Advances in Accident Tolerant Fuel Research by Doping of Uranium Nitride

Licentiate thesis, 2020

Nuclear energy is a carbon-free energy source alternative 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 assessed before it can be considered completely safe. The loss of cooling systems during the nuclear accident at Fukushima nuclear accident exposed the flaws of the fuel used today, UO2 encapsulated in a Zr alloy. Research into new types of improved fuels, also known as Accident Tolerant Fuels (ATF), has therefore became of great importance. Different alternative claddings and fuel materials have been explored in recent years. Amongst these, uranium nitride (UN) has shown to have very attractive thermomechanical properties. Nonetheless, UN reacts readily in oxidizing environments, making it undesirable for water-cooled reactors.

In this study, 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 (SPS). It was seen that the spherical shape was lost during sintering and densities between 77 and 98% of theoretical density were obtained, depending on the sintering parameters. For example, sintering at 1650 °C and 75 MPa for at least 5 minutes proved to produce pellets with densities close to 95% of theoretical densities, which are similar to densities used today in nuclear reactors.

Thorium and chromium were introduced as additives to form a protective oxide scale and improve the oxidation resistance of UN. It was seen that Th produced a homogeneous solid solution with uranium nitride between 0 and 20 % thorium molar metal ratio. Chromium, on the other hand, showed that there was a solubility limit in UN, and precipitation of a Cr-rich phase was observed. During exposure in air, the doped materials seem to reduce the oxidation kinetics, increasing onset temperatures and decreasing the reaction rates. Pellet exposure to water at high temperatures showed that 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.