Calculations of the consequences of the geological disposal of solid radioactive waste require models that apply a simplified description of the evolution of a repository. In work carried out to date for United Kingdom Nirex Limited (Nirex), models of repository performance have assumed that the contents of the repository are homogeneous. There is no direct representation in the models, for example, of the fact that all the waste will be contained initially within waste packages, with backfill surrounding the packages.

It is important that the assumption of a homogeneous repository is demonstrated to be cautious (i.e. the calculations of radiological consequence, based on a homogeneous model, should err in the direction of slightly overestimating any release of radionuclides). This paper introduces models that simulate the impact of specific heterogeneities on radionuclide release. These have been constructed and applied, in order to assess the appropriateness of the assumption of homogeneity.

Calculations using the NAMMU computer program have examined the release of radionuclides from within waste packaging. These calculations suggest that recent performance assessments overestimate the release of short-lived radionuclides. Calculations, including the use of the INHOMOG computer program, have examined the relative placement of different categories of waste packages within repository vaults (for the Nirex disposal concept). It is concluded from the calculations that, for a few radionuclides, consideration of the distribution of some waste packages may be appropriate in future performance assessment studies.