Local atomic structure of uranium ions and dissolution behavior of iron phosphate glass hosts to immobilize spent nuclear fuel

Abstract

Iron phosphate glasses with melting temperatures of ~ 1300 °C were developed to immobilize spent nuclear fuels. The glasses have densities of ~ 3.4 g/cm3 and glass transition temperatures of ~ 555 °C that are high enough to endure the temperatures in geological repositories. The waste loading of UO2 in the glass was ~ 33.73 wt%. Normalized elemental releases from the product consistency test were well below the regulated limit of 2 g/m2. Most of the U in the glass is in the 4 + state, which is more chemically durable than the 6 + state. UO8 polyhedra in the glass with five oxygens at the distance of 2.25 A and three at 2.87 A were formed. U atoms are connected to PO4 tetrahedra that form the phosphate glass network. Chemically-durable crystals (uranium pyrophosphate, UP2O7) were formed during the dissolution.