Table 3-5. Qualitative ranking

DOE-STD -3009-94

Table 3-5. Qualitative ranking.

Risk

Description

evaluation

No impact or beyond

e xtremely unlikely.

Low severity and

extremely unlikely.

Acceptable

Moderate severity and

extremely unlikely or low

severity and unlikely.

High severity and

extremely unlikely or low

severity and anticipated.

Marginal

Moderate severity and

Unlikely.

Moderate severity and

anticipated or high

severity and unlikely.

Unacceptable

High severity and

anticipated.

material releases. The logic behind Figure 3-2 is elaborated on in Tables 3-3

through 3-5, which provide a description of a four-by- four frequency and

consequence-ranking matrix. Although differing in presentation and structural

details, the philosophical basis and objectives for both examples are identical.

The ranking schemes are designed to separate the lower risk accidents that are

adequately assessed by hazard evaluation from higher risk accidents that may

warrant additional quantitative analysis if the phenomena involved are not

simplistic. A limited number of moderate risk accidents between the two

extremes may also be identified for assessment. Tables 3-3 through 3-5

provide typical descriptions of consequence and likelihood thresholds for

binning. Ranking should use broad bins. For example, frequency bins should

typically cover two orders of magnitude.

Although the exercise of binning is essentially qualitative, analysts often use a

simple numerical basis for judgments to provide consistency. For example, a

simple methodology for frequency binning would be to assign a probability of

1 to nonindependent events, 0.1 to human errors, and 0.01 to genuinely

independent failures. Another methodology would be to use a summary of

historical data. Likewise, before beginning the evaluation, a conservative

Gaussian plume estimation of the amount of material needed outside the

building to cause a certain dose might be performed to aid in defining

thresholds of significance. Briefly discuss or reference any such guidelines in

Section 3.3.1.2, "Hazard Evaluation." Note, however, that the ranking of

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