Knowing something of the failures which might occur and something of the response of the system to the accidents, it is important to classify the accidents according to:

(a) Probability. This is impossible to do exactly at present and engineer-

TABLE 3.3 (continued)

° See Budney (6).

ing judgment is used. As failure statistics improve, then it may be done for some of the plant components.

(b) Severity. This is a calculated severity with experimental confirma­tion where possible.

Light water reactors (LWR) also use accident classifications that range from those accidents and occurrences which are expected frequently (minor perturbations in reactivity or single failures of control elements) to those which are never expected to occur but which are nevertheless chosen as a design basis because their consequences are so severe (major pipe break, ejection of a control rod).

An accident classification may tie the probability and severity of the consequences together, without reference to criteria for the design of the plant. A more logical classification is proposed which in general may be applied to any nuclear system.