Outcome of VEGA program on radionuclide release from irradiated fuel under severe accident conditions

Hidaka, Akihide   

In VEGA program on radionuclide release from irradiated fuel under severe accident condition, totally 10 tests were performed at JAEA from 1999 to 2004 under inert and steam atmospheres including the highest pressure or temperature conditions. These tests showed that increase in release rate at high temperature around UO melting point and decrease in release rate under elevated pressure that was a first observation in the world. The mechanism of pressure effect on release was examined and the release model with pressure effect was proposed. The data on low-volatile radionuclide release, release from MOX fuel, the effect of fuel oxidation and eutectic reaction with cladding on release were obtained through the tests. In addition, the effect of obtained results on source term evaluation and effectiveness of the accident management measures were investigated. This paper summarizes the above described outcomes, limitations of VEGA program, and future issues.