Patent Number: 058825525
Section: description

DETAILED DESCRIPTION OF THE INVENTION It is well known that sintering aids, for example, Nb.sub.2 O.sub.5 and TiO.sub.2, increase substantially the grain size and plasticity of UO.sub.2 fuel pellets when being mixed with UO.sub.2 powder, pressed and sintered. But sintering aids have a negligible effect on the densification of UO.sub.2 green pellet. The published literature, entitled "UO.sub.2 fuel pellet microstructure modification through impurity additions" by K. C. Radford and J. M. Pope in Journal of Nuclear Materials 116(1983) 305-313, reported that the UO.sub.2 green pellet containing Nb.sub.2 O.sub.5, TiO.sub.2 or V.sub.2 O.sub.5 in a quantity of 0.05% to 0.5% by weight densified faster at a temperature of 1300.degree. C. to 1500.degree. C. than pure UO.sub.2 green pellet, but that its final density achieved at about 1700.degree. C. was slightly lower than pure UO.sub.2 green pellet, suggesting that the effect of sintering aids on the densification of UO.sub.2 green pellets is negligible. An attempt to increase the density of UO.sub.2 pellet through sintering aids has not been practically advantageous, because fresh UO.sub.2 powder has been inherently so sinterable as to produce a fuel pellet having a density of 95% TD. Sintering aids have not been used to recycle fuel scrap until now. We have found that a sintering aid increases enormously the sintered density of the green pellet consisting of fresh UO.sub.2 powder and recycled U.sub.3 O.sub.8 particles, of which densification is originally very poor. The invention according to our finding provides a method for recycling fuel scrap into the manufacture of new fuel pellets. Nuclear fuel pellets of UO.sub.2 alone or UO.sub.2 containing an oxide of plutonium, gadolinium or erbium are commonly produced through the manufacturing steps shown in FIG. 1. As described in "background of the invention", fuel scrap consisting of defective fuel pellets and grinding sludge is made in the sintering and grinding steps. If green pellets are defective in the pressing step, they should be sintered to make defective fuel pellets, which are a suitable form for the recycling. Defective fuel pellets are comminuted to fuel particles through oxidation in a furnace, and then a sintering aid is added in the mixing step to the sintering powder consisting of the recycled fuel particles, fresh fuel powder and grinding sludge. The sintering powder containing the recycled fuel particles in the range of about 10% to about 100% by weight is processed according to the normal manufacturing steps of fuel pellets: mixing, pressing, sintering and grinding. The sintered pellet so produced is loaded into a cladding tube and enclosed. Detailed description of the method for recycling fuel scrap consisting of defective pellets and grinding sludge is as follows: Defective fuel pellets of UO.sub.2 alone or UO.sub.2 containing an oxide of plutonium, gadolinium or erbium are heat-treated in boats in a furnace at a temperature in the range of about 300.degree. C. to about 800.degree. C. in an oxidizing gas, which is selected from the group consisting of air, oxygen, a mixture of air and inert gas and a mixture of oxygen and inert gas, until defective fuel pellets are comminuted to fuel particles of U.sub.3 O.sub.8 alone or U.sub.3 O.sub.8 containing an oxide of plutonium, gadolinium or erbium. Oxidation rate is naturally slow in lower temperatures and also progresses slowly in a higher temperatures due to the formation of dense and protective layer. It is preferred to oxidize defective pellets in a flowing air at a temperature in the range of about 350.degree. C. to about 700.degree. C., in which the sizes of recycled fuel particles become smaller as oxidation temperature decreases. Defective fuel pellets are easily comminuted to fuel particles through the above treatment, since large stress is generated during the oxidation due to