Station blackout anticipated transient without scram

The safety analysis of the TMSR-SF has also drawn much attention. Three types of transient conditions including ULOF, UOC, and UTOP were examined on the TMSR-SF by an FHR safety analysis code named the FHR Safety Analysis Code (FSAC Xiao et al., 2014). The station blackout anticipated transient without scram (SBO-ATWS) accident was analyzed by the modified RELAP5/MOD 4.0 code with the responses of the passive residual heat removal (PRHR) system (Jiao et al., 2015). 

SBO-ATWS Station blackout anticipated transient without scram... 

Criterion 20. Design extension conditions Limited set of events more severe than DBAs [station blackout, anticipated transients without scram (ATWS), aircraft impact, etc.] for design extension requirements. The design shall be such that design extension conditions that could lead to significant radioactive releases are practically eliminated. 

A long evolving use of PSA was for Anticipated Transients without Scram (ATWS) which extended over 15 years to culminate in NUREG-0460 which was upset by the Salem failure-to-scram incident and the subsequent SECY Letter 83-28. Other special studies have been (a) value-impact analysis (VIA.) studies of alternative containment concepts (e.g., vented containment, NUREG/CR-0165), (b) auxiliary feedwater studies, (c) analysis of DC power requirements, (d) station blackout (NUREG/CR-3220), and (e) precursors to potential core-damage accident.s (NUREG/CR-2497), to name a few of the NRC sponsored studies. 

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. 

Thus, the China syndrome led to a shift in emphasis from containment to prevention. As time passed, accident initiators other than the traditional large pipe break were identified as potentially leading to core melt. In particular, scenarios involving anticipated transients without scram, station blackout, other transients, and containment bypass would be evaluated, and regulated to reduce the probability of core meltdown. However, over the next decade, the emphasis was on the traditional design-basis... 

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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