Papers by Author: Adonis M. Saliba-Silva

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.

Abstract: - Ferromagnetic stainless steels (SS) produced by powder metallurgy (PM) techniques have been investigated as potential candidates for dental prosthesis applications in replacement of magnetic attachments made of noble and expensive alloys. Two SS were investigated: SS 17-4 PH produced by powder injection (PIM) and SS PM2000 obtained by mechanical alloying. In vitro cytotoxicity analysis of the two SS showed no cytotoxic effects. The magnetic retention force of both tested SS was also evaluated and they were comparable to noble commercially available material that is in use at the moment. The corrosion resistance of both SS was evaluated by electrochemical techniques in sodium phosphate buffer solution (PBS) at 37°C. The AISI 316L SS was also tested under the same conditions for comparison reasons. SS samples tested showed passive behaviour in the electrolyte, but they also presented susceptibility to pitting. The best pitting resistance was associated to the PM2000 whereas the 17-4PH PIM showed the highest pitting susceptibility among the tested steels. The results pointed out that the PM2000 SS might be considered a potential candidate for substitution of high cost magnetic alloys used in dental prosthesis.

Abstract: 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.

Abstract: Powdered uranium silicide (U3Si2) 20% U235 enriched is an intermetallic compound used as nuclear fuel material dispersed in aluminum to be the meat of fuel elements. U3Si2 powder is the state-of-the-art as nuclear fuel material mostly used in modern research reactors. Its recent established fabrication in IPEN replaced the previous ceramic powder U3O8 used in the fuel of IEAR1 (IPEN/CNEN, São Paulo, Brazil). The U3Si2 is a compound with 92.3%wtU and 7.7%wtSi. Its production is made by induction furnace melting using metallic uranium, produced by magnesiothermic reaction, and pure silicon. The induction furnace melts under argon controlled environment using zirconia crucible. Homogenization of liquid bath at 1800°C is a compromise between crucible resistance and homogenized melting, avoiding hazardous happenings. IPEN produced its first lot of enriched U3Si2 in September 2004, with a continuous fabrication ever since. This research work represents the ability of having fully Brazilian supply of this strategic and high cost nuclear material. The fuel quality meets the world quality standards required by International Atomic Energy Agency (IAEA) and RERTR standards. Brazilian production of U3Si2 powder not only closed the fuel cycle, from uranium mineral to fuel element, but also allowed higher productivity of nuclear medicine radioisotopes by IEA-R1.