Optimisation of non-destructive testing - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 222

Failure
probability

Lines of constant risk

Very
high

High

Medium

Low

Very
low

Numerical values
in brackets
are purely
illustrative

Failure
conse-
quences

Very low Low Medium High Very

high

7. 2 Risk matrix.

consideration. They symbolise the highest levels of risk and are not taken
seriously. The remaining sections are divided into three risk categories and
are marked with gray, light gray and white colours. 'Risk lines' represent
two different levels of risk.
In element D 3 there are the key elements needed to build the abscissa
of the risk matrix as a source of initial data, and the categories of potential
consequences are listed:
• frequency of the maximum initial release of fission products;
• frequency of emission of fission products;
• core damage frequency;
• as well as the cost factor in the case of loss of operational readiness
and the need for repairs.
The concept of defence in depth includes the probability of consecutive
violations of safety barriers which, consequently, to a large extent affects
the classification of the levels of consequences.
These categories of effects can be correlated with traditional regulatory
classification of the components as shown in element D 2 of the Main
scheme:
• absence of the failure probability of the reactor pressure vessel;
• class of equipment and pipelines 1;
• class of equipment and pipelines 2;

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