1.53. A probabilistic safety assessment should be started early in the design process. The probabilistic safety assessment should be used for identification of the core damage frequency (in a Level 1 probabilistic safety assessment) as well as for determination of plant damage states and their frequency (often called a Level 1+ probabilistic safety assessment). These probabilistic safety assessment data are significant in helping to determine the main threats to containment integrity. The probabilistic safety assessment approach is described in Ref. [1], para. 5.73.

1.54. For assessing the design of containment systems, especially with regard to mitigation of the consequences of a severe accident, the probabilistic safety assessment should be extended to Level 2 and a determination should be made of whether sufficient provision has been made to mitigate the consequences of severe accidents. The Level 2 probabilistic safety assessment would address the question of whether the containment is adequately robust and whether the mitigation systems, such as hydrogen control systems and measures for cooling a molten core, provide a sufficient level of protection to prevent a major release of radioactive material to the environment. (See Section 6 for design considerations for severe accidents.)