05000395/LER-2009-003
| Docket Number | |
| Event date: | 11-20-2009 |
|---|---|
| Report date: | 01-19-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(vii)(B), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3952009003R00 - NRC Website | |
I. EVENT DESCRIPTION
The Residual Heat Removal (RHR) system serves three functions in Mode 4. RHR provides heatup and cooldown capabilities for normal plant startup/shutdown, low temperature overpressure protection, as well as an injection and recirculation Emergency Core Cooling System (ECCS) function. VCSNS utilizes RHR relief valves for low temperature overpressure protection prior to decreasing below 300 degrees F. Per Technical Specification 3.5.3, during Mode 4 one train of ECCS is also required to be operable to provide core cooling in the event of a Loss Of Cooling Accident (LOCA).
In the early 1990s, it was recognized that when the RHR system was aligned to the RCS and operating in the cooldown mode, the suction piping could flash to steam when the system was realigned to the RWST in ECCS injection mode. This issue was the subject of NSAL 93 004, "RHRS Operation as Part of the ECCS During Plant Startup." To address the issue a temperature limit of 250 degrees F was established for the suction piping to ensure the RHR pump operability. Procedures were revised to place the operable RHR pump in PULL-TO-LOCK to prevent the pump from automatically starting on a safety injection signal and drawing high temperature RCS water into the RHR suction piping whenever RCS temperature was greater than 250 degrees F.
In the fall of 2009, Westinghouse issued NSAL 09 8, "Presence of Vapor In Emergency Core Cooling System/Residual Heat Removal System in Modes 3/4 Loss-of-Coolant Accident Condition." A review of the NSAL recommendations, site calculation, and operating procedures identified a potential for operating in an unanalyzed condition due to possible loss of the RHR system safety function. The temperature limit to manage flashing as addressed in NSAL 09 8 was conservatively determined to be less than or equal to 200 degrees F rather than 250 degrees F during realignment to the Refueling Water Storage Tank (RWST) or Reactor Building Sump.
A review of the last three years of RHR system operation determined that the system was vulnerable for 4.8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. In 2006, for Refueling 16 the plant was vulnerable in Mode 4 decreasing for 2.6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> with no hours ascending. In 2008, for Refueling 17 the plant was vulnerable in Mode 4 decreasing for 2.1 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and 0.1 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> ascending. The plant did not enter Mode 4 other than during the planned refueling outages. Vulnerability is characterized as the time when the protected train RHR pump was not in PULL TO LOCK between 250 and 200 degrees F.
II. EVENT ANALYSIS
Loss of RHR due to flashing and void formation challenges or prevents the safety function of the system thus placing the plant in an unanalyzed condition and is reportable per 10 CFR 50.73(a)(2)(ii)(B), 10 CFR 50.73(a)(2)(v(B), and 10 CFR 50.73(a)(2)(vii)(B).
The apparent cause of this condition was the failure to consider the most limiting boundary conditions when calculating the temperature limit to prevent flashing in the RHR system. The presence of vapor during a LOCA condition in Modes 3 and 4 has been a generic industry issue (GL-2008-01) and the subject of two Nuclear Safety Advisory Letters (NSAL-93-004 and NSAL-09 8). Knowledge of the key safety consideration associated with these events has evolved. Given the evolution of this industry issue, two causes contributing to this event are: VCSNS's original RHR system design and system operating guidance provided by Westinghouse were deficient, and the VCSNS did not fully understand the full scope of the issue when first evaluated in the early 1990s.
A review of the VCSNS calculation performed in 1993, indicated that the calculation was technically accurate for the conditions considered. The calculation was performed in response to an input request from Operations for the "plant specific temperature that can result in flashing at the suction of the RHR pumps when aligned to the RWST' as described in Abnormal Response Guideline (ARG)-2 for Shutdown LOCA. This calculation also addressed the concerns in NSAL-93-004. The ARG information, however, gave little information on explicit boundary conditions to be considered. NSAL-09-8 recommended the most limiting boundary conditions to include the use of the pump suction header elevation as opposed to the lower pump suction inlet elevation and lower RWST levels to cover Small Break Loss Of Cooling Accident (SBLOCA) scenarios where RHR pump start may be delayed. The failure to consider these more limiting boundary conditions resulted in the non-conservative temperature limit of 250 degrees F.
�NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER
III. SAFETY SIGNIFICANCE
A review of the RHR system operation for the past three years determined that the total time the system was vulnerable was 4.8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. A Probabilistic Risk Assessment (PRA) evaluation of the risk associated with a LOCA and no protected train (loss of both RHR trains) resulted in a Core Damage Frequency (CDF) of less than 1E-07 per year. In the event that the RHR system became inoperable, abnormal and emergency procedures exist that provide guidance to restore core cooling. Per operating procedures a charging pump would be aligned to the RWST with additional water supplied by the Reactor Makeup Water Supply System.
Additionally, the steam generators would be available with Emergency Feedwater providing a heat sink to aid in decay heat removal.
IV. CORRECTIVE ACTIONS
Immediate corrective actions were taken prior to the plant shutdown for RF18 due to the operating experience generated by the industry. Procedures were revised to ensure one dedicated train of RHR remained protected until Mode 5 entry (less than 200 degrees F). This approach ensured that the fluid in the protected train of RHR remained at or near ambient (-100 degrees F) conditions with no potential of flashing if the system was required to perform its ECCS function in Mode 4. The protected train concept was achieved by maintaining the protected pump in PULL-TO-LOCK until RCS temperature is less than or equal to 200 degrees F. This allowed the system to meet its safety function in accordance with Technical Specifications 3.5.3.
A formal calculation of allowable RHR water temperatures have since been completed and the results have been incorporated into VCSNS procedures to ensure RHR system remains operable for its ECCS safety function in Mode 4.
V. PREVIOUS OCCURRENCES
No previous occurrences or similar events have been identified for VCSNS.
�NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER