Physical Metallurgy of Uranium and its Alloys
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Uranium silicide compounds such as U 3 Si and U 3 Si 2 have different behavior under irradiation. While the first one presented in some cases break away swelling [ 18 ], U 3 Si 2 was finally selected to rich total uranium densities at the fuel plate meat of 4. This fuel is the last one that has been qualified for general uses in research reactors using LEU.
It is worthwhile to comment that U 3 Si is a ductile compound and centrifugal atomization [ 20 ] was specially dedicated to obtain a ductile compound in powder form. By , in a systematic study, several high density uranium compounds were revisited to be used as nuclear fuels with the intention of reducing even more the utilization of HEU in research reactor fuels and to look for a fuel more easily reprocessed than the U 3 Si 2 dispersions [ 21 , 22 ].
Uranium metallurgy - Wikipedia
UMo presents interface incompatibilities with aluminum at high irradiation fluxes when fission gases generate undesired porosity [ 28 — 30 ]; this drawback is diminished by the incorporation of silicon to the aluminum matrix [ 31 ]. They comprise a first step in which UMo foils are hot laminated to final dimensions and a second step is the incorporation of the aluminum cladding that can be performed by transient liquid phase bonding, friction stir welding or hot isostatic pressing.
Irradiation results can be obtained in more detailed overviews [ 35 ]. Several methods have been used to comminute UMo alloys: the more sophisticated centrifugal atomization already mentioned from KAERI, mechanical grinding performed initially in Canada and rotating electrode [ 36 ].
Studies of gaseous atomization with gold used as surrogate material to evaluate production performance were also performed [ 37 ].
The brittle hydride weakens the material that can easily be milled to particles of dimensions similar to the original grain size [ 38 , 39 ]. It could be observed that a new phase begun to form at the place where tensile stresses were bigger. Uranium was deoxidized using nitric acid and molybdenum with melted sodium hydroxide. A high frequency induction furnace was used for melting the alloy in magnesium oxide crucibles and an inert gas atmosphere.
The melt was poured in a graphite dish obtaining a circular plate of approximately 8 mm thick and mm diameter. Tension stresses at the top of the plate are retained since solidification begins at the bottom in contact with the graphite.
A flow meter was used to measure the incoming hydrogen needed to keep a constant hydrogen pressure during absorption. In the stressed sample, hydrogen can be allocated in interstitial positions in the crystal structure solubilization or in hydrogen traps in dislocations generated by tension stresses. After several hours, hydriding is completed. Additional temperature controls are needed during the process since the hydriding reaction is exothermic. This explains why in a cooling cycle hydrogen absorption is higher than in a heating one [ 42 ].
U10Mo has also been hydrided with similar results utilizing selected heating and cooling cycles such that hydriding conditions can be reached without initially incorporating residual stresses [ 45 , 46 ]. Finally, at room temperature, air is introduced in a temperature controlled way such that the hydride is passivated.
It is pyrophoric and burns with flame because of hydrogen liberation.
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When oxidized in air, it is dark brown. The XRD hydride density is This hydride must be milled in a sufficient inert atmosphere to avoid burning up. These particles were then passed once through a conical crusher in a dynamic inert atmosphere reducing the size , microns. Passivation is needed before exposing the particles to air.
Any gas evolution is followed by pressure measurements with a closed chamber; periodic evacuation is performed to reduce excess pressure. If, after hydriding, hydrogen in traps is removed as mentioned in 2. Hydrogen evolution during dehydriding. The heating chamber was vibrated to avoid the sintering of the particles. Controlled passivation is done by slow removal of the inert gas and air introduction. Dehydriding needs at least 5 hours.
The powder must be kept in an inert atmosphere to avoid oxidation. The surface absorption of HMD powder in an air atmosphere was studied by increasing the temperature in a closed vibrating chamber. A first run with 64 g showed an important pressure reduction up to a value of mbar.
