Additive Effect of Cation Exchanger on the Leaching of ¹³⁷Cs from Cement-solidified Cs-enriched Waste

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For the final safe disposal of ¹³⁷Cs-contaminated wastes generated by the Fukushima Daiichi nuclear accident, ¹³⁷Cs-enriched copper ferrocyanide produced during waste decontamination was calcined and subsequently solidified with ordinary Portland cement and natural inorganic cation exchangers. The addition of cation exchangers to a mixture of cement and copper ferrocyanide calcined at 550 °C effectively suppressed the leaching of ¹³⁷Cs from the hardened solid into both deionized water and seawater. The addition of natural mordenite, with an exchange capacity 1.6 times that of the Cs content in the calcined material, prevented 86% of Cs leaching into deionized water and reduced the leaching rate into seawater to 1/160–1/200. Analysis of the suppression effect with a reaction–diffusion equation in conjunction with ion-exchange theory revealed that the addition of a cation exchanger with high exchange capacity and Cs selectivity is very efficient for suppressing the leaching.