With respect to nitrides being considered as potential fast reactor fuels, research is conducted on the out-of-pile thermophysical properties, sintering and fabrication processes, gas migration mechanisms, self-diffusion and point defect behaviour of actinide nitrides, their surrogate materials, and the inert matrix material ZrN . The experimental research, carried out in the framework of qualifying fuel for the European Lead Cooled Training Reactor (ELECTRA), shows that sintered ZrN and (Dy,Zr)N pellet densities are influenced by the oxygen concentration in the material. The effect is confirmed in sintered (Pu,Zr)N pellets. Oxygen concentration also plays a role in the thermophysical properties of inert matrix nitride fuels, but does not have an impact on the electrical properties of these materials. With the fuel fabrication methods applied here, clean nitride powders can be synthesized. However, the subsequent fabrication phases, including milling and solid solution formation, increases the impurity levels significantly. Research of equal importance is performed on materials free of fabrication-induced impurities, whose properties are studied by employing first-principles methods. ZrN, UN and (U,Zr)N are studied, whereas the results from ZrN are expected to be applicable for actinide nitrides as a first approximation. The migration of noble gases in ZrN, on the atomic scale, confirms the experimentally observed tendency for noble gases with higher atomic number to be retained in the fuel matrix, while the majority of He is released to the fuel pin. Materials modelling implies that self-diffusion of nitrogen and metal atoms in inert matrix nitride fuels is accelerated under irradiation, since noble gas retention reduces migration barriers which govern self-diffusion. Unlike Kr and Xe, He has the capacity to be released into the fuel matrix, after having been trapped in a vacancy. The results are expected to aid in providing an explanation to the macroscopic diffusion phenomena in nitride fuels, as the diffusion behaviour of noble gases is sparsely studied. In addition, a study on the miscibility of ZrN and UN in a narrow composition range suggests solubility, based on the negative mixing energies. The results obtained from research on inert matrix nitride fuel underline several beneficial properties which are desirable in a fast reactor fuel. The relevance of these results is analyzed and contextualized in the thesis, from the perspective of current research and development in the field. / <p>QC 20130611</p>

In this work a manufacturing method for UN, ZrN and (U,Zr)N pellets was established at the nuclear fuel laboratory at KTH Stockholm/Sweden, which consists of the production of nitride powders and their sintering into pellets by spark plasma sintering. The nitride powders were produced by the hydriding-nitriding route using pure metal as starting material. This synthesis was performed in a stream of the particular reaction gas. A synthesis control and monitoring system was developed, which can follow the reactions in real time by measuring the gas flow difference before and after the reaction chamber. With the help of this system the hydriding and nitriding reactions of uranium and zirconium were studied in detail. Fine nitride powders were obtained; however, the production of zirconium nitride involved one milling step of the brittle zirconium hydride. Additionally uranium and zirconium alloys with different zirconium contents were produced and synthesized to nitride powders. It was found that also the alloys could be reduced to fine powder, but only by cyclic hydriding-dehydriding. Pellets were sintered out of uranium nitrides, zirconium nitrides, mixed nitrides and alloy nitrides. These experiments showed that relative densities of more than 90% can easily be achieved for all those powders. Pellets sintered from mechanically mixed nitride powders were found to still consist of two separate nitride phases, while nitride produced from alloy was demonstrated to be a monophasic solid solution both as powder and as sintered pellets.

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.