Thermal treatment of Cs-exchanged chabazite by hot isostatic pressing to support decommissioning of Fukushima Daiichi Nuclear Power Plant

Abstract

Ion exchange materials are used widely for the removal of radionuclides from contaminated water at nuclear licensed sites, during normal operating procedures, decommissioning and in accident clean-up, such as the ongoing recovery operation at the Fukushima Daiichi nuclear power plant. Framework silicate inorganic ion exchange materials, such as chabazite ((Na_0.14K_1.03Ca_1.00Mg_0.17)[Al_3.36Si_8.53O₂₄]•9.7H₂O), have shown particular selectivity towards ¹³⁷Cs uptake, but their safe storage poses a number challenges requiring conditioning into passively safe waste packages of minimal volume. We demonstrate the transformation of Cs-exchanged chabazite into a glass-ceramic wasteform by hot isostatic pressing to produce a durable consolidated monolith. The application of heat and pressure resulted in the collapse of the chabazite framework, forming crystalline Cs-substituted leucite (Cs_0.15(3)K_0.57(4)Al_0.90(4)Si_2.24(5)O₆) incorporated within a K₂O-CaO-MgO-Al₂O₃-SiO₂ glass. The Cs partitioned preferentially into the Cs/K-feldspar which incorporated ~77% of the Cs₂O inventory. Analysis of the chemical durability of the glass-ceramic wasteform revealed that the Cs release rates were comparable or lower than those reported for vitrified high level and intermediate level wastes. Overall, hot isostatic pressing was demonstrated to be an effective processing technology for conditioning spent inorganic ion exchange materials by yielding durable and passively safe wasteforms.

Keywords: Characterisation; Chemical durability; Hot isostatic pressing; Ion exchange materials; Nuclear waste management.