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Home , Uranium dioxide

u PATENT SPECIFICATION < > 1285 668 Q NO DRAWINGS Q (21) Application No. 59814/68 (22) Filed 16 Dec. 1968 H (23) Complete Specification filed 2 Dec. 1969 (45) Complete Specification published 9 Aug. 1972 Qf) (51) International Classification G21C 3/62 (52) Index at acceptance H G6C 734 740 CIA N36 (72) Inventors FRANK RIGBY and JOHN BRIAN AINSCOUGH

(54) IMPROVEMENTS IN FISSILE MATERIALS

(71) We, UNITED KINGDOM ATOMIC with powder of the material ENERGY AUTHORITY, London, a British prior to pressing and sintering of the nuclear Authority, do hereby declare die invention, fuel material to form solid bodies of the 50 for which we pray that a patent may be material. These known additives have been 5 granted to us, and the method by which it is found to be not as effective as desirable in to be performed, to be particularly described inhibiting the release of gaseous fission pro- in and by the following statement: — ducts from the nuclear fuel material when This invention relates to ceramic fissile irradiated in a , probably be- 55 materials. cause the additives are located mainly at 10 The invention is particularly concerned with the grain boundaries in the sintered nuclear ceramic fissile materials for us as fuel in a fuel material. nuclear reactor. When such ceramic fissile It is also known to provide small amounts materials as or dioxides of as an additive in oxide 60 or mixture of these are irradiated for nuclear fuel particles such as oxide 15 long periods of time at high temperatures or thorium- in order to improve stable gaseous fission products, in particular the dissolution properties of the fuel particles xenon and krypton, are produced by fission in acid media during chemical processing of and ate released from the ceramic material the fuel particles after irradiation in a nuclear 65 at a rate which is determined by the tempera- reactor. 20 ture of the material. Such gas release from According to the present invention a sin- ceramic nuclear fuel material is undesirable. tered polycrystalline ceramic fissile material For example in the case of a fuel element in is provided consisting essentially of uranium which the fuel material is held within an outer dioxide containing an additive soluble in a 70 gas tight container a high gas pressure will hyperstoichiometric form of uranium dioxide 25 be set up in the container and there is a risk but which is insoluble in hypostoichiometric that the container will be strained to failure. uranium dioxide and dispersed in fine particu- Also the formation of babbles of the gaseous late form within the grains of the uranium fission products at the grain boundaries in dioxide. 75 the fuel material to swelling of the This fine dispersion of the additive within 30 fuel material, with consequent straining and; the grains of the ceramic fissile material has possible failure of the container. Further it been found to be more effective in inhibiting is usual to include a quantity of helium gas the release of gaseous fission products from in the container in order to assist the transfer the material than is the case with the addi- 80 of heat from the fuel material to the con- tives previously proposed. The fine disper- 35 tainer. The pressure of released gases such as sion of the additive within the grains of the xenon and krypton is additive to the pressure ceramic fissile material is throught to act as of the helium. Also the addition of the re- pinning or nucleating sites for gas bubbles so leased gases to the helium results in a reduc- preventing movement of the gaseous fission 85 tion in its which may products to grain boundaries, bubble linkage 40 result in overheating of the fuel material. and consequent gas release. It is known to provide small amounts of additives such as , and neo- is a suitable additive in dymium oxides in ceramic nuclear fuel amount 0.15 to 3.7 weight percent, the pre- materials such as uranium dioxide in order to ferred range being 0.75 to 2.0 weight per- 90 45 inhibit the release of gaseous fission products cent. from such nuclear fuel materials. The addi- Magnesium oxide is a suitable additive in tive material is mixed in fine powder form amount 0.15 to 3.7 weight percent, the pre- 6 1,287,143 2

