05000364/LER-2004-004

LER-2004-004,

Docket Number
Event date:	04-11-2004
Report date:	06-04-2004
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3642004004R00 - NRC Website

Westinghouse -- Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX]

Description of Event

On April 11, 2004, at 1105, with Unit 2 at zero power critical conditions, the reactor tripped due to a logic failure unblocking the B Train Source Range High Flux Trip (P-6 interlock) which allowed a Source range high flux reactor trip signal to be generated from B Train source range detector. The A Train Reactor Trip Breaker did not automatically open and was manually opened by the operator. Post mortem indication revealed the reactor trip was due to the reset of the B Train source range block which resulted in the reenergization of source range channel N32. Once the source range block of N32 was reset, N32 experienced neutron flux higher than the trip set point and generated a B Train reactor trip signal.

The source range block on N31 remained intact and therefore no A Train source range trip signal was generated. Thus, the A Train Reactor Trip Breaker remaining closed was an expected condition.

Symptoms indicated a failure in the B Train SSPSPCJ. Engineering analysis of symptoms identified three potential suspect logic cards. Surveillance and bench testing were performed to identify failed cards from the suspect population. SSPS power supply cards were verified to be acceptable through testing. As a result, two cards (A408 and A411) indicating functional problems were replaced. B Train SSPS surveillance was conducted satisfactorily.

On April 12, 2004 the Unit 2 reactor was restarted. Approximately 30 minutes after blocking the source range high flux trip as a part of the normal startup sequence, the reactor tripped again with similar symptoms and system response.

Subsequent engineering analysis and troubleshooting was conducted. A special test procedure was developed and performed to provide a simulated Source Range input into the SSPS logic circuitry, while monitoring critical points in the logic circuit. No failures were identified. Source Range and Intermediate Range flux detectors were tested satisfactorily. SSPS surveillance testing was performed with no problems observed.

The special test procedure was conducted a second time with the SSPS cabinet doors closed which more closely reflects the conditions actually experienced during normal operation. A few hours into this test, while monitoring critical logic circuitry points, the Source Range P-6 interlock was found to have changed state. Additional testing with the cabinet doors closed was capable of recreating the intermittent card failure. The card (A406) which experienced the intermittent failure had previously been tested satisfactorily following the first reactor trip.

When the Source Range High Flux Trip is blocked (P-6 Interlock), circuits on the card are energized, thus increasing the temperature of the card. Bench testing of the defective card using an external heat source confirmed that the failure came in as the card temperature increased.

A review of work order history and of card components indicated that the card (A406) had components manufactured as early as 1974 and as late as 1985. The failed card (A406), as well as other cards (A408 and A411) in the source range high flux trip (P-6 interlock) were replaced. The special test procedure was conducted again simulating actual cabinet temperature. No problems were observed. SSPS surveillances were conducted to confirm proper functions.

Following a thorough review of investigative results and corrective measures, Unit 2 reactor was restarted.

Cause of Event

Both reactor trip events were due to an intermittent failure of universal logic card (A406) in the B Train Solid State Protection System (SSPS). The card failure was due to an increased sensitivity to heat believed to be the result of aging. The card was found to be overly sensitive to ambient temperature changes after energization of the Source Range Trip Block (P-6 interlock).

Safety Assessment All safeguards equipment functioned as designed following both trips.

The logic circuit failure in the B Train SSPS resulted in an inappropriate safety function actuation and did not result in the loss of a train of safety function.

The health and safety of the public were unaffected by this event.

This event does not represent a Safety System Functional Failure.

Corrective Action The failed card (A406) was replaced as well as other applicable cards (A408 and A411) in the P-6 interlock circuit.

The equipment reliability plan associated with the 7300 system will be revised by November 30, 2004 to incorporate SSPS card failure and aging issues identified during this event.

Additional Information

The following LER has been submitted in the past two years regarding aging of electronic circuits.