The article sets forth algorithms, created in the process of developing Russian Risk-Informed Decision Making (RIDM) Guideline for Nuclear Power Plants (NPPs):

1) algorithm of assessment of risk acceptability in conjunction with making a decision;

2) algorithm of assessment of decision acceptability from the point of view of its influence on the defence-in-depth.

The first algorithm takes into account such risk metrics as Base Severe Accident Frequency (SAF) and increase of SAF as a result of making a decision, instantaneous value of SAF, as well as Large Release Frequency (LRF) and its increase as a result of making a decision. Based on the combination of values of the abovementioned metrics and according to the developed algorithm, risk falls into one of the zones: acceptable risk (green zone), unacceptable risk (red zone) or zone of conditionally acceptable risk (yellow zone).

This article describes and explains those criteria, on the basis of which borderlines between the aforementioned zones are being established.

A nomenclature of threats to the defence-in-depth and of possible implementation mechanisms of the indicated threats have been proposed within the framework of the second algorithm.

For the decision under analysis it is necessary to evaluate for each of the nomenclature implementation mechanisms of threats to the Defence-in-Depth (DiD) whether the decision has influence on DiD according to the mechanism under consideration.

The article presents rules of assessment of level of changes in DiD vulnerability, caused by the decision under analysis, contained in the described algorithm.

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