ML15343A114

NRC Questions to Duke Energy Regarding Cable Testing in Support of Public Meeting on 12/15/15 (CAC Nos. MF7029, MF7030, and MF7031)

ML15343A114
Person / Time
Site:	Oconee
Issue date:	12/08/2015
From:	Office of Nuclear Reactor Regulation
To:	Duke Energy Carolinas
Whited, Jeffrey, NRR/DORL/LPLII-1
References
CAC MF7029, CAC MF7030, CAC MF7031
Download: ML15343A114 (2)

Text

Oconee Cable Testing - NRC Questions December 8, 2015 BACKGROUND: From November 2-6, 2015, NRC staff observed a series of cable fault tests conducted for Duke Energy at KEMA Laboratories in Pennsylvania. Duke commissioned the tests to determine the potential impacts of electrical faults in a medium voltage power cable with bronze tape as a metallic shield. The licensee conducted this testing, in part, in response to an Unresolved Item from the 2014 Oconee Component Design Basis Inspection, which is the subject of an ongoing TIA review. On November 18, 2015, in a public meeting with NRC staff, Duke outlined its plans to submit a licensing action to address cable separation issues, and this submittal will rely, in part, on the results of the cable fault testing. The following questions on the testing were developed by the NRC staff in preparation for a follow up public meeting with Duke Energy on December 15, 2015.

The test configuration used a 12 foot section of cable tray to house the test cables. In the test configuration, the power cable ends were not connected to any loads to pass current. One cable was shorted from the center conductor to the metallic shield with an 18 gauge copper wire at the approximate midpoint of the cable. During the short, current passed through the cable to the 18 gauge wire, then to the shield to ground. The shields at both ends of each cable were connected to each other, effectively splitting the fault current from the one cable between the three cable shields. NRC QUESTIONS: 1. In accordance with 10 CFR 50.54(jj) (2015), what quality standards were used to design the testing plan and to analyze the testing data? 2. Did the design of these tests meet the requirements of IEEE 279-1971? How?

3. How were the worst-case tested ground faults determined? What quality standards were used for this determination? 4. Why were three-phase faults not considered for testing? 5. Why did the testing not address cascading failures (i.e. circuit breaker failures that may result from the short circuit conditions)? 6. What analysis was done to ensure that each configuration bounded the worst case asymmetrical and symmetrical fault conditions for: a. Configuration 1? b. Configuration 2? c. Configuration 3?

d. Configuration 4? 7. What analysis was done to ensure that each configuration bounded the worst case arc flash duration for: a. Configuration 1? b. Configuration 2? c. Configuration 3? d. Configuration 4?

8. The "as installed" cable clamping configuration differs from that at the test laboratory. The test laboratory employed metal cable cleats designed for the forces encountered during electrical faults in accordance with IEC 61914 "Cable Cleats for Electrical Installations." The cleats were spaced at ~1 ft intervals and secured to a cable tray in an open environment. The installed condition used metal zip ties to strap the cables to Unistrut pegs approximately every 4 ft in an enclosed cable raceway. How does this difference address the impact of magnetic forces resulting from a worst-case fault condition as discussed in industry standards? 9. How does the use of new cables compare to the "as installed" cables, which can be in a more degraded condition due to variations in ambient conditions (temperature, moisture etc.), electrical transients, and variations in current flow? a. Is the assumption for a limiting condition single phase to ground fault appropriate for cables? What quality standards addressed this? 10. How were the configuration differences ("as-tested" vs "as installed") analyzed? What quality standards were used for this analysis? 11. What effects did the test configuration have on the test results? (i.e. the power cables were open-circuited and no operating loads were used for AC or DC) 12. How would the inductive and capacitive coupling effects be influenced when current is present on all phases of the power cables and the DC cables are energized? What quality standards were used to address this aspect? a. Were the concerns presented in Annex B of IEEE 603 investigated in relation to this question? b. Has the impact of increased cable length been investigated (i.e., as installed (4000 ft.) vs. as-tested (12 ft.))? 13. In some of the cable tests observed, the bronze tape shield melted partially. Has Duke evaluated the impact of such melting, if a worst-case fault is postulated? 14. Did Duke calculate the maximum magnetic force that would be exerted in the raceway system to CT4, which has approximately 4,000 feet of cable? The NRC staff's review of industry guidance indicates that cables in the concrete trench could be exposed to a substantial amount of force. It did not appear that these effects were simulated in the cable testing. If these magnetic forces were not modeled in the tests, how did the testing performed demonstrate that the existing cable configuration meets the ONS licensing basis and applicable ANSI standards?