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The major concern in nuclear waste management is its safe disposal. Models are used to predict the long-term fate and mobility of radionuclides and other contaminants through the environment. Information from these models is used to assess the long-term safety of a waste disposal site. To accurately predict the fate of contaminants these models must contain all the parameters that affect contaminant transport through the geosphere. Colloids can be found in all natural surface and ground waters and have large surface areas onto which contaminants can sorb. They have been identified as being potentially important in the transport of radionuclides in groundwater and could significantly affect any performance assessment of a repository. Colloid mediated transport is generally not included in the predictive models although their influence could be significant. This report describes firstly the results of a groundwater sampling campaign undertaken at the Drigg site in Cumbria which focussed on the colloidal content of the waters and their associated radionuclides.

The work performed and presented in this thesis forms a small part of a large, ongoing body of work currently being performed in France by the CEA in preparation of a dossier outlining possible strategies and processes for the reduction of the volume and radio-toxicity of high level nuclear wastes. In particular, this work has focussed on some elements and issues surrounding the option to transmute or incinerate high level nuclear waste pins in dedicated moderated target assemblies within an existing technology fast spectrum reactor. This approach enables the use of high fast reactor flux levels and high thermal neutron crosssections to optimise reaction rates within the target. In order to give some insight into the physics of llS4C, ZrH2 and CaH2 moderator materials within such an environment and provide initial validation of the deterministic fast reactor neutronics code, ERANOS, the COSMO series of moderated sub-assembly core experiments were performed in the MASURCA test reactor at CEA, Cadarache, France. This thesis has focussed on a calculational analysis of elements of the COSMO-I e IS4C moderator) and COSMO-3 (ZrH2 and CaH2 moderator) experiments which both contained moderated target sub-assemblies positioned at the centres of the simulated LMFR MASURCA core. It is important to note that the COSMO experiments did not contain any americium and as such were designed solely as a means to provide information on the physics of localised moderator materials within such a core and the development of appropriate calculational methodologies in support of the forthcoming ECRIX moderated target experiment in the PHENIX fast reactor. France. The work has demonstrated that ERANOS is suitable for calculations involving IlS4C moderators positioned at the centre of a fast reactor core but that a modification to the cross-section library generation routine will be required for cores containing ZrHz moderators to account for both the thermal incoherent inelastic and incoherent elastic scattering components at low energy. The same is not true for the case of a CaH2 moderator and existing scattering data based on hydrogen in water was found to be adequate. Contributions to discrepancies found to be due to 'ray' effects in the MASURCA geometry suggest that maximum detail is required in modelling such cores within and around the moderated assembly region.

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In recent years several models have been developed that have been used to predict the partitioning of a solute (metal or organic ligand) between water and a geological solid. These predictions are necessary for calculations of risk assessment associated with a deep underground nuclear waste repository. This thesis includes a review of international literature related to the reactions of metallic radionuclides with complexing organic ligands that may be present in and around a repository and the effect of these complexants on the sorption of radionuclides onto solids. The complexants include decontamination agents (EDTA, NTA and picolinic acid), degradation products of cellulose (ISA and gluconic acid), and humic and fulvic acids (HA and FA respectively) that may be present in the natural environment. The experimental work presented in this thesis uses a batch method to produce a comprehensive dataset of distribution ratios for the sorption of trace radioactive metals (Eu3+, Ni2+, Cd2+, Fe2+, and Cs+) in the presence, and absence, of organic complexants onto solid mineral phases (montmorillonite, kaolinite and goethite).