CALCULATION OF THE PROBABILITY OF DDT DURING SEVERE ACCIDENTS

摘要

In this paper, we are proposing a method to quantitatively calculate the likelihood of hydrogen explosions in the form of DDT (Deflagration to Detonation Transition) during a severe accident. Determination of the probability of the DDT during severe accident scenarios thus far has been subjected to qualitative expert opinions. There have been no proposed quantitative methods to calculate such probabilities to the best of the authors' knowledge. The proposed method enables the quantification of probabilities of DDT for any given severe accident sequence that can be simulated by a severe accident analysis code. Different accident sequences will have different characteristics of cladding oxidation, hydrogen production rate and hydrogen release rate, and therefore will have different probabilities of DDT. The proposed method can be used to objectively and quantitatively differentiate the DDT probability of different scenarios due to impacts of equipment availability or operator actions. The proposed definition of DDT probability is intended for two applications. First is the application in PSA level 2 assessments. The second potential application is to help assess the explosive conditions in the reactor building of Fukushima Daiichi units 1 and 3.