Fabrication method for UO2 pellets with large grains or a single grain by sintering in air

Preparation, sintering and leaching of optimized uranium thorium dioxides

“…Nevertheless, the use of wet chemistry methods for the preparation of (U,Th) mixed dioxides allows to lower significantly the sintering temperature. Indeed, the preparation of highly densified actinide dioxide pellets is generally reported between 1873 and 1973 K in the literature [17] while lower temperatures are only mentioned for the use of an oxidative atmosphere [18,19].…”

Mechanical Behavior of UO₂ at Sub-grain Length Scales: Quantification of Elastic, Plastic and Creep Properties via Microscale Testing

“…In this case, it was known that UO 2 heat treated under very high oxygen partial pressures (pO 2 ~0.2 atm) at high temperatures (up to 1700 °C) showed very rapid grain growth [2], but the temperatures required surpassed the limits of the initial setup available and was also shown to cause volatilization of uranium oxide [2], which increases the possibility of radioactive contamination. Furthermore, the grain sizes required to machine micro-cantilever beams was not as large as those reported in [2] and the presence of abnormal grain growth reported by these authors was initially thought to be a problem for this work, as many grains with different orientations were needed to obtain reliable statistics. On the other hand, Mansouri and Olander [3] reported significant grain growth in hyper-stoichiometric UO 2 heat-treated at 1200 °C after being irradiated to a burnup of ~10 -8 fraction of initial metal atom (FIMA).…”

“…Based on the kinetic equations provided in [3] a 5-hour heat treatment at 1250 °C would have resulted in grain growth from an initial grain size of ~8 µm to a final grain size of ~10 µm for an oxygen to metal ratio (O/M) of 2.2. Hence, heat treatments at temperatures lower than those used by [2] were performed to study grain growth kinetics under those conditions and evaluate their possible use in specimen production for the purposes of this work. Unfortunately, readings from the sample thermocouple used during the experiments were interrupted during the heating ramp due to thermocouple failure; subsequent experiments revealed actual sample temperatures being closer to 1100 °C.…”

“…This incident demonstrated the risk of extended intervals at elevated temperature, so the system was redesigned during quarter 8 to allow the use of atmospheres with greater oxygen content (up to ~10 -1 atm) [2]. Higher soak temperatures (>1530 °C) would result in shorter hold times (1-2 hours) to minimize potential equipment issues.…”

“…To fabricate an adequate supply of suitable specimens and meet the needs of project deliverables, arrangements were made with Areva of North America to procure high quality production UO 2 powder from each for the purposes of pellet fabrication. These powders were used to press green pellet compacts, ~6 mm in diameter and ~5 mm tall, which were then sintered under oxidative atmosphere to grow multi-and single-crystal samples based on [2]. Feedstock powders were used as-received to gather baseline data; later pellets were pressed with milled powder.…”