the fact that U.sub.3 O.sub.8 has a lattice volume larger by about 30% than UO.sub.2. The recycled fuel particle has a particle size in the range of about 3 .mu.m to about 100 .mu.m and a specific surface area in the range of about 0.1 m.sup.2 /g to about 1.0 m.sup.2 /g. The recycled fuel particles should be screened to remove large agglomerates. Comparing with defective UO.sub.2 pellets, the defective pellet which has a composition of not only UO.sub.2 but an oxide of plutonium, gadolinium or erbium oxidizes slowly and its recycled fuel particle becomes coarse. Oxidation temperature and time are properly adjusted depending on scrap materials. The recycled fuel particle is optionally treated again ; the recycled fuel particle is reduced to a lower oxidation state and/or the resulting oxide is oxidized and reduced at least once, or is mechanically milled. However, such extra treatments are not needed in normal practice according to the invention. The recycled fuel particle and the fresh fuel powder having a composition of UO.sub.2 alone or UO.sub.2 in a mixture of PuO.sub.2, Gd.sub.2 O.sub.3 or Er.sub.2 O.sub.3 constitute sintering powder, in which the amount of the recycled fuel particle is in the range of about 10% to about 100% by weight. Grinding sludge may be added to the sintering powder. A sintering aid which is an oxide and a compound containing an element selected from the group consisting of Nb, Ti, Li, V, Mg, Al, Sn, Si, Cr and mixtures thereof, is added in a quantity of about 0.02% to about 2% by weight, on said element basis, to the sintering powder. The sintering powder is then mixed uniformly. In case that UO.sub.2 fuel containing an oxide of plutonium, gadolinium or erbium is fabricated, the sintering powder is optionally milled to increase its sinterability. The sintering powder so produced is then pressed and sintered in the same way as normal fuel powder. The sintering powder is cold-pressed in a mold under a pressure of about 2 ton/cm.sup.2 to about 5 ton/cm.sup.2 to produce green pellets having about 40% to 65% TD (theoretical density). If the powder is not so flowable as to be pressed directly, it is pre-pressed under a lower pressure into slugs, which are broken up into granules having good flowability. Green pellets are heated to a temperature in the range of about 1500.degree. C. to about 1800.degree. C. and held for about 1 to about 20 hours in a sintering gas atmosphere. The sintering gas atmosphere should be reducing to make stoichiometric fuel, so it is selected from the group consisting of hydrogen, a mixed gas of hydrogen and inert gases such as argon and nitrogen, a mixed gas of carbon dioxide and carbon monoxide, and a mixed gas of hydrogen and carbon dioxide. Additionally the sintering gas atmosphere includes a small amount of water vapor to control the oxidation potential of sintering atmosphere. In case UO.sub.2 containing an oxide of plutonium, gadolinium or erbium is sintered, a sintering gas atmosphere is commonly humidified to increase grain size of the sintered pellet. The sintered pellet so produced has a density in the range of about 94% TD to about 97% TD and thus meets fuel specification. In addition, it has a grain size larger than about 20 .mu.m, so fission products will remain more trapped in the fuel pellets. The increase in density is below 1% TD after the resintered test carried out at 1700.degree. C. for 24 hours in hydrogen, so sintered pellets are thermally stable. In case the sintering aid is not added to the sintering powder consisting of fresh fuel powder and the recycled fuel particle, sintered density decreases linearly with the content of the recycled fuel particles, which is not allowed over about 10% by weight due to the density drop. The sintering atmosphere is controlled depending on the sintering aid. It has been found that Nb.sub.2 O.sub.5 which was added as a sintering aid does not dissolve completely in UO.sub.2 matrix