Patent Number: 056423900
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a Continuation of International Application Ser. No. PCT/EP94/02470, filed Jul. 26, 1994, published as WO95/04994, Feb. 16, 1995. BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The invention relates to a nuclear-fuel sintered pellet containing UO.sub.2, (U, Pu)O.sub.2, (U, Th)O.sub.2, (U, RE)O.sub.2, (U, Pu, Th)O.sub.2, (U, Pu, RE)O.sub.2, (U, Th, RE)O.sub.2 or (U, Pu, Th, RE)O.sub.2, wherein RE=rare earth. The invention also relates to a nuclear-reactor fuel assembly including a fuel rod having a cladding tube and such a uranium-containing nuclear-fuel sintered pellet in the cladding tube. The invention additionally relates to a method for treating such a uranium-containing nuclear-fuel sintered pellet. Published European Patent Application 0 239 843 A1, corresponding to U.S. Pat. No. 4,774,051, discloses a nuclear-fuel sintered pellet made of UO.sub.2, (U, Pu)O.sub.2 or (U, Th)O.sub.2. Boron is incorporated as a neutron poison in the chemical compound form UB.sub.x, wherein x=2; 4 and/or 12 and/or B.sub.4 C, in a sinter matrix of that nuclear-fuel sintered pellet. That known nuclear-fuel sintered pellet is obtained by producing a mixture of uranium oxide powder or uranium mixed oxide powder with uranium boride powder or boron carbide powder and pressing it to form pellets which are subsequently sintered in a sintering furnace under a reducing sintering atmosphere to form nuclear-fuel sintered pellets. In those nuclear-fuel sintered pellets, the boron is thereby uniformly distributed throughout the sinter matrix. From the neutron physics point of view, boron in uranium-containing nuclear-fuel sintered pellets is a burnable neutron absorber which loses its property as an absorber for thermal neutrons after those nuclear-fuel sintered pellets have been used in a nuclear reactor for a certain period of time. Nuclear-reactor fuel assemblies having fuel rods that contain uranium-containing nuclear-fuel sintered pellets are used in a nuclear reactor, for example, during four sequential fuel assembly cycles, generally being of equal durations. At the end of a fuel assembly cycle, some of the nuclear-reactor fuel assemblies in the nuclear reactor are in each case replaced by fresh, unirradiated nuclear-reactor fuel assemblies. The fresh, unirradiated nuclear-reactor fuel assemblies would cause a comparatively high reactivity in the nuclear reactor relative to the nuclear-reactor fuel assemblies that are already irradiated. However, the boron in the nuclear-fuel sintered pellets of those fresh, unirradiated nuclear-reactor fuel assemblies at first moderates the reactivity due to those nuclear-reactor fuel assemblies by initially absorbing thermal neutrons. The nuclear fuel in fresh and unirradiated nuclear-reactor fuel assemblies gradually burns out in the nuclear reactor through nuclear decay, but a burnable neutron absorber that is present in that nuclear fuel simultaneously burns out gradually due to the physical effects of neutrons, so that finally, that neutron absorber absorbs no thermal neutrons or only very few. In that way, even unirradiated nuclear-reactor fuel assemblies being newly loaded into the nuclear reactor may cause approximately the same reactivity in the nuclear reactor during their entire residence time in the nuclear reactor, as the nuclear-reactor fuel assemblies which have already spent a fuel assembly cycle in the nuclear reactor. Boron is advantageously used as a neutron absorber in a nuclear fuel as compared to other burnable neutron absorbers such as rare earths if the fuel assembly cycles are comparatively long, i.e., for example, longer than 12 months, since accumulation of heat in the nuclear fuel is avoided with boron. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a uranium-containing nuclear-fuel sintered pellet, a nuclear-reactor fuel assembly having a uranium-containing nuclear-fuel sintered pellet and a method for treating a uranium-containing nuclear-fuel sintered pellet, which overcome the hereinafore-mentioned disadvantages of the heretofore-known products and methods of this general type and in which an excessively fast and excessively high increase in reactivity is not caused upon startup of a nuclear reactor if the nuclear-fuel sintered pellet is newly loaded, in the unirradiated state, in the nuclear reactor. Since the surface of the uranium-containing nuclear-fuel sintered pellets in the cladding tube of a fuel rod in the nuclear reactor is kept relatively constant at a substantially lower temperature than the rest of the sintered pellet by a coolant flowing past the exterior of the cladding tube, whereas, however, in the case of the uranium-containing nuclear-fuel sintered pellet according to the invention most of the boron is situated in a surface layer, chemical reactions between UB.sub.x and UO.sub.2 take place only to a limited extent in that surface layer, so that the boron cannot emerge from the uranium-containing nuclear-fuel sintered pellet according to the invention, and a reactivity increase having a rate and amplitude that is moderated is thereby guaranteed. With the foregoing and other objects in view there is provided, in accordance with the invention, a uranium-containing nuclear-fuel sintered pellet containing UO.sub.2, comprising a sintered-pellet surface layer being formed of at least 80% by volume of a chemical boron compound UB.sub.x with at least one number x from a number set 2; 4 and 12, and a remainder of the sintered pellet containing at most 5% by volume of the chemical boron compound. With the objects of the invention in view, there is also provided a uranium-containing nuclear-fuel sintered pellet containing (U, Pu)O.sub.2, comprising a sintered-pellet surface layer being formed of at least 80% by volume of a chemical boron compound (U, Pu)B.sub.x with at least one number x from a number set 2; 4 and 12, and a remainder of the sintered pellet containing at most 5% by volume of the chemical boron compound. With the objects of the invention in view, there is additionally provided a uranium-containing nuclear-fuel sintered pellet containing (U, Th)O.sub.2, comprising a sintered-pellet surface layer being formed of at least 80% by volume of a chemical boron compound (U, Th)B.sub.x with at least one number x from a number set 4 and 6, and a remainder of the sintered pellet containing at most 5% by volume of the chemical