ENS 46397
ENS Event | |
|---|---|
04:25 Nov 5, 2010 | |
| Title | Four Average Power Range Monitors Outside Allowable Range |
| Event Description | Following a power reduction to 49 percent reactor thermal power at approximately 2312 [CDT] on November 4, 2010, 'B' high pressure feedwater heaters were isolated at approximately 2320 in preparation for repair of the 15B feedwater heater. Calculated core thermal power (CTP) rose by approximately 50 MWth. This resulted in all four average power range monitors (APRMs) failing to satisfy Technical Specification (TS) Surveillance Requirement SR 3.3.1.1.2 in that the absolute difference between the APRMs and the calculated power from the heat balance was greater than 2.0 percent rated thermal power (RTP). All four APRM channels were between approximately 3.5 and 4.0 percent lower than CTP. Since all four APRM channels were affected, the functions of the APRMs were inoperable and RPS trip capability had not been maintained. TS Conditions 3.3.1.1.A and 3.3.1.1.C were entered at 2325. All four APRM gains were adjusted and the TS Conditions were exited at 2349.
This event is reportable under 10CFR50.72(b)(3)(v) as an event that could have prevented the fulfillment of the safety function of a system needed to: (A) shutdown the reactor and (D) mitigate the consequences of an accident. This event notification is being submitted outside the 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> reporting requirement. The licensee notified the NRC Resident Inspector.
The following is an update to Event Notification 46397 made on 5 November 2010 concerning Average Power Range Monitors (APRMs) inoperability due to >2% deviation from calculated Core Thermal Power (CTP). During review of inputs to the CTP calculation, Operations staff determined that indicated power levels were not consistent with the indicated electrical output of the turbine generator. Subsequent investigation revealed an erroneous feedwater flow input from the isolated 'B' feedwater heater string. Actual CTP was less than indicated CTP by approximately 50 MWth. This was validated and verified through manual calculation. 50 MWth constitutes approximately 3% of rated thermal power. With the adjustments made to APRM on 4 November 2010, this resulted in all four APRMs being outside of the acceptable tech spec value. Following Operations review, at 1108 on November 7, 2010, all four APRMs were declared inoperable and tech. spec. 3.3.1.1.A and 3.3.1.1.C actions were initiated. Values consistent with the actual plant configuration ('B' feedwater heater string isolated) were input to the CTP calculation with subsequent indicated CTP returning to actual CTP. All four APRM gains were adjusted and APRMs returned to operable status. Tech Spec 3,3.1.1.A and 3.3.1.1.C conditions were exited at 1207 on November 7, 2010. The licensee has notified the NRC Resident Inspector. Notified R3DO (Giessner). |
| Where | |
|---|---|
Monticello  Minnesota (NRC Region 3) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | |
| Time - Person (Reporting Time:+8.27 h0.345 days <br />0.0492 weeks <br />0.0113 months <br />) | |
| Opened: | Mike Stidmon 12:41 Nov 5, 2010 |
| NRC Officer: | Pete Snyder |
| Last Updated: | Nov 7, 2010 |
| 46397 - NRC Website | |
Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (50 %) |
| After | Power Operation (50 %) |