The use of PSA approach as a model and its methods, as included in the existing standard procedures and tasks already highlighted in previous chapters, is driven for any PSA application by a series of generic and specific aspects. In this section the main generic aspects having a high impact on the use of PSA for EZ are summarized.
1. PSA goals and objectives – status and limitations
The PSA objectives and context are of high impact for its use on any application, including for EZ. In [47; 49] a set of results for various risk metrics in PSA studies is presented for all the period since early 1980’s. These surveys and the information on PSA referred in previous chapters present the PSA studies status. PSA studies are performed for various objectives and goals and with various limitations. Their intended use for various applications is also very diverse.
Therefore, for all those situations there are some limitations well known for PSA, which have a direct high impact if they are to be used for EZ application. Some of those limitations are as follows:
• The environmental and demographic aspects have limitations for an individual country, but also could be evaluated diverse approaches in different countries, which for cross-border accidents pose a serious problem of initial common databases.
• The calculations performed for a given plant have limitations and not yet commonly agreed approaches even if the question is about the same type of plant, as follows:
o Environmental source term and emergency plan;
o Approach to the consideration of core melt in shutdown states with open containment or containment bypass;
o Modelling of high pressure core melt is considered;
o Modelling of large early releases resulting from containment failure;
o Modelling of the mitigation of low pressure core melt and vessel melt-through;
o Interface between non PSA approaches and other methods used for the identification of severe accidents from methodology and acceptance criteria point of view.
• PSA technique issues in the use for EZ application require also evaluation of issues common to any approach in modelling severe accidents, as follows:
o In-containment source term and radiological releases; o Reliability of passive systems;
o Containment by-pass; o Hydrogen risk,
o Assumptions on corium cooling;
o In addition, some specific PSA type issues are to be solved, too, as for instance:
Definition of the PSA level and hence the risk metrics to be used;
Definition of scenarios and their evaluation in separate calculations;
Qualification of systems for severe accidents;
Requirements to probabilistic and risk metrics in the given regulatory environment.
In this context it is important to mention that the objectives of PSA, for instance of level 2, as described in [49] and which have previously been mentioned in section 5.4 of this report, will not change if they are to be used for EZ application.
2. PSA metrics
As it was shown in the previous paragraph, the present status of PSA and hence the level of PSA performed are expected to have an important impact on the EZ application. The existing situation of PSA studies is summarized in [49] for the whole period since PSA started to be developed. As it is shown in Figure 8.1, there was a continuous change of requirements to risk analysis and thus a certain evolution of risk metrics can be noticed. By risk metrics it is understood further mainly CDF (as the main result from L1 PSA ), LERF (as the main result from L2 PSA) and risk (as the main result from L3 PSA ).
Figure 8.1. Sample representation based on PSA published results [49] of the evolution of risk metrics.
Figure 8.1 is a sample representation of the evolution of risk metrics during the years (split into 7 periods since 1980 by now) as results from the PSA published results [49]. The evaluation of the level of requirements is done in a qualitative manner as represented on the vertical axis. The results show the possibility during all the periods to use various risk metrics in order to define requirements and targets for PSA analyses and their use.
However, if it is possible to have a choice on which risk metrics to use, it is also important to consider the fact (as shown later in this chapter) that the risk metric should be preferred.
The probabilistic criteria in the sense of probabilistic targets, which are defined in INSAG 3 (shown in comparison with the PSA studies published results in Figure 8.1) requirements are set for the risk metrics so that :
• The cumulated frequency CDF leading to a core melt has to be kept below 10-5 per reactor year for all plant states, considering internal as well as
external events;
• The cumulated frequency of sequences leading to unacceptable releases (i.e. in excess of maximum allowable releases for design extension conditions) has to be kept below 10-6 per reactor year;
• The cumulated frequency of sequences leading to early containment failure or to very large releases – LERF has to be kept below 10-7 per
reactor year.
In some specific cases [53] a set of risk based criteria are set by the regulatory environment and this is illustrated in Figure 8.2.
Figure 8.2. Risk levels for individuals in a Critically Exposed Group [53]. For all the world regions preparing for next generations of NPP it is expected to see a general move towards more intensive use of the risk as a risk metric for all NPP aspects and applications. In this context, for instance in USA expected future developments are (as shown in [48]) to move to the use of safety goals
including Quantitative Health Objectives, which require a more extensive use of risk for all NPP aspects including specific applications of PSA. It is expected to have this evolution as part of the existing risk informed regulatory environment. In some cases foreseen for future requirements, in Europe (like in EUR requirements case illustrated in more details in section 5.3 of this report) the use of risk targets and much stronger other risk metrics is expected to lead to no needs for expected EZ. However, this still definitely will be a matter of future analysis and clarification.
The clarifications and refinements for decisions on EZ radii are needed for various situations and aspects to be considered, as follows:
• The requirements for new reactors built at new sites;
• The requirements for new reactors built at old sites, close to other reactors of old generations, for which the EZ radii have already been defined; • The requirements for new NPP with multiple units at the same site;
• The agreed policies for EZ of NPP (new and/or old) in areas affecting more than one country.