05000458/LER-2014-004
| River Bend Station - Unit 1 | |
| Event date: | 07-30-2014 |
|---|---|
| Report date: | 09-26-2014 |
| 4582014004R00 - NRC Website | |
REPORTED CONDITION
On July 30, 2014, with the plant operating at 100% power, a review of an engineering analysis of the ultimate heat sink (UHS) (BS) determined that the UHS was in an unanalyzed condition that degraded plant safety. This condition was the result of a design basis deficiency for the UHS that did not account for the adverse effects of system leakage on compliance with the 30-day inventory required by Regulatory Guide 1.27. The system design basis requires that 30-day inventory be maintained, with the assumption that no replenishment of the UHS inventory occurs for the entire duration of the postulated event.
In support of the development of the engineering analysis, compensatory measures have been implemented which provide adequate assurance that the UHS will perform its design safety function. Corrective actions to restore full compliance with design basis requirements are in development.
This event is being reported in accordance with 10 CFR 50.72 (b)(3)(ii) as an unanalyzed condition that degrades the safety function of the UHS.
BACKGROUND
The service water system comprises a non-safety related loop, and two joined (but normally idle) safety-related loops supported by the two divisional emergency diesel generators (EDGs). The standby service water (SSW) subsystem, with the standby cooling tower and four divisional pumps, is referred to as the ultimate heat sink (UHS). The UHS, in the standby configuration, is isolated from the normal service water system by motor-operated valves. Upon an actuation signal, the UHS automatically starts and assumes the heat loads for all safety-related systems.
During a 2011 Component Design Basis Inspection, RBS received a non-cited violation concerning calculations related to the UHS.
Design changes were developed to correct the condition, but would require prior NRC approval for implementation. A license amendment request (LAR) was submitted in February 2014 to request approval for crediting replenishment of the UHS inventory approximately 22 days after the onset of a loss of offsite power / loss of coolant accident to account for both UHS out-leakage to the normal service water system and the operation of both divisions of SSW.
During acceptance review of the LAR, NRC raised concerns with the inability to meet the 30-day post-accident mission time without replenishment. The analysis that confirms compliance with the 30-day mission time assumes the operation of only one division of safety-related systems. This analysis is in compliance with the license basis as documented in the original Final Safety Analysis Report, and has been carried forth without revision into the current Updated Safety Analysis Report. Therefore, the RBS design for UHS inventory is in compliance with the licensing bases on the issue of divisional equipment operation.
The issue of leakage is not as clearly delineated in the licensing basis. The design basis accounts for evaporation and drift from the standby cooling tower, but does not take into consideration UHS out-leakage through the isolation valves. The UHS does not meet the 30-day mission time without replenishment when assuming this out-leakage. The current known leakage is 8.9 gpm for Division 1 and 6.3 gpm for Division 2. The basin does not contain adequate capacity at the Technical Specification minimum water level of 111' 10" to meet the 30-day mission time with the existing measured leakage without compensatory measures.
- 00 2014 - 004 == IMMEDIATE ACTIONS ==
In order to provide reasonable assurance of the ability of the UHS to fulfill its design basis function, several compensatory measures were instituted.
1. The standby cooling tower basin water level was raised approximately three feet.
2. A standing order was issued to direct the following:
a. Within four hours of the onset of the event, perform a qualitative assessment of the ability to provide at least 120 gpm of makeup flow to the UHS b. If insufficient makeup capacity exists, shut down either Division 1 or 2 EDG.
c. Enter the abnormal operating procedure for a malfunction of the SSW system.
CORRECTIVE ACTION TO PREVENT RECURRENCE
Various options are being evaluated to ultimately correct this condition and restore the UHS to a fully operable status. Resolution of this condition is being tracked in the station's corrective action program.
PREVIOUS OCCURRENCE EVALUATION
No similar conditions have been reported at River Bend Station during the past 3 years.
SAFETY SIGNIFICANCE
For the condition of the UHS prior to the institution of the compensatory measures described above, the UHS is considered to have been operable, but degraded. Various means of replenishing the UHS inventory have been described in abnormal operating procedures for over ten years. Therefore, there is reasonable assurance that the UHS would have been capable of supporting the response to a design basis event. With the new compensatory measures in place, the UHS is considered operable, and is capable of fulfilling its design safety function. Thus, this condition has been of minimal significance to the health and safety of the public.