Inorganic sulfides as radiolytic hydrogen production inhibitors

The auto-irradiation of nuclear wastes containing organic compounds produces radiolysis hydrogen which presents a potential risk. In the case of bituminous radioactive sludges packages, it has been shown that the presence of cobalt sulfide in the waste limits the hydrogen production. The aim of this work is then to define the structure of the cobalt compound which induces this phenomenon and to understand how it inhibits the hydrogen formation. Different cobalt compounds, crystallized or amorphous, have been synthesized and characterized. The study of their behaviour under irradiation (60Co) in presence of standard organic molecules has been carried out. In the same time, the gas-solid reaction of these solids with hydrogen has been followed too. In order to elucidate the solid-gas interaction mechanism, the solids have been characterized before and after reaction, by different analytical techniques (IR, elementary analysis, XRD, TGA, XPS, EXAFS, TEM, neutron inelastic diffusion) and the solid and gaseous phases have been analyzed by chromatography and mass spectroscopy. Only the cobalt hydroxy-sulfide, amorphous hydrated precipitate of a structure near those of the CoSOH.0.3H2O, is efficient to inhibit the hydrogen production. It acts as a hydrogen track with an equivalent consumption limit of about 0.5 molH2/molCo. With the results of the solids characterizations before and after track (TEM, EXAFS, neutron inelastic diffusion, isotopic exchange), it has been proposed a track mechanism in two successive steps. The first step consists in the opening of the S-S bridges initially present in the solid, to give -SH groups. The second step consists in the irreversible formation of the crystallized phase Co9S8. (O.M.)