05000220/LER-2017-002

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LER-2017-002, Manual Reactor Scram Due to Pressure Oscillations
Nine Mile Point Unit 1
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(B), System Actuation

10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 52624 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation
2202017002R00 - NRC Website
LER 17-002-00 for Nine Mile Point Nuclear Station, Unit 1 Regarding Manual Reactor Scram Due to Pressure Oscillations
ML17143A228
Person / Time
Site: Nine Mile Point Exelon icon.png
Issue date: 05/18/2017
From: Kreider R E
Exelon Generation Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
NMP1L3157
Download: ML17143A228 (8)


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 002 - 00

I. DESCRIPTION OF EVENT

A. PRE-EVENT PLANT CONDITIONS:

Prior to the event, Nine Mile Point Unit 1 (NMP1) was offline and reactor shutdown was in-progress.

Reactor power was approximately 4%.

B. EVENT:

On March 20, 2017 at 0227, during a scheduled reactor shutdown in support of the 2017 refueling outage, Operators at Nine Mile Point Unit 1 inserted a manual scram due to pressure oscillations exceeding procedural limits. NMP1 was offline with the scheduled reactor shutdown in-progress at the time of the scram. At approximately 4% reactor power, Control Room Operators recognized mechanical pressure regulator (MPR) oscillations causing pressure changes and turbine bypass valve movement. When pressure oscillations exceeded the procedurally required limit the Reactor Operator at The Controls manually inserted the reactor scram.

Nine Mile Point Unit 2 (NMP2) was and remained at 100% throughout the event.

Operations performed the ENS notification (#52624) required by 10 CFR 50.72(b)(2)(iv)(B) for the manual reactor scram due to pressure oscillations. This notification met the

4 hour
4.62963e-5 days
0.00111 hours
6.613757e-6 weeks
1.522e-6 months

reporting requirements.

This LER is being submitted pursuant to the requirements of 10 CFR 50.73(a)(2)(iv)(A).

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

THE EVENT:

None 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 002 - 00

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

March 19, 2017 19:25 — Operations transfers Reactor Pressure Control from the Turbine Mechanical Hydraulic Control System's Electronic Pressure Regulator (EPR) to the Mechanical Pressure Regulator (MPR) in support of reactor shutdown for 2017 refueling outage.

20:01 — Operations commences reactor shutdown March 20, 2017 00:01 — With the reactor at approximately 24% power the Main Turbine was taken offline.

00:38 — In support of surveillance testing the first of two turbine overspeed tests was performed. Reactor power was 18.8% at the time of the overspeed trip. Throughout the overspeed trip test the MPR responded as expected and reactor pressure oscillations were well within procedural limits.

02:06 — The second turbine overspeed test, "Back-up Overspeed Trip Test", was performed. Reactor power was at approximately 13.5% at the time of the second overspeed trip. The MPR response resulted in 2.0 to 2.5 psig reactor pressure oscillations.

02:10 — Operators entered the special operating procedure for Pressure Regulator Malfunction, due to reactor pressure oscillations of 2-3 psig.

02:27.— Operators inserted a manual scram of the reactor due to pressure oscillations exceeding procedural limits.

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

None

F. METHOD OF DISCOVERY:

Control Room Operations were closely monitoring the MPR and its associated critical parameters during and after turbine overspeed testing. The pressure oscillations and turbine bypass valve movement was promptly identified and Control Room Operations quickly verified that the indications were not the result of any ongoing field work.

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 002 - 00

G. MAJOR OPERATOR ACTION:

Based on the monitoring of plant conditions and in accordance with station procedures Control Room Operators performed a manual scram of the reactor.

H. SAFETY SYSTEM RESPONSES:

All safety systems responded as expected.

II. CAUSE OF THE EVENT:

The apparent cause of this event was a partial blockage within the MPR sensing line coupled with hysteresis found within the cylinder stroke of the Mechanical Hydraulic Control (MHC) Bypass Relay.

Crud collecting in the MPR pressure sensing bellows line resulted in overdamped feedback signals to the MPR. Overspeed testing performed at 13.5% power caused large pressure swings in the sensing lines, resulting in transport of crud through the pressure sensing bellows line. The over damped feedback caused the MPR to exhibit limit cycle behavior (pressure oscillations).

Additionally, there was hysteresis within the MHC Bypass Relay caused as the bypass relay linkage passed through a worn bushing resulting in a friction induced sticktion.

The partial blockage within the MPR sensing line and the Bypass Relay hysteresis, when combined with normal pressure oscillations at low power levels, lead to reactor pressure swings outside the procedurally acceptable limits.

III. ANALYSIS OF THE EVENT:

This event is reportable under 10 CFR 50.72 (b)(2)(iv)(B) and 10 CFR 50.73(a)(2)(iv)(A) as any event or condition that resulted in a manual or automatic actuation of any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B). Due to pressure oscillations exceeding the procedurally required limit, the RPS system was manually actuated, resulting in a reactor scram. There were no nuclear safety consequences or concerns associated with this event. All safety systems and balance of plant equipment responded as expected.

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 002 - 00

IV. CORRECTIVE ACTIONS:

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

STATUS:

1. The pressure sensing bellows line was replaced and the associated piping was flushed 2. The bypass relay was repaired.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

1. Implementation of a two year preventive maintenance activity for routine flushing and filling of the pressure sensing bellows line and associated piping with contingencies to replace when required.

2. Revision of associated procedures to include steps to flush and backfill sensing lines.

3. Revised the Turbine Trip Tests procedure to ensure turbine overspeed testing is completed above 21 percent power to minimize the possibility of pressure oscillations as a result of the overspeed test.

V. ADDITIONAL INFORMATION:

A. FAILED COMPONENTS:

Mechanical Hydraulic Control (MHC) Bypass Relay Mechanical Pressure Regulator (MPR) Pressure sensing bellows line

B. PREVIOUS LERS ON SIMILAR EVENTS:

99-04 — "Reactor Scram Due to Mechanical Pressure Regulator Suppressor Valve Failure and Mode Switch Position not in Conformance with Technical Specifications". The cause of the reactor scram was the failure of the MPR pressure suppressor valve (a J. A. Campbell Company Micro-Bean Valve). The vendor treated the valve with excessive corrosion inhibitor which led to an internal blockage in the MPR feedback loop.

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 - 002 - 00 A. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) COMPONENT FUNCTION

IDENTIFIER AND SYSTEM NAME OF EACH COMPONENT OR SYSTEM REFERRED TO

IN THIS LER:

Component IEEE 803 IEEE 805 Mechanical Pressure regulator (MPR) RG JJ Electric Pressure Regulator RG JJ Mechanical Hydraulic Control Bypass Relay RLY JJ Main Turbine TRB JJ Reactor Vessel RPV AD Suppressor Valve 4 V JJ .

B. SPECIAL COMMENTS:

None