05000293/LER-2013-007
| Pilgrim Nuclear Power Station | |
| Event date: | 07-16-2013 |
|---|---|
| Report date: | 9-16-2013 |
| Initial Reporting | |
| ENS 49196 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2932013007R00 - NRC Website | |
BACKGROUND:
Cape Cod Bay is the ultimate heat sink (UHS) for Pilgrim Nuclear Power Station (PNPS). The circulating water system (CWS) and the salt service water (SSW) system take sea water suction from Cape Cod Bay via the plant intake canal and Intake Structure and provide cooling water for various plant heat loads. These systems discharge the heated cooling water back into Cape Cod Bay via system discharge piping and the plant discharge canal.
The safety objective of the SSW system is to provide a heat sink for the reactor building closed cooling water (RBCCW) system under normal transient, and accident conditions. The SSW system has five (5) SSW pumps and is designed with sufficient redundancy so that no single active system component failure can prevent the system from achieving the safety objective. The system is designed to continuously provide a supply of cooling water to the secondary side of the RBCCW heat exchangers adequate to meet the requirements of the RBCCW system under all conditions. The RBCCW system provides the necessary cooling requirements for the residual heat removal (RHR) system and ultimately the reactor and primary containment.
The design and licensing basis maximum temperature of the UHS for PNPS is 75°F. This limit was established via license amendments #173 and #176 which were approved in 1997 and 1998 respectively. Technical specification (TS) 4.5.B.4.2 requires verification of sea water inlet temperature to be 575°F in order to assure UHS operability. If the UHS is not operable, the plant is required to be in cold shutdown within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> per TS 3.5.6.4.
During routine operation Pilgrim operators monitor sea water inlet temperature via temperature elements installed on redundant SSW piping feeds to the RBCCW heat exchangers. Routine surveillance is performed and alarms are provided in the Control Room to alert operators of increased SSW inlet temperature. For TS compliance purposes, operators will obtain instantaneous sea water inlet temperature readings locally in the Intake Structure in order to determine UHS operability when alarms indicate that SSW temperature at the RBCCW heat exchanger inlet has exceeded 74°F.
EVENT DESCRIPTION:
On Tuesday, July 16, 2013 at 1652 [EDT] and again on Wednesday, July 17, 2013 at 1054 [EDT] with the reactor at 100% core thermal power (CTP) the PNPS declared the UHS and SSW system inoperable due to high sea water inlet temperatures greater than 75°F. A maximum sea water inlet temperature reading of 75.5°F was observed and the maximum duration for either event was 5.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. The LCO action for TS 3.5.B.4 was entered then exited based on the rise and fall of sea water inlet temperature. Plant systems and components operated as required and no equipment failures occurred. The plant was not shutdown due to the short duration of the sea water temperature excursion.
CAUSE OF THE EVENT:
The cause of the high sea water inlet temperature readings was sustained increased sea water surface temperature in Cape Cod Bay due to hot summer weather conditions and the contribution from recirculation of water from the plant's outfall due to wind and tidal conditions.
Corrective action was completed to establish an ODMI action plan to reduce station power levels prior to reaching the TS UHS LCO temperature limit.
Additional corrective actions are planned to:
- Revise procedures to address operator actions to lower reactor power before reaching the UHS TS LCO temperature limit.
- Reassess engineering actions necessary to increase the established 75°F UHS limit to a higher temperature in accident and transient analyses.
Corrective actions are captured in the corrective action program under Condition Report, CR-PNP-2013-5246.
ASSESSMENT OF SAFETY CONSEQUENCES:
This condition posed no threat to the public health and safety.
The safety significance of this event is associated with the UHS temperature and systems relied on to provide cooling during accident and transient conditions. The safety function of SSW is to transfer heat from all systems and components cooled by the RBCCW system to Cape Cod Bay by continuously providing adequate cooling water flow to the secondary side of the RBCCW heat exchangers. Design and accident analyses for the station have been performed with an assumption that the seawater temperature is 75°F.
Although long term accident analyses do not address sea water temperatures above 75°F, an engineering evaluation was performed to address the reported events. This evaluation assumed a short duration event (i.e., 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />) where UHS inlet temperatures exceeded 75°F and remained less than 78°F. This evaluation concluded all structures, systems, and components (SSCs) would be capable of performing their safety functions with UHS/seawater temperatures of up to 78°F for short durations provided that average sea water temperature is less than 75°F for the 12 hour1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> time period evaluated.
Additionally, the LCO shutdown action statement identified in TS 3.5.6.4 (i.e., cold shutdown required within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />) addresses the reported events. Although plant shutdown was not required, the potential nuclear safety implications were previously evaluated by the approved technical specification which determined that plant shutdown would be accomplished in an orderly manner without challenging plant systems.
Therefore, this event does not involve any radiological or industrial safety concerns; and there are no radiological or industrial safety implications.
REPORTABILITY:
These events were initially reported to the NRC in accordance with 10 CFR 50.72(b)(3)(v)(D) as documented in Event Numbers #49196 and #49201. This LER is submitted based on NUREG 1022 Rev 3, Section 3.2.7 guidance which identifies that the requirements of 50.73(a)(2)(v)(B) and (D) apply due to the fact that a system (i.e., UHS and SSW) used to remove residual heat and/or mitigate the consequences of an accident was declared TS inoperable and no redundant system or equipment could be declared operable. The reported events did not involve plant equipment failure and the subsequent engineering evaluation indicates that the safety function of the UHS would have been satisfied based on the maximum temperature recorded and the short duration of each event.
inlet temperatures that exceeded the then specified Final Safety Analysis Report (FSAR) limit of 65°F.
Corrective action related to this LER resulted in increasing the UHS operability temperature limit to 75°F via License Amendment #176 in 1998.
Subsequent to revising the UHS operability temperature limit in 1998, no LERs related to high sea water inlet temperatures were issued. However, in 2003, 2006, 2008 and 2010 specific high sea water inlet temperatures were noted to have exceeded 74°F based on installed instrumentation. These events were evaluated under the corrective action program: See CR-PNP-2003-3256; CR-PNP-2006-2895; CR-PNP-2008-2611; and CR-PNP- 2010-2354.
ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES:
The ENS codes for Components and Systems referenced in this report are as follows:
SYSTEMS CODES
Ultimate Heat Sink (UHS) System (Cape Cod Bay) BS
REFERENCES:
Condition Report, CR-PNP-2013-5246, Technical Specification Limit Exceeded for UHS Temperature.