"Evaluating the Influence of Reactor Conditions on Swelling Rates in Uranium-Zirconium Fuels" 

Morgan Smith, MSE PhD Candidate 

Advisor: Professor Maria Okuniewski

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ABSTRACT

Uranium-zirconium fuels are used in fast reactors due to their higher melting temperatures and decreased susceptibility to fuel-cladding chemical interaction (FFCI). However, when U-Zr fuels undergo fission, gaseous and solid fission products are produced which leads to fission-gas swelling that degrades the thermomechanical properties of the fuel. While fuel swelling cannot be avoided altogether, different operating conditions and design factors can influence the swelling rates. The current work reviews the relative effects of three crucial factors: irradiation, temperature, and mechanical constraints. Burnup, a measure of irradiation, accelerates gas accumulation and pore formation, while temperature intensifies diffusion and phase transformations, creating interconnected porosity at elevated temperatures. The role of mechanical factors like smear density is assessed for their ability to alleviate cladding stress. While all of the factors identified contribute significantly to fuel swelling in U-Zr fuels, temperature emerges as the dominating factor. The conclusions from this review aim to establish a holistic understanding of the factors that affect fuel swelling rates and their influence.