05000458/LER-2017-008

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LER-2017-008, Automatic Reactor Scram due to Failure of Main Feedwater Regulator Transfer Relay
Event date: 08-18-2017
Report date: 10-12-2017
Reporting criterion: 10 CFR 50.73(a)(2)(iv), System Actuation
4582017008R00 - NRC Website

comments regarding burden estimate to the Information Services Branch (T-2 F43). U.S. Nuclear Regulatory Commission, Washington, DC 20555.0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to. the information collection.

REPORTED CONDITION

On August 18, 2017, at 8:55 p.m. CDT, an automatic reactor scram occurred while the plant was operating at 100 percent power. The operators promptly established control of reactor water level and pressure, and a controlled plant cooldown was commenced. No safety-related equipment was out of service at the time of the scram.

The initial scram signal was a flow-biased thermal trip on the average power range monitors (APRMs). This action closely followed a planned shift of the master feedwater [S.1) controller from channel "B" to channel "A." When the shift was made, all three main feedwater regulating valves unexpectedly moved fully open. At the same time, the reactor recirculation pumps shifted to slow speed.

This condition is being reported in accordance with 10 CFR 50.73(a)(2)(iv) as an event resulting in the automatic actuation of the reactor protection system.

INVESTIGATION and CAUSAL ANALYSIS At the time of the event, operators were performing a planned shift of the controlling channel of the master feedwater regulation system as part a scheduled surveillance test on the system.

Troubleshooting discovered that the feedwater level channel select relay (**83**) had failed, such that no signal was present on the "A" channel. When that channel was selected, the feedwater control system erroneously sensed that reactor water level was low, and caused all three feedwater regulating valves to move fully open. The operator took manual control of the system to restore the feedwater regulating valves to their original positions.

The false low water level signal was also sensed in the control circuitry for the reactor recirculation system. This initiated the automatic downshift of the recirculation pumps to slow speed, as designed. The decrease in core flow resulted in the flow-biased thermal trip in the APRMs.

The failure in the channel select relay was traced to degradation in the electrical contacts on the relay that caused a high resistance condition. This condition also masked an alarm that could have potentially alerted the operator to the fact that the "A" channel was inoperative.

The high resistance condition on the contacts was caused by the use of a relay model with silver contacts instead of gold contacts. Industry operating experience had shown that the Agastat GPI (silver contact) relay, when used in low-current applications, had a tendency to develop contact point oxidation that increased electrical resistance. A parts interchangeability evaluation had been performed in the past at River Bend to approve the Agastat GPIA with gold contacts as a replacement part. However, when that evaluation was performed, the part number for the model GPIA was added to the appropriate design documentation, but the part number for the GPI was not deleted.

comments regarding burden estimate to the Information Services Branch (T-2 F43). U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to NEOB-10202. (3150.0104). Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number. the NRC may not conduct or sponsor, and a person is not required to respond to. the information collection.

River Bend Station — Unit 1 05000-458 NUMBER NO 2017 008 00

CORRECTIVE ACTIONS TO PREVENT RECURRENCE

he failed GPI relay was replaced with a GPIA with gold contacts.

Design documentation for the feedwater regulation system will be updated to reflect the model GPIA as the required replacement part. This is being tracked in the corrective action program.

PREVIOUS OCCURRENCE EVALUATION

No events with a common root cause have been reported by River Bend Station in the last three years.

SAFETY SIGNIFICANCE

he plant response was bounded by the event described in the Updated Safety Analysis Report as a loss of the in-service eedwater level transmitter signal. This event was thus of minimal significance to the health and safety of the public.

(NOTE: Energy Industry Identification System component function identifier and system name of each component or system referred to in the LER are annotated as (**XX**) and [XX], respectively.)