05000416/LER-2005-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. N/A N/A | |
| Event date: | 04-27-2005 |
|---|---|
| Report date: | 06-27-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 4162005002R00 - NRC Website | |
On April 28 and 29, 2005 Grand Gulf Nuclear Station made eight hour non-emergency calls (EN#s 41645 and 41650) to the NRC pursuant to 10 CFR 50.72(b)(3)(v)(D), Accident Mitigation. The calls were the result of discovery that original assumptions used in development of the Air Operated Valve (AOV) program did not consider instantaneous loss of the non safety related Instrument Air [LD] system. Failure to include loss of Instrument Air in the original assumptions produced negative margin for secondary containment isolation valve (SCIV) closure under some postulated accident conditions. The subject valves were on the Main Steam Line [SB] drain line (1B21F113 and F114) and Reactor Core Isolation Cooling [BN] (RCIC) full flow test line (1P11F064 and F065). Also affected was the Residual Heat Removal (RHR) "C" connection to the Suppression Pool Cleanup System (SPCU) [CG] (1P60F001 and F021). However, this mode of RHR "C" has not been used in the past three years. In order to provide additional control of this line, valves 1P60F001 and F021 were administratively controlled closed.
Based on a thorough evaluation performed as part of the immediate actions the Main Steam Line drain valves were determined to have been operable. Only the RCIC full flow test line valves (1P11F064 and F065) and the suppression pool cleanup valves (1P60F001 and F021) to RHR "C" were determined to be inoperable when these functions are in service. All of the affected valves are flex wedge gate valves equipped with double acting Ralph A. Hiller Company actuators. It was determined that the actuator mounted accumulators would not provide sufficient air pressure to ensure positive closure of these SCIVs against postulated maximum expected differential pressure (MEDP) concurrent with loss of the Instrument Air system.
See the table in section "G", Additional Information, for a listing of operable but degraded and inoperable valves.
Based on the determination that certain SCIVs could not be assured to fully close in the event of loss of the Instrument Air system, this Licensee Event Report is submitted pursuant to 10 CFR 50.73(a)(2)(v)(D). This condition did constitute a safety system functional failure due to loss of function of Secondary Containment (accident mitigating system) caused by inoperable SCIVs.
B. INITIAL CONDITIONS
At the time of the event, the reactor was in OPERATIONAL MODE 1 with reactor power at approximately 100 percent. Moderator temperature, reactor pressure vessel (RPV) pressure, and RPV water level were at approximately 529 degrees F, 1033 PSIG, and 36 inches, respectively.
Attachment 1 On April 28, 2005 Grand Gulf Nuclear Station discovered that an assumption used in development of the AOV program resulted in negative safety margin. It was discovered that the actuator mounted accumulators would not provide sufficient air pressure to ensure positive closure of a certain population of Ralph A. Hiller Company actuator equipped flex wedge gate valves upon failure of the non safety related Instrument Air system. Based on engineering evaluation it appears that the SCIVs on the Main Steam Line drain lines may not positively close under all postulated conditions.
Further evaluation determined on April 29, 2005 that valves on the RCIC full flow test return line to the Condensate Storage Tank were also affected. This evaluation is based on 'worst case' assumptions for parameters such as air pressure to the valve actuator and maximum expected differential pressure MEDP across the valve.
Loss of Instrument Air would require that these AOVs perform their safety-function to close on valve accumulator pressure only. A population of these AOVs currently exist that do not have enough margin to overcome the currently calculated MEDP. This population of AOVs therefore cannot be relied upon to perform their design basis function under certain design accident conditions.
Operability for the entire population of negative margin Primary Containment Isolation Valves (PCIV) and SCIVs equipped with Ralph A. Hiller actuators was evaluated. The majority of the valves were determined to be operable (including those on the Main Steam Line drain lines), or operable but degraded and Generic Letter 91-18 criteria applied. The remaining small subset of valves affects the RCIC full flow test line (1P11F064 and F065) and the RHR "C" connection to the SPCU system (1P60F001 and F021). The RCIC system is affected because RCIC is not isolated from the Condensate Storage Tank on the high drywell pressure loss of coolant accident signal. Though the emergency SPCU mode of RHR "C" is affected, this particular mode of RHR "C" has not been used in the past three years. In order to provide additional control of this line, valves 1P60F001 and F021 were administratively controlled closed. Valves determined to be inoperable were closed per the Technical Specification (TS) action statement 3.6.4.2.B. Additionally, standing orders were issued to provide guidance for the operators concerning these conditions.
D. APPARENT CAUSE
The assumption which credited the non-safety related Instrument Air system as the primary source of air to operate the Ralph A. Hiller Company actuator equipped PCIVs and SCIVs during a design basis event is inconsistent with regulatory design documentation. This assumption was used in the calculations upon which the thrust capability of the subject valve actuators in the Grand Gulf AOV Program were developed. The assumption that non-safety related Instrument Air system could be credited for AOV closure was incorrect.
Attachment 1 Immediate Corrective Actions — Initiated Condition Reports CR-GGN-2005-01568, CR-GGNI-2005-01667, CR-GGN-2005-01681, and CR-GGN-2005-01815.
Long Term Corrective Actions — Condition Report CR-GGN-2005-01568 was issued.
F. SAFETY ASSESSMENT
Condition report CR-GGN-2005-01568 outlines component status and recovery plan for the affected valves. The Suppression Pool Cleanup Emergency Mode of RHR has not been used in the past three years. However, to provide additional control, the 1P60F001 and F021 valves were administratively controlled closed and information communicated to all operators in standing orders to maintain these valves closed to prevent operation of RHR in SPCU Emergency Mode. The RCIC full flow isolation 1E51F022 does not isolate on high drywell pressure unlike High Pressure Core Spray (HPCS). However the RCIC system was only in this configuration approximately eight hours per year. Therefore this operation did not present a significant increase in risk.
G. ADDITIONAL INFORMATION
The following table lists the inoperable and operable but degraded valves.
VALVE CONDITION COMMENTS
1P11F064 Inoperable Valve isolated and administrative controls implemented 1P11F065 Inoperable Valve isolated and administrative controls implemented 1P60F003 Operable but Degraded Administrative controls and MEDP adjusted 1P60F004 Operable but Degraded Administrative controls and MEDP adjusted 1P60F007 Operable but Degraded Administrative controls and MEDP adjusted 1P60F008 Operable but Degraded Administrative controls and MEDP adjusted 1P60F009 Operable but Degraded Administrative controls and MEDP adjusted 1P60F010 Operable but Degraded Administrative controls and MEDP adjusted 1P60F001 Inoperable Valve isolated and administrative controls implemented 1P60F021 Inoperable Valve isolated and administrative controls implemented Attachment 1