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Improvements in Ceramics Fissile Materials

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Improvements in Ceramics Fissile Materials 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 CERAMIC FISSILE MATERIALS (71) We, UNITED KINGDOM ATOMIC with powder of the nuclear fuel 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 nuclear reactor, 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 uranium or plutonium dioxides of magnesium oxide as an additive in oxide 60 or mixture of these oxides are irradiated for nuclear fuel particles such as thorium oxide 15 long periods of time at high temperatures or thorium-uranium oxide 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 leads 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 thermal conductivity 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 yttrium, iridium and neo- Magnesium oxide 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 carbon dioxide, at a temperature cent. in the range 1400°C to 1600°C, the material Another suitable additive is aluminium then being heated in a reducing atmosphere of oxide in amount 0.15 to 3.3 weight percent, hydrogen 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 aluminium oxide 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 oxygen 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.
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