05000282/LER-2012-002

LER-2012-002, Unplanned Actuation of 121 Motor Driven Cooling Water Pump

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
2822012002R00 - NRC Website

On April 2, 2012, while Prairie Island Nuclear Generating Plant (PINGP) Unit 1 was operating at 100% power, Operations performed a post-maintenance test (PMT) of the 22 Diesel Driven Cooling Water Pump (DDCLP) as part of a Work Order (WO) at approximately 02:24 CDT. However, because no work had been performed on the pump and the monthly surveillance procedure (SP) was not scheduled, 22 DDCLP was operated per Operations procedure C35, Cooling Water. The 121 Motor Driven Cooling Water Pump (MDCLP) auto started while shutting down 22 DDCLP even though 11 Cooling Water Pump (CLP) was running. The manual or automatic actuation of the CL system (equivalent to an emergency service water system) is reportable under 10 CFR 50.73(a)(2)(iv)(A).

EVENT ANALYSIS

At PINGP, the Cooling Water (CL) System2 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.

Five CL pumps are connected to a common pump discharge header that directs CL flow into two separate headers. There are three motor driven pumps (121 MDCLP, 11 CLP and 21 CLP) and two diesel driven pumps (12 DDCLP and 22 DDCLP). The 121 MDCLP can 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 April 2, 2012, 121 MDCLP was not aligned as a safeguards replacement pump. An autostart of 121 MDCLP occurred even though 11 CLP was running. Due to previous auto starts of 121 MDCLP, step 7.9.1.1 was added to SP 1106B (22 DIESEL COOLING WATER PUMP MONTHLY TEST), that states, "IF only one (1) motor driven cooling water pump is running, THEN start an additional motor driven cooling water pump per C35.

Procedure C35 was previously revised with a note that states an additional cooling water pump may be required. The additional step in SP1106B to require starting an additional pump prior to shutting down the 22 DDCLP provides more definitive guidance than C35 in this situation.

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 1 EIIS Component Identifier: P 2 EIIS System Code: BI automatically start before the DDCLPs. For this 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, there were no radiological, environmental, or industrial impacts associated with this event and PINGP did not affect the health and safety of the public. This event did not challenge nuclear safety as all plant systems responded as designed. Therefore, this event does not represent a safety system functional failure for Unit 1.

CAUSE

The Equipment Cause Evaluation (ECE) determined that low pressure in the header was reached due to having only one pump in service at the time of removing 22 DDCLP. If only one motor driven pump is supplying both headers while removing 12 or 22 DDCLP, the momentary drop in header pressure can auto start 121 MDCLP.

CORRECTIVE ACTION

The C35 Operating Procedure guidance and instruction will be revised to align with the changes made to the Surveillance Procedures.

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. The momentary drop in pressure was large enough to automatically start the 121 MDCLP while it was aligned for safeguards service.

the 121 Motor Driven Cooling Water Pump (MDCLP) when not aligned as a safeguards replacement pump is included in the list of systems in 10 CFR 50.73(a)(2)(iv)(B). As a result, an actuation of the 121 MDCLP on 12/23/2010 was determined to be reportable under 10 CFR 50.73(a)(2)(iv)(A).

Corrective actions to resolve the issue included performing a Cooling Water System review to determine methods and any single point vulnerabilities that can be performed to minimize the potential for auto starts of a cooling water pump. Operating procedures were evaluated to determine if procedural or operation period changes can be made to reduce the likelihood of auto starting a Cooling Water Pump.