Nitride fuel production by the internal sol gel process

Licentiate thesis, 2014

Abstract During operation of a fourth generation reactor system it is necessarily not desirable to utilize an oxide fuel matrix as in light water reactor systems. One type of chemical compounds that has been suggested is nitride based nuclear fuel. Nitride fuels possess several characteristics that make them interesting for reactor operation such as high melting point, high thermal conductivity and better chemical compatibility compared to oxides with liquid metal coolants such as sodium or lead. Nitride fuel can be produced either as pure actinide nitride fuel such as UN or as fuel where the fissile material is diluted in an inert matrix such as (Zr, Pu)N. A problem with nitride fuel is the production process. Nitrides are not stable in air and production must thus be performed under inert atmosphere to reduce oxygen uptake in the material. Also residual oxygen and carbon content from production and processing is an issue that needs to be addressed. Spontaneous oxidation in air together with radiological safety concern regarding handling fine powders of actinide nitrides such as PuN for example provides an incitement for evaluating potential dust free processes for nitride fuel production. Work has been performed on production of ZrN as an inert fuel matrix by the internal sol gel process and subsequent carbothermal reduction. Elemental carbon suspension sols have been produced and gelled into microspheres. Heat treatments of the produced microspheres in N2 + 5 % H2 gas mixture was performed during nitridation. Elemental investigation of the produced microspheres suggested an approximate composition of Zr(N0.7 0.8C0.2 0.3). Pelletization of the nitride microspheres produced pellets with blackberry structure and XRD measurements on sintered material also suggested a material composition of about Zr(N0.8C0.2). Keywords: Nitride, ZrN, internal sol gel, carbothermal reduction, sintering