Results 1 - 1 of 1
Results 1 - 1 of 1. Search took: 0.02 seconds
[en] Within the context of a global initiative to reduce the enrichment of nuclear fuels for research and test reactors, new fuel candidates with significantly higher uranium density must be developed and appropriate fabrication techniques need to be established. For the conversion of the FRM II research reactor and other high-performance research reactors worldwide to a lower enriched fuel element, a monolithic uranium-molybdenum (U-Mo) alloy fuel is the most promising candidate. In the proposed European fabrication process chain for such fuels, two major advancements must be made towards its further industrialization: (i) the demonstration of fabrication techniques for U-Mo foils with a thickness gradient, necessary to reduce thermal spikes at the edges of fuel plates, and (ii) the development of a pilot PVD coating process for the application of a zirconium interdiffusion barrier between the U-Mo foil and the surrounding aluminum cladding. Currently, no fabrication process for gradient U-Mo foils is available worldwide. Hence, a comprehensive assessment of possible techniques for a first-of-a-kind fabrication demonstration using non-radioactive surrogate materials was performed, focusing on the achievable precision, surface quality, material efficiency and expected radiological impact. In collaboration with industrial and research partners, numerous gradient foils of stainless steel AISI 316L and titanium alloy Ti-6Al-4V were produced. Hereby, gradient full-size foils (762 mm x 45 mm) conforming to specifications and compatible with the fuel plate fabrication process were produced by classical milling, and the collection of more than 98 % of the generated scraps for recycling was demonstrated. Gradient mini-size foils (82.5 mm x 19 mm) were successfully produced for the first time using additive manufacturing by selective laser melting, and post-production treatments like shot peening and hot isostatic pressing were applied to obtain foils compatible with the plate fabrication process. On the way towards its industrialization, the pilot PVD coating process for full-size foils follows the previously developed mini-size process at TUM. In parallel to its upscaling to full-size geometry, two key issues of the previous process were successfully resolved within this thesis. The ductility of the coating was significantly improved via an optimized substrate biasing approach, which aims to reduce the re-sputtering effect and structural damages in the deposited Zr layer. Therefore, the newly built PVD coating device was equipped with an unbalanced magnetron type 2 sputtering source, which allows a reduction of the substrate biasing voltage while simultaneously increasing the ion flux towards the substrate. A new substrate holder, which uses spring force to keep the foils flat throughout the coating process, allows the reliable coating of up to 800 mm long full-size substrates. An optimized set of process parameters was developed for this PVD coating device, aiming at the reproducible deposition of at least 8 μm Zr with defect-free adherence and high layer ductility at a high deposition rate. In a joint fabrication study, TUM’s pilot PVD coating process and Framatome-CERCA’s C2TWP cladding application process were successfully tested and specification-conform half-size plates were produced out of Zr-coated stainless steel substrates. By these first-of-a-kind demonstration experiments on the fabrication of gradient foils, and by successfully establishing the pilot PVD coating process at TUM’s Nuclear Fuel Fabrication and Characterization Laboratory, important advancements of the European fabrication process chain for monolithic U-Mo fuels were accomplished.