Patent Number: 056423900
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the steps of the method in detail, the following examples are described below: A plurality of ceramic nuclear-fuel sintered pellets made of UO.sub.2, which have the form of a solid cylinder with a diameter of 9.11 mm and a height of 10 mm were disposed next to one another to form a cylindrical column in an Al.sub.2 O.sub.3 boat on a ZrB.sub.2 powder bed that may, for example, contain at least one of the materials NH.sub.4 Cl, BaF.sub.2 and/or KBF.sub.4 admixed as a catalyst. Each of the nuclear-fuel sintered pellets had a sinter density of between 10.38 and 10.44 g/cm.sup.3. The nuclear-fuel sintered pellets were also completely covered with ZrB.sub.2 powder which may likewise, for example, contain NH.sub.4 Cl, BaF.sub.2 and/or KBF.sub.4 admixed as a catalyst. The boat with the nuclear-fuel sintered pellets was then disposed in an Al.sub.2 O.sub.3 tube and heated inside this tube in an electrically heated tube furnace for three hours at 1400.degree. C. under a treatment atmosphere being formed of 5% H.sub.2 and 95% He. After cooling, measurement of the nuclear-fuel sintered pellets through the use of X-ray diffractometry showed that these nuclear-fuel sintered pellets, insofar as they were situated between the nuclear-fuel sintered pellets at each end of the column, had a surface layer of virtually 100% UB.sub.4 and UB.sub.2 by volume under their external surface. The thickness of this surface layer was determined as 12 .mu.m on average by using a microscope for a transverse and a longitudinal ground section of the nuclear-fuel sintered pellets. The variation between maximum value and minimum value of this thickness was 6 .mu.m. The rest of the sintered pellets were formed virtually only of unaltered UO.sub.2 without a detectable boron content. When boron powder was used instead of ZrB.sub.2 powder for the powder bed, surface layers of virtually 100% UB.sub.2 and UB.sub.4 by volume with a thickness of 21 .mu.m.+-.5 .mu.m were produced under the external surface of the nuclear-fuel sintered pellets. In this case again, the rest of the sintered pellets were formed virtually of unaltered UO.sub.2 without a detectable boron content. In a further exemplary embodiment, use was made of an Al.sub.2 O.sub.3 tube which was disposed with a horizontal longitudinal axis in an electrically heated tube furnace. Two thirds of the empty volume of this Al.sub.2 O.sub. tube was filled with ZrB.sub.2 powder in which twelve ceramic nuclear-fuel sintered pellets made of UO.sub.2, likewise with sinter densities of between 10.38 and 10.44 g/cm.sup.3, were embedded. The nuclear-fuel sintered pellets likewise had the form of a solid cylinder with a diameter of 9.11 mm and a height of 10 mm. The Al.sub.2 O.sub.3 tube was rotated about its longitudinal axis at one revolution per minute, so that the powder, together with the nuclear-fuel sintered pellets, was circulated. In this case, the powder and the nuclear-fuel sintered pellets were heated for three hours at a treatment temperature of 1400.degree. C. under a surrounding atmosphere in the tube furnace being formed of 5% H.sub.2 and 95% He. After cooling, the nuclear-fuel sintered pellets had a surface layer of virtually 100% UB.sub.2 and UB.sub.4 by volume under their entire surface. This surface layer had, below the external surface of the nuclear-fuel sintered pellets, a thickness of 16 .mu.m.+-.4 .mu.m and, at the two end surfaces, a thickness of 7 .mu.m.+-.3 .mu.m. The rest of the sintered pellets was unaltered UO.sub.2 without a detectable boron content. In a variant of this exemplary embodiment, fifteen ceramic nuclear-fuel sintered pellets made of UO.sub.2, which likewise had the form of a solid cylinder with a diameter of 9.11 mm and a height of 10 mm, were mounted without a powder bed in the Al.sub.2 O.sub.3 tube, and this tube was likewise rotated in the tube furnace about its horizontal longitudinal axis at one revolution per minute. A gas mixture of diborane B2H.sub.6 and H.sub.2 was fed through a duct into the internal space of the tube, which was closed at both ends in gas-tight fashion, and fed out again through another duct. The flow rate of the gas mixture was 10 liters per minute, and the composition was 99.9 mole % H.sub.2 and 0.1 mole % B2H.sub.6. The nuclear-fuel sintered pellets made of UO.sub.2 in this case were kept at a temperature of 1050.degree. C. in the tube furnace for 90 minutes. After cooling, these nuclear-fuel sintered pellets made of UO.sub.2 had a surface layer with a thickness of 8 .mu.m that was formed of 100% by weight UB.sub.2 and UB.sub.4 under their entire surface. The rest of the sintered pellets was unaltered UO.sub.2 without a detectable boron content. The surface layer containing UB.sub.2 and UB.sub.4 can also be formed in the nuclear-fuel sintered pellet made of UO.sub.2 by embedding this nuclear-fuel sintered pellet in boron and/or a boron-containing chemical compound, which are in the molten state. It is expedient if the isotope B.sub.10 in the boron is enriched relative to the natural isotopic composition of boron, in the boron being used or in the boron-containing chemical compounds being used. This can be achieved in a known manner, for example by cyclotron enrichment, diffusion enrichment or separation nozzle enrichment. It is this isotope B.sub.10 that essentially absorbs the thermal neutrons. By virtue of the fact that it is enriched in the boron that is situated in the surface layer of the uranium-containing nuclear-fuel sintered pellet, the thickness of this surface layer can be selected to be comparatively small. In a similar way, it is even possible to treat uranium-containing ceramic nuclear-fuel sintered pellets that contain at least one of the chemical compounds (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 and (U, Pu, Th, RE)O.sub.2, since the other heavy metals in these mixed oxides all form borides structured identically or similarly to that which uranium forms. The rare earths RE may, in particular, be gadolinium, samarium, europium, erbium and dysprosium, which are all neutron poisons, but can exhibit a burnout behavior due to the physical effects of neutrons which is different from that of boron, and therefore can advantageously influence the reactivity control in a nuclear reactor, in combination with boron. It is advantageous to fit the uranium-containing nuclear-fuel sintered pellets according to the invention in a cladding tube of a fuel rod, wherein the cladding tube is generally made of a zirconium alloy or stainless steel, and to seal this cladding tube. This fuel rod is expediently a component of a nuclear-reactor fuel assembly for a nuclear reactor. Advantageously, such a nuclear-reactor fuel assembly is intended for a light water nuclear reactor, in particular for a pressurized water nuclear reactor or a boiling water nuclear reactor. Tests simulating the conditions in a nuclear reactor and being carried out with such a cladding tube showed not only that the boron-containing surface layer of the uranium-containing nuclear-fuel sintered pellets is firmly anchored in the crystal structure of these nuclear-fuel sintered pellets, but also that the boron does not escape from this surface layer, even at temperatures of 500.degree. C. and above.