ENS 42219
ENS Event | |
|---|---|
14:03 Dec 22, 2005 | |
| Title | Hpci Declared Inoperable |
| Event Description | The HPCI flow control current loop was found degraded in such a manner that HPCI would not perform its intended function. HPCI was declared inoperable per TS 3.5.E.2 (14 day LCO). Repair efforts have been initiated.
The licensee notified the NRC Resident Inspector.
On 12/22/05, the HPCI System flow indicator 'spiked' a number of times with the system in its normal standby line-up. Control Room Operators declared the HPCI system inoperable and entered a 14 day LCO per TS 3.5.E.2, to troubleshoot and repair the system as appropriate. The safety function of HPCI system is to provide and maintain an adequate coolant inventory inside the reactor vessel to prevent fuel clad conditions from exceeding 10CFR50.46 criteria as a result of postulated small breaks in the Nuclear System process barrier. To ensure that this safety function is met, the HPCI System must be able to deliver a minimum actual flow rate of 3570 gallons per minute (gpm) to the reactor vessel. The actual flow rate was reduced below the Technical Specification value of 4250 gpm to account for potential flow instrument string inaccuracies. The delivery of 3570 gpm of actual flow demonstrates that sufficient margin to safety function is maintained during accident conditions. In automatic operation, the HPCI flow controller uses the flow signal to maintain a flow rate of 4250 gpm and is designed to automatically adjust developed flow rate in response to changes in flow signals. System operation is tested quarterly by monitoring flow to ensure that the HPCI system can perform its safety functions. The Technical Specification surveillance requirement states that the HPCI system shall deliver at least 4250 gpm at normal reactor operating pressure when recirculating to the Condensate Storage Tank. The event investigation revealed that the cause of the flow spikes was a malfunctioning electrical component in the power supply module within the flow controller. Power to the flow transmitter is supplied by this module. The power supply would malfunction for a few seconds and then would return to normal operation, thereby resulting in step changes in the HPCI flow signal. It is conservative to assume the actual HPCI flow rate would have been reduced by the full amount of the flow rate, for the entire duration of the flow signal. Additional conservatism also exists because the HPCI system flow is unlikely to have been able to respond to the full value of a short duration flow spike. When calculating the average HPCI flow rate, the impact of the flow spikes decreases as a longer period is evaluated because the duration of all of the flow spikes added together is very short and comparably longer periods of stable performance exist between the individual spikes. Therefore, it is conservative to evaluate the shortest period of time that HPCI is required to perform its safety functions that is stated in the Design Bases Document as 1000 seconds. With HPCI operable, the worst case period occurred just prior to it being declared 'Inoperable'. HPCI was calculated to have developed an actual flow rate of greater than 4191 gpm during the worst case 1,000 second period. The calculation for 'HPCI Flow Control & Indication Loop Accuracy' provides total loop accuracy of plus or minus 216.2 gpm at 4250 gpm for Design Bases Accident conditions. A calculated flow rate of 4191 gpm, minus the flow instrument loop uncertainty of 216 gpm yields a worst case actual flow rate of 3975 gpm. A flow rate of 3975 gpm is greater than the 3570 gpm assumed in event calculations. The subject flow controller power supply module was removed and bench tested to demonstrate that sufficient operating margin existed to provide reasonable assurance that the power supply would not have failed if called upon to mitigate a design bases accident with sufficient margin. The testing simulated HPCI operation at 4250 gpm for more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. No flow spikes were recorded during this period. Based upon these test results and the analysis provided above, if the subject power supply component had remained in service and HPCI operation was required, the system would have performed its required safety function for a duration exceeding any analyzed event. ENS Event Number 42219, completed on 12/22/05, is being retracted. The licensee notified the NRC Resident Inspector. Notified R1 DO (J. Trapp) |
| Where | |
|---|---|
Vermont Yankee  Vermont (NRC Region 1) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | |
| Time - Person (Reporting Time:+-3.72 h-0.155 days <br />-0.0221 weeks <br />-0.0051 months <br />) | |
| Opened: | Kelly Robinson 10:20 Dec 22, 2005 |
| NRC Officer: | Jeff Rotton |
| Last Updated: | Feb 16, 2006 |
| 42219 - NRC Website
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Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |