Alternative Evolution Scenarios for the near surface radioactive waste disposal facility at Dessel, Belgium: Conceptual and mathematical model description and analysis of results.

AES scenarios are a limited number of scenarios that are sufficiently representative and enveloping for not expected but possible evolutions of a disposal system. They should be representative of the potential system behaviour, aiming at illustrating possible evolutions or processes that may occur in the disposal system domain, and that affect repository performance and/or radiological impact. The purpose of AESs is to evaluate the impact and/or the risk for not expected but possible evolutions. The timeframe for the evaluation of alternative evolution scenarios (AES) is, similar to the expected evolution scenario of the expected evolution (EES / RS), limited to 2000 years. In line with international practices, alternative evolution scenarios (AESs) are developed on the basis of a list of initiating FEPs, the combination of which resulted in the development of 10 AES scenarios which are described in Chapter 2. The report describes conceptualisation of the associated models (Chapter 3) and the mathematical description (Chapter 4), strongly building on the expected evolution and reference scenario modelling. Chapter 5 describes the results in terms of annual dose for each AES assessment case, while Chapter 6 evaluates other complementary indicators. Finally, Chapter 7 elucidate the results from the risk point of view, where first probability of an event is evaluated for each AES, which is the basis for risk evaluations. It is shown that the set of AES largely meets the 10-5a-1 risk constraint set by the regulatory body.