05000269/LER-2012-001
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 47810 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 2692012001R00 - NRC Website | |
EVALUATION:
BACKGROUND
This event is reportable per 10 CFR 50.73(a)(2)(ii)(B) as a condition that resulted in the nuclear power plant being in an unanalyzed condition and per 10 CFR 50.73(a)(2)(i)(B) as operation in a condition prohibited by Technical Specifications.
The Standby Shutdown Facility (SSF) [NB] at Oconee Nuclear Station (Oconee) is designed as a standby system for use under certain emergency conditions. The SSF includes a dedicated power system and provides additional defense-in-depth protection for the health and safety of the public by serving as a backup to existing safety systems. The SSF provides an alternate and independent means for all three of the Oconee Units to achieve and maintain a safe shutdown condition following a fire, sabotage, turbine building flood, station blackout (SBO), or Emergency Feedwater (EFW) [BA] system tornado missile damage. Failures in the SSF systems will not cause failures or inadvertent operations in other plant systems. The SSF requires manual activation and can be activated if emergency systems are not available.
The main components of the SSF are the SSF Auxiliary Service Water (ASW) System, SSF Portable Pumping System, SSF Reactor Coolant (RC) Makeup System, SSF Power System [EK], and SSF Instrumentation.
Per the Technical Specifications, the SSF is required to be operable in Modes 1, 2, and 3. On March 29, 2012, when this concern was first entered into the corrective action program, Units 1 and 2 were operating at full power, and Unit 3 was coasting down for a refueling outage.
EVENT DESCRIPTION
Duke Energy is performing a comprehensive design, licensing, and operational review of the Oconee Nuclear Station (ONS) SSF to ensure that systems, structures, and components associated with the SSF are capable of performing their design function. During conduct of this review, the SSF Comprehensive Review Team identified that unanalyzed conditions may exist for SSF-mitigated events. Technical Specification (TS) 3.10.1, Standby Shutdown Facility, requires that the SSF and associated systems be operable in Modes 1, 2, and 3. Existing thermal and hydraulic analyses which support the SSF only consider an event from 100% power for 4 EFPD (effective full power days) burn-up. Therefore, some initial operating conditions are not considered.
Lacking analyses, the ability of the SSF to mitigate events for which it is credited is in question.
Since discovery of this condition, Duke Energy has declared the SSF inoperable for the applicable unit when operating in an unanalyzed condition (i.e., Mode 3 up through 100%FP for 4 EFPD).
Note that the SSF TS does contain an exception to TS 3.0.4 that allows the affected unit to change modes when the Limiting Condition of Operation is not met. In the past three years, there were periods of time where this unanalyzed condition existed on each of the three units for greater than seven days, which is the allowed Required Action Completion Time. Therefore, this condition is also reportable as operation in a condition prohibited by Technical Specifications.
CAUSAL FACTORS
Duke Energy has determined that the apparent cause of this event is inadequate transfer of licensing information during the initial development of SSF Technical Specifications.
The original Safety Evaluation (SE) for the SSF, issued on April 28, 1983, described the SSF as a "'bunkered' facility which houses the systems and components necessary to provide an alternate and independent means to achieve and maintain a hot shutdown condition for one or more of the three Oconee units." At the time the SE was issued, hot shutdown was defined as the reactor subcritical by at least 1% delta k/k and T average at or greater than 525 degrees F. However, an August 14, 1987, License Amendment Request supplement introduced a discrepancy into the facility's licensing basis, changing the licensing basis temperature from 525 degrees F to 250 degrees F. On May 11, 1992, the NRC issued the amendment to add the SSF to the Oconee Technical Specifications with the 250 degrees F operability requirement. Analysis was not performed to support SSF operability from 250 degrees F.
Duke Energy guidance for correspondence with the NRC today is more formalized and the documentation requirements more stringent than was the case in the 1980s. Current guidance requires that written communication be appropriately validated for accuracy and completeness prior to submittal. Accordingly, with this guidance in place, submittal of a proposed technical specification without a corresponding analytical basis would not be expected to occur today.
CORRECTIVE ACTIONS
Immediate:
1. When this condition was determined reportable, Unit 2 was down powering to repair a cooling water system [CC] leak, and Unit 3 was in an end of cycle coast down. Duke Energy developed analyses to establish SSF operability during these changing plant conditions.
2. Declared the SSF inoperable for Unit 2 when power decreased below 85% on April 5, 2012.
Unit 2 entered Mode 3 on April 6, 2012, to support repair of a cooling water system [CC] leak.
3. Declared the SSF inoperable for Unit 3 when power decreased below 70% on April 13, 2012.
Unit 3 was shut down for normal refueling activities and exited the SSF Mode of Applicability on April 14, 2012.
Planned:
Initiate actions to resolve the SSF unanalyzed condition by providing appropriate analysis or licensing changes for SSF operability in Mode 3 through 4 EFPD.
SAFETY ANALYSIS
Duke Energy used a risk-informed approach to determine the risk significance associated with the unanalyzed conditions existing for the Standby Shutdown Facility (SSF) mitigated event.
The Conditional Core Damage Probability (CCDP) of this event was evaluated by considering the following:
- The risk increase for the event is during the period of vulnerability which is when a unit is subject to an increased chance of Pressurizer Safety Valve (PSV) liquid relief
- The short average duration of the exposure period
- The use of the average maintenance PRA model to determine the SSF challenge frequency The CCDP associated with this event was determined to be less than 1.0E-06 due to the average exposure period. Therefore, this event is considered to have a small risk impact. This event does not pose a threat to the health and safety of the public.
ADDITIONAL INFORMATION
Duke Energy is performing a comprehensive design, licensing, and operational review of the Oconee Nuclear Station Standby Shutdown Facility (SSF). The goal of the review is to ensure thdt systems, structures, and components associated with the SSF functions are capable of performing their design function. This comprehensive review identified the current concern.
Energy Industry Identification System (EMS) codes are identified in the text as [XX]. This event is considered reportable under the Equipment Performance and Information Exchange (EPIX) program. No component was selected because no specific equipment failures occurred. There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.