05000445/LER-2011-001
| Comanche Peak Nuclear Power Plant Unit 1 | |
| Event date: | 03-22-2011 |
|---|---|
| Report date: | 05-18-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii)(D), Common Cause Inoperability 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 4452011001R01 - NRC Website | |
10CFR50.73(a)(2)(v)(D) Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident" and 10CFR50.73(a)(2)(vii)(D) Any event where a single cause or condition caused two independent trains to become inoperable in a single system designed to mitigate the consequences of an accident'.
B. PLANT CONDITION PRIOR TO EVENT
On March 22, 2011, CPNPP Unit 1 and Unit 2 were both in Mode 1 operating at 100% power.
C. STATUS OF STRUCTURES, SYSTEMS, OR COMPONENTS THAT WERE
INOPERABLE AT THE START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
There were no inoperable structures, systems, or components that contributed to the
- event.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES
On March 22, 2011, as a result of a review of Information Notice (IN) 2010-11 and NSAL-09-8, it was determined that, while it appeared that controls had been established to adequately address the new concerns presented under NSAL-09-8, the basis for the 250 degree F temperature limitation previously imposed by the procedures could not be readily ascertained.
Therefore, an engineering evaluation was initiated to confirm the temperature limits previously implemented were acceptable, and to confirm the conclusion that the limits established in response to NSAL-93-004 appropriately addressed the NSAL-09-8 concerns. The engineering evaluation could not confirm that the 250 degree F limit currently applied to the RH [EllS: (BP)] system for alignment for ECCS injection was sufficient to prevent flashing/voiding in RH system suction piping when aligned to the RWST [El IS: (BE)(TK)] or to the ECCS recirculation sump but could result in flashing of liquids in the hot leg suction lines. This potential exists due to the elevation differences between the RH lines at the containment penetrations and the expected containment sump level combined with the postulated containment pressure at the established 250 degree F limit. The evaluation concluded that the RH system temperature must be reduced to 210 degrees F in order to eliminate the potential for flashing of hot water within the isolated hot leg suction piping during transfer to the RWST or the ECCS recirculation sump. This information affects the manner in which the RH system will be required to be operated in Mode 3 (>/= 350 degrees F) and Mode 4 (350 degrees F > T-average > 200 degrees F).
E. THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM FAILURE, OR
PROCEDURAL PERSONNEL ERROR
This condition was identified as a result of an engineering evaluation performed to confirm that temperature limits previously implemented in response to NSAL-93-004 were acceptable and appropriately addressed the concerns reported in IN-2010-11 and NSAL-09-8.
COMPONENT OR SYSTEM FAILURES
A. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE
Not applicable - No component failures were identified during this event.
B. FAILURE MODE, MECHANISM, AND EFFECTS OF EACH FAILED COMPONENT
Not applicable - No component failures were identified during this event.
C. SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AFFECTED BY FAILURE OF
COMPONENTS WITH MULTIPLE FUNCTIONS
Not applicable - No component failures were identified during this event.
D. FAILED COMPONENT INFORMATION
III.
Not applicable - No component failures were identified during this event.
ANALYSIS OF THE EVENT
A. SAFETY SYSTEM RESPONSES THAT OCCURRED
B.
Not applicable - No safety system responses occurred as a result of this event.
DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY
On March 29, 2011, a review of the past three years was performed to determine if CPNPP may have operated with both trains of RH shutdown cooling in service in Mode 4 (above 200 degrees F). Three occurrences were identified where both RH trains were placed into operation prior to reaching Mode 5 («/= 200 degrees F):
- Unit 2 refueling outage 2RF10 from 20:40 on 3/29/2008 to 00:05 on 3/30/2008 (approximately 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and 17 minutes)
- Unit 1 refueling outage 1RF13 from 20:13 on 9/27/2008 to 21:50 on 9/27/2008 (approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and 37 minutes)
- Unit 2 refueling outage 2RF11 from 21:57 on 10/7/2009 to 01:42 on 10/8/2009 (approximately 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and 45 minutes) Further review of plant computer data has identified one additional occurrence of this event:
- Unit 1 refueling outage 1RF14 from 18:53 on 4/3/2010 to 22:02 on 4/3/2010 (approximately 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and 9 minutes)
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
There were no actual safety consequences impacting plant or public safety as a result of this event. This issue addressed the potential for the development of steam voiding in the RH pump suction shutdown cooling piping of the RH system if the RH system had to be transitioned from shutdown cooling to the ECCS mode due to a LOCA occurring in Modes 3 or 4 at temperatures above 210 degrees F (Modes 3 and 4).
The potential exists for flashing/steam voiding of an RH system hot leg suction line if the RH system is aligned for ECCS injection or recirculation, following termination of shutdown cooling with an RH temperature that exceeds 210 degrees F, and the suction of the RH system is transferred to the RWST or the ECCS recirculation sump during a LOCA. Therefore, this is considered a safety system functional failure.
In the event that the RH system became inoperable, abnormal and emergency procedures exist that provide guidance to immediately secure any RH pumps aligned for shutdown cooling to prevent pump damage, to restore core cooling through alignment of a high head safety injection pump in injection mode, and restoration of the intermediate head safety injection pumps if necessary. Existing procedures also include steps to vent and refill the RH loops if necessary. In Modes 3 or 4, at least one charging pump is available and would be aligned to the RWST. Additionally, the steam generators would be available with auxiliary feedwater providing a heat sink to aid in decay heat removal.
When this issue was first identified by Westinghouse in NSAL-93-004, Westinghouse performed an Assessment of Safety Significance. Referencing prior work documented in WCAP-12476, "Evaluation of LOCA during Mode 3 and Mode 4 Operation for Westinghouse NSSS," which looked at the probabilities of a LOCA in Modes 3 or 4 and available guidance for actions to cope with a shutdown LOCA, Westinghouse concluded that this issue was not risk significant in regard to large LOCAs in Mode 3 and the relative risk was not much different whether or not flashing occurs in Mode 4. In the more recently issued NSAL-09-8, Westinghouse states that the conclusion of the previous safety significance assessments for this issue was based on the low risk of this event. This conclusion remains valid since no new information changes this condition. The consequences of RH system failure due to suction flashing in Modes 3 or 4 remained bounded by the core damage consequences of the Mode 1 LOCA events. This is because of the reduced pipe break probability due to the relatively low temperature and pressure that exists for the majority of time the plant is in these modes. It is also reflective of the time the plant is in these modes, which is very short relative to the time it is in Mode 1. Therefore, the risk significance of this event is considered to be low.
IV. CAUSE OF THE EVENT
The CPNPP organization failed to recognize the RH system limitations necessary to support all modes of RH operation (shutdown cooling, ECCS injection, and ECCS recirculation) without adversely impacting each other and consequently did not take steps necessary to preclude voiding in the RH system under all postulated system operating conditions.
V. CORRECTIVE ACTIONS
Station operating procedures were revised to prohibit both RH pumps from being aligned in the Shutdown Cooling Mode with RCS temperature 200°F. In addition, in accordance with the CPNPP Corrective Action Program, the RH system design basis document (DBD-ME-260) will be revised to address system limitations during cooldown.
VI. PREVIOUS SIMILAR EVENTS
There have been no previous similar reportable events at CPNPP in the last three years.