2. Safety analysis of NPP, the role of risk assessment, and measuring risk importance

In order to be able to “measure” the importance of a particular feature or component which has certain safety function in any technical system (NPP included), an appropriate technical definition needs first to be established for a quantitative measure of the system’s “safety”. One of the, arguably, best ways is to present it through its inversion, the “risk”, which is, in engineer’s terms, quantitatively defined by the famous “risk curve” representing the probability (or frequency) of exceedance as a function of magnitude of consequences, Figure 2-1. This presentation corresponds, for example, to the complementary cumulative distribution function used to depict the risk in NUREG-1150 [1].

Figure 2-1 Risk curve or definition of risk for an engineer.

The overall risk is defined by the area below the risk curve, i.e.:

The purpose of the “safety management” or “risk management” in NPP design and operation then, basically, is to minimize the area below the risk curve, or to suppress its “belly” as much as (practicably) achievable. This is illustrated by Figure 2-2, which also shows the two basic and most obvious principles of the risk/safety management. The third principle, which is not explicitly described in the picture but is mostly used in the practice, represents combination of the two.

Figure 2-2 Two basic principles of safety/risk management.

Ensuring low consequences by system design (principle a) is achieved through demonstration of the safety margins, which is a deterministic discipline and is implemented as a part of system’s design basis analyses. On the other hand, ensuring low likelihood of scenarios which may exceed the design basis and, hence, result in large consequences is done considering the results of probabilistic risk analyses. Deterministic design basis analyses and probabilistic risk analyses, therefore, are considered in ensuring the overall NPP safety and play complementary roles in the plant design and operation.

Safety importance of a particular component or feature within a system or plant also reflects these two aspects (or two dimensions, with respect to Figure 2-1 and Figure 2-2). The first one is importance with respect to the role in ensuring the appropriate safety margin within the design basis envelope. This importance is addressed deterministically by employing the principles of design bases and defense-in-depth reflected in “traditional” safety classification of equipment. The second aspect is addressed considering importance with respect to the risk, or risk importance, with regard to those scenarios which can lead to the conditions beyond design basis envelope. Generally, this is risk importance which is the subject of the discussions in this monograph.