No abstract

rradiation targets with 20% of 235U (Low Enriched Uranium - LEU) have been studied to replace HEU (Highly Enriched Uranium) targets in future nuclear reactors. These are used to produce the pair of radionuclides 99Mo / 99mTc, used for diagnostics in nuclear medicine. This work aims to develop an alternative route to produce LEU targets. It consists in hydrogenating and powdering metallic uranium and compacting the produced powder, followed by sealing it with nickel by electrodeposition. The deposited nickel should suppress the release of fission gases, and avoid a reactive contact of uranium with aluminum from the enclosure. In order to obtain the best conditions for deposition over uranium, in this work iron powder was compacted into small discs, with a diameter of 22mm and weight of 14g, simulating an equivalent volume of 10g of LEU uranium powder. As well, aluminum discs were used to ensure adhesion and uniformity of the nickel layer. Pulsed nickel electrodeposition was carried out over the compacts, employing current frequency of 900Hz, -0.84A/cm2 of peak current and duty cycle of 0.5 in Watts Bath. The electrical resistance of pulse Ni-plated layer was checked by experiments with impedance spectroscopy in plated samples using aluminum substrate, held in KCl (pH=6), giving EIS results after resting the discs in solution for 0h, 4h and 24h. The physical strength was evaluated qualitatively by treating the Ni covered compact at 600°C, developing a bump deformation on the original planar layer, up to the point to open the Ni-layer for gas relief. These results suggest an adequate mechanical strength of the Ni-layer for using under neutronic irradiation, sealing the radioactive gases, mainly 140Xe, produced during fission of 235U.

New nuclear fuel material with high density in uranium is envisaged for intense irradiation research reactors. The alloy U-Mo has been researched as a feasible candidate to be used in such reactors. This nuclear fuel is conceived to be used encapsulated in aluminum matrix. Nevertheless, there are interaction products of U-Mo/Al which form porosity during irradiation, leading to routine operation harms in research reactors. This interaction is due to solid solution interdiffusion of species, mainly of Al towards U-Mo region forming reaction products. This interaction could be studied by on-pile method, observing the occurrence of formed products during irradiation, but this method is costly and used only for long term experiments in very few reactors in the world. For this, several out-of-pile studies using heat treatments of diffusion pairs are carried out at adequate temperatures and times, just below the -phase eutectoid temperature to simulating the interdiffusion and formation U-Mo-Al phases. In the present study, it was employed a new developed assembling method to prepare interdiffusion pairs by immersing sliced U-10Mo sticks inside molten Al. These samples are made by induction furnace, in temperature range ~660-670 °C, under controlled argon atmosphere, in order to entrap molten Al around U-Mo sticks and so keeping this entangled structure after solidification. The interdiffusion pairs are then cut and prepared for treatments. This novel sample preparation guarantees full contact between the U-Mo and Al without oxidation contact, creating so, the ideal conditions for interdiffusion investigation of the interfaces of Al/U-Mo. Preliminary results to study interaction products where achieved by heat treatments during 5h at 550°C. Observations and calculations from SEM/EDS microstructures and XRD diffractograms revealed few microns interaction layer between the matrix and the fuel material, resembling phases reported in the literature for the interaction products between U-Mo-Al. This layer is mainly composed by Al and U, Mo phases, probably (U, Mo)Al3 and phases containing Si, as U3Si5 and a proposed one Al2Si3U3 that fits better to XRD spectrum of experimented diffusion pairs.

The U-Mo alloys are investigated with the goal to become nuclear material to fabricate high-density fuel elements for high performance research reactors. The enrichment level (20% 235 U) suggests that the U-Mo alloys should be between 6 to 10 wt. (%), which can reach up to 9 gU.cm -3 in fuel density. Nevertheless, the U-Mo alloys are very reactive with Al. Interdiffusion reaction products are formed since nuclear fission promotes chemical interaction layer during operation, leading to potential structural failure. Present studies were made with treated hot rolled diffusion couples of U-10Mo inserted in Al (AA1050). The U-10Mo/AA1050 pairs were treated in two temperatures (150 °C and 550 °C) with three soaking times (5, 40 and 80 hours). From microstructure analyses, rapid diffusion of Al happened inside U-10Mo in the first heating at 540 °C during 15 minutes, reaching 8 at% Al in a range of 170 µm towards U-10Mo. Longer time at 550 °C treatment maintain this level of Al-content up to 1000 µm inside U-10Mo. In this study, the results suggested the formation of a barrier made by residual elements, which promoted little interdiffusion phenomena between U-10Mo and alloy AA1050. Silicon co-diffusion with Al, along the interdiffusion line, is thought to be an important indication for this interdiffusion blockage.