Abstract: The invention relates to a method for producing nanoparticles of at least one oxide of a transition metal selected from Ti, Zr, Hf, V, Nb and Ta, which are coated with amorphous carbon, wherein said method includes the following successive steps: (i) a liquid mixture containing as precursors at least one alkoxyde of the transition metal, an alcohol, and an acetic acid relative to the transition metal is prepared and diluted in water in order to form an aqueous solution, the precursors being present in the solution according to a molar ratio such that it prevents or sufficiently limits the formation of a sol so that the aqueous solution can be freeze-dried, and such that the transition metal, the carbon and the oxygen are present in a stoichiometric ratio according to which they are included in the nanoparticles; (ii) the aqueous solution is freeze-dried; (ii) the freeze-dried product obtained during the preceding step is submitted to pyrolysis under vacuum or in an inert atmosphere in order to obtain the nano

Abstract: The invention relates to a confinement matrix for the storage or incineration of at least one long-life radioactive element, comprising at least one crystalline boron compound of a rhombohedral structure including the long-life radioactive element(s). The boron compound may be of the B3Si, B6O or B4C type.

Abstract: The invention relates to a neutron-absorbing material and to its production process.This material comprises a homogeneous, boron carbide matrix 1 in which are dispersed e.g. pseudospherical, 150 to 500 .mu.m , calibrated clusters 3 of boride such as HfB.sub.2, in order to prevent the propagation of cracks F in the material and improve its thermal shock resistance.