DOE OSTI2016
Large break loss-of-coolant accident (LOCA) evaluation models (EMs) have historically been focused on fresh fuel at or near beginning-of-life in water cooled reactors. However, these evaluation models did not account for fuel thermal conductivity degradation (TCD). With the explicit modeling of fuel TCD, it is possible that the limiting LOCA conditions could occur for higher burnup fuel. NRC-sponsored research conducted by ANL as part of the 10 CFR 50.46c rulemaking identified mechanisms for cladding embrittlement that exist in higher burnup fuel which were not previously well understood. Additionally, the explicit consideration of fuel pellet thermal conductivity degradation results in more severe fuel rod / assembly considerations for higher burnup fuel in the large break LOCA scenario. In aggregate, these embrittlement mechanisms and phenomena necessitate the consideration of all fuel in the core which undergoes a postulated LOCA, as the limiting fuel cannot be determined a priori for each of the analysis acceptance criteria. The Full Spectrum LOCA EM accounts for all assemblies in the core at the time of a postulated LOCA by defining representative hot assemblies which combine the limiting elements of the fuel assemblies which they represent. By doing so, it is ensured that the LOCA analysis result represents the more limiting assembly in the core for each criterion. Changes in the LOCA boundary conditions (fuel rod initialization, axial power distributions, etc.) that occur during a cycle of operation are also appropriately accounted for in the FSLOCA EM by integrating fuel rod design and nuclear design methods into the LOCA EM. As such, the FSLOCA EM is well-positioned to show compliance with the 10 CFR 50.46c rulemaking as the rule package is nearing completion.
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