Owing to its high uranium density and good thermophysical properties,

uranium nitride (UN) fuel has been considered as a potential Accident Tolerant

Fuel (ATF) candidate for use in Light Water Reactors (LWRs). However,

the main disadvantage of UN is its low oxidation resistance in water/steam

containing atmospheres at the operating temperatures of LWRs.

The main objective of this thesis is to investigate a concept of engineered

microstructure composites as means of improving the response of UN to waterside

corrosion. The methodology for incorporating the corrosion resistant

additives in the form of metals, nitrides and oxides into the UN matrix has

been developed and tested. The additives were proposed to produce coated

(no interaction with UN) or doped (incorporation of the additive into the

UN bulk) grains, which will be able to shield the UN from the oxidising environment

and slow down the oxygen diffusion through the bulk. The UN

composite pellets containing the selected additives were sintered using the

Spark Plasma Sintering (SPS) technique. The resulting microstructures of

the composite pellets were well characterised prior to subjecting some of the

engineered microstructure representative samples to oxidation testing in air

and steam containing environments.

The obtained results indicate that the response to air and steam oxidation

of the composite samples differs from that of pure UN. Moreover, a delay in

the oxidation onset was observed for the composite samples UN-20CrN^premix

and UN-20ZrN^premix in steam and for UN-20CrN^premix pellet in air. The

improved response to oxidation was accompanied by the formation of the

ternary oxides, an observation that could be applied to the screening process

of the additive candidates for waterproofing of UN.

The oxidation behaviour of the composite UN-AlN, UN-Cr 2 N/CrN and UN-AlN-Cr 2 N/CrN pellets in air and anoxic steam under thermal transient conditions was investigated and compared with the pure UN pellet. The composite pellets were manufactured to contain the engineered microstructure of coated (the addition of matrix-insoluble AlN) and doped (the addition of matrix-soluble Cr 2 N/CrN) grains. The composite powders were produced by powder metallurgy and sintered into pellets using the SPS method. Sintered composite pellets were subjected to a thermal transient up to 1273 K in an STA-EGA (TGA-DSC-Gas-MS) system, followed by crystallographic characterization by XRD and morphological and elemental analysis by FEG-SEM. Improved oxidation behaviour in air compared to pure UN was demonstrated by the UN-Cr 2 N/CrN composite pellet. The formation of the ternary oxide UCrO 4 from the ternary (U 2 Cr)N 3 phase (doped grain) was observed, consistent with the delayed oxidation onset and slower reaction rates. In an anoxic steam environment UN-Cr 2 N/CrN exhibited a higher onset oxidation temperature relative to UN, although the reaction progressed faster than for UN sample. Composite UN-AlN pellet oxidised at a lower temperature in both air and steam, compared to pure UN, due to internal stresses in the fuel matrix. A mechanism for degradation of the composite materials is proposed and the influence of the individual phases on the oxidation behaviour of the composites is discussed.

In this study the composite UN-AlN, UN-Cr, UN-CrN and UN-AlN-CrN pellets were fabricated, and the advanced microstructure with different modes of interaction between the phases was obtained. The dopants for this study were selected based on the results of the ab-initio modeling calculations, that identified the AlN phase as insoluble and CrN and Cr as soluble in the UN matrix. This method allowed to investigate the possibility of improving the corrosion resistance of UN by protecting the grain boundaries with insoluble AlN and by hindering the diffusion of oxygen through the bulk by adding soluble CrN and Cr. The UN powder was produced by hydriding-nitriding method and mixed with the AlN, CrN and Cr powders. High density (>90 %TD) composite pellets were sintered by Spark Plasma Sintering (SPS). The microstructure of the pellets was analysed using SEM coupled with EDS. The phase purity was determined by XRD. For the first time the presence of the ternary U2CrN3 phase was observed in the composite pellets containing Cr and CrN dopants. The results obtained in this study allowed to assess the methodology for fabrication of the UN composites with controlled microstructure.

