05000261/FIN-2011002-06
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Finding | |
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| Title | None |
| Description | TS 5.4.1, Procedures, required preventive maintenance be accomplished on safety related systems in accordance with Regulatory Guide 1.33, Revision 2, Appendix A, 1978. Contrary to this in 2004, during preventive maintenance on the B Inverter, the sync board was not replaced as planned and the work order was closed as completed. This resulted in the sync board remaining in service beyond the vendor 10 year replacement recommendations. The 10 year life expired in 2009. As a result, the B Inverter failed during surveillance testing on June 24, 2010. The failure resulted in a loss of RHR temperature control and inoperability of the B EDG. The cause of the violation was inadequate review of the work order which did not track the sync board replacement deviation. The licensee entered the issue into the corrective action program as AR 406834406834 Significance Determination Process (SDP) phase 1 screening for the finding determined that the finding required a phase 2 shutdown risk assessment in accordance with Inspection Manual Chapter (IMC) 0609 Appendix G. A loss of RHR(LORHR) event assessment was performed for the June 24, 2010 event. The event risk assessment period was <3 days while in mode 5 (Plant Operating State (POS-2)). A condition assessment was performed to cover the likelihood of an inverter failure conditional on a loss of offsite power (LOOP) event covering the period between the failure of inverter B and the last successful demonstration of inverter B to synchronize properly (March 28, 2010 June 23, 2010). The major assumptions of the LORHR event assessment were an initiating event likelihood (IEL) of 1.0, reactor coolant system (RCS) time to boil of 8.6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, time to core uncover of 13.9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br />, full operator and equipment credit for RHR recovery, refueling water storage tank (RWST) makeup and feed capability due to availability of safety injection and charging systems. The LORHR sequences were (1) LORHR with failure to recover RHR before RCS boiling, successful RCS injection, failure to recover RHR before RWST depletion and failure to accomplish RWST makeup leading to core damage, and (2) LORHR, failure to recover RHR prior to RCS boiling and failure of RCS injection leading to core damage. The major assumptions of the LOOP condition assessment were an IEL of 2 for the duration in POS 1, IEL of 3 for the duration in POS2, full credit for emergency ac power due to availability of EDG A, ability to manually load EDG B and the availability of steam generator (S/G) cooling due to motor driven and turbine driven auxiliary feed water trains. The dominant LOOP condition assessment sequences were (1) LOOP, failure of AC power, failure to maintain S/G cooling with a failure to recover offsite power prior to core damage and (2) LOOP, failure of AC power, successful gravity feed, with failure to recover offsite power or EDGs prior to core damage. The LORHR event risk was mitigated by the low decay heat present during the event, the availability of RHR trains and indications, and ease of diagnosis and restoration of the loss of instrument bus power allowing a recovery of RHR flow control from the main control room. The LOOP condition assessment risk was mitigated by the fact that the inverter B failure was conditional upon a LOOP event occurring during the evaluation period and the availability of EDG A during the evaluation period with EDG B available but requiring manual loading. The phase 2 SDP risk evaluation including the event and condition assessments resulted in a risk increase of <1E-6 in core damage frequency, a Green finding of very low safety significance |
| Site: | Robinson  |
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| Report | IR 05000261/2011002 Section 4OA7 |
| Date counted | Mar 31, 2011 (2011Q1) |
| Type: | NCV: Green |
| cornerstone | Mitigating Systems |
| Identified by: | Licensee-identified |
| Inspection Procedure: | |
| Inspectors (proximate) | J Hickey R Musser E Lea C Scott W Deschaine M Riches |
| INPO aspect | |
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Finding - Robinson - IR 05000261/2011002 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Robinson) @ 2011Q1
Self-Identified List (Robinson)
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