05000482/LER-2017-002

From kanterella
Jump to: navigation, search
LER-2017-002, Tornado Missile Vulnerabilities Result in Condition Prohibited by Technical Specifications
Wolf Creek Generating Station
Event date: 04-05-2017
Report date: 05-31-2017
Reporting criterion: 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications

10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor

10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident

10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
Initial Reporting
ENS 52666 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition, 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor, 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
4822017002R00 - NRC Website

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.

Wolf Creek Generating Station 05000- 482 2017 00

PLANT CONDITIONS PRIOR TO EVENT

Mode --1 Power -- 100% No structures, systems, or components (SSCs) were inoperable at the start of this event which contributed to this condition.

BACKGROUND DOCUMENTS

Nuclear Regulatory Commission (NRC) Enforcement Guidance Memorandum (EGM) 15-002, "Enforcement Discretion for Tornado-Generated Missile Protection Noncompliance," provides guidance to exercise enforcement discretion when an operating power reactor licensee does not comply with a plant's current site-specific licensing basis for tornado-generated missile protection. Specifically, discretion would apply to the applicable Technical Specification (TS) Limiting Condition(s) for Operation (LCO) that would require a reactor shutdown or mode change in the event a licensee could not meet TS LCO required action(s) within the TS completion time.

NRC Interim Staff Guidance DSS-ISG-2016-01, "Clarification of Licensee Actions In Receipt of Enforcement Discretion Per Enforcement Guidance Memorandum EGM 15-002," provides interim staff guidance to facilitate staff understanding of expectations for consistent oversight associated with implementing enforcement discretion for tornado missile protection noncompliance(s) per EGM 15-002.

Appendix A to DSS-ISG-2016-01 provides guidance for acceptable initial and comprehensive compensatory measures for licensee use in implementing the enforcement discretion outlined in EGM 15-002. The licensee should declare (log) the utilization of EGM 15-002, inform the resident inspector, and enter the issue into the corrective action program. For initial compensatory measures, it is expected that the measures listed are already in place at sites that may be affected by severe weather, such as tornados and/or hurricane force winds. The measures should be verified as current and readily deployable within a very short timeframe.

DESCRIPTION OF EVENT

Wolf Creek Generating Station (WCGS) has 2 emergency diesel generators (EDGs) [EIIS Codes: EK, DG] contained in the Seismic Category I Diesel Generator Building [EIIS Code: NB]. Each EDG provides emergency AC power to its associated Class 1 E electrical bus [EIIS Codes: EB, BU] to maintain cold shutdown and to mitigate the consequences of a design basis accident (DBA) whenever offsite power sources are unavailable. The configuration of both trains of EDG and support equipment are essentially the same. Each EDG has a day tank located in the Diesel Generator Building which contains approximately one hour of fuel, and a storage tank located below grade which provides enough fuel to supply that EDG for continuous full-load operation for seven days [EIIS Code: DC]. A transfer pump is provided inside each storage tank. This pump supplies fuel from the storage tank to the day tank through a transfer line. This transfer line is a safety-related (SR) 2" Schedule 80 carbon steel pipe located entirely within the Diesel Generator Building. However, this transfer line has a non-safety related (NSR) truck connection which penetrates one wall of the Diesel Generator Building approximately 8 inches to the outside environment. The function of the truck connection is to provide a pathway to pump out the storage tank, if necessary, using the transfer pump as the motive force.

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.

05000- NUMBER NO.

_002 00 Wolf Creek Generating Station 482 2017 - On April 5, 2017, during the evaluation of tornado missile protection of SR equipment, WCGS personnel identified an unanalyzed condition related to the truck connections to the fuel oil transfer lines. The NSR truck connections penetrate the wall of the Diesel Generator Building to the outside environment, and are not protected from tornado missiles. It was discovered that tornado missiles could strike the NSR truck connections in such a way as to cause damage to the SR transfer lines inside the Diesel Generator Building. There is currently no analysis demonstrating that if a tornado missile were to strike one of the NSR truck connections the SR transfer line, to which that truck connection is attached, would still be able to perform the function of allowing fuel oil to be transferred from the storage tank to the day tank. In that case, the associated EDG would be incapable of performing its safety function of mitigating the effects of a DBA and allowing the plant to achieve safe shutdown conditions in the event of a loss of offsite power.