during sintering in a dry hydrogen and consequently does not enhance the densification of green pellets. In order to dissolve Nb.sub.2 O.sub.5, hydrogen gas should include a small amount of water vapor which is equivalent to a dew point of about 20.degree. C. If Al.sub.2 O.sub.3 or MgO is a sintering aid being added, the oxidation potential of sintering atmosphere is kept higher to dissolve it, which can be controlled by the mixed gas having a composition in the range of about 5% to about 40% carbon dioxide by volume with the balance being hydrogen. DESCRIPTION OF PREFERRED EMBODIMENTS The following examples illustrate a preferred method for recycling defective UO.sub.2 fuel pellets by using a sintering aid. However, these examples should be understood to in no way limit the scope of the invention which is defined by the appended claims. EXAMPLE I Defective UO.sub.2 pellets of about 150 g were put in a boat, heated in a lab furnace to 400.degree. C. under a flowing air, and held for 4 hours. Temperature measured by the thermocouple attached to the boat indicated that temperature rose to 450.degree. C. due to the exothermic oxidation of UO.sub.2 to U.sub.3 O.sub.8. The defective pellets were comminuted to U.sub.3 O.sub.8 particles, which were then passed a 200 mesh sieve to remove large agglomerates. The recycled U.sub.3 O.sub.8 particle has an average particle size of about 8 .mu.m and an average specific surface area of 0.6 m.sup.2 /g. SEM micrographs of recycled U.sub.3 O.sub.8 particle indicated that the defective pellet was comminuted by intergranular cracking. UO.sub.2 powder was used which was obtained through the AUC (Ammonium Uranyl Carbonate) route. The UO.sub.2 powder had an average particle size of about 20 .mu.m, and a particle consisted of many primary crystallites of smaller than about 0.1 .mu.m. This powder had a spherical shape and was free flowing enough to be pressed directly into green pellets without granulation. Starting powders were prepared by mixing UO.sub.2 powder with recycled U.sub.3 O.sub.8 particle in so-called "Turbula" for 1 hour, in which the amounts of recycled U.sub.3 O.sub.8 particle were 0%, 10%, 20%, 30%, 40%, 60%, 80% and 100% by weight. These starting powders were pressed and sintered for the control pellets. In addition, Nb.sub.2 O.sub.5 powders in quantities of 0.3% and 0.5% by weight were added to the starting powders (8 compositions), respectively, and mixed again. Powders were directly pressed at a pressure of 3 ton/cm.sup.2 in a mold whose wall was deposited with zinc stearate for lubrication, and green pellets so produced were in good mechanical condition. Green pellets were heated in a furnace to 1680.degree. C. at the rate of 5.degree. C./min under a flowing hydrogen, held for 4 hours and cooled down. It was found that Nb.sub.2 O.sub.5 did not completely dissolve in UO.sub.2 matrix during the sintering under a dry hydrogen gas, so it was preferred that hydrogen gas had a dew point of about 20.degree. C. The density of sintered pellet was determined by water immersion method. In Table I, green and sintered densities for example I are given. In the case of the control pellets, the sintered density decreases linearly with the amount of recycled U.sub.3.sub.O.sub.8 particle in the starting powder, so the UO.sub.2 pellets produced from the starting powders which contain U.sub.3 O.sub.8 particle more than 10% by weight have lower density than the density limit (94% TD) set by fuel specification. In the case of the addition of Nb.sub.2 O.sub.5 in quantities of 0.3% and 0.5% by weight, the fuel pellets produced from starting powders which contain the recycled U.sub.3 O.sub.8 particle more than 10% by weight have densities higher than 94% TD (10.30 g/cm.sup.3). The fuel pellets containing 0.3 weight % Nb.sub.2 O.sub.5 are have a grain size in the range of 25 .mu.m to 30 .mu.m (linearly intercepted), and those containing 0.5 weight % Nb.sub.2 O.sub.5 