Surface alteration and chemical durability of hollandite (Cr, Al and Ti) consolidated by spark plasma sintering in acid solution

For the development of pyrochemical reprocessing of spent fuels in which a substantial amount of Cs-containing waste is confronted, appropriate management of the long-lived radioactive Cs is required. In the present study, solid state reaction and Spark plasma sintering (SPS) were applied to prepare three types of dense hollandite samples as advanced waste form matrix including Ba_1.23Al_2.46Ti_5.54O₁₆, Ba_1.15Cr_2.3Ti_5.7O₁₆ and Ba_1.15Ti_2.3Ti_5.7O₁₆ with a relative density higher than 95%. The thermogravimetric analysis (TGA) results show that the three hollandite dense pellets prepared by SPS have good thermal stability and high temperature oxidation resistance, which can be used as potential waste forms for 135Cs. Ba_1.15Cr_2.3Ti_5.7O₁₆ demonstrates the best Ba immobilization among three pellets due to its best chemical durability based on a 240-h leaching test, with a cumulative release amount of 0.62 mmol/m2 for Ba at semi-dynamic leaching (pH = 2). The leaching rates of elements decrease continuously with time, and finally reach a dynamic equilibrium as the long-term leaching rates tend to be constant. It is because the dominant leaching mechanism switches from initial rapid dissolution and surface effects to the long-term diffusion. The Ba_1.15Cr_2.3Ti_5.7O₁₆ sample surface was corroded after 14 days of static leaching in HCl solution with pH = 2, with corrosion pits found on the grain surface of specific crystal orientations, which demonstrates that different crystal orientations have different corrosion resistance. Ba_1.15Cr_2.3Ti_5.7O₁₆ displays good thermal stability and high corrosion resistance, and thus could be used as a potential nuclear waste form for the immobilization of the long-lived and highly mobile Cs.