Realistic small- and intermediate-break loss-of-coolant accident analysis using WCOBRA/TRAC

Since the 1988 Appendix K Rulemaking change, there has been significant interest in the development of codes and methodologies for "best-estimate " analysis of loss-of-coolant accidents (LOCAs). Most development has been directed toward large-break (LB) LOCAs (LBLOCAs), since for most pressurized water reactors (PWRs), the LBLOCA generates the limiting peak cladding temperature (PCT). As plants age, are uprated, and continue to seek improved operating efficiencies, the small break (SB) LOCA (SBLOCA) and intermediate-break (IB) LOCA (IBLOCA) can become a concern. Modifications have been made to the WCOBRA/ TRAC-MOD7A code to enable it to make realistic calculations of SBLOCAs and IBLOCAs in a Westinghouse PWR. The MOD7A version has recently been approved for use as part of the Westinghouse best-estimate LOCA methodology for LBLOCAs. Thus, the modifications and improvements potentially allow LOCA calculations ranging from SBLOCAs to LBLOCAs using a single code version. The WCOBRA/TRAC-MOD7A, Rev. 4 SB02 version was used to calculate the transient response of a four-loop PWR for a range of break sizes located at the bottom of one of the cold legs. The break sizes ranged from a 0.051-m (2-in.) to a 0.406-m (16-in.) equivalent hole diameter. Each calculation was performed assuming American Nuclear Society (ANS) 1979 decay heat. The plant input assumed the loss of one train of safety injection as well as a power shape that was highly top skewed, which imposed some conservatism on the calculations but allowed a meaningful comparison to Appendix K-type analysis results. The realistic SBLOCA and IBLOCA results showed significantly reduced PCTs compared to those typically obtained from Appendix K LOCA calculations. The realistic results also can be categorized into four separate types of breaks, from a conventional slowly draining SBLOCA to an LBLOCA.