Advancing the understanding of volatile fission-product diffusion in reactor-fuel matrices

Project title: Advancing the understanding of volatile fission-product diffusion in reactor-fuel matrices
Main applicant: Robert J.W. Frost, Division of Applied Nuclear Physics
Co-applicants: Eduardo Pitthan Filho and Daniel Primetzhofer, Division of Applied Nuclear Physics
Grant amount: SEK 5 000 000 for the period 2024-2028

In the proposed project, a PhD student (to be based at Uppsala University) will be recruited to investigate the diffusion properties of volatile elements such as Xe, Kr, I and Cs, in a range of different nuclear-fuel types, under a range of thermal conditions. The core of the methodology, by which this will be achieved, is ion-implantation of the volatiles in unirradiated nuclear-fuel followed by elemental depth-profiling with time-of-flight elastic recoil detection analysis. Fuel samples will then be annealed and the effect of the annealing assessed by additional depth profiling. This is a novel methodology within this field of research. The effect of running the ion implantation at different sample temperatures will also be examined, as well as performing in-situ depth profiling of fuel samples while they are annealed. These general procedures will be further supported by a range of other material categorisation techniques, which will provide correlated information on changes in the microstructure of the samples. Finally, this multidimensional analysis will be coupled with detailed computer modelling of the physics involved. The three main scientific questions to be answered by this project are:

• What is the interplay between fuel microstructure and thermal diffusion of volatile fission products?
• What is the interplay between fuel chemistry and the thermal diffusion of volatile fission products?
• What shortcomings can be identified in the present atomistic models of the diffusion process used for fuel qualification?

The PhD project will combine periods spent in industrial placement at Westinghouse, Vattenfall and OKG, experimental work in the laboratories at both KTH Royal Institute of Technology and Uppsala University, and modelling. The proposed project involves research in the areas of nuclear physics, radio chemistry and materials science, with a strong emphasis on application of the results to the industrial stakeholders. Due to the highly collaborative and multidisciplinary nature of the project, a supervisory team with uniquely combined skill sets has been assembled to support the PhD student, representing both the academic and the industrial sides of the collaboration. The PhD student is to be hired by the Division of Applied Nuclear Physics at Uppsala University, with KTH Royal Institute of Technology and Westinghouse also playing supervisory roles. Vattenfall and OKG will be directly involved in the project for its full duration. The data that this project will produce will have direct implications for both current and future reactor operation, the development of future fuel technologies, and the long-term storage of fuel assemblies in Sweden's final repository. As such, a strong line of communication will be held with both SSM and SKB throughout.