05000255/LER-2005-002

LER-2005-002,

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
2552005002R01 - NRC Website

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) �I Palisades 05000-255 2005 -- 002 -- 01

EVENT DESCRIPTION

On February 17, 2005, with the plant operating at approximately 100% power, a latent design inadequacy was discovered which affects the emergency diesel generator (EDG) 1-2 [DG;EK] load analysis. For certain postulated post-accident scenarios, it was determined that the pressurizer heaters [PZR/EHTR;AB] that are powered from 2400V safety bus 1-D [BU;EB], may automatically re-energize after initially being shed. As a result of the additional load, EDG 1-2 would become overloaded, and could trip on over-current.

Breaker 152-211 [BKR;EB], supplies power to the pressurizer heaters from bus 1-D. The breaker automatically opens in response to a sustained undervoltage condition on bus 1-D, and is not automatically reconnected to the bus via the load sequencer, when the bus is subsequently re­ powered by EDG 1-2. However, following a review of plant drawings and documents, it was concluded that if certain conditions exist, primarily restoration of pressurizer level after an initial low pressurizer level condition, the breaker will automatically close, adding the pressurizer heater load to EDG 1-2. The added load was not previously identified in the EDG 1-2 load calculations.

Certain Final Safety Analysis Report (FSAR) design basis accident scenarios may result in an initial lowering of pressurizer level, with subsequent pressurizer level restoration (e.g. small break loss of coolant accident or main steam line break). For these scenarios, concurrent with other postulated design basis accident assumptions of loss of offsite power and failure of the opposite EDG, EDG 1-2 would be relied upon for powering safety related equipment, but would become overloaded by the pressurizer heater load.

This condition applies only to EDG 1-2. Pressurizer heaters cannot be powered from the redundant 2400V safety bus 1-C and EDG 1-1 without manual action.

This is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by the plant's Technical Specifications, and 10 CFR 50.73(a)(2)(v)(D) as a condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.

CAUSE OF THE EVENT

Original plant design incorporated automatic isolation of pressurizer heater breaker 152-211 when a safety injection signal (SIS) was present. Automatic closure of the breaker was blocked until the SIS actuation signal was reset.

In December 1986, a circuit modification was implemented to eliminate the SIS blocking feature for automatic re-closure of pressurizer heater breaker 152-211. Consideration for the possibility of U.S. NUCLEAR REGULATORY COMMISSION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Palisades 05000-255 2005 -- 002 -- 01 overloading EDG 1-2 with the added load of the pressurizer heaters was not satisfactorily addressed. Based on review of documentation associated with the modification, the only accident scenario that was considered was a large break loss of coolant accident (LBLOCA). In a LBLOCA, pressurizer level is assumed to not recover sufficiently to allow for automatic re-closure of pressurizer heater breaker 152-211.

SAFETY SIGNIFICANCE

The safety significance of the event is considered to be minimal, based on a qualitative review of FSAR safety analyses that credit emergency diesel generator operation for event mitigation. The review identified that for the accident analyses that could result in the described condition, alternate mitigating strategies are procedurally directed which enable successful response to the events. The alternate mitigating strategies include continued heat removal via feeding and steaming the steam generators [SG;AB], and passive makeup provided by the safety injection tanks [TK;BPj. In addition, the described condition would not be expected to occur until after peak fuel cladding temperature has occurred. Diagnosis and restoration of the EDG are procedurally directed, allowing recovery of emergency onsite power to mitigating equipment trains.

CORRECTIVE ACTIONS

EDG 1-2 was declared inoperable when the design inadequacy was confirmed.

A temporary modification was subsequently completed to preclude pressurizer heater breaker 152-211 from automatically re-closing when pressurizer level recovers.

A review of bus 1-C and bus 1-D loads was completed to confirm the loading assumptions for the EDGs. No additional discrepancies were identified.

PREVIOUS SIMILAR EVENTS

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