ferred range being 0.75 to 2.0 weight per- volume in dioxide, at a temperature cent. in the range 1400°C to 1600°C, the material Another suitable additive is then being heated in a reducing atmosphere of oxide in amount 0.15 to 3.3 weight percent, at a temperature in the range 5 the preferred range also being 0.75 to 2.0 1200°C to 1400°C. 70 weight percent. The following are examples of the method The invention also relates to a method for of the invention relating to the production of preparing a sintered polycrystalline ceramic bodies of sintered uranium dioxide containing fissile material consisting essentially of additions of magnesium oxide. 10 uranium dioxide containing an additive which is dispersed in fine particulate form within EXAMPLE I 75 the grains of the ceramic material. Uranium dioxide powder intimately mixed The invention is based on the discovery with 0.75 weight percent magnesium oxide that in the case of certain ceramic fissile powder is pressed into pellets and the pellets 15 materials which can exist in more than one are sintered at 1400°C for 2 hours in an more an additive can be specified which is atmosphere of 50% hydrogen/50% carbon 80 soluble in one form of the ceramic material dioxide by volume to produce pellets of den- and insoluble in another form of the material. sity above 10 grams/cubic centimetre. This In particular in the case of uranium dioxide, atmosphere is slightly oxidising so that the 20 the cations of which can exist in two valency uranium dioxide is oxidised to the hyper- states, an additive can be specified which stoichiometric form. Magnesium oxide is 85 is soluble in the oxidised hyperstoichiometric soluble in hyperstoichiometric uranium dioxide form of uranium dioxide but insoluble in the so that the magnesium oxide is taken into reduced hypostoichiometric form of the di- solution by the uranium dioxide. The sintered 25 oxide. It may also be insoluble in the stoichio- pellets are subsequendy heated in pure hydro- metric form. The uranium ion can exist in gen for twelve hours at 1200°C which reduces 90 the tetravalent or hexavalent state. In the the hyperstoichiometric uranium dioxide to tetravalent state the uranium ion has an ionic the stoichiometric form. As magnesium oxide radius of 0.97 Angstroms whilst in the hexa- is insoluble in stoichiometric uranium dioxide 30 valent state the ionic radius is 0.80 Ang- this causes precipitation of the magnesium stroms. Thus in hypo-stoichiometric (or oxide predominantly within the grains of 95 stoichiometric) uranium dioxide with the the uranium dioxide, on a fine scale of uranium ions wholly or partly in the tetra- approximately 1016 particles/cubic centimetre. valent state certain other oxides such as mag- The reduction temperature of 1200°C is 35 nesium oxide or are in- sufficiently low for grain growth not to occur soluble, one of the reasons for this insolu- as otherwise the precipitate of magnesium 100 bility being the fact that their cations are oxide may be swept from within the grains too large with respect to that for tetravalent to the grain boundaries as grain growth occurs. uranium for solution to be possible. If how- 40 ever the uranium dioxide is oxidised to the EXAMPLE II hyperstoichiometric form with the uranium Uranium dioxide powder intimately mixed ions wholly or partly in the hexavalent state with 1.0 weight percent of magnesium oxide 105 this difference between the ion sizes becomes powder is pressed into pellets and the pellets less and solution becomes possible. The ex- are sintered at 1600°C for 24 hours in an 45 tent of solubility appears to be dependent on atmosphere of 5% hydrogen/95% carbon di- the extent to which the tetravalent uranium oxide by volume. The sintered pellets are sub- ions have been oxidised to the hexavalent sequently heated in pure hydrogen for twelve 110 state. hours at 1400°C producing as in Example I According to this aspect of the invention a pellets having a fine dispersion of magnesium 50 sintered polycrystalline ceramic fissile material oxide predominantly within the grains of the comprising uranium dioxide containing an uranium dioxide. additive which is dispersed in fine particu- late form within the grains of the uranium EXAMPLE III 115 dioxide is prepared by sintering uranium Uranium dioxide powder intimately mixed 55 dioxide containing an additive soluble in with 2.0 weight percent magnesium oxide hyperstoichiometric uranium dioxide but in- powder is pressed into pellets and the pellets soluble in hypo-stoichiometric uranium di- are sintered at 1600°C in pure carbon di- oxide in an oxidising atmosphere to bring oxide, it being ensured that is not 120 the additive into solution in hyperstoichio- present as a significant impurity in the car- 60 metric uranium dioxide and the sintered bon dioxide. The sintered pellets are subse- material is then heated in a reducing atmos- quently heated in pure hydrogen for twelve phere to convert the uranium dioxide to a hours at 1400°C. form in which the additive is insoluble. The above examples cover the preferred 125 Sintering may be carried out in an oxidis- range of 0.75 to 2 weight percent magnesium 65 ing atmosphere of 0 to 50% hydrogen by oxide additions to uranium dioxide although 6 1,286,<568 3

