05000352/LER-2002-002
| Docket Number | |
| Event date: | 02-04-2000 |
|---|---|
| Report date: | 05-23-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3522002002R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 97% power. Unit 2 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On February 4, 2000, the Unit 1 "Core Spray Line Internal Break" alarm (EIIS:PDA) spuriously annunciated during the "HPCI Pump, Valve and Flow" surveillance test. An investigation of this condition determined that the instrument setpoint zero offset was invalid for the as-found value of negative 0.5 psid for Core Spray (EHS:BM) Header differential pressure. The instrument setpoint zero offset values were later modified to permit operation with differential pressures between 0.0 psid and negative 2.5 psid. The Unit 1 instrument setpoint was modified on July 10, 2000 and the instrument was declared operable. The Unit 2 instrument setpoint was modified on July 11, 2000.
This issue was identified as reportable on March 26, 2002. An NRC inspector had identified this historical issue and requested a review for reportability since a similar condition had been reported in 1998 (LER 1-98-015). The requested station reportability review of the issue determined that there was firm evidence that a condition prohibited by technical specifications existed before the point of discovery on February 4, 2000. This was not identified when the Unit 1 alarm was declared inoperable resulting in the late reporting of this condition.
Technical Specification 3.5.1, ECCS — Operating, requires this alarm to detect differential pressure in excess of 4.4 psid between the "A" and "B" Core Spray headers. The alarm was inoperable since it was not capable of detecting all Core Spray line breaks internal to the reactor pressure vessel. TS compliance has been maintained since the point of discovery on February 4, 2000. However, this event involved an operation prohibited by Technical Specifications.
Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(i)(B).
Analysis of the Event
There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal. The alarm would have actuated for a line break in the "A" loop but may not have actuated for a line break in the "B" loop. Inspections of welds on the core spray lines are included in the reactor pressure vessel (RPV) inservice inspection (ISI) program.
A difference in the configuration of the Core Spray headers inside the RPV (EIIS:RPV) core shroud results in the "A" loop being more susceptible to steam void accumulation. The lower ("B" loop) ring header spray nozzles are mounted on the top of the header, which inhibits steam entry. The upper ("A" loop) ring spray header nozzles are mounted on the bottom of the header, which facilitates steam entry. The 0.25 inch vent hole at the high point of the "A" loop header inlet pipe does not vent enough steam to keep the header full at all times resulting in different pressures being sensed by the Core Spray Line Internal Break instrumentation. The sensed differential pressure has been verified to be between 0.0 psid and negative 2.5 psid after accounting for indicator accuracy limits. The negative 2.5 psid value corresponds to the expected value for complete voiding of the "A" header with no voiding of the "B" header. The value of Core Spray header differential pressure equals "B" header pressure minus "A" header pressure.
The instrument setpoints have been revised to account for this normal variation in the sensed pressure. This ensures TS 3.5.1 ECCS compliance will be maintained for all conditions.
General Electric (GE) Service Information Letter (SIL) 300 was issued in 1979 to address Core Spray Line Break Detection instrumentation that is downscale during normal operation. The SIL provided a range of expected differential pressures across the core shroud for use in calibration of the instrument. This SIL addressed velocity and process noise effects but did not consider possible voiding effects. The expected differential pressure at full power is approximately 6 psid.
Sensing a differential pressure of approximately 6 psid would be indicative of a Core Spray header failure in the area outside the shroud.
Since a design bases Loss of Coolant Accident (LOCA) would result in RPV coolant flashing to steam, including the coolant in the header, the voided condition has been previously evaluated and core reflood time is not affected. The potential for water hammer damage due to voiding in the header has been evaluated as acceptable. The nozzle opening areas are approximately equal to the pipe diameter resulting in the equivalent of an open pipe.
Cause of the Event
This event occurred due to a previously unidentified condition that causes a variation of the zero offset value for the sensed differential pressure. The previous modification of the instrument field setpoints to account for apparent power rerate flow based zero offset did not allow for possible variation in this parameter due to accumulation of steam voids in the piping.
Corrective Action Completed The Core Spray Line Internal Line Break alarm field setpoints have been modified to allow for the expected range of instrument zero offset values.
Previous Similar Occurrences Alarm on September 28, 1998.
Component information:
Component: � Switch, Indicating, Differential, Pressure Manufacturer: � Rosemount Nuclear Instruments Inc.
Model number: � 710DUOTT