05000282/LER-2011-001
| Prairie Island Nuclear Generating Plant Unit 1 | |
| Event date: | 12-23-2010 |
|---|---|
| Report date: | 10-07-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2822011001R02 - NRC Website | |
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U.S. NUCLEAR REGULATORY COMMISSION i. FACILITY NAME 05000 282 YEAR 2011 - 001
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EVENT DESCRIPTION
At the Prairie Island Nuclear Generating Plant (PINGP), the Cooling Water (CL) System' is a shared system common to both Units 1 and 2 that provides a heat sink for the removal of process and operational heat from safety related components during a Design Basis Accident or transient. During normal operation and shutdown, the CL System also provides this function for various safety related and non-safety related components.
The CL System consists of a common CL pump discharge header for the five CL pumps (two non- safeguards, two safeguards, and 1 interchangeable) that direct flow into two separate CL headers.
Normal operation utilizes two non-safeguards horizontal pumps with the vertical motor-driven cooling water pump (121 MDCLP) as a standby. In addition, two safeguards vertical diesel driven cooling water pumps (DDCLPs) are provided for emergency operation. The 121 MDCLP will start upon a safety injection signal, but will trip off when both diesel driven pumps reach operating speed.
The 121 MDCLP may also function as a safeguards replacement when a diesel driven pump is taken out of service. In this configuration, the pump is aligned manually to the appropriate train of safeguards power and motor operated valves are administratively disabled in accordance with technical specifications.
On December 23, 2010, 121 MDCLP was not aligned as a safeguards replacement pump. Units 1 and 2 were operating at 100% power in MODE 1. An autostart of 121 MDCLP occurred due to a loss of train 'A' header pressure. The loss of header pressure was initiated by a sudden increase in flow demand when the 11 Auxiliary Building and Containment Chiller2 tripped off line, causing the Containment Fan Coil Units (CFCUs) and Control Rod Drive Mechanism. Cooling Coils (CRDM) to shift to Cooling Water. The autostart of the 121 MDCLP due to a loss of header pressure did not constitute a Maintenance Rule Functional Failure and is not reportable to EPIX.
During a January 2011 review of MSPI Unavailability data, the CL system engineer noted that the autostart of the 121 MDCLP occurred in December 2010. Due to recent changes in the understanding of the PINGP licensing basis for the CL system associated with postulated high energy line break (HELB) scenarios, the system engineer questioned whether the autostart might constitute a reportable event and initiated a corrective action request. During the subsequent evaluations, on March 3, 2011, the autostart was determined to be reportable.
The reported condition represents an automatic actuation of emergency service water systems that do not normally run and that serve as ultimate heat sinks. This is reportable under 10 CFR 50.73(a)(2)(iv)(A).
I EIIS System Code: 131 2 EIIS System Code: KM
NRC
-2 0 FORM 366A LICENSEE EVENT REPORT (LER) N0 0
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--, U.S. NUCLEAR RaULATORY COMMISSION 05000 282 YEAR 2011 - 001
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EVENT ANALYSIS
The determination that an autostart of the 121 MDCLP, when not aligned as a safeguards replacement pump, should be reported under 10 CFR 50.73(a)(2)(1v)(A) is a change from previous station position. This is due to recent changes in the understanding of what equipment is required to mitigate a HELB at PINGP. The 121 MDCLP Is an extra pump that is available to supplement the two safeguards DDCLPs. Previously, unless the pump was aligned as a safeguards replacement pump, it was not required to mitigate accident scenarios.
In 2010, additional analysis determined that the 121 MDCLP would be needed to mitigate the consequences of a HELB even when it is not aligned as a safeguards replacement pump. LER 50- 282/2010-001-01 (Accession Number MU 01830384) was submitted to the NRC on July 2, 2010 regarding this postulated HELB scenario.
Since the 121 MDCLP pump is now required to mitigate this HELB scenario, on March 3, 2011, PINGP determined that the December 23, 2010 autostart of the pump meets the criterion for NRC reporting under 10 CFR 50.73(a)(2)(iv)(A).
The reported condition demonstrated that the 121 MDCLP is capable of performing its Specified Function. Thus, the condition described in this LER does not represent a Safety System Functional Failure.
SAFETY SIGNIFICANCE
The 121 MDCLP and the two DDCLPs automatically start on low cooling water header pressure. The water header pressure setpoint that starts the 121 MDCLP is slightly higher to ensure that it will automatically start before the DDCLPs. For this reported event, the 121 MDCLP started on the low pressure signal as expected. The 121 MDCLP and other equipment operated as intended during the event, therefore, this condition did not affect the health and safety of the public.
CAUSE
The causal evaluation determined that, on December 23, 2010, the 121 MDCLP autostart was due to operation of the in service 21 Motor Driven Cooling Water Pump at near run out pump curve condition. When 11 Containment and Auxiliary Building Chiller tripped on low evaporator temperature (from being operated under low load conditions) the Cooling Water System cooling water demand increased when the containment fan coil cooling water valves switched to the Cooling Water System.
The increased cooling water flow demand dropped cooling water header pressure down to the autostart setpoint of 121 Cooling Water Pump. 121 Cooling Water Pump started as designed.
NRC -2GFORM 366A LICENSEE EVENT REPORT (LER) (1010)
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U.S. NUCLEAR REGULATORY COMM! ION 05000 282 YEAR 2011 - 001
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- 02 3. PA t Initially, it was thought that leakage across the divider plait from a gasket contributivd to the 121 MDCLP autostart. Further analysis determined that a failure of the divider-plate gasket could not be the cause of the chiller trip and that the chiller tripped on low evaporator temperature.
CORRECTIVE ACTION
The 11 Containment and Aux Building Chiller was shut down for the remaining 2010 and 2011 Winter period to correct the potential for another CL pump autostart from a chiller low flow condition.
The Cooling Water System will be reviewed to determine any single point vulnerabilities and methods that can be performed to minimize the potential for autostarts of a cooling water pump.
Operating procedures will also be evaluated to determine if procedural or operation period changes can be made to reduce the likelihood of autostarting a Cooling Water Pump.
The need for providing Operations personnel with additional training wilt be assessed and conducted if necessary.
PREVIOUS SIMILAR EVENTS
MDCLP when it was aligned as a safeguards pump. The actuation occurred following planned maintenance of the 12 DDCLP. During post maintenance testing, 12 DDCLP was tripped per procedure. This resulted in a transient of the cooling water system pressure. Thd momentary drop in pressure was large enough to automatically start the 121 MDCLP while it was aligned for safeguards service.