Production of ceramic nuclear fuel pellets

Abstract: Uranium dioxide nuclear fuel pellets are produced using a ceramic grade uranium dioxide powder derived from the reaction of uranium hexafluoride with steam and hydrogen. The powder is pelletized and the green pellets are transported through a furnace and subjected to fast firing typically at temperatures of the order of 2000-2200°C and for a time interval between 50 to 500 seconds. It has been found that fast-fired sintering of such powders under these conditions enables densities of the order of 95% theoretical density to be achieved, with resinter density changes of the order of 2% and lower, without requiring any mechanical compression of the hot pellets. The inclusion of seed crystals of uranium dioxide in the pelletized powder increases the grain size of the fuel pellets.

Description: This invention relates to the production of ceramic nuclear fuel pellets. We are aware of Us Patent No 4562017 which discloses the production of ceramic fuel pellets, containing fissile and/or fertile material in oxide form, by heating green pre-molded pellets of UO₂ or UO₂/PuO₂ powders to temperatures up to 2500°C at a rate less than 30°C second⁻¹ and subsequently above 1200°C at a rate greater than 30°C second⁻¹, and rapidly transferring the heated pellets into a die in which they are compressed to desired dimensions within a few milliseconds and then ejected. The patent recommends the use of powder having the smallest possible sintering activity and specifically refers to ground sintered scrap and a powder produced through thermal decomposition of uranyl nitrate or uranyl-plutonium nitrate. The present invention, by contrast, employs a ceramic grade powder derived from the reaction of uranium hexafluoride with steam and hydrogen. With such a powder it has been ground possible to achieve, by fast firing, pellet densities well in excess of 90% theoretical density (TD) without resorting to the complication of die pressing the heated pellets, ie densification is achieved solely by sintering. The powder employed in the present invention may incorporate an additive or additives; for example a fugitive pore former such as ammonium oxalate (as described in our prior British Patents Nos 1461263 and 1461264 to which reference should be made for further details). The additive may comprise a plutonium oxide (for example in an amount of the order of 20% by weight) having suitable ceramic properties amenable to fast firing to achieve sintered pellet densities above 90% TD without the need for die pressing of the hot pellets. The green pellets may include seed crystals which are essentially single crystals and may comprise uranium dioxide. The fast firing may be carried out within a temperature range of 1900°C-2500°C, preferably 2000°C-2200°C, for an interval of time which is no greater than 10 minutes, preferably between 50 seconds and 500 seconds and typically 200-500 seconds. In conventional processes for the production of nuclear fuel pellets using powder derived from the reaction of uranium hexafluoride with steam and hydrogen, the sintering process is carried out at about 1700°C for 3-4 hours or longer. For example, US Patent No 4020146 describes a three stage fluidised bed process in which uranium hexafluoride is reacted with steam and hydrogen to produce a ceramic grade uranium dioxide powder having low residual content of fluorides and other foreign materials. The resulting powder is comminuted in an attrition or other suitable mill and compacted into pellets at high pressure. These pellets are sintered at 1600 to 1800 degrees centigrade in a hydrogen atmosphere to obtain pellets having a theoretical density of from 90 to 96% with a final fluorine value of less than 10 ppm. However, to achieve these values it is necessary to sinter the pellets for 4 hours. In accordance with the present invention the sintering process is executed much more rapidly, eg less than 10 minutes compared with 3-4 hours, the temperature and time for sintering being selected such that the end product is sintered to a density above 90% TD, preferably at least 94% TD, and such that the density change on resintering the sintered pellets of 1700°C for 24 hours in a hydrogen atmosphere is of the order of 2% or less, preferably no greater than 1%. According to the present invention in a method of producing ceramic nuclear fuel pellets said method comprising forming green pellets comprising uranium dioxide powder, and introducing the pellets into a heating zone in which they are subjected to fast firing, the powder comprises a ceramic grade powder having a specific surface area of at least 1m²g-¹ and derived from the reaction of uranium hexafluoride with steam and hydrogen, and the fast firing is performed at a sintering