A scoping analysis of containment venting at a BWR using THALES-2

The integrated severe accident analysis code THALES-2 code, developed at JAERI for use in Probabilistic Safety Assessment (PSA), is now being used for analysis of accident management measures. Merits of THALES-2 are that it is relatively fast running and that it can simulate various system effects. System effects here means the effects of complex interactions between various plant systems or between plant systems and physical phenomena on accident progression and fission product (FP) source terms. A series of scoping analyses on accident progression and FP source terms were performed aiming at obtaining better understanding on the controlling factors of the source terms of containment venting and demonstrating the usefulness of THALES-2 for planning or reviewing of containment venting strategies. The accident conditions assumed were the TQUV sequence (loss of feedwater transient followed by failure of high pressure and low pressure injection systems) at a BWR with a Mark-I containment. Containment sprays were assumed to be failed. The containment venting was assumed to be done with a hardened vent line from the wetwell. Two sets of sensitivity calculations were made for examining the effect of scrubbing model and the effect of revaporization of volatile species on the source terms. The calculation results confirmed that the containment venting through the wetwell reduces source terms. The sensitivity calculations on scrubbing model indicated that the original model of the ART module of THALES-2, which is similar to the theoretical model of the SPARC code developed in US, gives more conservative estimation than other two models based on experiments performed in Japan. These sensitivity calculations highlighted the strong dependency of the efficiency of the pool scrubbing on aerosol size and suggested that models that influence aerosol size should be carefully examined and be validated by experimental data as far as possible. (J.P.N.)