Damage Evolution and Countermeasure Performance in the Fukushima Daiichi Accident

Analyses addressing the 2011 Fukushima Daiichi accident were carried out to explain the variability of the damage evolution with emphasis on containment performance and to derive related insights into the performability of countermeasures in the domain of accident management (AM). The explanatory conditions concerning essential stages of the damage evolution (loss of containment integrity, significant release, hydrogen explosion) include the exposures to loss of containment cooling before and after core uncovering, loss of core cooling, core uncovering, and high containment pressure and temperature (close to or above the design limit). Besides the size (abs. value) of a containment load, the duration of this load appears to be a damage-influencing factor as well. A key insight derived from the analyses of countermeasure performability issues is that the inference from performability to performance requires the assessment of decision-making. Overall, the findings from the analyses confirm the good practice to predict the damage evolution on the basis of simulations of limiting cases of accident sequences, and to inform the identification of limiting cases by human reliability analyses of the countermeasures challenged to fight against the damage evolution.