ENS 44228
ENS Event | |
|---|---|
00:56 May 21, 2008 | |
| Title | Decay Heat Removal Cooling Interupted During Core Reload |
| Event Description | At 1956, during reactor core reload with a full refueling cavity, power was lost to the #2 non-vital instrument bus. This power loss resulted in closure of the shutdown cooling temperature control valve, HCV-341. The closure of HCV-341 interrupted the cooling capability of the in service shutdown cooling loop.
While cycling a condenser motor operated valve a 480 volt ground occurred which resulted in tripping the feeder breaker to motor control center MCC-4B2. MCC-4B2 was supplying power to Instrument Bus 2 via the Inverter 2 test transformer. The test transformer was powering Instrument Bus 2 due to Inverter 2 replacement per plant modification. The loss of the #2 Instrument Bus resulted in HCV-341, the shutdown cooling heat exchangers temperature control valve, failing closed. HCV-341 was manually opened to restore cooling at 2019. At 2049 power was restored to Instrument Bus 2 and the shutdown cooling system was returned to automatic operation. Flow through the core was maintained throughout the event, as the shutdown cooling heat exchanger bypass valve responded by opening to maintain flow. At the time shutdown cooling was lost, 44 of 133 assemblies had been loaded into the vessel and shutdown cooling temperature was approximately 88 degrees F. Time to boil was conservatively calculated to be 22.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> per plant procedures which assume decay heat from all 133 assemblies. Shutdown cooling temperature rose approximately one degree during this event. Technical Specification 2.8.1(3)(1) was entered due to no Shutdown Cooling loop in Operation. No reactor coolant boron reductions were in progress. Irradiated fuel assembly loading into the reactor core was secured and actions to restore a Shutdown Cooling loop were being initiated. [Ft. Calhoun] entered] a 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> LCO to close all containment penetrations providing direct access from the containment to the outside atmosphere. The licensee notified the NRC Resident Inspector.
Following a detailed review of the event of May 20, 2008, Fort Calhoun station determined that the safety function of removing residual heat from the reactor coolant system was available throughout the entire event. The system is largely manual and the manual functions were not affected by the event. At the time of the loss of power the core was being reloaded. The heat load was very small and the temperature control valve (HCV-341) was closed to allow the system to increase in temperature. When control power to HCV-341 was lost the valve did not change position. Since the ability of the shutdown cooling system to remove residual heat was not impacted by the loss of power and plant procedures have provisions to control the system locally, the safety function of removing decay heat was not lost. Therefore this notification is being retracted. The licensee notified the NRC Resident Inspector. Notified R4DO (William Jones) |
| Where | |
|---|---|
Ft Calhoun  Nebraska (NRC Region 4) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat | |
| Time - Person (Reporting Time:+1.55 h0.0646 days <br />0.00923 weeks <br />0.00212 months <br />) | |
| Opened: | Scott Lindquist 02:29 May 21, 2008 |
| NRC Officer: | Pete Snyder |
| Last Updated: | May 27, 2008 |
| 44228 - NRC Website
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Unit 1 | |
|---|---|
| Reactor critical | Not Critical |
| Scram | No |
| Before | Cold Shutdown (0 %) |
| After | Cold Shutdown (0 %) |