Ambient pressure-dependent radionuclide release from fuel observed in VEGA tests under severe accident condition and influence on source term evaluation

Hidaka, Akihide   

In VEGA program on FP release from fuel during severe accidents, 10 tests were conducted under the highest pressure and/or temperature conditions. Tests with PWR fuel at 1.0MPa showed first that Cs release was suppressed by about 30% compared with that at 0.1MPa. This was reproduced by 2-stage diffusion model in UO grains & pores, and a simplified 1/P**0.5 CORSOR-M model. In BWR and MOX fuel tests, however, this effect was not observed clearly due to higher fuel temperature during normal operation and differences in test conditions. The pressure effect may affect PWR source terms and AM measures such as intentional depressurization. Analyses with THALES-2 suggested that the depressurization has many advantages such as delay in accident progression and mitigation of source terms at early CV failure despite increase in FP release into RCS. The effect of pressure on consequences needs to be evaluated systematically for various accident sequences with AM measures.