05000282/LER-2011-001, For Prairie Island, Unit 1, Regarding Unplanned Actuation of 121 Motor Driven Cooling Water Pump
| ML111170271 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 04/26/2011 |
| From: | Schimmel M Xcel Energy, Northern States Power Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-PI-11-028 LER 11-001-00 | |
| Download: ML111170271 (6) | |
| 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 |
| 2822011001R00 - NRC Website | |
text
@ Xcel Energy*
April 26, 201 1 L-PI-11-028 10 CFR 50.73 U S Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Prairie Island Nuclear Generating Plant Units 1 and 2 Dockets 50-282 and 50-306 License Nos. DPR-42 and DPR-60 LER 50-2821201 1-001 -00, Unplanned Actuation of 121 Motor Driven Coolinn Water Pump Northern States Power Company, a Minnesota corporation (NSPM), doing business as Xcel Energy, herewith encloses Licensee Event Report (LER) 50-2821201 1-001 -00.
Summary of Commitments This letter contains no new commitments and no changes to existing commitments.
Mark Schimmel Site Vice President, Prairie Island Nuclear Generating Plant Northern States Power Company - Minnesota Enclosure cc:
Administrator, Region Ill, USNRC Project Manager, Prairie lsland Nuclear Generating Plant (PINGP), USNRC Resident Inspector, PINGP, USNRC Department of Commerce, State of Minnesota 1717 Wakonade Drive East Welch, Minnesota 55089-9642 Telephone: 651.388.1 121
ENCLOSURE LICENSEE EVENT REPORT 50-282i2011-001-00 4 Pages Follow
LICENSEE EVENT REPORT (LER)
(See reverse for requ~red number of digitslcharacters for each block)
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20 2203(a)(3)(11) 50 73(a)(2)(11)(A)
Cj 50 73(a)(2)(vlll)(A) 20 2203(a)(1) 20 2203(a)(4)
Cj 50 73(a)(2)(11)(8) 50 73(a)(2)(v111)(8) 20 2203(a)(2)(1)
Cj 50 36(c)(l)(l)(A) 50 73(a)(2)(111) 61 50 73(a)(2)(1x)(A) 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).
The 121 MDCLP autostart was due to low header pressure. This low pressure condition was due to the failure of a header gasket on a non-safeguards chiller. The gasket failure was due to an over-torque condition, gasket material, and the conditions under which the chiller was operating at the time of the failure. Corrective actions to resolve the issue include using more rigid gasket material, applying correct torque values in accordance with existing Prairie Island Nuclear Generating Plant procedures, and providing additional training on refrigeration principles.
NRC FORM 366 (10-2010)
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EVENT DESCRIPTION
I At the Prairie Island Nuclear Generating Plant (PINGP), the Cooling Water (CL) system' is a shared system common to both Units I and 2 that provides a heat sink for the removal of process and operating heat from safety related components during a Design Basis Accident or transient. During normal operating and shutdown, the CL System also provides this function for various safety related and non-safety related components.
j 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 vertical motor-driven pump 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 'A1 header pressure. The loss of header pressure was initiated by a sudden increase in flow demand when the 11 Auxiliary Building and Containment ~hille?
tripped off line, causing the Containment Fan Coil Units (CFCUs) and Control Rod Drive Mechanism Cooling Coils (CRDM) to shift to Cooling Water. Additional evaluation of the chiller trip is underway to determine whether the failure is reportable to EPIX. A supplement to this LER will be provided to provide the equipment failure information.
During a January 201 1 review of MSPl 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, 201 1, 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).
1 2 Ells System Code: BI Ells System Code: KM
Prairie Island Nuclear Generating Plant Unit 1
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)(iv)(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 50-2821201 0-001 -01 (Accession Number ML101830384) was submitted to the NRC on July 2,201 0 regarding this postulated HELB scenario.
Since the 121 MDCLP pump is now required to mitigate this HELB scenario, on March 3, 201 1, PI NGP determined that the December 23, 201 0 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
In the event of a loss of offsite power, the loss of the motor driven pumps would result in a rapid loss of discharge header pressure. At a low pressure setpoint, the 121 MDCLP and two DDCLPs would start. The start setpoint of the 121 MDCLP is slightly raised 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.
CAUSE
Equipment supplied by the 11 Containment and Auxiliary Chiller on December 23, 2010 included Containment Fan Coil Units (CFCUs) and Control Rod Drive Mechanism Cooling Coils (CRDMs). In the event of a SI or failure of the chillers, the chilled water supply to the CFCUs and CRDMs automatically isolates. This equipment is then automatically aligned to the CL system.
The trip of the 11 Containment and Auxiliary Chiller was due to a no flow condition in the chiller evaporator. The no flow condition was primarily due to a leak in the evaporator water box head gasket which allowed water to bypass the evaporator. This leak was due to the materials used for the evaporator head gasket and the gasket being over-torqued. The tripping of the chiller is to prevent freezing of the chiller evaporator tubes due to the no flow condition. When the CFCUs and CRDMs
I chilled supply transferred to the CL system, a low cooling water pressure resulted. This low pressure 1
I caused the autostart of the 1221 MCDLP.
2 The apparent cause for the I I Containment and Auxiliary Chiller trip was the gasket material and over-torquing of the evaporator head gasket, In addition to the no flow condition in the chiller evaporator, a low chiller loading condition was also recognized as a contributing cause.
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CORRECTIVE ACTION
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Procedural changes have been initiated to change the evaporator head gasket to a more rigid L I 6
material. Torque values will be assigned in accordance with existing PINGP procedures. Additional 1
training on refrigeration is also requested for Operations personnel.
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PREVIOUS SIMILAR EVENTS
LER 1-09-02 was submitted on May 18, 2009. This LER reported the automatic actuation of 121 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.