boron compound. With the objects of the invention in view, there is further provided a uranium-containing nuclear-fuel sintered pellet containing (U, RE)O.sub.2 (RE=rare earth), comprising a sintered-pellet surface layer being formed of at least 80% by volume of a chemical boron compound (U, RE)B.sub.x with at least one number x from a number set 4; 6 and 12, and a remainder of the sintered pellet containing at most 5% by volume of the chemical boron compound. With the objects of the invention in view, there is also provided a uranium-containing nuclear-fuel sintered pellet containing (U, Pu, Th)O.sub.2, comprising a sintered-pellet surface layer being formed of at least 80% by volume of a chemical boron compound (U, Pu, Th)B.sub.x with at least one number x from a number set 2; 4; 6 and 12, and a remainder of the sintered pellet containing at most 5% by volume of the chemical boron compound. With the objects of the invention in view, there is additionally provided a uranium-containing nuclear-fuel sintered pellet containing (U, Pu, RE)O.sub.2 (RE=rare earth), comprising a sintered-pellet surface layer being formed of at least 80% by volume of a chemical boron compound (U, Pu, RE)B.sub.x with at least one number x from a number set 2; 4; 6 and 12, and a remainder of the sintered pellet containing at most 5% by volume of the chemical boron compounds. With the objects of the invention in view, there is further provided a uranium-containing nuclear-fuel sintered pellet containing (U, Th, RE)O.sub.2 (RE=rare earth), comprising a sintered-pellet surface layer being formed of at least 80% by volume of a chemical boron compound (U, Th, RE)B.sub.x with at least one number x from a number set 4 and 6, and a remainder of the sintered pellet containing at most 5% by volume of the chemical boron compound. With the objects of the invention in view, there is also provided a uranium-containing nuclear-fuel sintered pellet containing (U, Pu, Th, RE)O.sub.2 (RE=rare earth), comprising a sintered-pellet surface layer being formed of at least 80% by volume of a chemical boron compound (U, Pu, Th, RE)B.sub.x with at least one number x from a number set 4 and 6, and a remainder of the sintered pellet containing at most 5% by volume of the chemical boron compound. In accordance with another feature of the invention, there is provided at least 90% by volume of the chemical boron compounds in the sintered-pellet surface, and at most 2% by volume of the chemical boron compounds in the remainder of the sintered pellet. In accordance with a further feature of the invention, there is provided at least 98% by volume of the chemical boron compound in the sintered-pellet surface layer, and at most 1% by volume of the chemical boron compound in the remainder of the sintered pellet. In accordance with an added feature of the invention, the remainder of the sintered pellet is without a detectable boron content. In accordance with an additional feature of the invention, the boron-containing sintered-pellet surface layer has a thickness of from 2 to 40 .mu.m. In accordance with yet another feature of the invention, the boron-containing sintered-pellet surface layer has a thickness of from 5 to 20 .mu.m. In accordance with yet a further feature of the invention, the isotope B.sub.10 in the boron of the chemical boron compound is enriched relative to a natural isotopic composition. With the objects of the invention in view, there is also provided a nuclear-reactor fuel assembly, comprising a fuel rod having a cladding tube, and such a uranium-containing nuclear-fuel sintered pellet in the cladding tube. With the objects of the invention in view, there is additionally provided a method for treating a uranium-containing nuclear-fuel sintered pellet with boron or a boron-containing chemical compound, which comprises treating a uranium-containing nuclear-fuel sintered pellet with boron or a boron-containing chemical compound at a treatment temperature being high enough to form uranium-containing boride in a surface layer of the nuclear-fuel sintered pellet. In accordance with another mode of the invention, there is provided a method which comprises carrying out the treatment step in the presence of hydrogen-containing inert gas. In accordance with a further mode of the invention, there is provided a method which comprises carrying out the treatment step in the presence of at least one hydrogen-containing inert gas selected from the group consisting of helium, argon and nitrogen. In accordance with an added mode of the invention, there is provided a method which comprises embedding the uranium-containing nuclear-fuel sintered pellet in a boron and/or a boron-containing chemical compound. In accordance with an additional mode of the invention, the boron or boron-containing chemical compound contains an admixed catalyst. In accordance with yet another mode of the invention, the boron or boron-containing chemical compound is in the form of a powder. In accordance with yet a further mode of the invention, there is provided a method which comprises circulating the powder. In accordance with yet an added mode of the invention, the boron or boron-containing chemical compound is in molten form. In accordance with yet an additional mode of the invention, there is provided a method which comprises selecting the boron-containing chemical compound as at least one gas from the group consisting of borane, boron halide and boron alkyl. In accordance with again another mode of the invention, there is provided a method which comprises selecting the powder as at least one material from the group consisting of boron carbide, silicon boride and metal boride, preferably zirconium diboride. In accordance with again a further mode of the invention, there is provided a method which comprises setting the treatment temperature from 850.degree. to 1600.degree. C. and preferably from 1100.degree. to 1450.degree. C. In accordance with a concomitant mode of the invention, there is provided a method which comprises setting a treatment time of from 10 minutes to 6 hours and preferably of from 1 to 4 hours. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a uranium-containing nuclear-fuel sintered pellet, a nuclear-reactor fuel assembly having a uranium-containing nuclear-fuel sintered pellet and a method for treating a uranium-containing nuclear-fuel sintered pellet, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific examples.