The goal of TEX-MOX experiments is to fill the gaps in the criticality safety validation basis in intermediate energy range. 3 experiments were designed to answer this purpose to be executed at NCERC (National Criticality Experiments Research Center) in USA on the Planet vertical lift machine. These experiments were designed to provide a high keff sensitivity to 239Pu and 240Pu in the intermediate energy range, comparable to the real case sensitivities, corresponding to the criticality safety validation needs in French nuclear facilities. The decay heat produced by the mixed uranium-plutonium metallic plates to be used in the experiments challenged the mechanical design that was adopted to handle this issue. Final design of the experiments will be presented in this paper along with the main characteristics as expected uncertainties, keff sensitivities and mechanical design of the cooling system.