RECENT ADVANCES OF THERMAL-HYDRAULIC RESEARCHES IN SEVERE ACCIDENT

Research on severe accident for light water reactors has been initiate d after the accident of Three Mile Island in 1979 and accelerated all over the world after the Chernobyl accident in 1986. For the severe ac cident management Nuclear Safety Commission of Japan recommended elect ric utilities to investigate the possible measures in 1992. In respons e to this they proposed accident management strategies based on their investigations in 1994 to be voluntarily implemented in several years by reflecting findings from severe accident research. Since the severe accident phenomena largely involve the thermal-hydraulic aspects, it is of great importance to evaluate the thermal-hydraulic behaviors of severe accident in order to reduce the associated uncertainties. In th e present article thermal-hydraulic behaviors during severe accident a re described in terms of phenomenology, severe accident management, ke y issues, research methodology and remaining research items. The main phenomena include core melt progression, in-vessel core melt behaviors , molten core coolant interaction, molten core concrete interaction, d irect containment heating, hydrogen behaviors, and fission product beh aviors.