05000265/LER-2001-002
| Event date: | |
|---|---|
| Report date: | |
| 2652001002R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 2511 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
Potential Common Cause Inoperability of Emergency Diesel Generator Fuel Oil Transfer Systems
A. CONDITION PRIOR TO EVENT
Unit: 2 � Event Date: May 1, 2001 � Event Time: 2036 hours0.0236 days <br />0.566 hours <br />0.00337 weeks <br />7.74698e-4 months <br /> Reactor Mode: 1 � Mode Name: Power Operation � Power Level: 100% Power Operation (1) - Mode switch in the RUN position with average reactor coolant temperature at any temperature.
B. DESCRIPTION OF EVENT
On May 1, 2001, at 2036 hours0.0236 days <br />0.566 hours <br />0.00337 weeks <br />7.74698e-4 months <br />, approximately 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> after initiation of the 24-hour surveillance test of the Unit 2 Emergency Diesel Generator (EDG) [EK], an alarm was received in the control room for low level in the diesel generator fuel oil day tank [DC]. The test was stopped and the situation was investigated. The investigation considered several potential causes for the low level including that a solenoid valve [FSV] in the fuel oil transfer line from the fuel oil transfer pump [P] to the day tank [TK] had failed to open when required. The solenoid valve assembly was removed and the valve was overhauled. The solenoid valve assembly was reinstalled and on May 3, 2001, the test was run again.
At 2012 hours0.0233 days <br />0.559 hours <br />0.00333 weeks <br />7.65566e-4 months <br /> on May 3, 2001, approximately 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> into the second test, the alarm was received in the control room for low level in the diesel generator fuel oil day tank. The test was continued, and the situation was investigated. Again, it was believed that the solenoid valve in the fuel oil transfer line might have failed to open. The operator investigating the alarm opened a drain valve in the fuel oil transfer line and the solenoid valve then opened. The test was completed without further incident. The solenoid valve was subsequently replaced with a new solenoid valve rated for a larger wattage. The test was performed one final time without any problems, after which the system was returned to service and the EDG was declared operable.
C. CAUSE OF EVENT
The root cause of this condition is the inadequate original design of the fuel oil transfer system in that thermal effects in the piping volume between the discharge check valve and solenoid valve were not accounted for, allowing a pressure increase in the isolated volume beyond the operating capability of the fuel oil transfer solenoid.
The design of the fuel oil transfer piping on the Unit 1 and Unit 1/2 EDGs is similar to the Unit 2 design. Although the solenoid valves on the Unit 1 and Unit 1/2 EDGs had a larger actuator (15.4 watts versus 6 watts), and have been shown in tests to be capable of opening against a larger differential pressure, these valves may still have been susceptible to failure to open due to thermal pressurization. Note that no failures of the solenoid valves in the Unit 1 and Unit 1/2 EDG fuel oil transfer systems have been experienced.
D. SAFETY ANALYSIS
The safety significance of this event was minimal. As noted in the Updated Final Safety Analysis Report, fuel oil can be manually transferred to the day tanks from the main diesel oil tanks. The actions to manually provide fuel oil to the EDG day tank are delineated in a normal station operating procedure. Operators are directed to this procedure by the annunciator response procedure for the Diesel Generator Day Tank low level alarm. No troubleshooting or diagnosis is required to perform the procedure.
This issue does not impact the automatic start and loading of the EDG. Approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of fuel oil are available in the day tank. Performance of this procedure was previously time validated and shown to take less than 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to perform. The low day tank level alarm annunciates with 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> of fuel oil remaining in the day tank. A discussion of the method for manually providing fuel oil to the EDG day tanks is included in a lesson plan that is presented biennially in the classroom to operators.
The equipment required to manually provide fuel oil to the EDG day tank is pre-staged in accordance with station procedure. Therefore, in the event the fuel oil transfer system was to fail fuel oil would be manually transferred to the EDG day tank without impacting operation of the EDG.
E. CORRECTIVE ACTIONS
Immediate Corrective Actions:
The Unit 2 solenoid valve was replaced with a solenoid valve with a higher rating.
A formal root cause investigation and laboratory analysis was initiated.
Completed Corrective Actions:
The Unit 1 and 2 EDG fuel oil transfer solenoid valves have been replaced with normally open manual valves.
The Unit 1/2 EDG fuel oil transfer solenoid valve has been removed. There is no requirement to isolate this line for the Unit 1/2 EDG.
Corrective Actions to be Completed:
Safety-related and risk-significant systems will be reviewed for solenoid valves in piping sections that are susceptible to thermal pressurization.
Case studies will be performed and reviewed with design engineers to illustrate the effects of human performance errors in design.
F. PREVIOUS OCCURRENCES
No LERs were identified in the past 5 years that involved the EDG fuel oil transfer system or thermal pressurization.
G. COMPONENT FAILURE DATA
The solenoid valve that failed to open was a 6 watt ASCO model HT8211C89.