[en] Objective is to define the nuclear criticality research program needed in support of the HTGR fuel cycle outside reactors. An experimental plan and supportive calculational efforts are presented. Currently, some data on well-moderated ²³⁵U and ²³³U exist which are applicable to thorium--uranium fuel mixtures. However, experimental data are needed for undermoderated ²³⁵U and ²³³U systems. Data are needed also on ²³⁵U and ²³³U containing soluble neutron poisons and in the presence of fixed neutron poisons. For future fuels and processes in which thorium and uranium cannot be separated, there would be a large economic incentive for including the thorium in the criticality considerations. The research program presented covers all aspects of the experiments needed to provide data in the above areas