Patent Number: 059463643
Section: description

The invention will now be described with reference to the following non-limiting example. EXAMPLE A particular example of practicing this invention is as follows. Sintering uranium dioxide nuclear fuel pellets, in the form of right circular cylinders and containing 0.25 wt % of alumina/silica in the ratio of 0.4 Al.sub.2 O.sub.3 /0.6 SiO.sub.2 are prepared by blending alumina and silica powders in a weight ratio of 0.4 Al.sub.2 O.sub.3 /0.6 SiO.sub.2 with uranium dioxide powder to achieve a total addition of 0.25 wt % of the alumina/silica with 99.75% uranium dioxide. The blended powders are dry-pressed to a green density of approximately 5.6 gm/cm.sup.3 to form powder compacts in the form of right circular cylinders for sintering to fuel pellets. The dry pressed pellets are sintered using a furnace feed gas of 75% hydrogen -25% nitrogen which has been moisturized by passing the gas through a water bubbler with the temperature of the water in the bubbler maintained at 55.degree. C. and a total furnace gas pressure of 1 atmosphere (760 mm Hg). At 55.degree. C., the vapor pressure of water is 118 mm Hg, the hydrogen and nitrogen gas pressures of the furnace feed gas are 481.5 and 160.5 mm Hg, respectively, and the H.sub.2 O to H.sub.2 ratio of the furnace gas atmosphere is 118/481.5=0.245. The sintering furnace temperature profile is maintained to provide prolonged (.about.4 hours) sintering at 1750.degree. C. in the hot or working zone of the sintering furnace. At that sintering temperature, for the H.sub.2 O to H.sub.2 ratio noted above, the oxygen free energy in the hot zone of the sintering furnace is maintained at about -70 kcal/mole, the O/U ratio of the uranium oxide during the sintering operation is maintained at about 2.005, and the vapor pressure of SiO is maintained at about 10.sup.-5 (0.00001) atmospheres. For these sintering conditions, the desired final fuel pellet density of 10.5 gm/cm.sup.3 is achieved, and the aluminum and silicon content of the final sintered pellets are within acceptable ranges of the initial amount added. The sintered pellets are tested for resistance to density changes by maintaining them in the temperature range 1700-1725.degree. C. with a furnace feed gas of 75% hydrogen-25% nitrogen which has been moisturized by passing the gas through a water bubbler with the temperature of the water in the bubbler maintained at 55.degree. C. and a total furnace gas pressure of 1 atmosphere (760 mm Hg). At 55.degree. C., the vapor pressure of water is 118 mm Hg, the hydrogen and nitrogen gas pressures of the furnace feed gas are 481.5 and 160.5 mm Hg, respectively, and the H.sub.2 O to H.sub.2 ratio of the furnace gas atmosphere is 118/481.5=0.245. At that test temperature, for the H.sub.2 O to H.sub.2 ratio noted above, the oxygen free energy in the hot zone of the testing furnace is maintained at about -70 kcal/mole, the O/U ratio of the uranium oxide during the sintering operation is maintained at about 2.005, and the vapor pressure of SiO is maintained at about 10.sup.-5 (0.00001) atmospheres. During cooling, in this test atmosphere, the O/U ratio of the uranium oxide re-equilibrates to near 2.000. For these test conditions, the desired final fuel pellet the weight loss of the pellets as a result of SiO loss is negligible, and the density changes are essentially only a result of pore volume elimination from continuation of the sintering process. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.