ENS 42512
ENS Event | |
|---|---|
14:45 Apr 19, 2006 | |
| Title | Postulated Scenario Where Containment Spray System May Be Unable to Fulfill Design Safety Function |
| Event Description | During a review of the operation of the plants emergency cooling system for the containment an unanalyzed single failure was discovered. The identified single failure scenario could result in the containment spray system being unable to fulfill its design safety functions.
In the event of loss of offsite power occurring after the initiation of an accident signal, the 480V undervoltage relays serve to trip open containment spray pump breakers (as well as other ESF breakers) in order to prepare the breakers for resequencing after the diesel generator output breakers have closed onto their associated buses. The undervoltage trip bypass function performed by the sequencer timer relay contacts serves to prevent tripping ESF breakers due to inadvertent actuation of the undervoltage trip circuits and allows ESF breakers to trip only when sequencers have been reset by a loss of voltage at the 4160V bus level. In situations where a loss of power occurs at the 480 volt level without a coincident loss of power at the associated 4160 volt level, ESF loads supplied from the lost 480 volt bus, such as containment spray pumps, do not receive a trip signal due to the undervoltage blocking feature of the sequencing relays. For most ESF loads, this is not a problem and can be considered part of a single failure scenario affecting only one train of ESF equipment. In the case of containment spray pumps SI-3B and SI-3C, however, the failure of the associated breakers to trip during a single failure of bus 1B4B, results in the operation of a single spray pump, SI-3A with two containment spray valves open. This results in one pump operation to two containment spray headers. Operating the containment spray system in a one pump, two header configuration creates the possibility of inadequate system performance. This configuration may result in overloading the running pump (due to runout) and inadequate NPSH to the running pump. This condition was intended to be prevented by a modification which installed an interlock between spray pumps SI-3B and SI-3C and spray valve HCV-344. The modification apparently failed to consider the single failure of specific 480 Volt buses. Spray valve HCV-344 has been disabled in the closed position so that no external signals will allow the valve to be opened. Disabling this valve places the plant in a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> LCO (Technical Specification 2.4.2.d) starting at 0950 CDT. The licensee notified the NRC Resident Inspector.
On April 19, 2006, Ft. Calhoun station reported that an unacceptable single failure scenario had been identified that could result in the containment spray system being unable to fulfill its design safety functions. The failure scenario required that there be a loss of power at the 480 volt AC level without a coincident loss of power at the associated 4160 volt AC level. This resulted in the potential for one pump operation to two containment spray headers. Following a review of failures that would cause this it was determined that of all the possible failure mechanisms that could impact the capability of affected 480 volt AC bus, such that both trains of containment spray pump could be adversely affected, the only failure mechanism that has unacceptable consequences is a failure of multiple phases of AC power failing in an open circuited manner without a fault occurring and, consequently, without supply breakers opening. This failure mechanism has been determined not to be credible. Therefore, it is not necessary to assume that this type of failure could occur as part of a design basis event. Therefore, no credible single failure could result in the failure of the containment spray system to perform its intended safety function. The report of April 19, 2006 is being retracted June 1, 2006. The licensee notified the NRC Resident Inspector. R4DO (Spitzberg) notified. |
| Where | |
|---|---|
Ft Calhoun  Nebraska (NRC Region 4) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | |
| Time - Person (Reporting Time:+1.22 h0.0508 days <br />0.00726 weeks <br />0.00167 months <br />) | |
| Opened: | Erick Matzke 15:58 Apr 19, 2006 |
| NRC Officer: | Bill Huffman |
| Last Updated: | Jun 1, 2006 |
| 42512 - NRC Website
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Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |