Accident-tolerant uranium nitride

Licentiate thesis, 2017

After the nuclear accident at Fukushima-Daiichi in 2011, a search for alternatives to a current fuel-clad standard UO2-Zircaloy was encouraged by government and industry and novel fuel-clad systems were selected. These systems are often referred to as Accident Tolerant systems, because of an improved response to severe accidents. In addition, a better economy of operation of Light Water Reactors is also a motivation to change the UO2-Zircaloy for another fuel-clad type. UN is one possible accident tolerant fuel, mostly due to its high thermal conductivity, and higher fissile atom density compared to UO2. However, UN is not stable in hot water/steam, where it hydrolyzes and converts into oxide. Such processes can be described as corrosion mechanisms. In this research, the possibility to improve the corrosion resistivity of UN fuel was investigated, by doping the UN matrix with a protective component e.g. chromium (III) oxide, aluminum (III) oxide, nickel (II) oxide or thorium (IV) oxide was investigated. All UN based materials were produced using a internal sol-gel technique followed by a carbothermic reduction. Due to the ease of preparing aqueous solutions of metal nitrates and their intrinsic advantages to form homogeneous solid products compared to powder processes. In the first series of experiments, carbon nano powder was used as a carbon source, in the second series glucose was used instead and urea was omitted from the internal sol-gel process. A simple dissolution test on pellets made of UN doped materials was conducted in order to identify, if any of the dopants have an effect on the corrosion resistivity. Results showed that a pellet sintered from nitrided microspheres doped with chromium of 40% TD did not disintegrate in water after 5 hours of boiling at normal pressure. A reference pellet of pure UN and 75.6% TD dissolved after 2 hours in boiling water.