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 similar to 1300 degrees C were developed to immobilize spent nuclear fuels. The glasses have densities of similar to 3.4 g/cm(3) and glass transition temperatures of similar to 555 degrees C that are high enough to endure the temperatures in geological repositories. The waste loading of UO2 in the glass was similar to 33.73 wt%. Normalized elemental releases from the product consistency test were well below the regulated limit of 2 g/m(2). 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.