The purpose of this research is to develop a methodology for a thorium fuel recycling analysis that provides results for isotopics and radio-toxicity evaluation and analysis. This research is motivated by the need to reduce the long term radiological hazard in spent nuclear fuel, which mitigates the mixing hazard (radiotoxicity and chemical toxicity) and decay heat load on the repository. The first part of the thesis presents comparison of several once-through cases with uranium and thorium fuels to show how transuranics build up as fuel is depleted. The once-through analysis is performed for the following pairs of comparison cases: low enriched uranium dioxide (UOX) vs. thorium dioxide with 233UOX (233U-ThOX), natural uranium dioxide mixed with transuranic oxides (U-TRUOX) vs. thorium dioxide mixed with transuranic oxides (Th-TRUOX), natural uranium dioxide mixed with weapons grade plutonium dioxide (U-WGPuOX) vs. thorium dioxide mixed with weapons grade plutonium dioxide (Th-WGPuOX), natural uranium dioxide mixed with reactor grade plutonium dioxide (U-RGPuOX) vs. thorium mixed with reactor grade plutonium dioxide (Th-RGPuOX). The second part of the research evaluates the thorium fuel equilibrium cycle in a pressurized water reactor (PWR) and compares several recycling cases with different partitioning schemes. Radio-toxicity results of the once-through cycle and multi-recycle calculations demonstrate advantages for thorium fuel and reprocessing with respect to long term nuclear waste management.

Nuclear power plants provide about 11 percent of the world's electricity production. For fission process is uranium fuels used with varying percentage of enrichment 235U for most of nuclear reactors. Uranium reserves are reducing and their mining cost increases. Therefore, the thorium fuel is discussed as revolution fuel for current and future nuclear power plants. This diploma thesis deals with possibility of thorium fuel utilization at various types of nuclear reactors with a focus on light water reactors. The practical part of the thesis is focused on simulation and calculations of various uranium dioxide and thorium dioxide layers at the fuel rods. Model of WWER 440 reactor was developed for the calculations with the addition of thorium fuel. The model simulates burning out of fuel for 5 years, with monitoring of fuel behavior and tracking changes of each material. The thesis tries to define the suitable ratio and parameters of layers combination of uranium and thorium fuel. For these ratios and parameters the thesis tries to give sufficient amount of computational analyzes.

Metallic fuels were produced through arc-melting. As-cast phases, microstructuresand selected mechanical properties were investigated for UZr,U-Th, and U-Th-Zr systems. For each system, two compositions wereinvestigated, with approximately 5 at. % and 20 at. % solute material, for atotal of six alloys. As-cast alloy microstructures were assessed in the contextof their equilibrium systems and compared to relevant published works whereapplicable. Mechanical testing revealed increased hardness with increasingsolute concentration, compared to the reference materials. The results supportthe conclusion that solid solution strengthening is the primary mechanismenabling this change in each binary system.Additionally, (U,Zr)N fuel was synthesized. This work exemplified aprocess to produce fuel with a homogeneous distribution of zirconium in thefuel matrix, thus representing a simulated burn-up distribution of zirconium.Refinements can be made to further improve this process in future work. Thesefindings will support a broader separate effects testing campaign underway bythe SUNRISE centre / Metalliska bränslen framställdes genom bågsmältning. Som gjutna faser,mikrostrukturer och utvalda mekaniska egenskaper undersöktes för U-Zr-, UTh-och U-Th-Zr-system. För varje system undersöktes två sammansättningar,med cirka 5 at. % och 20 at. % löst material, för totalt sex legeringar.Mikrostrukturer av gjutna legeringar diskuterades i samband med derasjämviktssystem och jämfördes med literattur. Mekanisk testning visadeökad hårdhet med ökad halt lösta atomer, jämfört med råvarorna. Denprimära mekanismen som möjliggör denna förändring föreslogs vara solidlösningsförstärkning.Vidare syntetiserades (U,Zr)N-bränsle. Detta arbete exemplifierade enprocess för att producera bränsle med en homogen fördelning av zirkoniumi bränslematrisen, vilket representerar en simulerad utbränningsfördelning avzirkonium. Denna process kan förbättras. Resultaten stödjer en bredare separateffekttestningskampanj som SUNRISE-centret arbetar med.