1PIE: postulated initiating event.

2The term ‘accident progression event tree’ is also used by some practitioners for this part of the Level 2 PSA.

3The attributes listed in Table 3 should be adjusted, as appropriate, for plants with structures that provide a confinement function rather than pressure retaining containments.

4This section addresses several key parts of a Level 2 PSA. The order in which they are presented here is not an indication of their relative importance or the order in which they should be carried out within a PSA project.

5Each alternative C matrix within this set may in fact have, dependent on the nature of the events in the containment event tree, elements whose values are 1 or 0 and the baseline C matrix will have elements whose values are the weighted averages of the C matrix values over the whole set of alternative matrices.

6Some Level 2 PSAs have developed parametric source term models on the basis of calculations performed with codes such as MAAP [26] or MELCOR [27] and this approach enables the uncertainties in the source term parameters to be combined with the integrated process for uncertainty assessment and uncertainty propagation.

7For example, many calculations of accident sequences involving ‘station blackout’ for several reactor designs can be found in the open literature. However, there are many variations of station blackout, depending on the particular system configuration of a plant. In some cases, sufficient DC power might be available to operate a small group of components (e.g. relief valves) or systems (e.g. steam driven pumps) in some plants that are not available in other plants. Such differences should be carefully considered before calculated results from the literature are applied to the plant under study.

8The main report is intended for use by specialized PSA analysts and peer reviewers. The main report and all of the appendices should include sufficient information to support fully the conclusions of the Level 2 PSA.

9The objective for large off-site releases requiring short term off-site response is 1 x 10^–5 per reactor-year for existing plants. Reference [34] does not specify a numerical value for a large off-site radioactive release for future plants, but states the following qualitative objective: “Another objective for these future plants is the practical elimination of accident sequences that could lead to large early radioactive releases, whereas severe accidents that could imply late containment failure would be considered in the design process with realistic assumptions and best estimate analyses so that their consequences would necessitate only protective measures limited in area and in time.”