Matrices for Isolation of Actinide Wastes in a Deep Well Repository

Radioactive nuclear waste containing long-lived actinides (Np, Pu, Am, and Cm) is proposed to be placed in well repositories with a depth of up to 5 km. The optimum form of such wastes is crystalline phases that are capacious in relation to radionuclides and stable in mineralized groundwater (brines) heated due to heat generation in the waste and due to the geothermal gradient. In order to find possible phases, we have studied samples of the Nd–Ti–Zr–O system, where Nd³⁺ acts as an imitator of the rare-earth-actinide fraction of highly radioactive wastes of reprocessed nuclear fuel. The samples were obtained by induction melting in a cold crucible with subsequent melt crystallization. It has been concluded that the Nd_2–x(Ti,Zr)₂O_7–1.5х and Nd₄Ti₉O₂₄ phases are promising as potential matrices for these wastes. The structure of NdO_1.5–TiO₂–ZrO₂ system at a high temperature has been revealed.