The second run, curve g bis, was performed after cooling and reintroducing air in the chamber. These last two runs showed that pressure equilibrium was achieved without reaching saturation. A similar scenario will be present inside a picture and frame ensemble before hot rolling.
Surrogate powders were used to initially set up equipment and coverage conditions; silicon, aluminum, magnesium and silicon were used with chemical vapor deposition and dip coating techniques [ 50 , 51 ]. Physical vapor deposition [ 52 ] is another possibility of coverage technique.
The coverage of ductile UMo particles with brittle materials is thermomechanically incompatible in layers thicker than 1 micron. Probably ductile nickel and niobium seem to be better candidates as they have been tested previously in other nuclear fuel developments. New technologies are now available that consist in particles spheroidization and coverage using an induction couple plasma equipment that nowadays can be purchased in a commercial form [ 53 ]. The brittle transgranular fractures produced during comminution of the hydride gives as result polyhedral shaped particles.
The irregular shape of HMD particles favors the mixing with Al powder, diminishes flow segregation—compared with spherical particles—and green compacts practically have no shredding.
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Dispersed UMo particles in an aluminum matrix miniplates were also colaminated using silicon covered U7Mo HMD powder and centrifugal atomized particles with LEU and natural uranium [ 51 ]. The size of the big particle is approximately microns. Holes between particles were fundamentally produced during polishing. Monolithic UMo meats can be obtained by powder metallurgy. The use of a UMo monolithic meat elaborated by powder metallurgy allows an easier way of obtaining different meat widths in the same plate using conformational dies, incorporation of powdered neutron moderators such as high temperature stable hydrides, burnable poisons and also nanosized porous powders to adsorb fission gases at grain boundaries so as to reduce overall swelling of fuel plates [ 54 ].
UMo and aluminum have interlayer incompatibility under irradiation and different thermomechanical properties that complicates a monolithic plate fabrication. Hot rolling colamination needs several deformation steps while coextrusion is performed at higher temperatures in only one deformation step. The interaction zone growth kinetics between UMo and Zry is well known [ 40 ] and more recently it was shown by calculation that there exists a low interaction between Zr and UMo alloys [ 57 ].
Finally, a fabrication technique was developed [ 51 ], and two fuel miniplates were produced, irradiated and the post irradiation examination PIE was performed [ 43 , 54 , 58 , 59 ]. Total welding in the sandwich hot colamination can only be achieved through sequential deformation steps.
Physical Metallurgy Uranium
Thickness measurements were replaced by a very fast measurement of the plate length with a ruler from which the reduction pass was calculated. After several steps, the contact surfaces are totally welded. Welding between meat and frame was checked by destructive metallographic techniques. Miniplates final size was x 25 mm with a total thickness of 1 mm; nominal meat thicknesses were 0.
The three at the right extreme are in an intermediate step of surface oxide removal. The LEU uranium molybdenum alloy for the MZ25 and MZ50 miniplates was melted in the same way as described for powder production in section 2. Finished frames, lids and coupons of MZ25 and MZ50 ready to be stacked and welded to conform the two sandwiches.
To minimize the oxide layer growth the heating time between passes was set up at minimum values that could guarantee tension relieving and recrystallization of UMo. Totally welded interfaces ND near final thickness were reached after eight passes. Mean reduction step was After the hot colamination, the plates were straightened in a multi roller machine and the oxide layer must be removed. Wet silicon carbide sand papers were used for this last purpose.
The final thickness was reduced in 0. Cutting to obtain final dimensions was done with a guillotine. Meat and cladding thickness are 0. The estimated beginning of life BOL thermal conditions, operating parameters, peak surface heat flux, power density generated at the beginning and end of irradiation, minimum and maximum clad surface temperatures, fission density, internal and external temperatures, heat flux and total swelling were previously shown [ 43 , 59 — 63 ]. Plate swellings were 3. The average oxide layer thickness after irradiation was 2.