Exploring system behaviour and consequence analysis Deterministic and probabilistic approaches

Different and complementary approaches are used to explore system behaviour and evaluate the consequences of the different types of uncertainty discussed above. On the one hand determi-nistic analyses are conducted for a broad range of cases, while, on the other hand, probabilistic methods are used to explore systematically the consequences of different combinations of para-meters that fall within the ranges of uncertainty. In the present study, the main emphasis is on deterministic analyses for a broad range of cases that are representative for all realistically con-ceivable possibilities for the characteristics and the evolution of the system. The objectives are to develop system understanding, to illustrate the possible radiological consequences of the repository, to evaluate uncertainties and design / system options in terms of their impact on the

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radiological consequences of the disposal system, and to determine whether existing uncertain-ties are acceptable, or need to be addressed in the course of future stages of the programme.

These deterministic calculations are, however, complemented by probabilistic calculations that aim to enhance system understanding, ensure that no unfavourable combinations of parameters are overlooked, and to test whether there are sudden or unexpected changes in performance as parameters are varied, which might not be detected using a deterministic approach. The probabilistic calculations also provide input to the evaluation of the compliance of the repository performance with protection objectives.

The Reference Case and sensitivity analyses

The starting point for exploring system behaviour and for consequence analyses is to define and analyse a Reference Case. The Reference Case is based on the reference design / system and on the assumption that the likely / expected broad evolutionary path of the disposal system is followed (this is termed the Reference Scenario). It is also based on a number of assumptions regarding the conceptualisation for modelling purposes of key FEPs associated with the various system components (the Reference Conceptualisation), together with a reference set of para-meters. The Reference Conceptualisation and reference parameter set are generally based on the expected characteristics and evolution of the system, but some pessimistic or conservative conceptual assumptions and parameter values are also used (see above). Quantitative models are used to examine the fate of radionuclides in the Reference Case, and to perform deterministic as well as probabilistic sensitivity analyses, both within and beyond the constraints of the Refe-rence Scenario and the RefeRefe-rence Conceptualisation. The sensitivity analyses provide under-standing of the behaviour of the system with respect to perturbations and the extent to which deviations from the likely / expected characteristics and evolution of the disposal system affect overall performance and the performance of individual system components. They provide insight into the robustness of the system chosen, guide the definition of alternative assessment cases and assist in the interpretation of results. Probabilistic analyses around the Reference Case provide also an indication of compliance with regulatory criteria taking into account the combined effects of uncertainties.

Groupings of alternative assessment cases

Alternative cases address other possibilities and stylised conceptualisations and are divided into a number of groups, according to the issues or uncertainties that they address. In particular, there are cases that address:

• the range of possibilities arising from uncertainties affecting the disposal system, where this range can be bounded with reasonable confidence on the basis of available scientific understanding,

• "what if?" cases,

• design / system options, and

• different (stylised) possibilities for future human actions and for the characteristics and evolution of the surface environment (the biosphere).

Like the Reference Case, each alternative case is defined in terms of a scenario (the broad evolutionary path that the disposal system follows), a number of conceptual assumptions for modelling key FEPs, and a set of parameters. Issues and uncertainties are assessed as to whether they (i), significantly affect the broad path of evolution of the disposal system described by the Reference Scenario, in which case they generate alternative scenarios, or whether they only

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affect (ii), the conceptualisation of FEPs within a given scenario, or, (iii), the assignment of parameter values within a given conceptualisation of a scenario. The result is a number of scenarios, within each of which there may be alternative conceptualisations of particular FEPs.

Furthermore, for each conceptualisation, there may be a range of alternative parameter sets, including a Base Case parameter set. This hierarchy of scenarios, conceptualisations and parameter sets is illustrated in Fig. 3.7-3.

Combinations of multiple, highly unlikely possibilities are, for the most part, excluded from the assessment cases, although their consequences are screened by the use of probabilistic analyses.

Fig. 3.7-3: The hierarchy of scenarios, conceptualisations and parameter sets

"What if?" cases

A "what if?" case is an assessment case set up to test the robustness of the disposal system. In a

"what if?" case, a particular set of assumptions and / or parameter values is adopted which lies outside the range of possibilities supported by scientific evidence. In order to limit the number of such cases, they are restricted to those that test the effect of perturbations to key properties of the pillars of safety. It is not the aim to derive a comprehensive list of all conceivable "what if?"

cases, but rather to select and analyse a few typical cases in order to illustrate system behaviour under extreme conditions. A limited number of probabilistic sensitivity analyses are carried out in which "what if?" assumptions and / or parameter values are assumed, with other parameters being varied within the range of possibilities supported by scientific evidence.

Analysing the assessment cases

A chain of computer codes (the reference model chain) is used to analyse the Reference Case and most of the alternative cases, as well as to perform probabilistic analyses. Parameter values required by the codes are assigned reference values in the Reference Case, and alternative

Different scenarios to

illustrate scenario uncertainty Different conceptualisations of a scenario, illustrating

conceptual uncertainty

Different parameter sets to illustrate parameter uncertainty

for one conceptualisation Reference parameter set Alternative parameter set Alternative parameter set Alternative parameter set Reference conceptualisation

Alternative conceptualisation

Alternative conceptualisation Alternative conceptualisation Reference

scenario Alternative

scenario

Alternative

scenario Alternative scenario

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(generally more pessimistic) values in the alternative cases. Parameters that are considered in probabilistic analyses are also assigned probability density functions (PDFs). In these analyses, large numbers of calculations are performed using parameter values sampled at random from the PDFs. The reference and alternative parameter values and the PDFs are presented in Appendix 2. All of the models, codes and data used in the present study in order to analyse the assessment cases and perform probabilistic analyses are described in detail in Nagra (2002c).

For a few cases, alternative conceptualisations are considered that do not fall within the scope of the reference model chain, and alternative codes or analytical solutions are employed. Further-more, in order to examine particular aspects of Reference Case system performance in more detail, and to investigate sensitivity to key system properties, results using the reference model chain are complemented by the results of simplified models that focus on a limited number of FEPs. These are termed "insight models", and are also described in Nagra (2002c).

In order to keep the amount of information to be processed to a manageable size, all release and dose calculations are limited to safety-relevant radionuclides, i.e. radionuclides which have the potential to make a significant contribution to overall radiological releases to the environment.

These are identified by carrying out simplified release and consequence calculations which are described and documented in Nagra (2002c).