05000327/LER-2017-001
| Sequoyah Nuclear Plant Unit 1 | |
| 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)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| Initial Reporting | |
| ENS 52597 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident, 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3272017001R00 - NRC Website | |
| ML17117A495 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 04/26/2017 |
| From: | Williams A L Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 17-001-00 | |
| Download: ML17117A495 (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.
I. Plant Operating Conditions Before the Event
At the time of the event, Sequoyah Nuclear Plant (SQN) Unit 1 and Unit 2 were in Mode 1 at 100 percent rated thermal power.
II. Description of Event
A. Event Summary:
As a result of fire protection [EIIS: KF] piping inspections, the fire protection for the Fuel Handling Exhaust Fan (FHEF) [EIIS: FAN] Filter [EIIS: FLT] Enclosure was authorized to be isolated under a clearance. The isolation required a continuous fire watch to inspect the inside of the FHEF Filter Enclosure. With the FHEF in service, excessive differential pressure across Auxiliary Building [EIIS: NF] Door [EIIS: DR] A212 hindered opening the door for the inspection. Therefore, a fire protection impairment permit was generated to breach Door A212; however, the door was not evaluated as a breach of the auxiliary building secondary containment enclosure (ABSCE) boundary. On March 3, 2017, at 2232 eastern standard time (EST), Door A212 was breached to facilitate the continuous fire watch.
An 8-hour non-emergency event notification (EN 52597) was made to the NRC in accordance with 10 CFR 50.72(b)(3)(v) as an event or condition that could have prevented fulfillment of a safety function of structures or systems that are needed to: (C) control the release of radioactive material and (D) mitigate the consequences of an accident. This LER documents the reportable event under 10 CFR 50.73(a)(2)(v)(C) and 10 CFR 50.73(a)(2)(v)(D).
Additionally, it was determined that both trains of ABGTS were inoperable from March 3, 2017, at 2232 to March 7, 2017, at 0949. LCO 3.7.12, Condition B requires restoration of the ABSCE boundary within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Upon failure to meet the Required Action and associated Completion Time of Condition B, LCO 3.7.12, Condition C requires the unit to be in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Because both trains of ABGTS were inoperable for approximately 83.3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and remained in Mode 1, this is a condition prohibited by TS and is therefore being reported in accordance with 10 CFR 50.73(a)(2)(i)(B), as any operation or condition which was prohibited by the plant's TS.
On March 7, 2017, at 0830 EST, a senior reactor operator (SRO) discovered Door A212 blocked open during a walk down of the Auxiliary Building. The open door created a breach of the ABSCE boundary. The identified breach exceeded the allowed ABSCE breach margin. As a result, both units entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.7.12, Condition B for two trains of the Auxiliary Building Gas Treatment System (ABGTS) [EIIS: VF] inoperable due to an inoperable ABSCE boundary in Mode 1, 2, 3, or 4. At 0949 EST, on March 7, the door was closed and both units exited LCO 3.7.12, Condition B.
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B. Status of structures, components, or systems that were inoperable at the start of the event and contributed to the event:
No inoperable structures, components, or systems contributed to this event.
C. Dates and approximate times of occurrences:
D. Manufacturer and model number of each component that failed during the event:
There was no component that failed during the event.
E. Other systems or secondary functions affected:
There were no systems or secondary functions affected by this event.
F. Method of discovery of each component or system failure or procedural error:
While performing a walk down of the Auxiliary Building, an SRO discovered Door A212 blocked open.
G. The failure mode, mechanism, and effect of each failed component, if known:
There was no component that failed during the event.
H. Operator actions:
Both trains of ABGTS were declared inoperable due to the identified breach exceeding the allowed ABSCE breach margin. Both units entered LCO 3.7.12, Condition B.
I. Automatically and manually initiated safety system responses:
There were no automatic or manual system responses associated with this event.
Date/Time (EST) Description 03/03/17, 2232 Door A212 was breached to facilitate a continuous fire watch.
03/07/17, 0830 An SRO discovered Door A212 breached. The identified breach exceeded the allowed ABSCE breach margin. Both units entered LCO 3.7.12, Condition B.
03/07/17, 0949 Door A212 was closed. This restored both the ABSCE boundary and both trains of the ABGTS to operable status. Both units exited LCO 3.7.12, Condition B.
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V. Assessment of Safety Consequences
III. Cause of the Event
A. Cause of each component or system failure or personnel error:
There was no component or system failure associated with the event.
B. Cause(s) and circumstances for each human performance related root cause:
There was no identified human performance related root cause.
IV. Analysis of the Event:
The ABGTS is a standby system that consists of two independent and redundant trains. Each train consists of a heater, a prefilter, a high efficiency particulate air filter, an activated charcoal adsorber section for removal of gaseous activity, and a fan. The system initiates filtered ventilation of the auxiliary building following receipt of a high radiation signal from the fuel handling area radiation monitors, a high radiation signal from the train-specific Auxiliary Building exhaust vent monitor, a Phase A containment isolation signal from either reactor, or a high temperature signal from the Auxiliary Building air intakes.
The ABGTS filters airborne radioactive particulates from the area of the fuel pool following a fuel handling accident or loss of coolant accident (LOCA). In Mode 1, 2, 3, or 4, the ABGTS is required to be operable to provide fission product removal associated with Emergency Core Cooling System leaks due to a LOCA and leakage from containment and annulus.
The breached door exceeded the allowed ABSCE breach margin rendering the ABSCE inoperable. This configuration could have prevented the ABGTS from maintaining a pressure greater than or equal to -0.25 inches water gauge with respect to atmospheric pressure during the post accident mode of operation.
There were no actual safety consequences as a result of this event. The SQN probabilistic risk assessment model does not specifically credit the use of the ABGTS when calculating the probability of core damage or large early release. Accordingly, the risk associated with this event is considered to be very small.
An evaluation determined the cause to be a less than adequate single barrier breaching standard exists at SQN. A contributing cause was an inconsistent approach to entry into the barrier breaching process. Individuals enter into the barrier breaching process using the applicable standard that they are most familiar with rather than using the barrier breaching governing procedure.
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VI. Corrective Actions
A. Immediate Corrective Actions:
Door A212 was closed and both units exited LCO 3.7.12, Condition B.
B. Corrective Actions to Prevent Recurrence or to reduce probability of similar events occurring in the future:
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event:
C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from discovery of the failure until the train was returned to service:
B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident:
Corrective actions include revising the breaching procedure to address all possible breaches and include a matrix for doors and their associated impacts. Additionally, a performance analysis worksheet will be performed to identify knowledge deficiencies associated with the ABSCE and door postings.
There were no components or systems that failed during the event.
The event did not occur when the reactor was shutdown.
Both trains of ABGTS were inoperable for approximately 83.3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />. The elapsed time from discovery of both trains of ABGTS being inoperable until both trains were restored to operable status was approximately 79 minutes.
Corrective Actions are being managed by the Tennessee Valley Authority (TVA) corrective action program under Condition Report 1269767.
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VII. Previous similar events at the same plant:
VIII. Additional Information
None.
IX. Commitments:
None.
A review of SQN LERs identified an event in which a penetration affecting the ABSCE was breached without required compensatory measures. LER 327/2013-01, associated with the event, identified the root cause as ineffective procedures for controlling containment penetration breaches during Modes 5 and 6. A corrective action was to develop and implement a governing procedure for controlling breaches of the shield building, ABSCE, control room boundaries, and design basis flood barriers.
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