Reactive sintering of the thorium phosphate-diphosphate. Study of physical, thermal, and thermomechanical properties and chemical durability during leaching tests

Taking into account some attractive properties of the thorium phosphate-diphosphate (Th 4 (PO 4 ) 4 P 2 O 7 ), called TPD, we focused our research on its potential use as a ceramic for the long term storage of some radionuclides including trivalent and tetravalent actinides. In this objective, we already investigated with success several ways of preparation of the pure TPD and solid solutions with tetravalent actinides either by wet or dry chemical methods. In the field of nuclear applications and considering handling operations on the radioactive materials, the preparation of this compound in the ceramic form was developed. Therefore, a precursor of TPD was especially synthesized in order to get sintered samples after performing an uniaxial pressing at room temperature followed by a heat treatment at high temperature. The synthesis, the characterization, and the sintering procedure of the precursor are described in this paper. The influence of the heating temperature on the specific area of the powder is examined and then correlated with the granulometry and the densification of the solid. For all of the samples, the apparent and effective relative densities obtained are in the range of 87-99% of the value calculated from XRD data. Moreover, some physical properties such as thermal diffusivity, thermal conductivity, and specific heat were measured from room temperature up to 1000 °C. The main results are given in this paper. Finally, the durability of the sintered TPD samples during leaching tests was studied. The apparent dissolution rates remain low even in very corrosive media (1.25(2) x 10 -3 g d -1 in 5 M HNO 3 at 25 °C) which shows that the TPD can be considered as a promising candidate for the immobilization of actinides and some fission products for a long-term storage.