temperature of at least 1900°C for an interval of time up to 10 minutes, whereby densification above 90% theoretical density is achievable solely by sintering without die pressing the hot pellets. The fast firing step may be carried out in vacuo or in a non-oxidising atmosphere - for example a reducing environment. The reducing atmosphere may be a hydrogen-containing atmosphere which preferably contains some moisture - for example 99% hydrogen and 1% moisture which may be generated by addition of CO₂ in the hydrogen-containing atmosphere. The fast firing may be carried while the pellets are transported through a furnace, eg continuously, or may be carried out in a batch furnace. The starting powder employed in the method of the invention is preferably derived by the process described in our prior British Patent No 1320137 or 2064503. The starting powder preferably has a specific surface area of between 2 and 3 m²g⁻¹. Where the UO₂ starting powder is combined with a fugitive pore former, such as ammonium oxalate (typically in an amount of the order of up to 1% by weight) or a lubricant to aid pressing of the green pellets, the pellets may be pre-heated slowly at relatively lower temperatures eg 250°C-800°C depending on the properties of the lubricant to decompose and drive off the pore former or lubricant before being subjected to fast firing at temperatures of at least 1900°C at a rate of up to 1300°C min⁻¹. In many cases such a pre-treatment will be unnecessary, and the rate of heating might be from 300°C min⁻¹ up to 700°C min⁻¹, for example in temperature ranges of between 300 and 400°C min⁻¹, or 600 and 700°C min⁻¹. When the fast firing process is carried out by transporting the pellets continuously through an oven in which the temperature is maintained at least at 1900°C, typically 2000°C-2200°C, the rate of travel of the pellets is such that the time during which the pellets are exposed to the fast firing temperature in the hot zone of the oven is sufficient to achieve a density above 90% TD and a density change on resintering, under the previously stated conditions of the order of 2%, and more preferably 1%. In experimental work carried out, a UO₂ powder derived from the reaction of UF₆ with steam and hydrogen was granulated and pressed at pressures of 3.1 tonnes cm⁻², with die wall lubrication, to produce green pellets. The pellets were rapidly heated from room temperature to various sintering temperatures at a rate of 300°-400°C min⁻¹ and maintained at the selected sintering temperature of 200 seconds in each instance. The following densities were obtained: Sintering Temp °C Density (% TD) Resinter Density (% TD) Density change on resinter (% TD) 1700 95.4 99.3 3.9 1800 96.7 99.3 2.6 1900 97.3 99.4 2.1 2000 97.8 99.2 1.4 2100 98.1 99.0 0.9 It can be seen that at a sintering temperature of 1900°C, the density change on resinter was of the order of 2% TD (in fact 2.1% TD) which is an acceptable level for some applications. Seeding of the green pellets might have advantages, for example to increase grain size as described in the following Example. EXAMPLE Precursor green pellets (about 11mm diameter x 11mm long) or uranium dioxide where heated to 2100°C at 20°C min⁻¹ in a dry hydrogen environment, and held at this temperature for one hour. The pellets were then cooled at 20°C min⁻¹ between 2100°C and 450°C and subsequently allowed to cool naturally to ambient temperature. During cooling the pellets disintegrated into a powder which was sieved, and fractions less than 37 microns were retained - essentially single crystal seeds. The seeds were added to a matrix uranium dioxide powder at 2% and 5% level (eg 2g seeds and 98g matrix powder). The resulting mixed powder was granulated and the granules pressed into pellets using die wall lubrication at 4 tonnes cm⁻². The pellets were fast fired from ambient temperature to a sintering temperature of about 2100°C in a reducing environment - hydrogen +1% CO₂. The rate of heating was between 600°C min⁻¹ and 700°C⁻¹. The pellets were held at the sintering temperature for between 0 and 500 seconds and then rapidly cooled between 2100°C and 1600°C at a rate between 850°C and 1000°C min⁻¹. The results obtained are displayed in the following Table: Densities were obtained by an immersion technique using water as the liquid. Grain sizes were established by the ASTM mean linear intercept method. Resintering took place at 1700°C in a hydrogen environment for 24 hours to assess thermal stability of the fast fired material. The advantages from the use of seed crystals are apparent from the Table, particularly in run 3 and 4 where relatively large grain sizes were obtained. Further information relating to the use of seed crystals may be obtained from British Patent Specification No 2177249A.

