05000410/LER-2017-001

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LER-2017-001, 1 OF 5
Nine Mile Point Unit 2
Event date: 08-05-2017
Report date: 10-04-2017
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(B), System Actuation

10 CFR 50.73(a)(2)(iv)(A), System Actuation
4102017001R00 - NRC Website
LER 17-001-00 for Nine Mile Point Nuclear Station, Unit 2, Regarding Automatic Reactor Scram due to High Reactor Pressure
ML17296A158
Person / Time
Site: Nine Mile Point Exelon icon.png
Issue date: 10/04/2017
From: Kreider R E
Exelon Generation Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
NMP2L 2655
Download: ML17296A158 (7)


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.

3. LER NUMBER

2017 001 - 00

I. DESCRIPTION OF EVENT

A. PRE-EVENT PLANT CONDITIONS:

Prior to the event, Nine Mile Point Unit 2 (NMP2) was in Mode 1 and operating at 84% reactor power. Turbine controls troubleshooting and overspeed trip system repairs had been completed, followed by power ascension from 20% power to 84% power. The plant was stabilized at 84% power to support quarterly turbine valve testing.

B. EVENT:

On August 5, 2017, at approximately 2235, during performance of quarterly turbine valve testing, an automatic Reactor scram occurred. The RPS system actuation was due to high reactor pressure. The pressure increase occurred when the turbine control valves (TCV) began closing while stroking turbine stop valves (TSV) 1 and 2. All systems responded as expected to the reactor scram signal.

Based on the turbine bypass valves (TBV) fast opening and TCVs ramping closed, a loss of control valve demand was determined to be the initiator of the event. The TSV limit switch circuit was tested for grounds or stuck limit switches after the scram but none could be found.

Troubleshooting identified an erroneous control valve demand signal resulting in fast closure of the control valves and subsequent increase in reactor pressure. After instrumenting the load control circuit, an anomaly on the load limit amplifier card was identified. The card was replaced and sent out for failure analysis. To further protect against latent failures, a temporary modification to lift the leads on the stop valve load limit switches was implemented. Lifting the leads minimizes the risk of fast closure of the TCVs in the event of a stuck load limit switch during future turbine valve testing.

The failure analysis of the load limit amplifier card determined the card was functioning properly and was not the cause of the event. This further supports that the spurious actuation of load limit relays KL186 and KL187 was caused by an intermittent ground and/or an induced voltage within the load limit circuit.

Nine Mile Point Unit 1 (NMP1) was unaffected by the scram at NMP2.

The event has been entered into the plant's corrective action program as IR 4039435.

C. INOPERABLE STRUCTURES, COMPONENTS, OR SYSTEMS THAT CONTRIBUTED TO

THE EVENT:

No other systems, structures, or components contributed to this event.

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.

001 - 00 Nine Mile Point Unit 2 05000410 2017

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES AND OPERATOR

ACTIONS:

The dates, times, and major occurrences and operator actions for this event are as follows. All dates are 2017.

August 5 1500 Began power ascension from 20% after completion of turbine controls troubleshooting and overspeed trip repairs.

2040 Power ascension placed on hold at 84% power and started TSV and TCV testing per N2-OSP-RPS-0001.

2235 During step 8.8.3, when testing the last set of TSVs, the TCVs started closing, the TBVs opened and the reactor scrammed on high reactor pressure. EOP-RPV was entered due to low reactor water level, as well as high reactor pressure. N2- SOP-101C was entered due to the reactor scram.

2323 Exited EOP-RPV and N2-SOP-101C.

E. METHOD OF DISCOVERY:

This event was self-revealing during the performance of the surveillance test when relays were heard actuating unexpectedly and a reactor scram occurred.

F. SAFETY SYSTEM RESPONSES:

All systems operated as expected. After the scram, there were no safety system actuations.

II. CAUSE OF EVENT:

The definitive root cause of the equipment failure was not located but was bound to spurious actuation of load limit relays KL186 and KL187. The spurious action was caused by an intermittent ground and/or an induced voltage within the load limit circuit. This is a result of the non-fault tolerant original design of the Electro-hydraulic Control (EHC) system.

III. ANALYSIS OF THE EVENT:

The automatic reactor scram is reportable under 10 CFR 50.72(b)(2)(iv)(B) and 10 CFR 50.73(a)(2)(iv)(A). It is defined under paragraph 10 CFR 50.73(a)(2)(iv)(A) as any event or condition that resulted in manual or automatic action of any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B). The RPS system actuation, including reactor scram, is listed in 10 CFR 50.73(a)(2)(iv)(B).

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 NE08-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. It 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.

3. LER NUMBER 1. FACILITY NAME 2017 001 - 00 The definitive root cause of the equipment failure was not located but was bound to spurious actuation of load limit relays KL186 and KL187. The spurious action was caused by an intermittent ground and/or an induced voltage within the load limit circuit. The relay actuation resulted in the TCVs closing, the TBVs opening and a reactor scram on high reactor pressure.

All control rods fully inserted and there were no safety system actuations.

Based on the above discussion, it is concluded that the safety significance of this event is low and the event did not pose a threat to the health and safety of the public or plant personnel.

Troubleshooting identified an erroneous control valve demand signal resulting in fast closure of the TCVs and subsequent increase in reactor pressure. After instrumenting the load control circuit, an anomaly on the load limit amplifier card was identified. The load limit amplifier card was replaced. Failure analysis did not identify any anomalies with the card.

To further protect against latent failures, leads have been lifted on the stop valve load limit switches to prevent fast closure of the TCVs in the event of a stuck load limit switch during future valve testing.

This event does affect the NRC Regulatory Oversight Process Indicator for unplanned scrams per

7000 hours
0.081 days
1.944 hours
0.0116 weeks
0.00266 months

of critical operation.

IV. CORRECTIVE ACTIONS:

A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL

STATUS:

The load limit amplifier card was replaced.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

To prevent recurrence, replacement of the current obsolete and single point vulnerable NMP2 Turbine EHC system with a fault tolerant Digital EHC system is scheduled for the next refueling outage.

To further protect against latent failures, a temporary modification to lift the leads on the stop valve load limit switches was implemented. Lifting the leads minimizes the risk of fast closure of the control valves in the event of a stuck load limit switch during future turbine valve testing. Additionally, to minimize risk prior to replacement of the system, the testing method for TSVs is being revised prior to the next performance.

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.

3. LER NUMBER 1. FACILITY NAME 2017 001 - 00

V. ADDITIONAL INFORMATION:

A. FAILED COMPONENTS:

The definitive root cause of the equipment failure was not located but was bound to spurious actuation of load limit relays KL186 and KL187. The spurious action was caused by an intermittent ground and/or an induced voltage within the load limit circuit.

The load limit amplifier card was replaced, but troubleshooting determined it was not the single cause of the event. Potentially stuck limit switches have been disabled.

B. PREVIOUS LERs ON SIMILAR EVENTS:

None C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) COMPONENT FUNCTION

IDENTIFIER AND SYSTEM NAME OF EACH COMPONENT OR SYSTEM REFERRED TO

IN THIS LER:

COMPONENT

Electro Hydraulic Control System Load Limit Amplifier Card Load Limit Relay Limit Switch Turbine Stop Valve Turbine Control Valve Turbine Bypass Valve Main Turbine Reactor Protection System

D. SPECIAL COMMENTS:

None