A Carbon Composite Based on Pyrolyzed Diphthalocyanines for Immobilization of High-Level Waste from Nuclear Industry

A new procedure for immobilization of high-level waste from spent nuclear fuel (SNF) of nuclear power plants (NPP) is suggested. The HLW immobilization process consists in the radionuclide transfer into the form of acetates, followed by synthesis of their diphthalocyanines and their subsequent pyrolysis in an inert medium. Real raffinate solutions from reprocessing of SNF of Novovoronezh NPP were used as HLW. The efficiency of the immobilization of radionuclides (mainly REE and minor actinides) was found to be higher than 99%. The thermal stability, chemical durability (leaching with aqueous solutions with pH from 2 to 9), and radiation resistance of the samples were evaluated using alpha-, beta-, and gamma-ray spectrometry. The evolution of the structure of the pyrolyzed diphthalocyanines in the temperature range 800-1600 degrees C was studied by small-angle neutron scattering, X-ray diffraction analysis, and atomic force microscopy. The results obtained have confirmed the hypothesis that the pyrolysis of diphthalocyanines yields a branched network of carbon atom nanoclustes of size from 5 to similar to 150 nm, ensuring efficient retention of radionuclides in the host material. The host material is comparable in the thermal stability, i.e., in the degree of radionuclide retention at 1000-1200 degrees C, with borosilicate or phosphate glasses, which are the most widely used host materials for radioactive waste immobilization, and considerably surpasses them in the resistance to leaching and radiation.