We have a long established interest in the materials used for the immobilisation of nuclear waste from the domestic power industry and legacy waste from various defence programmes. We are currently investigating the site substitution preferences for various surrogate waste cations in beta-tricalcium phosphate, using a range of techniques, in order to understand the limits on loading of nuclear waste in this host. In the case of Al3+ substitution, the evidence from XRD and NMR is that there is considerable disorder in the material which is a function of substitution level and heat treatment. This is a consequence of (a) incompatibility of the small Al3+ ion with the existing cation sites and (b) the creation of cation vacancies. We wish to perform total scattering experiments to determine the nature and distribution of the defects, particularly the Al environment.