05000323/LER-2017-001

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LER-2017-001, Relief Valve Leakage Resulting in Inoperable Pressurizer Power Operated Relief Valve
Diablo Canyon Power Plant, Unit 2
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications

10 CFR 50.73(a)(2)(v), Loss of Safety Function
3232017001R00 - NRC Website
LER 17-001-00 for Diablo Canyon, Unit 2, Regarding Relief Valve Leakage Resulting in Inoperable Pressurizer Power Operated Relief Valve
ML17276B863
Person / Time
Site: Diablo Canyon Pacific Gas & Electric icon.png
Issue date: 10/03/2017
From: Welsch J M
Pacific Gas & Electric Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
DCL-17-085
Download: ML17276B863 (6)


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. 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.

2017 001 Diablo Canyon Power Plant, Unit 2 05000323 00

I. Reportable Event Classification

This event is reportable pursuant to the following criteria:

  • 10 CFR 50.73(a)(2)(v)(A & D), Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to shut down the reactor and maintain it in a safe shutdown condition, and mitigate the consequences of an accident.

II. Plant Conditions

At the time of the event, Diablo Canyon Power Plant (DCPP) Unit 2 was in Mode 1 (Power Operation), operating at 100 percent power.

III. Probkyrn Description

A. Background

The DCPP Unit 2 pressurizer is equipped with three Power Operated Relief Valves (PORVs), two safety-related (PCV-455C and PCV-456) and one non safety-related (PCV-474). The pressurizer maintains Reactor Coolant System (RCS) pressure and volume through the surge line during operation, and limits pressure changes during transients. During plant load reduction or increase, reactor coolant volume changes arc accommodated in the pressurizer via the surge line, pressurizer sprays and/or heaters, and the PORVs. The PORVs are normally operated by instrument air. When instrument air is isolated, the backup nitrogen supply provides the motive force to operate the PORVs. The safety-related PORVs and associated backup nitrogen accumulators are credited to mitigate the Feedwater Line Break (FLB) event, the Spurious Safety Injection (SSI) event, the Steam Generator Tube Rupture (SGTR) event and the Low Temperature Overpressure Protection (LTOP) event.

An FLB event generates a Safety Injection (SI) signal and Phase A containment isolation signal, which isolates instrument air and nitrogen supply to the containment. The backup Nitrogen accumulator is therefore credited to provide an adequate capacity to cycle the PORV at least 300 times following a FLB event. This provides adequate time for the operators to take action to terminate the Emergency Core Cooling System (ECCS) injection now and re-establish a bubble in the pressurizer.

The safety-related PORVs and operator actions are credited to mitigate the SSI event for pressurizer overfilling.

If the pressurizer overfills, the pressurizer PORVs are available to relieve water inventory from the RCS, as 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.

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2017 SEOUENI1AL 001 00 long as nitrogen is available from the DCPP Design Class I backup nitrogen accumulators (instrument air is not available).

The safety-related PORVs are credited to open and depressurize the RCS as part of the SGTR event.

When an LTOP transient occurs, a pressurizer PORV opens until the pressure returns to acceptable limits.

B. Event Description

On July 28, 2017, at 1206 PDT, with DCPP Unit 2 operating at 100 percent power, an Alert notification was declared due to low oxygen levels inside the containment. The cause of the low oxygen level was a nitrogen leak inside the containment. The nitrogen source was isolated and the containment atmosphere was restored to ormal conditions. The Alert notification was terminated on July 28, 2017, at 1819 PDT. During an investigation of the nitrogen leak inside the containment, RV-355 was found to be leaking. The leak caused the le ressure in the back up nitrogen accumulator supply to PORV PCV-455C to decrease to a level that made the ORV inoperable.

ased on a review of trend data for nitrogen usage in the containment, it is conservatively assumed that RV-355 I ad been degraded since December 1, 2016, rendering the PORV inoperable for longer than permitted by Technical Specifications. Technical Specification 3.4.11.B requires an inoperable PORV to be restored to operable within

72 hours
8.333333e-4 days
0.02 hours
1.190476e-4 weeks
2.7396e-5 months

. Therefore, the event is reportable as an event or condition prohibited by Technical Specifications. In addition, because the other safety-related PORV PCV-456 had been declared inoperable for esting multiple times since December 1, 2016, this event is reportable as an event or condition that could have .revented the fulfillment of a safety function.

C. Status of Inoperable Structure, Systems or Components That Contributed to the Event one.

1). Other Systems or Secondary Functions Affected None.

E. Method of Discovery

During an investigation of the nitrogen leak inside the containment, RV-355 was found to be leaking. The leak caused the pressure in the back up nitrogen accumulator supply to PORV PCV-455C to decrease to a level that made the PORV inoperable.

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 Infocallects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, 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.

Diablo Canyon Power Plant, Unit 2 YEAR 05000323 2017 001 00

F. Operator Actions

Operators declared PCV-455C inoperable and entered Technical Specification 3.4.11.E on July 29, 2017, at 0457 PDT. RV-355 was replaced and Technical Specification 3.4.11.6 was exited on July 30, 2017, at 1533 PDT.

U. Safety System Responses

None.

IV. Cause of the Problem

The presumptive cause was inadequate instructions provided in plant procedures for placing a new nitrogen supply bottle in service. These instructions did not provide a sequence that assures system pressure transients are mitigated. This may have caused excessive pressure excursions resulting in multiple lifts of RV-355 which resulted in damage to the RV 0-ring seat and a nitrogen leak path.

V. Assessment of Safety Consequences

DCPP assessed the Unit 2 risk significance of the inoperability of PCV-455C using Probabilistic Risk Assessment and the Significance Determination Process. The assessment concluded that the PORV would be available for the most risk significant functions. An incremental conditional core damage probability associated with this event was estimated to be less than 1.0E-06.

VI. Corrective Actions

A. Immediate Actions

1. Relief Valve RV-355 was replaced on July 29. 2017.

B. Other Corrective Actions

1. Plant procedures will be revised to provide instructions for placing nitrogen supply bottles in service to minimize pressure transients on the nitrogen system.

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.

3. LER NUMBER

L 00 2017 001

VII. Additional Information

A. Failed Components

Component: Relief Valve Model: 951282ME Manufacturer: Anderson Greenwood Crosby

B. Previous Similar Events

Internal Operating Experience A similar event occurred on January 5, 2000, while DCPP Unit 2 was at 100 percent power. A failure of RV-355 caused nitrogen pressure to decrease rapidly, resulting in PORV PCV-455C being inoperable.

RV-355 was replaced and PCV-455C was returned to service.