05000263/LER-2002-004

LER-2002-004, Unplanned Loss of Both Trains of Control Room Ventilation During Auto Start Testing Due to Timing Circuit Relay Failure

Docket Number
Event date:	04-01-2002
Report date:	05-29-2002
2632002004R00 - NRC Website

Description While operating at 100% power, at 1436 on April 1, 2002, both trains (A & B) of the Control Room Ventilation (CRV)1 System became inoperable for a short period of time. The loss of both trains of CRV occurred during a scheduled annual test. The test demonstrates the low system flow auto start feature of CRV and Emergency Filtration Treatment (EFT) via system logic and instrumentation manipulation, one train and one system (CRV versus EFT) at a time.

In accordance with the annual test, one train of CRV and EFT is made inoperable to induce a low system flow to allow the testing of the auto start on low system flow feature of the redundant train of CRV and EFT. The air conditioning unit receives the auto start signal for the CRV system (V-EAC-14A for train A and V-EAC-14B for train B). By the same procedure, similar auto start testing is separately performed on the EFT system fans (V-ERF-11 for EFT Train A, V-ERF-12 for EFT Train B) and applicable Limiting Conditions of Operations (LC0s) are entered.

At 1401 the A train of CRV was made inoperable (entering a 30 day LCO TS 3.17.A.2.a) and the B train of CRV successfully auto started on a low system flow signal. At 1430 the A train of CRV was restored to operability and the 30 day LCO was exited. At 1434 the B train of CRV was made inoperable (entering the 30 day LCO) in preparation to test the auto start on low flow for train A. The A train of CRV did not start on low system flow. In response, after 153 seconds, the attendant control room operator manually started the A train of CRV in accordance with the test procedure and the test was discontinued. With both trains of CRV inoperable, an unplanned 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> LCO (TS 3.17.A.3.a) was entered at 1436.

At 1600, following system restoration, the B train of CRV was declared operable. The plant exited the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> LCO, but remained in the 30 day LCO due to A train inoperability.

On April 2, 2002, following troubleshooting, replacement and operability testing of the A train CRV auto start relay2 , the 30 day LCO for A train of CRV inoperability was exited at 1350. The annual test of CRV-EFT low flow was completed at 1825.

Event Analysis

Analysis of Reportability The event is reportable under 10 CFR 50.73(a) (2)(v)(D), as both trains of a system designed to control the consequences of an accident were unavailable.

1 EIIS System Code: VI 2 Component Identifier: 2 The event is a safety system functional failure.

Safety Significance

The safety significance of the event is considered low. The auto start function on low system flow was tested successfully on the B train of CRV initially, demonstrating the operability of the auto start feature of that train. The loss of the auto start feature for both trains of CRV occurred per procedure for a short time interval between when the testing began on April 1, 2002 until the B train of CRV was restored to operable status (less that two hours).

The PRA group reviewed the event and concluded the event was of low safety significance.

The basis of this conclusion was: the threat of a chemical or radiological air contamination issue affecting control room habitability was very small, significant radiological habitability issues for the main control room would only occur following some core damages scenarios and therefore Core Damage Frequency (CDF) was unaffected by this event, and the Large Early Release Frequency (LERF) was unaffected because the automatic initiation of the EFT upon a high radiation conditions remained functional and the standby EFT train was available by manual initiation.

Cause

The cause of the event was failure of the timing circuit relay failure in the A train of CRV.

Corrective Actions

The relay was replaced and the system was successfully tested for auto start on low system flow.

Test method changes to eliminate the susceptibility of the loss of both trains are being investigated for the annual low flow test.

Failed Component Identification Struthers Dunn Timing Circuit Relay Model Number 236ABX138NE

Previous Similar Events

None event. The function and type of the involved relays were different in the earlier event.