|2772003001R00 - NRC Website|
Unit Conditions Prior to the Event Unit 2 was in Mode 1 and operating at approximately 100% rated thermal power when the event occurred. At the time of the event, there were no activities in progress related to Main Steam Isolation Valves (MSIVs) (EIIS: ISV). There were no structures, systems or components out of service that contributed to this event.
Description of the Event
on 4/12/03, Unit 2 scrammed as a result of a high reactor pressure condition. The high pressure condition resulted from the closure of the 'D' Outboard Main Steam Isolation Valve (MSIV). The MSIV closed as a result of a failed instrument air line (EIIS: TBG) that provides a pneumatic supply to the valve control manifold (EIIS: PSX).
As a result of the high pressure condition, the Recirculation Pumps tripped and the Alternate Rod Insertion (ARI) system actuated. The mode switch was promptly moved to the 'Shutdown' position and plant cool down was commenced using the normal heat sink (Bypass valves to the condenser). As expected, PCIS Group II and III isolations were received, as a result of reaching the Level 3 reactor water level set point subsequent to the scram. All isolation valves operated as designed.
The PCIS Group II / III isolations were reset by approximately1900 hours
. NRC prompt notifications were completed at approximately2055 hours
. A half scram ('A' Channel) signal was generated on 4/13/03 at0039 hours
due to erratic oscillations in the '2A' Wide Range Neutron Monitor (EIIS: MON). The monitor was bypassed and the half-scram signal was reset. The plant was maintained in the hot shutdown condition (Mode 3) until the mode switch was placed in the Startup position (Mode 2) on 4/13/03 at1545 hours
Analysis of the Event
There were no actual safety consequences as a result of this event. All control rods inserted on the reactor scram signal. The Group II / III PCIS isolations resulted in the primary containment isolation safety function being met. ARI and the Recirculation Pump Trip functions operated due to a high reactor pressure condition. This is a conservative plant action that provides backup actions for the Reactor Protection System.
Post-scram cooling was accomplished using the Main Steam Bypass Valves to the Condenser.
A Conditional Core Damage Probability (CCDP) study was performed. This analysis determined that core damage could occur once in approximately every one million events. Therefore, this event did not have any risk significance. This event is bounded by the design basis event entitled, 'Isolation of One Main Steam Line'.
During this event, the plant safety systems responded as necessary. This event did not involve operations that exceeded the design basis. The single MSIV closure did cause a rise in neutron flux, however, the flux rise did not reach the bounding Hi Flux Scram set point as described in the Updated Final safety Analysis Report (UFSAR). The SCRAM on high reactor pressure (which reached approximately 1080 psig) rather than on high neutron flux was evaluated and is considered appropriate for the given transient conditions. This was based on lower void coefficients in the currently licensed fuel and a slower pressure transient than the bounding MSIV failure analyses in the UFSAR.
The MSIV accumulator pneumatic supply is safety related for both the inboard and outboard MSIVs. The MSIV solenoid valve control manifold pneumatic supply to the inboard MSIVs is required to be safety related since they are not isolated from the safety related pneumatic supply to the accumulator. The outboard MSIV pneumatic supply to the control manifolds are not fed from within the safety related pneumatic supply boundary and therefore, are not considered as safety related.
Cause of the Event
The cause of the Unit 2 scram was due to a failed instrument air line that supplies the solenoid valve control manifold for the 'D' Outboard MSIV. It was observed that the instrument line failed at the ferrule area of the tubing fitting at the solenoid valve control manifold. The failed 'D' Outboard MSIV instrument air line was sent to a failure analysis laboratory to determine the exact cause of the failure. It was determined that high cycle, vibration induced fatigue was the cause of the instrument air line failure.
Further engineering review determined that the 'D' Outboard MSIV instrument tubing was vulnerable to fatigue failures due to the method of supporting the instrument air lines. The instrument line that failed is a 3/8" diameter Type 304 stainless steel tubing utilizing a tubing fitting for connection to the solenoid valve control manifold. The instrument air lines that supply the solenoid valve control manifold are not safety related and non-seismic. The tubing that failed contained an expansion loop and the unsupported tubing length was approximately 100".
FACILITY NAME (1) DOCKET (2) LER NUMBER (6) PAGE (3)
- Cause of the Event, (continued) The spurious half-scram generated by the '2A' Wide Range Neutron Monitor (General Electric, Model 304A3712G005) is believed to be due to electromagnetic interference and is being further evaluated through the corrective action program.
The 'D' Outboard MSIV instrument air tubing was replaced. Similar tubing on the other three Outboard MSIVs was also replaced. An engineering evaluation was performed on the Outboard MSIV instrument tubing supports. This resulted in several additional tubing supports being added to the four Outboard MSIV instrument air lines. Similar upgrades are planned for the Unit 3 Outboard MSIVs when plant conditions permit maintenance.
A walk down of Unit 2 was performed to assess the adequacy of pneumatic control lines. Appropriate minor repairs were made to some pneumatic lines. Additional Unit 3 walk downs will be performed when plant conditions permit to assess other instrument air lines for degradation and tubing support adequacy.
Engineering performed a review of the Unit 2 and Unit 3 Inboard MSIV instrument nitrogen supply lines. It was concluded that these lines are more robust than the Outboard lines and do not require additional supports. The Inboard MSIV nitrogen supply lines are safety related.
Appropriate additional grounding of the '2A' Wide Range Neutron Monitor is being considered in accordance with the corrective action program.
Previous Similar Occurrences There were no previous events identified involving a scram due to MSIV closure caused by a failed instrument air line.