Fabrication and Testing of Doped Uranium Nitride as an Accident Tolerant Fuel Alternative

Doctoral thesis, 2023

Nuclear energy is a carbon-free energy source often considered less harmful to the environment than fossil fuels. However, accidents have shown that there are some safety concerns regarding nuclear energy that need to be continuously assessed and further improved. Research into new types of improved fuels, also known as Accident Tolerant Fuels, has therefore become of great importance. Different alternative claddings and fuel materials have been explored in recent years. Amongst these fuels, uranium nitride (UN) has very attractive thermomechanical properties. Nonetheless, UN is easily oxidized in thepresence of air or water, making it undesirable for water-cooled reactors.

In this thesis, UN microspheres were manufactured through a sol-gel method, followed by carbothermic reduction and nitridation. The as-produced microspheres were pressed and sintered into pellets using spark plasma sintering. Thorium, chromium, and aluminum were studied as additives to improve the oxidation resistance of UN. It was observed that Th produced a homogeneous solid solution with UN between 0 % and 20 mol-% thorium metal content. Chromium showed that there was a solubility limit in the UN. Depending on the synthesis conditions, the resulting material can be manufactured to either contain a ternary phase (U2CrN3) or metallic chromium. No solubility of aluminum nitride was detected in the UN matrix.

Doping with Th and Cr proved to be efficient in improving the oxidation in air, by increasing the onset oxidation temperatures and decreasing the reaction rates of the pellets. In most cases, the high porosity of the microspheres counterbalanced any protective effect caused by the dopant. Aluminum-containing samples showed the worst oxidation resistance in air due to poor solubility of AlN in the UN. Steam interaction of Cr-doped pellets also showed a delay in the hydrolysis of the UN when Cr is present. The last exposure environment was water, and it was shown that undoped UN pellets can survive at 100 °C and 1 bar pressure with zero mass change. However, at higher temperatures and pressures, 200 °C and 15 bar or 300 °C and 85 bar, pellet disintegration into a UO2 powder was observed. An incomplete reaction was also observed for the Th-doped pellet in the exposure test at 200 °C, indicating that no improvement in the corrosion resistance of UN in water was achieved by doping with thorium. On the other hand, Cr-doped pellets exposed to water at 200 and 300 °C showed partial crumbling. The resulting material was unreacted UN with some UO2 byproduct.