have a grain size in the range of 40 .mu.m to 45 .mu.m. The resintered test carried out at 1700.degree. C. for 24 hours in hydrogen showed that the increase in density ranged from 0.6% TD to 0.9% TD, indicating that the sintered pellets were thermally stable. TABLE I __________________________________________________________________________ Sintered densities of UO.sub.2 pellets and Nb.sub.2 O.sub.5 -doped UO.sub.2 pellets produced from the starting powder consisting of AUC-UO.sub.2 and recycled U.sub.3 O.sub.8. Content of Density Sintered density of Sintered density of recycled U.sub.3 O.sub.8 in of green Sintered density 0.3 wt % Nb.sub.2 O.sub.5 -doped 0.5 wt % Nb.sub.2 O.sub.5 -doped starting powder pellet of control pellet UO.sub.2 pellet UO.sub.2 pellet (wt %) (g/cm.sup.3) (g/cm.sup.3) (g/cm.sup.3) (g/cm.sup.3) __________________________________________________________________________ 0 wt % 5.79 10.53 10.60 10.71 10 wt % 5.79 10.31 10.52 10.65 20 wt % 5.79 10.06 10.45 10.61 30 wt % 5.75 9.85 10.49 10.64 40 wt % 5.73 9.46 10.49 10.64 60 wt % 5.70 8.94 10.33 10.59 80 wt % 5.66 8.45 10.35 10.59 100 wt % 5.59 break 10.32 10.64 __________________________________________________________________________ EXAMPLE II The method was repeated as in EXAMPLE I except that TiO.sub.2 in quantities of 0.1% and 0.2% by weight, instead of Nb.sub.2 O.sub.5, were added. The sintered densities for EXAMPLE II are given in TABLE II, in which the TiO.sub.2 -doped UO.sub.2 pellets produced from the starting powders containing the recycled U.sub.3 O.sub.8 particle more than 10% by weight have densities higher than 94% TD. TABLE II __________________________________________________________________________ Sintered densities of UO.sub.2 pellets and TiO.sub.2 -doped UO.sub.2 pellets produced from the starting powder consisting of AUC-UO.sub.2 and recycled U.sub.3 O.sub.8. Content of Density Sintered density of Sintered density of recycled U.sub.3 O.sub.8 in of green Sintered density 0.1 wt % TiO.sub.2 -doped 0.2 wt % TiO.sub.2 -doped starting powder pellet of control pellet UO.sub.2 pellet UO.sub.2 pellet (wt %) (g/cm.sup.3) (g/cm.sup.3) (g/cm.sup.3) (g/cm.sup.3) __________________________________________________________________________ 0 wt % 5.79 10.53 10.72 10.75 10 wt % 5.79 10.31 10.62 10.66 20 wt % 5.79 10.06 10.54 10.60 30 wt % 5.75 9.85 10.36 10.42 40 wt % 5.73 9.45 10.31 10.38 60 wt % 5.70 8.94 10.32 10.34 80 wt % 5.66 8.45 10.30 10.34 100 wt % 5.59 break 10.32 10.37 __________________________________________________________________________ The grain sizes of these pellets are in the range of 30 .mu.m to 60 .mu.m (linearly intercepted), decreasing with the content of recycled U.sub.3 O.sub.8 particle in the starting powder. The resintered test carried out at 1700.degree. C. for 24 hours in hydrogen showed that the increase in density ranged from 0.6% TD to 0.9% TD, indicating that the sintered pellets were thermally stable. If an excessive amount of TiO.sub.2 is added, a considerably second phase which seems to be a liquid phase at a sintering temperature, forms in grain boundary. Massive formation of this phase will be detrimental to in-reactor fuel performance since material transport would be accelerated through this phase. EXAMPLE III The method was repeated as in EXAMPLE I except that Li.sub.2 O in quantities of 0.1% and 0.2% by weight, instead of Nb.sub.2 O.sub.5, were added. The sintered densities for EXAMPLE III are given in TABLE III, in which sintered densities of Li.sub.2 O-doped UO.sub.2 pellets are higher than those of undoped UO.sub.2 pellets. Li.sub.2 O has an advantage that it vaporizes during sintering and thus remains in the sintered pellet with much smaller quantity than the initially added. Li.sub.2 O-doped UO.sub.2 pellets have grain sizes larger than 100 .mu.m. TABLE III __________________________________________________________________________ Sintered densities of UO.sub.2 pellets