Station Blackout Rule

NRC staff studies and analyses concerning seal leakage are documented in NUREG/CR-5167 Cost benefit Analysis for Generic Issue 23. This report identifies several modes of RCP seal leakages which may be in excess of that assumed in station blackout rule (lOCFR 50.63)... 

Also, all the nuclear power plants are required to comply with the Station Blackout Rule (loss of all offsite and onsite power supplies) required by 10CFR50.63, "Loss of All Alternating Current Power." The nuclear power plants in the US may use either two startup transformers or use one startup transformer as one source and a unit auxiliary transformer via main stepup transformer (where the main generator circuit breaker is used) as the other offsite power source. In a multi-unit plant, there are generally two startup transformers per nuclear unit and each startup transformer is sized to simultaneously start (block loading) all the accident loads of its safely bus, in case of an accident (LOCA) if a sequencer is not used. If a sequencer is used, each startup transformer is sized for the full loads of its safety bus plus the starting of the largest load on that safety bus. In plants where there are two startup transformers for the two units, the staff has traditionally required that each startup transformer should be sized to supply the accident loads of one unit and safe shutdown of the other unit. 

The station blackout rule 10 CFR 50.63, which became effective on July 21, 1988, was promulgated to reduce the risk of severe accidents resulting from station blackout by ... 

It should be noted that station blackout was not deemed to constitute an undue risk without the station blackout rule. It was recognized that even with the rule, station blackout may still remain an important contributor to residual risk. The station blackout rule was developed to enhance safety by accident prevention and thereby reduce the likelihood of a core damage accident being caused by a station blackout. Like the ATWS rule (Section 2.7.1) it... 

FR 23203, Statement of Considerations for Final Station Blackout Rule, June 21, 1988. 

The completion of PSA analysis for a large number of plants has allowed a more comprehensive and systematic assessment of the safety of the plants. Systems and situations to vdiich not much attention was paid before, are now considered relevant to safety (i.e. analysis of accidents in modes other than full power, anticipated transients without scram (ATWS), station blackout, spent fuel systems, specific configuration situations, maintenance rule, external hazards). Issues with an analysis source include GL 4, SS 8, CS 3, AA 4, AA 5, AA 6, AA 7, MA 11, EP 3, TR 3, FS 2. 

Since that time, NRC has been using risk assessment and directing the issuance of decisions on complex items associated with or related to safety, such as (a) total loss of power (station blackout) (b) Anticipated Transients Without reactor Shutdown (ATWS) (c) Pressurized Thermal Shock events (PTS) and (e) Maintenance Rule. 

The next three subsections describe the role that quantitative risk estimates played in addressing and resolving three important regulatory issues Anticipated Transients Without Scram, Auxiliary Feedwater System Reliability, and Station Blackout. Following these discussions, current policies and practices of the NRC regarding the use of quantitative risk estimates are discussed in subsections addressing the Safety Goal Policy, the Backfit Rule, and Individual Plant Examinations. 

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