PNNL

Abstract

The immobilization of used nuclear fuel (UNF) may be desirable for storage and permanent disposal. Ceramics are viable candidates for immobilization for an entire UNF assembly, as ceramic phases such as pyrochlore and fluorite incorporate target elements (i.e., U, Pu). In this work, titanate and zirconate ceramics were formulated to account for light water reactor UNF compositions. They were fabricated using Ce and Gd as analogues for U and the other actinides in nonradioactive formulations and U or U/Pu for radioactive formulations using similar processing conditions. Ceramics were characterized with powder X-ray diffraction, microscopy techniques (e.g., SEM-EDS, EBSD), and X-ray absorption near edge structure spectroscopy. For nonradioactive titanate ceramics, perovskite, rutile, and zirconolite were detected when Ce was used as an analogue, and pyrochlore and zirconolite were formed when Gd was used. For the radioactive titanate ceramics, pyrochlore, perovskite, and fluorite (including UO2) phases formed. Only pyrochlore was formed for zirconates using Gd analogues but required high temperatures and long dwell times to produce. When Si was added as a sintering aid to lower temperatures and dwell times, fluorite and apatite phases formed on Gd zirconates. Fluorite, perovskite, and pyrochlore phases were observed in a U based zirconate using Si as a sintering aid. Overall, the nonradioactive ceramics were more consolidated than the radioactive ceramics; future work should focus on improving processing conditions for radioactive formulations.