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Table 3-3. Qualitative severity classification table
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Preparation Guide for U
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Accident Analysis


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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