05000306/LER-2007-001

LER-2007-001,

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function
3062007001R01 - NRC Website

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) � 05000306 YEAR 07� 01 1 The root cause evaluation (RCE) of this event determined that the equipment root cause was due to high contact resistance on the contact of the safety injection relay which did not allow enough current to reach the reset coil of the relay and the relay did not reset. This caused a safety injection actuation and reactor trip when the system was taken out of test. The RCE further determined that the organizational root cause was due to lack of developing and implementing a preventive maintenance strategy for the MG-6 style relays in the RPS. Historically Prairie Island had not experienced failure of these relays with resulting adverse consequences and had not applied the resources necessary to develop and implement a preventive maintenance strategy.

EVENT DESCRIPTION

On April 5, 2007, Prairie Island Nuclear Generating Plant (PINGP) Unit 2 was operating at 100% power. At approximately 0908 CDT, during surveillance testing of Unit 2 Train A safeguards logic at power, a spurious Train A Safety Injection (SI) actuation occurred resulting in Reactor Protection System2 (RPS) actuation. Train A SI was in "Test" at the time and should not have caused the RPS trip. At approximately 0913 the operating crew manually actuated Train B SI as required by emergency operating procedures. All automatic actions for a reactor trip and safety injection occurred as required. Reactor Coolant System3 (RCS) pressure momentarily decreased below the shutoff head of the high head Emergency Core Cooling System (ECCS) pumps during the transient, resulting in ECCS discharge to the RCS. At approximately 0920 safety injection was terminated per emergency operating procedures. All systems operated as expected and operator response and recovery actions were as expected.

EVENT ANALYSIS

The trip of the Unit 2 reactor and the actuation of the emergency core cooling system are required to be reported per 10 CFR 50.73(a)(2)(iv)(A).

Impact on Safety System Functional Failure Performance Indicator This event did not result in a loss of the safety injection system since the Unit 2 Train B safety injection was manually actuated and performed as expected. Therefore, this event does not represent a loss of safety function. Consequently, this event is not reportable per 10CFR 50.73(a)(2)(v).

1 EIIS System Code: BQ 2 EIIS System Code: JC 3 EIIS System Code: AB _ _ FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) 07 -- 01 -- 1

SAFETY SIGNIFICANCE

The plant was stabilized in Mode 3 after the trip and all systems performed as expected in response to the reactor trip. Therefore, this event did not affect the health and safety of the public.

CAUSE

Initial investigation of the cause of the automatic reactor trip was determined to be a deficiency with the safety injection relay equipment. Instrument and Control technicians discovered a safety injection relay4 with high contact resistance. The high resistance contact caused the relay to not reset when exiting the safeguards logic test. With the relay not reset, and safeguards logic not in test, a spurious SI actuation occurred. The defective relay was replaced and tested to verify proper operation prior to the Unit 2 reactor startup.

The root cause evaluation (RCE) of this event determined that the equipment root cause was due to high contact resistance on the contact of the safety injection relay which did not allow enough current to reach the reset coil of the relay and the relay did not reset. This caused a safety injection actuation and reactor trip when the system was taken out of test. The RCE further determined that the organizational root cause was due to lack of developing and implementing a preventive maintenance strategy for the MG-6 style relays in the RPS. Historically Prairie Island had not experienced failure of these relays with resulting adverse consequences and had not applied resources necessary to develop and implement a preventive maintenance strategy.

CORRECTIVE ACTION

Immediate:

1. The defective relay was replaced and tested.

Subsequent:

2. Safeguards logic test procedures have been revised to preclude a similar event.

Planned:

3. Replace the remaining MG-6 style relays in the safeguards racks during the next refueling outage on each unit.

4. Complete development and implementation of a preventive maintenance strategy for the MG-6 style relays in the safeguards racks.

5. Complete development and implementation of a preventive maintenance strategy for all plant equipment classified as critical.

4 EDS Component Code: RLY FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) 07 -- 01 -- 1

PREVIOUS SIMILAR EVENTS

Both Unit 1 and Unit 2 have experienced unplanned reactor trips in the past. The only other unplanned reactor trip in the last three years occurred in 2006. The 2006 reactor trip was not caused by the safety injection system. There were no other reportable events in the past three years that were related to relay failures or inadvertent SI actuation.