At 1400 Central Daylight Time (CDT), on April 5, 2017, both EDGs were declared inoperable due to the potential tornado vulnerability. WCGS entered TS LCO 3.8.1, Conditions B (one EDG inoperable) and F (two EDGs inoperable). At 1445 CDT, initial compensatory measures were put in place consistent with EGM 15-002. These included verifying procedures related to severe weather and FLEX support are current and readily deployable, ensuring that training was current for those procedures, and taking measures to establish a heightened level of station awareness and preparedness relative to the tornado vulnerability. Upon completion of the initial compensatory measures, both EDGs were considered operable but non-conforming. TS LCO 3.8.1 Conditions B and F were exited at this time, within their required action completion times. At 1632 CDT, an 8-hour, non-emergency notification to the NRC was completed (event notice #52666) for the unanalyzed condition.

BASIS FOR REPORTABILITY

This condition is reportable as required by:

10 CFR 50.73(a)(2)(i)(B) for a condition that is prohibited by Technical Specifications.

10 CFR 50.73(a)(2)(ii)(B) for an event or condition that results in the plant being in an unanalyzed condition that significantly degrades plant safety.

10 CFR 50.73(a)(2)(v)(A) for a condition that at the time of discovery could have prevented the fulfillment of a safety function of structures or systems needed to shut down the reactor and maintain it in a safe shutdown condition.

10 CFR 50.73(a)(2)(v)(D) for a condition that at the time of discovery could have prevented the fulfillment of a safety function of structures or systems needed to mitigate the consequences of an accident.

CAUSE OF EVENT

This condition is an original plant design legacy issue. Due to the historical nature of this vulnerability, a specific cause has not been identified.

CORRECTIVE ACTIONS

These vulnerabilities have been permanently eliminated by the completion of design change package 15264.

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 Infocollects.Resource@nrc.goY, 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.

U.S. NUCLEAR REGULATORY COMMISSION 05000- Wolf Creek Generating Station 482

SAFETY SIGNIFICANCE

As documented in EGM 15-002, tornado missile scenarios that may lead to core damage are very low probability events because SR SSCs are typically designed to withstand effects of tornados. For a tornado missile-induced scenario to occur, a tornado would have to hit the site and result in the generation of missiles that would hit and fail vulnerable, unprotected SR equipment, and/or unprotected SR subcomponents in a manner that is non-repairable and non- recoverable. In addition, because plants are designed with redundancy and diversity, the tornado missiles would have to affect multiple trains of safety systems and/or means of achieving safe shutdown.

The NRC has completed a generic risk analysis of potential tornado missile protection noncompliances to examine the risk significance of these scenarios. This assessment documents a conservative, bounding-type analysis of the risk significance for plant facilities. The generic analysis assumed that core damage would occur if a tornado hit a plant located in the most active tornado region in the country and that it caused a tornado-generated missile to fail all emergency core cooling equipment at the plant with no ability to recover. Given this conservative assumption, the staffs study established that the core damage frequency (CDF) associated with tornado missile-related noncompliances are well below CDFs requiring immediate regulatory action. In summary, the generic bounding risk analysis performed by the NRC concluded that this issue is of low risk significance.

During a postulated design basis tornado, the conditions documented could have resulted in a loss of safety function for the EDGs. The EDGs are used to mitigate the effects of a loss of offsite power by providing an emergency AC power source. WCGS also has 3 Station Blackout Diesel Generators (SBO DGs) which are available to provide reliable AC power to one of the Class lE busses. Though the SBO DGs are NSR, they (and all equipment necessary to operate them) are protected by missile shields. In the highly unlikely event that tornado conditions caused a loss of offsite power, and made both EDGs non-functional, the SBO DGs would have been available to provide emergency AC power to one Class 1 E bus to complete and maintain safe shutdown of the reactor. As WCGS has not experienced an actual tornado missile event, this condition had no actual safety consequences impacting plant or public safety. The vulnerabilities of the truck connections to tornado missile strikes have been eliminated with the completion of design change package 15264.

Therefore, this condition had a very low safety significance.

OPERATING EXPERIENCE/PREVIOUS EVENTS

None