A two-stage PSA method for nuclear multi-stage risk assessment is introduced. To this end, the present invention (a) core damage expansion step; (b) a plant damage group allocating step of grouping according to a plant damage state based on a core damage accident derived by step (a); (c) converting the tree to the fault based on the previously stored mapping table; And (d) calculating a fraction of the source term emission group, wherein the pre-stored mapping table is a branch probability table for branching each of the power plant damage groups to the source term emission group.
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