PhD Thesis

Read here: https://kth.diva-portal.org/smash/record.jsf?pid=diva2%3A2060123&dswid=1189

Abstract

Uranium mononitride (UN) is a promising advanced nuclear fuel for Gen-IV fast-spectrum systems and small modular reactors due to its high uranium density and superior thermal conductivity compared to conventional oxide fuels. However, its behaviour under irradiation, particularly the coupled effects of fission product (FP) chemistry and radiation damage on thermophysical and mechanical properties, remains insufficiently understood.

This thesis presents an experimental investigation of the thermophysical, mechanical, and dimensional response of pure and doped UN within a separate-effects testing framework. Simulated burn-up fuels (SIMFUEL) were employed to study the role of chemical disorder, while ion irradiation and implantation were used to introduce controlled radiation damage and selected FP chemistry.

Chemical modification was achieved through powder mixing and arc-melting routes, enabling the incorporation of zirconium and thorium as solutes. Structural characterization confirmed the formation of solid solutions, and secondary phases as well as microstructural variations depending on processing route. Thermal diffusivity measurements revealed that chemical disorder leads to a systematic reduction in thermal transport, primarily attributed to enhanced phonon scattering resulting from mass and strain fluctuations in the lattice.

Irradiation effects were investigated using proton irradiation and ion implantation of selected FP (Zr, Ba, Kr, Xe). Defect formation, clustering, and irradiation-induced cracking were observed, with clear dependence on damage level and implanted species. Near-surface thermal diffusivity measurements demonstrated a degradation of heat transport with increasing irradiation dose. Mechanical characterization revealed irradiation-induced hardening, while microstructural observations showed crack initiation and propagation associated with defect accumulation and stress development.

In addition, a surface-based methodology was developed to quantify irradiation-induced dimensional changes at the microscale. Both swelling and shrinkage were observed, highlighting the complex relation between defect evolution and microstructural conditions.

The combined results establish clear relationships between chemical disorder, irradiation damage, and property degradation in UN. The findings demonstrate that both compositional modification and defect accumulation significantly influence thermal transport, mechanical response, and dimensional stability.

This work provides new experimental insight into the behaviour of UN and contributes to the understanding required for its future application in advanced nuclear energy systems. The results motivate further studies combining chemistry and irradiation with in-reactor validation to fully capture fuel performance under realistic reactor conditions.