additions of magnesium oxide from a lower ing preferred temperature range of 1200°C— 65 limit of 0.15 weight percent, up to the 1400°C for the hydrogen reduction step. theoretical limit of solubility of 3.7 weight In general species of oxide which will be percent in uranium dioxide are possible. suitable additions to uranium dioxide will 5 As for the gas compositions used for the probably have metal ion radii in the range sintering atmosphere a range of 0—85% 0.50 to 0.75 Angstroms, will have high melt- 70 hydrogen by volume in is pos- ing/boiling points and will have one valence sible although the above examples cover the state. Multivalent ions raise the complication preferred range of 0—50% hydrogen by that when the uranium oxide is oxidised or 10 volume in carbon dioxide. The preferred range reduced they may follow suit changing ion of sintering temperatures is 1400°—1600°C size as they do. Another suitable oxide addi- 75 although sintering is possible in the range tion meeting the above requirements is thought 1200°C—2000°Cj the temperature of 2000°C to be oxide. being the upper limit of sintering temperature 15 as liquid phases are thought to occur in the WHAT WE CLAIM IS: — magnesium oxide/uranium oxide system at 1. A sintered polycrystalline ceramic fissile about 2100°C. material consisting essentially of uranium di- 80 The above examples also cover the pre- oxide containing an additive soluble in a ferred range of hydrogen reduction tempera- hyperstoichiometric form of uranium dioxide 20 tures ie 1200°C—1400°C having in mind but insoluble in hypostoichiometric uranium that the reduction step must be carried out dioxide and dispersed in fine particulate form at a lower temperature than the sintering step. within the grains of the uranium dioxide. 85 As a further example of the method of the 2. A ceramic fissile material as claimed invention relating to the production of bodies in claim 1 comprising uranium dioxide con- 25 of sintered uranium dioxide containing addi- taining as the additive magnesium oxide in tions of aluminium oxide, ammonium di- the range 0.15 to 3.7 weight percent. is coprecipitated with 1.0 weight per- 3. A ceramic fissile material as claimed in 90 cent aluminium hydroxide by the addition of claim 1 comprising uranium dioxide con- ammonia to a solution of uranium nitrate and taining as the additive magnesium oxide in 30 aluminium nitrate. The precipitate is cal- the range 0.75 to 2.0 weight percent. cined at 800°C and reduced in hydrogen at 4. A ceramic fissile material as claimed in 700°C and the resulting powder which con- claim 1 comprising uranium dioxide contain- 95 sists of uranium dioxide containing approxi- ing as the additive aluminium oxide in the mately 1.0 weight percent aluminium oxide range 0.15 to 3.3 weight percent . 35 is pressed into pellets and the pellets are 5. A ceramic fissile material as claimed in sintered for 24 hours in an atmosphere of claim 1 comprising uranium dioxide contain- 5% hydrogen/95% carbon dioxide by volume ing as the additive aluminium oxide in the 100 at a temperature of 1600°C. The aluminium range 0.75 to 2.0 weight percent. oxide is taken into solution by the hyper- 6. A method for preparing a sintered poly- 40 stoichiometric uranium dioxide which is crystalline ceramic fissile material comprising formed. The sintered hyperstoichiometric uranium dioxide containing an additive which pellets are subsequently heated in pure hydro- is dispersed in fine particulate form within 105 gen for 6 hours at 1400°C which reduces the grains of the uranium dioxide wherein the hyperstoichiometric uranium dioxide to the uranium dioxide containing an additive sol- 45 stoichiometric form. As aluminium oxide is uble in hyperstoichiometric uranium dioxide insoluble in stoichiometric uranium dioxide but insoluble in hypostoichiometric uranium this causes precipitation of the aluminium dioxide is sintered in an oxidising atmosphere 110 oxide predominantly within the grains of the to bring the additive into solution in hyper- uranium dioxide in the form of very small stoichiometric uranium dioxide and the sin- 50 particles approximately 20 Angstroms in tered material is then heated in a reducing diameter. atmosphere to convert the uranium dioxide to a form in which the additive is insoluble. 115 As in the case of magnesium oxide addi- tions the preferred range of aluminium oxide 7. A method for preparing a ceramic fissile additions is 0.75 to 2.0 weight percent, oxide material as claimed in claim 6 wherein 55 although additions of aluminium oxide from the additive is magnesium oxide in amount a lower limit of 0.15 weight percent up to the 0.15 to 3.7 weight percent. theoretical limit of solubility of 3.3 weight 8. A method for preparing ceramic fissile 120 percent are possible. oxide material as claimed in claim 6 wherein the additive is aluminium oxide in amount Also the preferred range of gas compositions 0.15 to 3.3 weight percent. 60 for sintering is 0—50% hydrogen by volume 9. A method for preparing a ceramic fissile in carbon dioxide. oxide material as claimed in claim 7 wherein 125 Sintering temperatures may be in the range the material is sintered in an oxidising atmos- 1200°C—1900°C, the preferred range again phere comprising 0 to 85% hydrogen by being 1400°C—1600°C, with a correspond- volume in carbon dioxide in the temperature 6 1,286,<568 4

range 1200°C—2000°C and the material is volume in carbon dioxide in the temperature then heated in a reducing atmosphere of range 1400°C to 1600°C and then being hydrogen at a temperature below the tempera- heated in a reducing atmosphere of hydrogen ture of sintering to convert the material to at a temperature lower than the sintering tem- 25 5 the stoichiometric form. perature in the range 1200°C—1400°C. 10. A method for preparing a ceramic 12. A method for preparing a ceramic fissile oxide material as claimed in claim 8 fissile oxide material as claimed in claim 6 wherein the material is sintered in an oxidis- wherein the additive is aluminium oxide in ing atmosphere comprising 0 to 85% hydro- amount 0.75 to 2.0 weight percent, the 30 10 gen by volume in carbon dioxide in the tem- material being sintered in an oxidising atmos- perature range 1200°C—1900°C and the phere comprising 0 to 50% hydrogen by material is then heated in a reducing atmos- volume in carbon dioxide in the temperature phere of hydrogen at a temperature below range 1400°C—1600°C and then being the temperature of sintering to convert the heated in a reducing atmosphere of hydrogen 35 15 material to the stoichiometric form. at a temperature lower than the sintering tem- 11. A method for preparing a ceramic perature in the range 1200°C—1400°C. fissile oxide material as claimed in claim 6 wherein the additive is magnesium oxide in amount 0.75 to 2.0 weight percent, the J. Y. LE MASURIER 20 material being sintered in an oxidising atmos- Chartered Patent Agent phere comprising 0 to 50% hydrogen by Agent for the Applicants Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa, 1972. Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which, copies may be obtained.