HIPed Tailored Hollandite Waste Forms for the Immobilization of Radioactive Cs and Sr

Hot isostatically pressed tailored hollandite waste forms were used to demonstrate the immobilization of Cs and Sr are separable from spent nuclear fuel, as well as Ba and Rb. Four hollandite formulations were investigated, two samples with ～12 wt% waste loading (on an oxide basis) and two with ～18 wt% waste loading. Two of the samples were Al-substituted and the other two contained Mg. The hollandite in the Al-substituted samples contained all the waste cations, as designed, but this was not the case in the Mg-substituted samples. The hollandite in the Mg-substituted samples did not contain all the waste cations, with ～50% of the Sr forming SrTiO₃ as a secondary phase. This resulted in waste forms that were not as durable, with respect to Cs, as their Al counterparts. The formation of SrTiO₃ had little effect on the Sr release rates and was not detrimental to the Mg-substituted hollandite waste form. For the Al-substituted samples, the MCC-1 normalized release rates were <0.06 g·(m²·day)^?1 at 0–28 days for all elements, while the Cs release rates remained at 2.0 g·(m²·day)^?1 at 0–28 days for the Mg-substituted samples.