Validation of the Vault computer model ''VERMIN'' for the post-closure behaviour of repositories in geological formations

Electrowatt Engineering Services (UK) Limited, under contract to the Department of the Environment, have developed a computer code entitled VERMIN (Vault Exudations Resulting in the Migration of Nuclides). The model simulates the time dependent behavior of radionuclide releases from shallow (LAND2) and deep (LAND3) repository types. It also provides the source term for the probabilistic radiological assessment code SYVAC. Electrowatt's responsibility in the PACOMA project was to validate the computer code VERMIN. Where necessary, development was carried out, within the constraints of VERMIN's role within SYVAC. A validation methodology was proposed and applied. Validation of various nuclide migration processes was achieved by comparison of the model predictions with experimental results. The processes included in the validation exercise were leaching from the waste matrix, diffusive transport and advective transport combined with sorption. Suggestions were made for further experimental studies of processes currently modelled in VERMIN but not yet validated. The results of the validation exercise were presented. VERMIN was used to simulate the behavior of a repository in a clay formation, and thus provide a source term for input to the geosphere model NAMSOL, which was being used within the PACOMA project by AERE Harwell Laboratories. It was decided by the PAGIS/PACOMA Committee to use the radionuclides inventories provided by NRPB and CEA. The inventories represent the intermediate wastes generated by the production of 30 GW of electricity from a Pressurized Water Reactor (PWR). The wastes are assumed to be encapsulated in a cement matrix. As a basis for comparison, it was also decided to use an inventory, developed for the UK Department of the Environment (DoE) in the context of an illustrative probabilistic assessment of a hypothetical deep inland repository in the UK