Patent Number: 044302762
Section: description

The following examples further illustrate this invention. EXAMPLE I This example describes the addition of dopant oxides or salts to highly sinterable UO.sub.2 (whose characteristics were defined previously). In the case of the dopant being introduced as a salt, the addition was made by mortar and pestle mixing of the salts with the UO.sub.2 powder for 30 minutes in aqueous media. These slurries were then dried in a vacuum oven at 60.degree. C., with the resultant cake being granulated and "slugged" and pressed into pellets as given below. Salts, e.g., nitrates, of Ti, V, Al, Ca, Mg, Nb, and mixtures of Ca and Ti, in the range of 0.05 to 0.15 mole% cation produced the proper density, thermal stability, and tailored microstructure. In the case of the oxide dopants, these were added to the UO.sub.2 powder by roll blending for 15 minutes before slugging to 4.5 g/cm.sup.3. The slugs were granulated through a 14 mesh screen and roll blended with 0.2 wt.% zinc stearate (die lubricant) for 10 minutes prior to pressing pellets to a green density of 5.8 g/cm.sup.3. Oxide additions including Ti, V, Al, Ca, Mg, Nb, and mixtures of Ca plus Ti, in the approximate ranges 0.05 to 1.7 mole% with respect to the UO.sub.2, were found to produce the aforementioned special pellet features upon sintering. The pellets were sintered at 1780.degree. C. for 1 hour in H.sub.2 saturated with room temperature water vapor. The density, thermal stability (or change in density on resintering for extended times up to 33 hours at 1780.degree. C.), associated grain size, and quantitative porosity were measured. Undoped UO.sub.2 pellets had an as-sintered density of 97.8% of theoretical, a grain size of 7.9 .mu.m and contained fine porosity, primarily &lt;1 .mu.m, and little or none &gt;5 .mu.m. On resintering, the density increased to greater than 99% of theoretical. In contrast, the doped UO.sub.2 pellets had a controlled density of .about.95% of theoretical, which satisfies current LWR fuel specifications. Importantly, these pellets were stable on resintering, showing less than a 1% increase in density. This stability, crucial for good in-reactor fuel performance, is attributable to the relatively large grain size (.about.15-30 .mu.m) and large porosity (generally, &gt;5 .mu.m size) produced in the doped pellets. Furthermore, these microstructural features are known to be valuable for good fission gas retention during reactor operation. If an excessive amount of dopant is added, evidence of a second phase begins to be apparent primarily as a grain boundary phase. This situation is to be avoided since enhanced grain boundary mobility due to a liquid phase would result in excessive deformation during reactor operation due to creep. EXAMPLE II This example describes the addition of the soluble dopant compound as an aqueous solution dissolved in the liquid uranyl fluoride. An intimate co-mix of ADU incorporating the dopant is then precipitated. Titanium nitrate in an amount corresponding to 0.15 mole% with respect to the uranyl fluoride was added to the uranyl fluoride while stirring. The concentration of uranium in the starting uranyl fluoride solution was 159 g/l, and it contained 2 moles of HF and 2 moles of NH.sub.4 F for each mole of UO.sub.2 F.sub.2. A dispersant, Tamol 731, manufactured by Rohm and Haas, was also added at a concentration of 0.24 g/l to minimize agglomeration during precipitation. Subsequent co-precipitation from this solution of the ADU containing the dopant was effected by adding an excess of ammonium hydroxide. The conditions of precipitation included a pH of 10.2 obtained with a NH.sub.3 /U molar ratio of 26/1, a temperature of .about.29.degree. C., and a residence time of about 8 minutes. The co-precipitate was filtered from the reaction mixture and rinsed with deionized water. The washed filter cake was calcined at 550.degree. C. for 3 hours in a steam/hydrogen mixture of 50:1 ratio. The resulting UO.sub.2 powder containing the dopant was fabricated into pellets as described previously. Sintering was performed at 1780.degree. C. for 8 hours in H.sub.2 saturated with water vapor. Undoped (control) pellets had an average sintered density of approximately 97% of theoretical and a grain size of about 15-20 .mu.m. Most of the porosity appeared to be less than 1 .mu.m in size and practically none was greater than 5 .mu.m. The pellets containing the additive(s) were notably superior. A controlled density of nearly 94% of theoretical, which meets the density criterium in light water reactors, was measured. Furthermore, the grain size was considerably larger, falling in the range 30 to 40 .mu.m, and the porosity appeared to be relatively large and primarily greater than 5 .mu.m in size. Limited resintering studies suggested that these microstructural characteristics would lead to excellent thermal stability in the pellets as would be expected and was the case in Example I. EXAMPLE III Example II was repeated using vanadium fluoride at a 0.05 mole% concentration instead of titanium nitrate. EXAMPLE IV Example II was repeated using 0.15 mole% niobium chloride instead of titanium nitrate. EXAMPLE V Example II was repeated using 0.15 mole% aluminum nitrate instead of titanium nitrate. EXAMPLE VI Examples, II, III, IV and V were repeated using uranyl nitrate instead of uranyl fluoride. In this case the same amount of the dopant cited in Examples II, III, IV and V was added. The uranium concentration in the uranyl nitrate starting solution was 160 g/l and it had a specific gravity of 1.298. Precipitation was performed at about 34.degree. C. and using a NH.sub.3 /U molar ratio of 28 to give a pH of .about.9.5. The residence time for the co-precipitate was approximately 4 minutes. EXAMPLE VII Examples I and II were repeated using less or greater concentrations of dopants. In the case of too little dopant the LWR density specification range (93.5 to 96% of theoretical was not achieved), but rather an excessive density was reached. Furthermore, the pellets were not thermally stable due to their relatively small grain size (.about.10-15 .mu.m) and their fine porosity (.about.1 .mu.m) which persisted as in the undoped fuel. When excess dopant was used, the density suppression generally was excessive to the extent that the pellets did not meet the minimum LWR requirement. Moreover, the grain size can be non-uniform or can exhibit discontinuous growth during sintering. EXAMPLE VIII This example describes the addition of dopant(s) as insoluble compounds to the wet ADU filter cake obtained from either the uranyl fluoride or the uranyl nitrate as recounted above. In one case niobium oxide in an amount equal to 0.20 mole% relative to the ADU was uniformly distributed in the ADU filter cake in a blending operation. The dopant-ADU mixture was then calclined as before to leave the dopant in intimate and uniform contact with the resultant UO.sub.2. Pelleting and sintering followed with the same results as described in Example II. EXAMPLE IX Example VIII was repeated using 0.05 to 1.50 mole% titanium oxide (relative to the ADU) instead of the niobium oxide. EXAMPLE X Example IX was repeated using calcium oxide instead of titanium oxide in the same amount. EXAMPLE XI Example VIII was repeated using a mixture of calcium and titanium oxide in equal proportions and totaling from 1.0 to 2.0 mole% instead of the niobium oxide. EXAMPLE XII Example XI was repeated using calcium nitrate instead of calcium oxide. EXAMPLE XIII In the event these dopant levels are not reached or are exceeded, then the undesirable pellet properties described in Examples I and VII will be obtained causing the UO.sub.2 fuel to be unacceptable.