ISP-47 'containment thermal-hydraulics': lessons learned from MELCOR calculations

Full text of publication follows: In 2002, OECD launched the 47. International Standard Problem (ISP 47) focused on some thermal hydraulic aspects of accident progression, namely on steam condensation and lighter-than-air gas behavior in containment of a nuclear power plant. The first step of the project (2002-2003) consisted of experiments performed on the French TOSQAN and MISTRA facilities, and benchmark calculations with various lumped-parameter or CFD codes. Nuclear Research Institute (UJV) participated in the ISP 47 with the lumped-parameter MELCOR code. The first UJV calculations (performed in 2003 and submitted to ISP 47) did not agree very well with Step-1 experiments, especially in case of large steam injection. Therefore, new calculations were performed in 2004 with the aim to identify possible reasons of discrepancies. It was found that the choice of a flow type around walls (INTERNAL or EXTERNAL) contributes significantly to the pressure overestimation in case of large steam injection. Even if the condensing surface is INSIDE a vessel, the calculations with EXTERNAL option were much closer to the experimental results. Other parameters had only limited influence (if any) on the results. Several types of nodalization were also tested and the results are discussed. The lessons learned from these sensitivity and nodalization studies may be useful for future calculations of other experiments where thermal-hydraulics play an important role (e.g. aerosol behavior). Applicability of results to calculations of prototype VVER containments is discussed, too. (author)