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Sintering Uranium Oxide Using a Preheating Step

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Sintering Uranium Oxide Using a Preheating Step United States Patent [19] [ii] 4,052,278 Jensen et al. [45] Oct. 4, 1977 [54] SINTERING URANIUM OXIDE USING A mum fluoride removal from the compacted pellets PREHEATING STEP along with optimum sintering in a single process. The preheating step of this process is conducted in a temper- [75] Inventors: Neil John Jensen, Mountain View; ature range of about 600° to about 900° C and the pellets Yogesh Nivas, San Jose; Douglas are held for at least twenty minutes, and preferably Randall Packard, Sunol, all of Calif. about 60 minutes, in an atmosphere having a composi- [73] Assignee: General Electric Company, San Jose, tion in the range of about 10 to about 75 percent hydro- Calif. gen by volume with the balance being carbon dioxide. The sintering step is conducted at a temperature in the [21] Appl. No.: 560,534 range of about 900° C to; 1500° C in the presence of an [22] Filed: Mar. 20, 1975 atmosphere having a composition in the range of about 0.5 to about 90 percent hydrogen by volume with the [51] Int. C1.2 G21C 3/58 balance being carbon dioxide. The reduction step re- [52] us. ri . 252/301.1 R: 264/ 5 duces the oxygen to metal ratio of the pellets to a range [58] Field of Search 264/.5; 176/89; of about 1.98 to 2.10:1 and this is accomplished by grad- 252/301.1 R ually cooling the pellets for about 30 to about 120 min- [56] References Cited utes from the temperature of the sintering step to about 1100° C in an atmosphere of about 10 percent to about U.S. PATENT DOCUMENTS 90 percent hydrogen by volume with the balance being 3,786,120 1/1974 Hollander et al 264/.5 carbon dioxide. Thereafter the pellets are cooled to 3,808,145 4/1974 Packard et al 264/.5 about 100° C under a protective atmosphere and in one 3,872,022 3/1975 Hollander et al 264/. 5 preferred practice the same atmosphere used in the Primary Examiner—Leland A. Sebastian reduction step is used in the cooling step. The preheat- Attorney, Agent, or Firm—Ivor J. James, Jr.; Sam E. ing, sintering and reduction steps may also be con- Laub; Samuel E. Turner ducted with their respective atmospheres having an initial additional component of water vapor and the [57] ABSTRACT water vapor can comprise up to about 20 percent by Compacted pellets of uranium oxide or uranium oxide volume. with one or more additives are heated in a kiln in a process having a preheating step, a sintering step, a reduction step, and a cooling step in a controlled atmo- sphere. The process is practiced to give a range of tem- perature and atmosphere conditions for obtaining opti- 21 Claims, 2 Drawing Figures 300L Kiln Length 03 sr n> n> to 20 O 17. 18 2! X 16 x 19 « /5\ £2 to 4 / 12 3 ff /J\ V sy 1 H=HJ|C ji\ <n in jn / o /J Ul // N> Preheati ng I Sintering II Reduction Coo Iing U) U> Zone r~ Zone Zone Zone o F/0 / 4.0 C/5 3.8 & 3.6 0) 3.4 ft a» k- 3.2 a> c 3.0 o <n o 2.8 « (fc- CI 2.6 v© cn -4 c 2.4 -a c 2.2 a E 2.0 ai>. 0) TJ 1.8 GO w tr o 1.6 n> 3 <-»n>• li- 1.4 to o o c 1.2 i-h K) u> w 1.0 O "o .8 .0s 0 .6 .4 o - .2 v* .0 10 20 25 30 35 40 45 55 U> % H2 in Unreacted State-Preheat Zone o 2 ,052,330 ^ 1 about 5 hours followed by cooling the pellets in an SINTERING URANIUM OXIDE USING A atmosphere having an oxygen partial pressure in the PREHEATING STEP range of about 10-4 to 10-18 atmospheres of oxygen with a preferred value being in the range of 10-12 to BACKGROUND OF THE INVENTION 5 10-18 atmospheres of oxygen. In another embodiment This invention relates to the production of sintered there is disclosed a process using a furnace having three uranium oxide containing compositions. One of the zones of temperature consisting of a preheating zone, a very important utilities of uranium oxide, especially sintering zone and a cooling zone. The first region uranium dioxide, is in nuclear power plants as a fuel in through which the boat passes is a preheating zone the generation of electric power. The uranium dioxide, 10 which has an increasing temperature as the boat moves either alone or in a mixture with other ceramics such as closer to the sintering zone. In the preheating zone the gadolinium oxide or plutonium oxide, is compacted to a atmosphere of the kiln is a mixture of hydrogen and given size and shape and sintered to achieve dense bod- carbon dioxide. Fluoride impurities are expelled from ies for use in a nuclear fuel rod. The