielded cable- insulation resistance* *- Some members have used tan delta to test three conductor, nonshielded MV cables with modified acceptance criteria 17

18 EPRI Research on Wet/Submerged Cables Low Voltage Cables Low voltage wet susceptibility research Evaluate operating experience/forensic analysis Medium Voltage Cables Failure mechanism research Tan delta test data evaluation Submergence Qualification 18

19 Submergence Research - Low Voltage Cables 19

20 Researching Low Voltage Cable Wet Aging Low voltage wet susceptibility study: Do low voltage cables wet age? 1 year aging at 90 C water, under AC and DC voltage stress published in June, 2016 The results were that cross-linked polyethylene and ethylene propylene rubber insulations did not degrade/fail after 1 year The jackets, chloro-sulfonated polyethylene and Neoprene, both were badly degraded The results support that these low voltage cable insulations did not degrade in wet/submerged environments Insulation resistance measurement can detect degraded insulation in wet/submerged locations 20

21 Submergence Research - Medium/High Voltage Cable 21

22 Background A submergence qualification would definitely support General Design Criteria 4: Structures, systems, and components important to safety shall be designed to accommodate the effects of and to be compatible with the environmental conditions associated with normal operation, maintenance, testing, and postulated accidents, including loss-of-coolant accidents. 22

23 Qualifying MV Cables For Submergence MV Cable Submergence Qualification- Supplemental Project Attempting to accelerate aging via high frequency and high voltage to obtain accelerated aging factor for submergence Brown/Black ethylene propylene rubber cable is 2+ years aging, complete in 2016 or 2017 Pink ethylene propylene rubber cable complete in 2018 Qualified Life= (Operating Time) + (Accelerated Aging Time X Aging Factor) 23

24 Submergence Qualification Research Research questions? Can ethylene propylene rubber aging in water be accelerated? Can a type test (years under test multiplied by acceleration factor) combined with operating experience qualification be established for submergence? Results showed that acceleration was possible, but the acceleration factor was insufficient to obtain long qualified life Reports issued ,

25 Summary 25

26 Summary EPRI research results Temperature and radiation levels in most plant areas are not severe Thermal degradation is main concern for low voltage cable insulation Low Voltage cables typically do not degrade in wet/submerged conditions Medium/High Voltage cables develop water trees Tan delta testing can be used to manage cable reliability 26

27 Summary OE and research results do not indicate failure of wet LV cable is a concern except for a few know insulations (Raychem, Vulkene) Takeaway: Monitor the cables in adverse thermal (mainly low voltage cables) and submerged (medium/high voltage cables) environments. So long as they are in good condition then the cables in more benign environments will provide reliable service through the end of plant operating life. If the adverse environment cables degrade, look at the cables in the next harshest environment and monitor those. 27

28 Questions 28

29 Together Shaping the Future of Electricity 29