Claims: A method of producing ceramic nuclear fuel pellets said method comprising forming green pellets comprising uranium dioxide powder, and introducing the pellets into a heating zone in which they are subjected to fast firing, characterised in that the powder comprises a ceramic grade powder having a specific surface area of at least 1m²g⁻¹ and is derived from the reaction of uranium hexafluoride with steam and hydrogen, and the fast firing is performed at a sintering temperature of at least 1900°C for an interval of time up to 10 minutes, whereby a densification above 90% theoretical density is achievable solely by sintering without die pressing the hot pellets. A method as claimed in Claim 1, wherein the uranium dioxide powder is combined with an additive. A method as claimed in Claim 2, wherein the additive comprises plutonium dioxide. A method as claimed in any one of Claims 1-3, wherein the uranium dioxide powder has a specific surface area between 2 and 3m²g⁻¹. A method as claimed in any one of Claims 1-4, wherein the sintering is carried out within the temperature range of 1900°C-2500°C. A method as claimed in Claim 5, wherein the sintering is carried out within the temperature range 2000°-2200°C. A method as claimed in any one of Claims 1-6, wherein the fast firing is carried out for an interval between 50 seconds and 500 seconds. A method as claimed in any one of Claims 1-7, wherein the fast firing is carried out for an interval between 200 and 500 seconds. A method as claimed in any one of Claims 1-8, wherein the rate of heating the green pellets is from 300°C min⁻¹ to 1300°C min⁻¹. A method as claimed in Claim 9, wherein the rate of heating is between 300°C and 400°C min⁻¹. A method as claimed in Claim 9, wherein the rate of heating is between 600°C and 700°C min⁻¹. A method as claimed in Claim 9, wherein the green pellets are pre-heated slowly at between 250°C and 850°C to decompose and drive off any fugitive pore former and lubricant therein, before fast firing takes place. A method as claimed in any one of the preceding Claims, wherein the green pellets include seeds which comprise essentially single crystals. A method as claimed in Claim 13, wherein the single crystals comprise uranium dioxide. A method as claimed in Claim 14, wherein the seed crystals are formed by heating green precursors comprising uranium dioxide to about 2100°C in a reducing environment and holding the precursor pellets at that temperature for about one hour, disintegrating the precursor pellets by cooling the precursor pellets at about 20°C min⁻¹ between 2100°C and 450°C and then naturally from 450°C to ambient temperature, and selecting seeds from the disintegrated pellets comprising essentially single crystals of less than 37 microns. A method as claimed in any one of Claims 13-15, including mixing the seeds with a matrix powder comprising uranium dioxide to form the green pellets therefrom. A method as claimed in Claim 16, wherein the seeds comprise up to about 5% by weight of the green pellets. A method as claimed in any one of the preceding Claims, wherein after fast firing the pellets are rapidly cooled. A method as claimed in Claim 18, wherein the pellets are cooled between 2100°C and 1600°C at a rate between 850 and 1000°C min⁻¹. A method as claimed in any one of the preceding Claims, wherein fast firing is effected by moving the pellets continuously through the heating zone.

Inventors: WOOD GEOFFREY ALAN, WOOD, GEOFFREY ALAN
Applicants: BRITISH NUCLEAR FUELS PLC
Application year: 1988
Main classification: G21C3
IPC codes: G21C21, G21C3