Patent Number: 045029873
Section: description

DETAILED DESCRIPTION Spherules of enhanced product quality containing mixtures of uranium, plutonium, thorium, and mixtures thereof are derived by control of the crystallite size of the phases therein by heat treatment of the HMTA-urea feed solution to be used in the formation of the spherules. The product spherules are suitable for formation of nuclear-reactor fuels capable of being directly loaded into fuel elements by vibration-packed or gel-derived-pellet technologies. Preferably, it has been found that high density spherules with small crystallites make the best vibra-packed fuel forms and that moderate density spherules with large crystallites make the best gel-derived-pellet fuel forms. The method of the present invention is directed to the optimization of crystallite growth in mixed-fuel spherules of nuclear-reactor fuels and is effected by heat treating the concentrated HMTA-urea feed solutions. Basically, three steps are combined in the heat-treating procedure. The first step comprises heating the feed solution to boiling, i.e., about 104.degree. C., within about thirty minutes. The second step comprises maintaining this boiling temperature for a sufficient duration without causing excessive urea decomposition. Heating the solution to a temperature less than boiling will not provide the desired control of the crystallite size. The third step comprises cooling the boiled solution from boiling down to about ambient temperature. Preferably, the heating step takes place within about thirty minutes so that the solution is taken very rapidly from about ambient temperature to boiling. Typically, the duration at which the solution then boils will vary according to the desired end use of the product spherules and will normally be in the range of about 0 to 60 additional minutes yielding a total heating and boiling duration of about 30 to 90 minutes. The shorter durations are preferred for denser spherule formation, hence, the vibra-packed fuels, while the longer durations are preferred for the moderately dense spherules. Total durations for heating and boiling of much below about 30 minutes accomplish little or no benefit and total durations in excess of about 90 minutes are unnecessary to cause the desired effect and may result in excessive urea decomposition. The third step of the heat-treating procedure comprises cooling the resultant solution in about 30 to 40 minutes to approximately ambient temperature, i.e., 25.degree. C..+-.5.degree. C. before mixing with the metal solution. It has been found that the heat-treatment, or boiling-cooling procedure, of this invention significantly and beneficially effects the control of the crystallite size of the phase or phases present in the resultant air-dried spherules. For example, crystallites of about 1200 .ANG. to 3000 .ANG. may be derived in urania-plutonia spherules (Pu/U+Pu=0.25), by simply heating and boiling the HMTA-urea feed solution for about 90 minutes. The resultant spherules have been found to have a tap density of about 0.92 g/cc and are especially suited for forming gel-derived pellets of about 93 to 95% theoretical density. Micrographic examination of such pellets have revealed good-quality ceramic pellets characterized by the absence of visible voids and defects or structural remnants of the spherules. In comparison, urania-plutonia spherules prepared without heat treatment of the HMTA-urea feed solution have been found to be characterized by crystallites within the range of about 500 .ANG. to 1400 .ANG. and by high tap densities of about 1.31 g/cc. Pellets prepared from these denser spherules were of about 84% theoretical density. Since 88% theoretical density is considered to be the absolute minimum density for loadable fuel pellets for nuclear-reactor applications, heat-treated spherules prepared herein are more suitable for making gel-derived-pellet fuel forms. EXAMPLE In a series of experiments, the effect of heat treatment on HMTA-urea feed solutions provided to an internal gelation process was evaluated. Spherules were tailored within a range of HMTA-urea preparations to establish the relationship between duration and extent of heat treatment as well as ultimate calcined pellet density. The first experiment was designated a control test wherein the HMTA-urea feed solution was not heat-treated prior to deployment in spherule forming operations of the aforementioned copending patent application of assignee. The prepared spherules were found to have a tap density of about 1.29.+-.0.02 g/cc after air drying at 110.degree. C. Electron micrographs of specimens of these high-density spherules revealed a large population of crystallites within the size range of about 500 .ANG. to 1400 .ANG. with a maximum urania crystallite size of about 1500 .ANG.. Sintered pellets prepared from these spherules were not of good ceramic quality and were only of about 83.7% theoretical density. Micrographic examination of sections of these pellets revealed structural remnants of the pressed spherules which is undesirable. In a second experiment, the HMTA-urea feed solution was heated to boiling (104.degree. C.) in about thirty minutes but not maintained at that temperature for any duration. The prepared spherules were of about 1.24 g/cc tap density. Thus, little difference can be effected in spherule density by heat treatment of the feed solution without sustained boiling. In each subsequent experiment, a 500 ml solution of 3.2M HMTA and 3.2M urea was heat treated under reflux conditions to the solution boiling point of about 104.degree. C. for varying periods of measured time within the range of about 30 to 90 minutes and then rapidly cooled to about ambient temperature in about 30 minutes. A standard heat-up time of about thirty minutes to boiling was applied in each experiment so that only the actual time of boiling was varied. Thereafter, each HMTA-urea feed solution was combined with a solution containing a metal selected from the group consisting of uranium, plutonium, thorium, or mixtures thereof. The results of boiling durations as a function of sphere density are graphically depicted in the figure of the drawing. As demonstrated in the figure, the effect of heat treatment produces an almost linear function in spherule tap density with more boiling time producing spherules of lower densities. When the feed solution was heat treated for about 40 minutes, the product spherules were found to have a tap density of about 1.04 g/cc which is much below that found in the first and second tests with no or only minimal heat treatment. Electron micrographs of these spherules revealed a large population of crystallites within the size range of about 1200 .ANG. to 3000 .ANG. with a maximum urania crystallite size of about 3000 .ANG.. Sintered pellets prepared from these spherules appeared to be of good ceramic quality and of about 95.4% theoretical density. Visual examination of the microstructures of these pellets revealed no voids or defects and the complete absence of structural remnants from the pressed spherules. From the foregoing, it can readily be concluded that tap density of air-dried spherules to be used in the manufacture of nuclear-reactor fuels may be varied within the range of about 1.31 to 0.92 g/cc by heat treating the HMTA-urea feed solution for a duration in the range of about 30 minutes to 90 minutes, respectively. By varying spherule tap densities within this range, the ultimate quality of the nuclear-reactor fuel form may be effectively determined. In addition to a better understanding of the reasons for variant product qualities heretofore encountered in the art, the method of the present invention has the added advantages of providing greater process flexibility to the method described in the aforementioned copending application including the probability for commercial production of high plutonium content mixed fuels (Pu/U+Pu=0.35) in pellet form, a highly tolerant system for NH.sub.4 NO.sub.3 present in nuclear fuel reprocessing streams which formerly required pretreatment for removal, and a simplified system with minimal reagent requirements for processing or waste treatment .