Behaviour and fate of uranium in a high-level nuclear waste processing system

Abstract

). Heterogeneous crystallization of sodium diuranate (Na 2 U 2 O 7 ) occurs in tandem with sodium aluminosilicate phases within the evaporators used in the waste liquor concentration in the temperature range 30-140°C. Typically, the HLNW liquor comprises high ionic strength (6-10 M) caustic solutions containing sodium hydroxide, silicate, aluminate, nitrite, and nitrate ions, trace amounts (approx. 300 mg·dm -3 ) of radionuclides (i.e., uranium-235, plutonium-238) and radio-toxic species (e.g., caesium-137). The supersaturation of Na ion-paired tetrahydroxo Al(III), SiO 2 , and uranyl complex species in solution (the limiting reactants) created by continual water evaporation, together with the intense heating applied, synergistically act to facilitate the polycondensation and crystallization of SAS and