In the present study, physical properties of the ternary phase U2CrN3 are evaluated experimentally and by modeling methods. High density pellets containing the ternary phase were prepared by spark plasma sintering (SPS). The microstructural and crystallographic analyses of the composite pellets were performed using scanning electron microscopy (SEM), standardised energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). Evaluation of the mechanical properties was performed by nanoindentation test. The impact of temperature on lattice properties was evaluated using high temperature X-ray diffraction (XRD) coupled with modeling. Progressive change in the lattice parameters was obtained from room temperature (RT) to 673 K, and the result was used to calculate average linear thermal expansion coefficients, as well as an input for the density functional theory (DFT) modeling to reassess the degradation of the mechanical properties. The ab-initio calculation provides an initial assessment of electronic configuration of this ternary phase in a direct comparison with one of UN phase. For this goal, modeling was also employed to evaluate point defect formation energies and electronic charge distribution in the ternary phase. Results indicate that the U2CrN3 phase has similar mechanical properties to UN (Young's, bulk, shear moduli, hardness). No preferential crystallographic orientation was observed in the composite pellet. However, charge electron density distribution highlights the significant directionality of chemical bonds, which is in agreement with the anisotropy and non-linear behaviour of the obtained thermal expansion (α¯(aa) = 9.12 × 10−6/K, α¯(ab) = 5.81 × 10−6/K and α¯(ac) = 6.08 × 10−6/K). As a consequence, uranium was found to be more strongly bound in the ternary structure which may delay diffusion and vacancy formation, promising an acceptable performance as nuclear fuel.

Thermophysical properties and oxidation behaviour of the composite pellet UN-20vol%ZrN were investigated experimentally and compared with the behaviour of the pure UN pellet. A compound of one phase, a solid solution of the average composition U_0.8Zr_0.2N, was obtained by Spark Plasma Sintering (SPS) of the powders UN and ZrN. Crystallographic and microstructural characterisation of the composite was performed using Scanning Electron Microscopy (SEM), standardised Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). Nano hardness and Young’s modulus were also measured by the nanoindentation method. High Temperature X-ray diffraction (XRD) was applied to obtain the lattice expansion as a function of temperature (room temperature to 673 K). Thermogravimetric Analysis (TGA) was applied to evaluate oxidation behaviour in air. Results demonstrate that the fabrication method results in a matrix of solid solution with homogeneous composition averaged to U_0.8Zr_0.2N. The mechanical properties of such solution are uniform, with variation only due to the crystallographic orientation of the grains of the solution phase, similarly to pure UN. The obtained value for the average linear thermal expansion coefficient is  = 7.94*10^-6/K, which compares well to UN ( = 7.95*10^-6/K) for the same temperature range. The degradation behaviour of the composite pellet UN-20vol%ZrN in air shows a lower oxidation onset temperature, compared to pure UN, with the final product of oxidation being mainly U₃O₈. Smaller crystallites in the product of corrosion of the composite pellet indicate that the mechanism of degradation of the solid solution phase U_0.8Zr_0.2N is accompanied by the formation of two distinct oxides and their interaction.

Thermophysical properties and oxidation behaviour of the composite pellet UN–20 vol%ZrN were investigated experimentally and compared with the behaviour of the pure UN pellet. A compound of a single phase, a solid solution of the average composition U0.8Zr0.2N, was obtained by Spark Plasma Sintering (SPS) of the powders UN and ZrN. Crystallographic and microstructural characterisation of the composite was performed using Scanning Electron Microscopy (SEM), standardised Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). Nano hardness and Young's modulus were also measured by the nanoindentation method. High-Temperature X-ray diffraction (XRD) was applied to obtain the lattice expansion as a function of temperature (room temperature to 673 K). Thermogravimetric Analysis (TGA) was applied to evaluate oxidation behaviour in air. Results demonstrate that the fabrication method results in a matrix of solid solution with homogeneous composition averaged to U0.8Zr0.2N. The mechanical properties of such solution are uniform, with variation only due to the crystallographic orientation of the grains of the solution phase, similar to pure UN. The obtained value for the average linear thermal expansion coefficient is α¯ = 7.94 × 10-6/K, which compares well to UN (α¯ = 7.95 × 10-6/K) for the same temperature range. The degradation behaviour of the composite pellet UN-20 vol%ZrN in air shows a lower oxidation onset temperature, compared to pure UN, with the final product of oxidation being mainly U3O8. Smaller crystallites in the product of corrosion of the composite pellet indicate that the mechanism of degradation of the solid solution phase U0.8Zr0.2N is accompanied by the formation of two distinct oxides and their interaction.

The use of U15N fuel in water cooled reactors permits to increase the fuel average residence time by 40-60%1,2. This corresponds to a reduced cost for manufacture of fuel assemblies and waste packages, as well as a reduced cost for refuelling. The industrial application of UN fuels requires to improve the resistance of the fuel to steam corrosion and to develop methods for manufacture that are commercially competitive. In this contribution we describe the activities carried out at LeadCold, KTH and Promation Nuclear towards these goals.