Improving prompt temperature feedback by stimulating Doppler broadening in heterogeneous composite nuclear fuel forms

Highlights: • Proof of concept of thermal resistance to enhance prompt temperature feedback. • Multi-physics coupling of short timescale heat and neutron transport. • Neutron spectrum analysis yielding time evolution of resonance absorption. • Monte Carlo with on-the-fly Doppler broadening and sampling of S(α,β) data. - Abstract: Nuclear fuels with similar aggregate material composition, but with different millimeter and micrometer spatial configurations of the component materials can have very different safety and performance characteristics. This research focuses on modeling and attempting to engineer heterogeneous combinations of nuclear fuels to improve negative prompt temperature feedback in response to reactivity insertion accidents. Improvements in negative prompt temperature feedback are proposed by developing a tailored thermal resistance in the nuclear fuel. In the event of a large reactivity insertion, the thermal resistance allows for a faster negative Doppler feedback by temporarily trapping heat in material zones with strong absorption resonances. A multi-physics simulation framework was created that could model large reactivity insertions. The framework was then used to model a comparison of a heterogeneous fuel with a tailored thermal resistance and a homogeneous fuel without the tailored thermal resistance. The results from the analysis confirmed the fundamental premise of prompt temperature feedback and provide insights into the neutron spectrum dynamics throughout the transient process.