Study of the Effect of Impurities from Nuclear Fuel Reprocessing on the Neutronics of a Fast-Spectrum Generation IV Nuclear Reactor

Abstract

Green, sustainable, and clean are ubiquitous terms in modern energy discourse, yet their meaningful realization remains a profound challenge. Throughout history, electrification has been a catalyst for human progress—symbolizing not only access to dignity but also the foundation for economic and intellectual growth. While renewable energy sources dominate global decarbonization strategies, nuclear power currently offers the most reliable and scalable pathway to large-scale emissions reduction. However, its full sustainability is constrained by persistent concerns across the nuclear fuel cycle, particularly the long-term radiotoxicity and heat generation of spent nuclear fuel. Closing the fuel cycle is therefore central to aligning nuclear energy with the United Nations Sustainable Development Goal 7—“ensure access to affordable, reliable, sustainable, and modern energy for all”—by addressing both resource conservation and intergenerational equity. This master’s thesis investigates one crucial dimension of that challenge: the impact of reprocessinginduced impurities on the neutronic behavior of sodium-cooled fast reactor fuel assemblies, with a focus on the SPX Superphénix reference design. Building on the CHALMEX separation process and the internal gelation route for fuel fabrication, the work examines how residual actinides, fission products, and trace elements carried over from reprocessing alter the final fuel composition and performance. A literature review is combined with microscopic cross-section analysis of selected impurities using the OECD/NEA JANIS database. Lattice depletion and branch calculations are then performed with the Monte Carlo code Serpent 2 on SPX-type assemblies, with and without impurities, to quantify their effects on reactivity coefficients, neutron flux distributions, absorption behavior, and burnup characteristics. By identifying tolerance thresholds and impurity-driven deviations in key neutronic parameters, this thesis assesses the operational viability of reprocessed fuels and proposes directions for optimizing separation and fabrication processes. The results support the safe, efficient, and genuinely sustainable deployment of Generation-IV fast reactors within a closed fuel cycle framework, contributing evidence-based insights for future reactor development and fuel cycle strategies. i