05000458/LER-2009-003
| River Bend Station - Unit 1 | |
| Event date: | 09-29-2009 |
|---|---|
| Report date: | 11-23-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4582009003R00 - NRC Website | |
On September 29, 2009, at approximately 11:27 a.m. CDT, the main control room operators manually started the low pressure coolant injection (LPCI) function of the residual heat removal (RHR) (BO) system in response to a decrease of upper reactor cavity water level. At the time of the event, the plant was in a refueling outage. The reactor pressure vessel (RPV) was disassembled, and the reactor cavity water level was greater than 23 feet above the vessel flange. No handling of irradiated fuel or control blades was in progress at the time.
This event is being reported in accordance with 10CFR50.73(a)(2)(iv)(A) as a condition involving the unplanned actuation of an ECCS system.
BACKGROUND
A Division 1 integrated emergency core cooling system (ECCS) surveillance test was in progress at the time of the event. At approximately 10:37 a.m. CDT, when the test signal was initiated, one of the expected responses was the actuation of the Division 1 primary containment isolation logic. That actuation caused the closure of primary containment isolation valves in various systems, one of which was the service air system. When the service air system (LF) containment isolation valve (**ISV**) closed as designed, pressure in the piping inside primary containment began to decrease. Among the components being served by the service air system was the main steam line (MSL) plugs in the RPV.
These plugs are installed during refueling outages to keep water in the RPV from entering the main steam (SB) piping.
The MSL plugs have as their primary sealing feature a mechanical device powered by an integral pneumatic motor that expands a segmented plug against the inner diameter of the RPV nozzle leading out to the steam line. This mechanism compresses an 0-ring against the inside of the nozzle, forming the primary seal. The backup seal is formed by a separate inflatable seal supplied with compressed air from the plant service air system.
Part of the scheduled maintenance affecting the main steam system is the periodic replacement of main steam system safety-relief valves (SRVs). At the time of this event, refurbished SRVs had been installed. However, the bolts in the SRV mounting flanges had not been fully tightened. When the service air pressure at the MSL plugs decreased, the plug in the "A" main steam line began to leak.
Water entering the main steam lines leaked from the loose SRV mounting flanges. The SRVs are located inside the drywell.
Approximately seven minutes after the initiation of the ECCS test, operators noted that water level indication in the drywell floor drain sumps was offscale high. At about the same time, it was also noted that upper reactor cavity water level was decreasing. Operating data for the waste water processing system indicated that approximately 5000 gallons of water was pumped out of the drywell floor drain system during this event.
At 11:26 a.m., the containment isolation valve in the service air system was opened, restoring air to the MSL plugs. The leakage into the main steam system then stopped.
Operators implementing the abnormal operating procedures for the decreasing water level in the upper reactor cavity manually started RHR pump "C" to restore the cavity level. The injection lasted for approximately 90 seconds, and was terminated when the upper cavity water level was returned to normal.
CAUSAL ANALYSIS
The investigation of this event found that the MSL plug in the "A" main steam line was not installed correctly. The mechanical seal had not been completely expanded, leaving a small gap between the 0-ring and the nozzle wall. In this condition, the inflatable seal pressurized by the service air system was the only barrier keeping water out of that main steam line. When the service air header in the primary containment was depressurized, the seal deflated and began to leak.
There were also the following contributing causes that led to this event:
1) The procedure for RPV disassembly did not have adequate instructions for installation of the MSL plugs. It did not have sufficiently detailed instructions and acceptance criteria for verifying that the plugs are properly installed.
2) When the use of MSL plugs of a different design was begun at River Bend Station in 2001, part of the change involved using the service air system for the inflatable seal instead of the instrument air system. The investigation of this event discovered that several procedure changes needed to accurately reflect this aspect of the change were not made. One of the documents that should have been revised was the procedure for the integrated ECCS test. That procedure contains provisions for the prompt restoration of the instrument air supply to primary containment following verification of the required isolation, but not for the service air system. The prompt restoration of the instrument air supply was to be performed to support the function of the backup seal in the MSL plugs.
CORRECTIVE ACTION TO PREVENT RECURRENCE
The following actions are being taken to prevent a recurrence of this event. These actions are being tracked in the station's corrective action program:
1) The procedure for RPV disassembly will be revised to add specific instructions and acceptance criteria on the correct installation of the MSL plugs. Additionally, a method to test the primary sealing mechanism for leakage will be developed and proceduralized.
2) A method to maintain air pressure to the backup inflatable seal will be developed and implemented.
For operations with reactor cavity water level greater than 23 feet above the RPV flange (and the reactor cavity gate open), there is no requirement in River Bend Technical Specifications to have ECCS systems operable. However, prior to reaching a level of 23 feet above the RPV flange, the operators had verified that sufficient ECCS systems were in standby to maintain compliance with Technical Specifications for operation at less than 23 feet. The operating crew determined a predetermined cavity level at which to start an available ECCS pump to level greater than 23 feet above the RPV flange. As soon as the level decreased to 23 feet, the "C" RHR pump was started to restore water level. Immediately prior to starting the pump, the operators recorded the minimum water level as 22 feet 11.9 inches.
There was no handling of irradiated fuel or control blades in progress at the onset of this event. This event was of minimal significance to the health and safety of the public.
(NOTE: Energy Industry Component Identification codes are annotated as (**XX**).)