Remodeling of computer codes to evaluate tornado-borne missile hazard for tornado PRA

Tornado PRA is an effective method to quantitatively evaluate tornado hazard impact on a nuclear power plant relative to the overall plant risk. Tornado hazard is generally classified into three categories, i.e., wind pressure load, negative pressure load, and impact load of tornado-borne missiles. The risk owing to tornado-borne missile impact on components and structures is supposed to be most significant among them, because the effects of the wind pressure load and the negative pressure load are usually negligible on components and structures with seismic resistance design, which are rigid enough to withstand postulated seismic load in Japan. Therefore, the authors have developed a probabilistic tornado-borne missile analysis code, TONBOS-pro, and an evaluation code of annual strike probability of tornado-borne missiles, TOMAXI-pro, which are intended to be used for tornado PRA. In this paper, we explain the primary features and sample results of both codes, with describing the future direction of their application to tornado PRA. (author)