uranium present in the pellets in this zone before the ceramic boat reaches uranium dioxide must be enriched with the U-235 iso- 15 the sintering zone. When the boat reaches the sintering tope for use in light water reactors. Presently this is zone, the temperature is maintained in the range of 900° done in a gaseous state, the preferred practice being to to 1500° C during which a substantial proportion of the use uranium hexafluoride. After enrichment it is neces- sintering practiced in this invention occurs. The atmo- sary to convert the uranium hexafluoride to uranium sphere in the sintering zone according to this practice is dioxide. The resulting uranium dioxide can contain 20 the same as that in the preheat zone and is in the range undesired fluoride ion concentrations and an oxygen to of about 0.5 to about 90 percent by volume hydrogen metal ratio above the desired ratio of about 1.98:1 to with the balance being carbon dioxide. When a boat about 2.10:1. reaches the cooling zone the temperature falls as the The sintering of uranium dioxide structures has been distance from chemical reduction is increased. U.S. Pat. used as the step in the process to attempt to reduce the 25 No. 3,872,022 is assigned to the same assignee as the oxygen and the fluoride content of the uranium dioxide. present application. The current practice has been the use of wet hydrogen A process using an atmosphere of a mixture of hydro- atmospheres at temperatures preferably greater than gen and carbon dioxide for defluorinating and control- 1600° C to achieve dense bodies of uranium dioxide. ling the oxygen to metal ratio of a particulate composi- Past experience indicates a certain amount of water 30 tion is disclosed in U.S. patent application Ser. No. vapor mixed with the hydrogen is required to remove 358,738, filed May 9,1973 in the name of Yogesh Nivas, the fluoride from compacted ceramic structures during entitled "Ceramic Defluorination and Reduction Pro- sintering, but the wet hydrogen process has not been cess." This application is assigned to the same assignee satisfactory when the ceramic has high fluoride concen- as the present application. trations. 35 Another process using an atmosphere of a mixture of Another method presented in U.S. Pat. No. 3,375,306 hydrogen and carbon dioxide along with an additive of for sintering dense uranium dioxide structures with or water vapor as introduced to the furnace is disclosed in without ceramic additives is to heat the compressed U.S. Pat. No. 3,808,145. This process is for defluorinat- powder at a temperature of 1300° to 1600° C in a sinter- ing and controlling the oxygen-to-metal ratio of a par- ing atmosphere of carbon dioxide or a mixture of car- 40 ticulate composition containing uranium compounds. bon dioxide and carbon monoxide and cooling the sin- Sintering at temperatures of about 1600° C or higher tered structure in a reducing atmosphere which varies to produce uranium dioxide structures is a relatively as the composition of the structure varies. Where the expensive process due to high rates of energy consump- structure being sintered is uranium dioxide the cooling tion. Lower sintering temperatures under controlled gas is dry hydrogen, wet hydrogen or a mixture of 45 atmospheres have the advantage of yielding a con- carbon dioxide and carbon monoxide. Where the struc- trolled oxygen to metal ratio of the sintered structure ture is uranium dioxide with an additive of plutonium and more effectively removing undesirable impurities dioxide, the cooling gas is steam or carbon dioxide from the sintered structure such as undesirable impuri- mixed with carbon monoxide. The use of a mixture of ties of fluoride ions. carbon dioxide and carbon monoxide is more costly 50 Lower sintering temperatures, in addition to saving than use of wet hydrogen but enables the use of lower energy, represent a considerable cost savings in cheaper temperatures to achieve sintered structures of high den- building materials for kiln construction, a longer func- sity. However this carbon monoxide-carbon dioxide tional life for the sintering kiln and its associated fixtures sintering atmosphere does not appreciably decrease the and less corrosion of the kiln components. fluoride content of the uranium dioxide structures. 55 The use of an atmosphere of a mixture of hydrogen In U.S. Pat. No. 3,872,022, issued Mar. 18,1975, in the and carbon dioxide does enable the use of a lower tem- names of W.
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