05000338/LER-2013-003
| North Anna Power Station , Unit 1 | |
| Event date: | 11-21-2013 |
|---|---|
| Report date: | 01-16-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(C), 50.54(x) TS Deviation 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3382013003R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT During a review of EE-0684, Casing Cooling Level Channel Statistical Allowance (CSA), it was identified that the final casing cooling tank level would be below the top of the Casing Cooling pump suction nozzle (approximately 0.75% or 1 inch below the top of the pipe), when accounting for the full CSA on the low level isolation bistable, allowance for setting the setpoint at 3.25% (in 1/2-ICP-RS-L-103A/203A) and closure time for 1/2-RS-MOV- 1/200A/B. The setpoint also does not account for necessary submergence to avoid vortexing in the pump suction.
Technical Report ME-0178, Revision 0 and Revision 1 were written to address concerns raised in Generic Letter 2008-01. The section of the report that addressed the potential for vortexing identified calculation 01040.0910-M-001, which accounted for an acceptable submergence level as the basis for acceptable void prevention. The review did not take the extra step to ensure that instrumentation setpoints had incorporated the requirements of the usable volume calculation. In 1995, due to a CSA increase, the Casing Cooling tank low-low setpoint needed to be increased to ensure the level remained above the top of the pipe. ET-CME-95-005 had evaluated the acceptability of a level setpoint less than the required 3.3 ft submergence to preempt vortexing. However, ET-CME-95-005 did not drive a revision to the original calculation 01040.0910-M-001 to cross reference the ET to note the usable volume calculation assumption of 3.3 ft of submergence was not used for the low-low level setpoint.
The Casing Cooling system provides cold water to the suction of the Outside Recirculation Spray (ORS) pumps to improve Net Positive Suction Head (NPSH). As a result of the above review, the ORS pumps for both units were declared inoperable and Technical Specification (TS) 3.6.7.F was entered for each unit at 2006 hours0.0232 days <br />0.557 hours <br />0.00332 weeks <br />7.63283e-4 months <br /> on 11/21/2013, which directed entry into TS 3.0.3. A prompt report was made to the NRC and an Operability Determination (OD) was completed with compensatory actions to isolate the Casing Cooling tank on a low level at less than or equal to 10% versus the current setpoint of 4%.
TS 3.0.3 was exited at 2217 on 11/21/13 upon approval of the OD.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS No significant safety consequences resulted from this event because no accident requiring the use of the Recirculation Spray (RS) system occurred. However, the vortexing concern could have adversely impacted the performance of the ORS pumps. As such, this event could have prevented the fulfillment of a safety function.
3.0 CAUSE The lack of integration of diverse technical documents provided a latent error where the latest usable volume calculation conflicted with the original calculation. This resulted in the inadequate level setpoint not being identified as part of the NRC Generic Letter 2008-01 review.
4.0 IMMEDIATE CORRECTIVE ACTION(S) An Operability Determination, OD556, was completed which included compensatory actions. Specifically, the control room operator is to initiate the Emergency Operating Procedure (EOP) actions for isolating the casing cooling tank on a low level at less than or equal to 10% versus the current setpoint of 4%. With a 10% maximum indicated EOP setpoint, the safety analysis assumption of 100,000 gallons for minimum tank usable water volume will be met and vortexing avoided.
5.0 ADDITIONAL CORRECTIVE ACTIONS An Engineering Information Bulletin (EIB) will be prepared to emphasize procedural guidance regarding configuration management and latent errors to emphasize the need to verify design documents are reflective of the current design basis. The EIB topic will include how to address old active calculations and other documents used as design inputs and to evaluate superseding or voiding those documents when found.
6.0 ACTIONS TO PREVENT RECURRENCE Associated calculations will be updated to include the NPSH evaluation for the casing cooling pumps. An Engineering Technical Evaluation (ETE) will be prepared to document an acceptable level setpoint and a design change will be implemented to revise the setpoint.
7.0 SIMILAR EVENTS No similar events have occurred at North Anna Power Station.
8.0 ADDITIONAL INFORMATION Unit 1 and Unit 2 continued operating in Mode 1, 100 percent power during this event.