Fixation of radioactive chemical sludge in a matrix containing cement and additives

The radioactive chemical sludge stored in a clariflocculator (CF) at CWMF has to be fixed in cement before disposal in Near Surface Disposal Facilities (NSDF). The gross β–γ and ¹³⁷Cs content associated with the sludge were typically of the order of 10³–10⁴ Bq/g while the α content was about 10² Bq/g. The total solid content of the sludge varied in the range 3–13% by weight. Experiments were carried out to optimize the ratio of sludge to cement, study the effect of additives like vermiculite and bentonite and characterize the resulting cement blocks with respect to leaching and compressive strength. Results indicated that the blocks obtained on mixing the sludge only with cement in the ratio 1.0:1.5 by weight showed good compressive strength but poor leaching characteristics in tap water. Addition of bentonite or vermiculite (5% of weight of cement) reduced the compressive strength but improved the leaching behaviour. The effects were subdued in case of bentonite but were very significant in case of vermiculite. The effect of addition of both bentonite and vermiculite (each 5% of weight of the cement) was comparable to that of vermiculite alone and hence, addition of only vermiculite to cement was recommended in order to keep the leaching minimum.