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 cations in beta-tricalcium phosphate in order to understand the limits on loading of nuclear waste in this host. In the case of Zn substitution (Ca3-xZnx(PO4)2), the evidence from XRD is that the unit cell contracts as Zn replaces Ca up to x = 0.3, as might be expected from the significantly smaller radius of the Zn ion compared with Ca. However, further increase in x produces an increase in the c-axis, whilst the a-axis remains unchanged. The existing model for substitution cannot explain the observed behaviour. We wish to perform neutron total scattering experiments to determine the detailed structural changes in Ca3-xZnx(PO4)2.