Cements in radioactive waste disposal: some mineralogical considerations

Cementitious matrices are being assessed for immobilization of radioactive wastes. This paper dis- cusses some mineralogical aspects of cement chemistry and the uses of siliceous minerals as selective sorbants to enhance immobilization potential. Studies of sorption and leaching of caesium from pulverized fuel ash (PFA), blast furnace slag, tobermorite, xonotlite, and clinoptilolite are reported. The role of incorporation of these additives in cement and the effect on the nature of the composite matrix on caesium behaviour has been investigated. Specific mechanisms of the interaction of additives with highly alkaline cement environment are described. While slags, PFA, and clinoptilolite undergo reaction at different rates, tobermorite and xonotlite appear to be stable in cement. SPENT fuel elements may undergo a lengthy and complex reprocessing cycle before disposal of the resulting waste streams can be accomplished; moreover, storage, transportation, and reprocess- ing operations give rise to other waste streams. The bulk of these are non-heat generating. The esti- mated United Kingdom arisings from all sources have been estimated to the year 2000 and classified according to volume and radioactivity (Duncan and Brown, 1982). It is apparent that cement and concrete will find much application in waste disposal, mainly of non-heat generating wastes. We exclude from con- sideration its structural applications, e.g. in silos, tanks, bunkers, tunnel linings, and concentrate instead on non-structural applications, e.g. as a primary matrix for immobilization of wastes or in secondary barriers such as grouts or sealants. Cements are tolerant of water, resistant to the action of certain salts, have a long history of use and lend themselves to remote handling. Moreover, they are cheap. However, they may constitute a chemically reactive, microporous matrix and the interactions between cements and waste form, or cements and their environment, or both, have many features