05000425/LER-2004-002
| Docket Numberisi | |
| Event date: | 05-11-2004 |
|---|---|
| Report date: | 08-04-2004 |
| 4252004002R00 - NRC Website | |
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3)
A. REQUIREMENT FOR REPORT
This event is reportable per 10 CFR 50.73 (a)(2)(v)(B) because a condition existed that could have prevented the fulfillment of the safety function of a system needed to remove residual heat. It is also reportable per 10 CFR 50.73 (a)(2)(i) because the unit operated in a condition prohibited by the Technical Specifications.
B. UNIT STATUS AT TIME OF EVENT
At the time of this event on May 11, 2004, Unit 2 was in Mode 4 (hot shutdown) and Mode 3 (hot standby) heating up to operating temperature, at 0 percent of rated thermal power, coming out of the 2R10 refueling outage. Other than that described herein, there was no inoperable equipment that contributed to the occurrence of this event.
C. DESCRIPTION OF EVENT
On June 8, 2004, an evaluation was completed that determined a safety system functional failure could have occurred following the 2R10 refueling outage when unit heat-up occurred on May 11, 2004.
Specifically, it was determined that the containment building held debris in an amount adequate to block the containment sump screens, which could result in inadequate net positive suction head and possible failure of the residual heat removal (RHR) pumps to perform at design limiting conditions. The condition existed for a period of 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> and 53 minutes after the unit entered into Mode 4 and Mode 3.
It also represented operation of the unit in a condition prohibited by the Technical Specifications because the surveillance task for verifying containment cleanup prior to Mode 4 entry was inadequately performed. Walkdowns to verify containment cleanliness removed the miscellaneous loose debris.
D. CAUSE OF EVENT
The cause of this event is low available margin in the sump screens such that a very small amount of debris left in containment is sufficient to block the sump screens based on the very conservative assumptions used in determining the potential for sump blockage. SNC will pursue design changes to increase sump screen margin. In the interim, prior to the implementation of design changes, SNC will pursue improvements to containment building lighting (temporary lighting fixtures) and materials visibility (high visibility tools, etc.) to minimize containment debris to the extent possible.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3)
E. ANALYSIS OF EVENT
At the time of this event, more than one-third of the subcritical core consisted of new fuel. The remainder had been removed from the previous core, and the fission process, for nearly 4 weeks. Based on these factors, the reactor core possessed a minimal amount of decay heat.
In addition, an assumption was made that 100% of the steam generator insulation material destroyed during a design basis LOCA would migrate to the sump. This assumption is overly conservative as documented in recent NUREGs. Design engineering believes the insulation debris created during a LOCA at the operating temperature and pressure experienced during heatup for this event would be reduced from that of a design basis LOCA. Insulation destruction is based on testing from a jet blast from a water/steam source at 590°F and 1595 psi. Testing information for insulation destruction at the reduced operating temperature and pressure existent during this event is not available. Additionally, much of the insulation debris will not transport to the sump as it will be caught up on grating, equipment and pocket areas in containment.
Finally, an analysis was performed to determine the increases in annualized core damage frequency (CDF) and large early release frequency (LERF) as a result of this event. The increase in CDF was determined to be 1.71E-7 events per year, and the increase in LERF was determined to be 4.95E-9 events per year. Applying the criteria from the NRC Significance Determination Process, it was found that the potential for a containment sump blockage event was of low risk significance.
Based on these considerations, there was no adverse effect on plant safety or on the health and safety of the public as a result of this event.
This event represents a safety system functional failure.
F. CORRECTIVE ACTIONS
1) Sufficient debris was removed from containment on May 11, 2004, to enable net positive suction head requirements to be met for the RHR pumps, and the unit was returned to power operation.
2) By the next refueling outage, temporary lighting fixtures and highly visible tools will be made available for use in the containment buildings.
3) Personnel and materials entering the containment buildings will be inspected to ensure only high visibility consumables are utilized, whenever possible. Additionally, the containment buildings will be inspected more frequently during refueling outages to ensure housekeeping requirements are being met.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) 4) By February 25, 2005, housekeeping requirements for containment building entry will be added to the "all hands" ALARA briefings for outage containment entry.
5) Design changes will be implemented to increase sump screen margin.
G. ADDITIONAL INFORMATION
Failed Components:
None
Previous Similar Events:
adequate to prevent the event of May 2004.
Energy Industry Identification System Code:
Residual Heat Removal System — BP Containment Sumps - NH