Thermohydraulics and fuel rod behavior during reflood/quench situations

At the Forschungszentrum Karlsruhe (FZKA) the Institute for Reactor Safety (IRS) supports the experimental activities focused on the safety features of existing and advanced light water reactors by plant calculations using SCDAP/RELAP5 (S/R5) mod3.2 and RELAP5 (R5) mod 3.3. These codes are continuously validated by comparison with various experiments. In the area of design basis safety R5 mod3.3.b code validation and code application for the high performance LWR (HPLWR, 5th European Framework Programme is ongoing), extending the capability of RELAP5 to simulate conditions of supercritical water. Best estimate safety analyses for severe accidents with the in-house version S/R5 mod 3.2.irs were performed successfully and documented for the European Pressurized Water Reactor. (FZK-6299, -6315, -6567, http://bibliothek.fzk.de/zb/berichte). Analytical support for bundle experiments in the QUENCH facility at FZK is ongoing with S/R5 mod3.2.irs to define test parameters and to support experimental analyses. A fine discretization with 5 cm meshes has been developed to better simulate reflood conditions. The improved shattering model works fairly well for an intact rod geometry, but is not adequate for damaged fuel rods. Furthermore, S/R5 is used for test analyses especially for QUENCH-06, basis for the OECD/NEA/CSNI International Standard Problem No. 45 ISP-45, and for QUENCH-07 as contribution to EU-program COLOSS. A code extension to simulate B4C oxidation is planned, but postponed until 2003 due to limited man power caused by the OECD/NEA ISP-45 benchmark. Besides, ICARE2V3 is used to support S/R5 analyses with respect to QUENCH and Phebus analyses. The blind phase of the OECD/NEA/CSNI International Standard Problem No. 45 ISP-45 (QUENCH-06) is finished, the preparation of the OECD Interpretation and Comparison report is under way. The current state of code to code comparison reveals strong variation within the same code family probably attributed to user effects. The pre-flood phase agrees rather well with the experimental results whereas during reflood strong discrepancies were found. For in-vessel late phase situations code validation using Phebus FP experimental data is ongoing with S/R5 mod3.2 and ICARE2V3. The experiments FPT0, FPT1 and FPT2 were analyzed to support the Phebus team and to validate S/R5. (see ''Analyses of QUENCH and Phebus FP experiments using ICARE2'', W. Sengpiel, ICARE/CATHARE Users Club, 19 - 20. Nov. 2001). Deficiencies have been identified in the new code version S/R5 mod 3.3, especially because FZK extensions for experimental facilities (CORA, QUENCH, PHEBUS; FZKA 6566) as well as some error corrections are not included. Therefore the new version is not yet our workhorse. (authors)