Thermal decomposition of fission product nitrates and their reaction with glass batch additives Part A

Thermal decomposition of fission product nitrates constituting a 'Purex' waste and their reaction with silica, alumina and aluminosilicate (Al2O3.2SiO2) have been studied. Oxides of nitrogen may be released from fission product nitrates in the temperature range (150 - 850degC) either due to their thermal decomposition or due to the reaction of alkali nitrates with silica or alumina of the glass batch additives. Fission product nitrates, CaNO3 and RbNO3 excepting, react very slowly with silica, the principal glass forming additive for a silicate glass, at temperatures of 1000degC. Complete reaction of Gr.II-Gr.IV metal nitrates with silica may be achieved at temperatures of 1400degC which is too high for a fission product immobilisation process. The alkali nitrates react very rapidly with boric oxide, silica, alumina and metakaolin (Al2O3.2SiO2) at subsolidus temperatures forming the alkali metaborate, alkali disilicate, alkali monoaluminate and alkali alumino disilicate respectively. A phenomenological description of the heating-up processes for fission product nitrate with silicate glass batch additives has been included. It has been very strongly recommended, that, if a silicate matrix is selected for incorporation of fission products, the glass melting should be carried out until a homogeneous or nearly homogeneous product is obtained and should not be stopped at a stage where a sintered or semivitreous mass is the resultant product. (author)