A comprehensive, structured approach to identifying failure scenarios, constituting a conceptual and mathematical tool for deriving numerical estimates of risk. Three levels of PSA are generally recognized. Level 1 comprises the assessment of plant failures leading to the determination of the frequency of core damage. Level 2 includes the assessment of containment response leading, together with Level 1 results, to the determination of frequencies for failure of the containment and release to the environment of a given percentage of the reactor core's inventory of radionuclides. Level 3 includes the assessment of off-site consequences leading, together with the results of Level 2 analysis, to estimates of public risks. (See, for example, Ref..) Living Probabilistic Safety Assessment (PSA): A PSA which is updated as necessary to reflect the current design and operational features, and is documented in such a way that each aspect of the PSA model can be directly related to existing plant information, plant documentation or the analysts' assumptions in the absence of such information