Iron Phosphate Glass as Optional Final Waste Matrix for High-Level Radioactive Waste

Abstract

The study investigated experimentally a basic capability of iron phosphate glass (IPG) matrix to vitrify the high level radioactive waste (HLW) in the commercial reprocessing. IPG matrix is a mixture of iron oxide and phosphorus oxide, and Fe/P mole ratio is one of important conditions to form a vitrified product with good chemical and physical properties. Moreover, the corrosion of IPG matrix is more rigorous than that of borosilicate glass matrix, and an appropriate furnace material should be selected for the durability of equipments. The composition of HLW used in experiments was simulated, based on that of HLW generating from commercial reprocessing for a standard spent fuel of light water reactor. Dependence of Fe/P mole ratio on both crystallinity of product and loading of simulated HLW was investigated. At Fe/P mole ratios of 0.43 and 0.33, crystals of Nd and Zr phosphate, appeared in a part of Fe-P matrix zone of vitrified products containing 30wt% of the HLW. The products at Fe/P ratios of 0.18 and 0.27 become a homogeneous glassy state with increasing solubility of crystals of Nd and Zr phosphate. It means that the low Fe/P mole ratio is better to form the vitrified product of high waste loading. Concerned with water resisting property, products vitrified at Fe/P = 0.18,0.23 have larger amount of dissolution rates than that at Fe/P = 0.33,0.43. An appropriate amount of Fe needs to form vitrified product of IPG with good water resisting property, although the homogeneity in micro-structure of the product slightly decreases. Fe/P mole ratio in IPG, 0.3 is an appropriate condition to form vitrified product containing HLW with less dissolution and leaching rate. It is totally concluded that Fe/P mole ratio in IPG, 0.3 is an appropriate condition to form vitrified product containing HLW with less dissolution and leaching rate. This study focused on a special material, ZrB2, which has electric and heat conduction equivalent to iron metal in order to estimate an adoption of furnace material. The corrosion rate of ZrB2 in the melting of IPG matrix was as low as that of Alumina, and there was no rigorous corrosion on the surface of crucible. It was shown that ZrB2 is able to apply a furnace material for IPG vitrification with an induction-heated ceramic melter.