05000346/FIN-2007007-05
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Finding | |
|---|---|
| Title | Concern Regarding SAFETY-RELATED Battery Electrical Isolation |
| Description | During a review of the 125/250 Vdc safety-related distribution system, the inspectors determined that the safety-related buses supplied power to non-safety-related loads. In particular, the inspectors observed that some of the loads, such as the reactor coolant pump back-up oil lift pumps and lighting panel L49E1, were potentially subject to HELB/LOCA environments. The inspectors expressed a concern that, under such environments, the non-safety-related loads could become grounded and impose added loads on the DC buses from which they were powered. This concern was also expressed by the licensee in CR 05-01849. This CR addressed the automatic transfer of emergency lighting panel L49E1, an in-containment panel, from its non-safety-related AC source to the safety-related DC system. Among others, the CR expressed the concern that the transfer might result in unintentional grounds being placed on the DC system. In their evaluation of this CR, the licensee performed a root cause analysis, but concentrated primarily on the impact that such faults would have on the containment penetration. Issues associated with multiple grounds were also addressed by CR 04-07150. In this CR, the licensee recognized that their earlier evaluation of IN 88-86, Operating with Multiple Grounds in Direct Current Distribution Systems, and Supplement 1 to IN 88-86 may have not considered the impact of the multiple grounds issues. Therefore, the licensee recommended that such an evaluation be made, but also indicated that the condition was within the design bases of the plant. The recommended evaluation was performed, and each non-safety-related load on the DC system was reviewed; however, the evaluation was primarily focused on the affected components rather than on the impact of grounded non-safety-related components on the power supplying battery. As a result, no penalty for multiple grounds was taken in the DC calculation, C-EE-002.01-010. The inspectors review also determined that five automatic transfer switches transfer their non-safety-related loads between non-safety-related inverters YVA and YVB. The loads in question included the station annunciator, the plant computer, the non-nuclear instrumentation channels X and Y, and the integrated control system channels X and Y. Although these inverters are not safety-related and, hence, power other non-safety-related loads, they are in turn powered by the safety-related station batteries. Therefore, faults on the five automatic transfer switches and their loads could be transferred from one DC power source to its redundant DC power source, thereby, potentially impacting the ability of the safety-related batteries of both divisions to perform their safety function. The use of the automatic transfer switches and their compliance with Safety Guide 6 was discussed in CR 04-07151 and again in CR 04-07761. In CR 04-07151, the licensee recognized that an analysis of the automatic transfer of loads between divisions had not been performed and that the transfer might not meet the intent of Safety Guide 6. In their investigation of this CR, however, the licensee concluded that, because the applicability section of the USAR pertaining to Safety Guide 6 had not specifically referenced the DC system, the design met the licensing bases. Therefore, the licensee proposed to revise the USAR and clarify their position. In CR 04-07761, the licensee concluded that Regulatory Positions 4.b. and 4.c. of Safety Guide 6 were applicable to Davis-Besse and that the use of the automatic transfer switches was inconsistent with the USAR text. Therefore, CR 04-07761 proposed that the review of the condition described in CR 04-07151 be performed using NG-NS-00808 and NOP-LP-4003, Evaluation of Changes, Tests, and Experiments. In the Introduction section, Safety Guide 6 states: General Design Criterion 17 requires that onsite electrical power systems have sufficient independence to perform their safety functions assuming a single failure. This safety guide describes an acceptable degree of independence between redundant standby (onsite) power sources and between their distribution systems. That the Safety Guide was applicable to both AC and DC was evident in several sections of Safety Guide 6. For instance, Position 1 of the Guide states: The electrically powered safety loads (a-c and d-c) should be separated into redundant groups. Regarding the specific concern addressed in the licensees CR, Safety Guide 6 Positions 4.b. and 4.c. state, respectively that, No provisions should exist for automatically connecting one load group to another load group and No provisions should exist for automatically transferring loads between redundant power sources. Therefore, the inspectors concluded that Safety Guide 6 was applicable to the DC loads as well as the AC loads and that the use of automatic transfer switches to transfer loads between redundant power sources did not meet the intent of Safety Guide 6 or General Design Criterion XVII. As indicated previously, the inspectors concern was that the indiscriminate addition of nonsafety- related loads to the safety-related 125 Vdc buses, i.e. without a detailed analysis of the impact of multiple grounds on the non-safety-related components and system, could induce faults on the safety-related buses and either reduce the margin of safety on the design of the safety-related system or prevent the safety-related system from performing its intended safety function. Furthermore, the use of automatic transfers of non-safety-related loads from one Division to the redundant Division could cause the same faults to impact both safety-related DC sources at the same time. This item is unresolved pending further review by the NRC of the documentation to be provided by the licensee pertaining to the design and licensing bases of the plant (URI 05000346/2007007-05). |
| Site: | Davis Besse  |
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| Report | IR 05000346/2007007 Section 1R21 |
| Date counted | Dec 31, 2007 (2007Q4) |
| Type: | NCV: |
| cornerstone | No Cornerstone |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | A Wilson C Brown D Kimble J Beavers J Cameron J Neurauter M Holmberg M Mitchell P Smagacz R Jonesa Dahbura Dunlop A Stone C Baron C Brown J Neurauter N Della Grec |
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Finding - Davis Besse - IR 05000346/2007007 | |||||||||||||||||
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Finding List (Davis Besse) @ 2007Q4
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