and Li.sub.2 O-doped UO.sub.2 pellets produced from the starting powder consisting of AUC-UO.sub.2 and recycled U.sub.3 O.sub.8. Content of Density Sintered density Sintered density of Sintered density of recycled U.sub.3 O.sub.8 in of green of undoped 0.1 wt % Li.sub.2 O-doped 0.2 wt % Li.sub.2 O-doped starting powder pellet UO.sub.2 pellet UO.sub.2 pellet UO.sub.2 pellet (wt %) (g/cm.sup.3) (g/cm.sup.3) (g/cm.sup.3) (g/cm.sup.3) __________________________________________________________________________ 0 wt % 5.79 10.53 10.47 10.31 10 wt % 5.79 10.31 10.43 10.33 20 wt % 5.79 10.06 10.26 10.25 __________________________________________________________________________ EXAMPLE IV This example describes the case that UO.sub.2 powder obtained through the ADU (Ammonium Diuranate) route is used. This ADU-UO.sub.2 powder is different from the AUC-UO.sub.2 powder in that it has an average particle size of about 2 .mu.m and is much agglomerated. The ADU-UO.sub.2 powder can not be pressed directly due to the lack of flowability, so it needs granulation prior to pressing into green pellets. U.sub.3 O.sub.8 particle was produced in the same manner as that in EXAMPLE I. Starting powders were prepared by mixing fresh UO.sub.2 powder with the recycled U.sub.3 O.sub.8 particle in so-called "Turbula" for 1 hour, in which the amounts of recycled U.sub.3 O.sub.8 particle were 0% and 40% by weight. These starting powders were pre-pressed, pressed and sintered for the control pellets. In addition, Nb.sub.2 O.sub.5 powder in a quantity of 0.3% and TiO.sub.2 powder in a quantity of 0.1% by weight were added to the starting powders (2 compositions), respectively, and mixed again. Powder mixtures were pre-pressed at a pressure of 1 ton/cm.sup.2 into slugs, which were broken up on the 35 mesh sieve and then passed it. Granules so produced were mixed with zinc stearate in a quantity of 0.2% by weight, and then pressed into green pellets in a mold at a pressure of 3 ton/cm.sup.2. Green pellets were then heated in a sintering furnace to 1680.degree. C. at the rate of 5.degree. C./min under a flowing hydrogen and held for 4 hours and cooled down. The sintering procedures in the case of Nb.sub.2 O.sub.5 addition and TiO.sub.2 addition are the same as those in EXAMPLE I and EXAMPLE II, respectively. In Table IV, green and sintered densities for EXAMPLE IV are given. In the case of the control pellets sintered density decreases quite a lot due to the recycled U.sub.3 O.sub.8 particle in the starting powder. When TiO.sub.2 and Nb.sub.2 O.sub.5 were added in quantities of 0.1% and 0.3% by weight, respectively, the fuel pellets produced from the starting powder containing the recycled U.sub.3 O.sub.8 particle in a quantity of 40% by weight have densities higher than 94% TD. If the sintered density will be higher than the limit set by fuel specification, a pore former is additionally added to decrease the density, which is a normal practice in sintering the UO.sub.2 powder produced from the ADU route. The resintered test carried out at 1700.degree. C. for 24 hours in hydrogen showed that the increase in density was less than 1% TD. TABLE IV __________________________________________________________________________ Sintered densities of UO.sub.2 pellets and Nb.sub.2 O.sub.5 -doped and TiO.sub.2 -doped UO.sub.2 pellets produced from the starting powder consisting of AUC-UO.sub.2 and recycled U.sub.3 O.sub.8. Content of Density Sintered density Sintered density of Sintered density of recycled U.sub.3 O.sub.8 in of green of undoped 0.3 wt % Nb.sub.2 O.sub.5 -doped 0.1 wt % TiO.sub.2 -doped starting powder pellet UO.sub.2 pellet UO.sub.2 pellet UO.sub.2 pellet (wt %) (g/cm.sup.3) (g/cm.sup.3) (g/cm.sup.3) (g/cm.sup.3) __________________________________________________________________________ 0 wt % 5.54 10.70 10.84 10.80 40 wt % 5.73 10.03 10.76 10.57 __________________________________________________________________________