05000249/LER-2010-003

LER-2010-003, Steam Leak Results in HPCI Inoperability

Docket Number
Event date:	11-26-2010
Report date:	01-25-2011
Reporting criterion:	10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
2492010003R00 - NRC Website

PLANT AND SYSTEM IDENTIFICATION

Dresden Nuclear Power Station (DNPS) Unit 3 is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].

A. Plant Conditions Prior to Event:

Unit: 03 Event Date: 11-26-2010 Event Time: 1734 hours0.0201 days <br />0.482 hours <br />0.00287 weeks <br />6.59787e-4 months <br /> CST Reactor Mode: 1 Mode Name: Power Operation Power Level: 020 percent

B. Description of Event:

On November 26, 2010, Dresden Unit 3 restarted from a refueling outage. As a part of startup activities, the High Pressure Coolant Injection (HPCI) [BJ] system is required to be tested in accordance with Technical Specification (TS) 3.5.1. TS Surveillance Requirement (SR) 3.5.1.7 requires that the HPCI system develop a flow rate greater than or equal to 5000 gpm against a system head corresponding to reactor pressure. This test is performed with the reactor vessel pressure less than or equal to180 psig. Additionally, if maintenance has been performed which can affect system performance, SR 3.5.1.6 requires that the HPCI system develop a flow rate greater than or equal to 5000 gpm against a system head corresponding to reactor pressure. This test is performed with the reactor vessel pressure between 920 and 1005 psig. In each test, the HPCI system is started and allowed to recirculate water to the Condensate Storage Tanks [KA].

On the morning of November 26, 2010, the low pressure test had been successfully completed. No leaks had been observed during this test. Following the successful completion of the low pressure test, operations personnel continued with plant startup. Once reactor vessel pressure had reached 920 psig, personnel prepared for the HPCI high pressure test.

At approximately 1700 hours0.0197 days <br />0.472 hours <br />0.00281 weeks <br />6.4685e-4 months <br />, plant personnel commenced the high pressure HPCI test to complete the required in-service testing following maintenance. Steam was supplied to the turbine and the pump ramped up to speed. At approximately 1734 hours0.0201 days <br />0.482 hours <br />0.00287 weeks <br />6.59787e-4 months <br />, the Main Control Room received fire detection [IC] alarms for the Unit 3 HPCI pump room. In-field operations personnel reported to the Main Control Room that steam was leaking in the HPCI pump room. The HPCI turbine was manually tripped and the test was suspended. It was discovered that a large steam leak had developed between the control valve rack and the turbine steam chest.

Following the failure of HPCI system, the required actions of TS 3.5.1 Condition G were taken. As a part of the required actions, Operation personnel verified that the Isolation Condenser (IC) [BL] system was operable.

The HPCI system was taken out of service to allow maintenance to be performed. A maintenance work package was developed to re-torque the HPCI control valve rack. During the re-torque activity, Maintenance personnel identified four (4) nuts which were tightened an additional two flats. The remaining thirty-six nuts were found tight. Following completion of there-torque activity, the HPCI system was returned to service. On November 27, 2010, the HPCI high pressure test was successfully completed and the system declared OPERABLE.

C. � Cause of Event:

During the Unit 3 refueling outage, maintenance was performed on the HPCI turbine control valves as a part of twelve year preventive maintenance to inspect the HPCI turbine. The control valve rack was disassembled and removed from the turbine steam chest to allow internal inspections. Based on the inspections, no repairs were required to be performed.

The control valve rack was re-installed onto the steam chest and all nuts were initially torqued to 150 ft-lbs in order to seat the sealing surfaces. Once the seating pass was complete, all nuts were torqued to 1510 ft-lbs. The nuts were torqued using a hydraulic torque wrench which had been calibrated prior to use.

The low pressure HPCI test was performed following reassembly of the HPCI turbine and control valves and there was no evidence of a steam leak during or following the test. However, later in the day, the HPCI turbine developed the steam leak during the high pressure post maintenance test.

Personnel that had reassembled the control valve rack and steam chest stated that 1510 ft-lbs were applied until there was no movement on any nut. This torque range was in accordance with package work instructions.

Personnel that performed the re-torque torqued all nuts to 1585 ft-lbs, which is in accordance with work instructions, and found little or no movement on thirty-six of the forty nuts. The remaining four nuts were tightened approximately two flats.

The cause of the steam leak is inadequate torque on four nuts connecting the control valve rack to the turbine steam chest.

Based on the subsequent investigation, there are three potential factors that could have resulted in less than adequate torque. Possible causes are:

➢ During heat up and the subsequent cool down that occurred between the high and low pressure HPCI test, the bolting relaxed causing improper torque.

➢ Improper use or set up of the hydraulic torque wrench by the Maintenance personnel.

➢ During the final pass following the initial seating pass, no additional torque was applied to the four nuts.

D. � Safety Analysis:

The safety significance of this condition is low. The Automatic Depressurization and all Low Pressure Emergency Core Cooling Systems were available to provide makeup to the reactor vessel inventory in the event of an accident or transient. The IC system was available for reactor vessel pressure control if use had been required. Therefore, health and safety of the public was not compromised as a result of this condition.

E. Corrective Actions:

The HPCI control valve rack was successfully re-torqued to the turbine steam chest and the high pressure HPCI test completed satisfactorily.

System Engineering is evaluating whether to re-torque the control valve rack to the steam chest connection between the low pressure and high pressure HPCI tests.

System Engineering is evaluating including the HPCI turbine and its components in the Operational Critical Component Program for future maintenance.

A Training Request is being generated to perform refresher training to all qualified personnel on hydraulic torque wrenches and bolted connections.

F. Previous Occurrences:

A review of DNPS Licensee Event Reports (LERs) for the last three years did not identify any LERs associated with HPCI inoperabilities or failures as a result of steam leaks.

G. Component Failure Data:

Not Applicable