05000261/LER-2010-008
| H. B. Robinson Steam Electric Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2612010008R00 - NRC Website | |
I. DESCRIPTION OF EVENT
At 1545 hours0.0179 days <br />0.429 hours <br />0.00255 weeks <br />5.878725e-4 months <br /> EDT on October 5, 201Q;with H. B. Robinson Steam Electric Plant (HBRSEP), Unit No. 2, in Mode 1 at approximately 100% power, it was determined that during past plant configuration in Mode 4, Hot Shutdown, the Residual Heat Removal (RHR) System, was not OPERABLE per Technical Specification requirements for RHR's Emergency Core Cooling System (ECCS) function for low-head safety injection during Mode 4.
Corrective Actions in response to NSAL-09-08, Presence of Vapor in Emergency Core Cooling System Residual Heat Removal System in Modes 3/4 Loss-of-Coolant Accident Conditions, resulted in procedure changes that precluded the use of the RHR system under certain conditions for low-head safety injection from the Refueling Water Storage Tank to mitigate a Mode 4 Loss of Coolant Accident.
These changes were made to protect the system from gas voiding concerns. At the time, these changes were justified based on technical information that concluded that one high-head Safety Injection (SI) pump provided sufficient flow to mitigate a Mode 4 LOCA during the injection phase, and an interpretation that the Technical Specifications did not specifically require that both ECCS subsystems (RHR and SI) were required operable for both the injection and recirculation modes.
Based on subsequent information, it was concluded on October 5, 2010, that the Technical Specifications do require the RHR System to be operable for the ECCS injection phase in Mode 4 and therefore the procedure changes that had been implemented resulted in a condition prohibited by Technical Specifications, which is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B).
II. CAUSE OF EVENT
This event has been investigated using the HBRSEP, Unit No. 2, Corrective Action Program (CAP) and is documented in Nuclear Condition Report 425136. The root cause was determined to be unclear wording in the Technical Specification Bases combined with an inadequate 10 CFR 50.59 evaluation.
Although Technical Specification 3.5.3, "ECCS — Shutdown," clearly requires one SI train and one RHR train to be operable in Mode 4 for the ECCS function, the Technical Specification along with its associated Bases do not clearly designate if the RHR system needs to be operable for both the ECCS injection phase (injection of water from the RWST) and the recirculation phase (recirculation of water from the containment sump). Since the Technical Specifications did not provide clear requirements for operability, the determination of operability was based on Mode 4 LOCA mitigation requirements as determined in technical report WCAP-12476, Revision 1, "Evaluation of LOCA during Mode 3 and Mode 4 Operation for Westinghouse NSSS." This report shows that one SI pump is sufficient for Mode 4 LOCA mitigation during the injection phase. Based on the interpretation of the Technical Specifications and the use of the technical analysis requirements, procedural changes were implemented under 10 CFR 50.59 that precluded the use of RHR for injection under certain conditions for a Mode 4 LOCA. The changes were intended to protect the RHR system from gas voiding concerns and maintain sufficient ECCS capability to mitigate a Mode 4 LOCA. Subsequent information has led to the conclusion that these procedure changes resulted in a condition prohibited by the TS.
Additionally it was determined that the 10 CFR 50.59 evaluation was inadequate and should have concluded that NRC approval was required for the changes.
III. SAFETY SIGNIFICANCE
The procedure changes did not result in the SI system being inoperable for either the injection phase or the recirculation phase. The procedure changes did not impact the operability of RHR for the recirculation phase. Based on analyses, these capabilities would ensure the ECCS ability to mitigate a Mode 4 LOCA. The unit was in Mode 4, during startup from Refueling Outage 26, from about 1857 hours0.0215 days <br />0.516 hours <br />0.00307 weeks <br />7.065885e-4 months <br /> on July 12, 2010, until about 0048 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> on July 14, 2010. This event report is based on non compliance with the TS and not a loss of the ECCS safety function. Therefore, the safety significance of this event is considered to be low.
IV. CORRECTIVE ACTIONS
Completed Corrective Actions:
- Procedures were revised to ensure that at least one train of RHR remains operable for ECCS injection during Mode 4.
Planned Corrective Actions:
- The Bases for Technical Specification 3.5.3 will be revised to make it clear that the RHR System ECCS function must be operable for both the injection phase and the recirculation phase during Mode 4. This action is currently scheduled to be completed by January 14, 2011.
- Additional guidance will be added to the procedure for performing 10 CFR 50.59 evaluations.
This action is currently scheduled to be completed by January 14, 2011.
V. PREVIOUS SIMILAR EVENTS:
License Event Reports for the past five years at HBRSEP, Unit No. 2 were reviewed and the following events were considered to be similar:
Licensee Event Report 2009-002-00 This LER also represents a condition prohibited by TS as TS required actions were not met. The cause was attributed to insufficient work instructions to highlight the TS impact of repair activities and is therefore not considered similar to the cause of this event.
Licensee Event Report 2010-003-00 This LER is similar as it is related to the inoperability of RHR during the injection phase for a shutdown LOCA. This LER is reporting the gas voiding potential that the procedure changes discussed above were intended to preclude. Therefore, the cause is not considered similar to this event.