Patent Number: 055966152
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method of manufacturing a fuel assembly applicable for a core of a nuclear reactor using a fuel containing Pu.sup.239, members constituting the fuel assembly (sometimes referred to herein as "fuel assembly elements"), and alloys used for the members. In particular, the present invention concerns a method of manufacturing a fuel assembly applicable for a reactor core in which a water-uranium fuel volume ratio is 1.5 or less and the conversion ratio from U.sup.238 to Pu.sup.239 is high, members constituting the fuel assembly, and alloys used for the members. As for members constituting a fuel assembly used for nuclear power generation, those for a light water reactor use a zirconium alloy; and those for a fast breeder reactor use a stainless steel. A high conversion reactor acts as a bridge between a light water reactor and a fast breeder reactor, and has a feature of effectively converting non-fissionable U.sup.238 contained in natural uranium to fissionable Pu.sup.239 usable for power generation. The non-fissionable U.sup.238, which has been not used in a light water reactor, can be used by the high conversion reactor, resulting in the effective utilization of uranium resource. The stored Pu.sup.239 can be effectively used as a fuel for a fast breeder reactor, or a fuel for a high conversion reactor and a general breeder reactor. In a conventional light water reactor and a high conversion reactor, the reduction in the exhaust amount of a spent fuel by increasing an operation cycle and the burn-up of fuel contributes to an economic merit, for example in reducing a power generation cost. However, when the operation cycle is increased and the burn-up of fuel is enhanced, the staying period of a fuel assembly in a reactor is increased. This further accelerates the corrosion of the surfaces of members constituting the fuel assembly in water at a high temperature/high pressure. Moreover, the effective conversion from U.sup.238 to fissionable Pu.sup.239 is mainly due to resonance neutrons having an energy higher than that of thermal neutrons. As a result, neutron spectrum in a reactor core is hardened (a large number of neutrons having high energy exist), thus accelerating the damage of the material due to neutrons. A further problem is that the zirconium alloy (normally used as a high corrosion resisting alloy) has a tendency to become brittle by fast neutron irradiation. Further, in the environment of a BWR (Boiling Water Reactor), a member constituting a ZIRCALOY fuel assembly generates a local oxidization called the nodular corrosion, and the corrosion portion propagates with time. A method of reducing this corrosion has been known, wherein a heat-treatment of heating a zirconium alloy for a short period of time in a temperature range of (.alpha.+.beta.) phase or .alpha. phase and quenching the alloy is inserted in the downstream step in a member manufacturing process (for example, Unexamined Japanese Patent Publications Nos. SHO 51-110411 and SHO 51-110412, and Examined Japanese Patent Publications Nos. SHO 60-59983 and SHO 63-31543). This known technique is called (.alpha.+.beta.) quenching or .beta. quenching, which is applied to alloys used for the existing light water reactor: ZIRCALOY-2 (Sn: 1.2-1.7 wt %, Fe: 0.10-1.20 wt %, Cr: 0.05-0.15 wt %, Ni: 0.03-0.08 wt %, O: 0.06-0.14 wt %, and the balance: Zr); and ZIRCALOY-4 (Sn: 1.2-1.7 wt %, Fe: 0.15-1.24 wt %, Cr: 0.05-0.15 wt %, O: 0.06-0.14 wt %, and the balance: Zr). Of the above alloy components, Fe, Cr and Ni are elements for improving corrosion resistance, and Sn is an element of improving strength. Fe, Cr, Ni precipitate as intermetallic compounds within crystal grains and crystal boundaries. These intermetallic compounds are refined by the (.alpha.+.beta.) quenching or .alpha. quenching; and further when the cooling rate is sufficiently large, they are dissolved in solid even in the matrix. The mechanism of enhancing the corrosion resistance is not fully understood, but it is generally considered that the refining of precipitations and the increase in the concentration of solid-solution of Fe, Ni, and Cr contribute to the increase in the corrosion resistance. The improvement of the alloy composition and alloy components leads to the enhancement of the corrosion resistance. Various improved alloys have been known as follows: an alloy improved in corrosion resistance which has the same composition of that of the existing ZIRCALOY but is optimized in the added amounts of the alloy elements (Unexamined Japanese Patent Publication No. SHO 62-228442); an alloy having the composition of ZIRCALOY which is further added with the fifth element such as Nb, Mo, W, V, Te, Ta, Si, Ru, Rh, Pd, Pt, or An (Unexamined Japanese Patent Publication Nos. SHO 60-36640, SHO 63-33535, SHO 64-73037, SHO 64-73038, and HEI 1-242747); an alloy having the composition of Zr--Nb alloy which is further added with elements of Sn, Mo, Cr, Ni, Fe, V, W, and Cu in a slight amount (Unexamined Japanese Patent Publication Nos. SHO 50-148213, SHO 51-134404, SHO 61-170552, SHO 62-207835, and HEI 1-119650); a Zr--Bi alloy (Unexamined Japanese Patent Publication No. SHO 63-290234); and a Zr--Sn--Te, Mo alloy (Unexamined Japanese Patent Publication No. SHO 63-290233). These zirconium alloys are intended to be used for a light water reactor, and thereby they are difficult to be used as they are for a high conversion type future reactor in which neutron spectrum is shifted on a high energy side as compared with the existing light water reactor. As described above, in the high conversion type future reactor, non-fissionable U.sup.238 is effectively convened into fissionable Pu.sup.239 and is used for power generation. The nuclear transformation is generated by allowing resonance neutrons (energy: 10.sup.0 to 10.sup.4 eV) to absorb U.sup.238. In such a reactor core, it is required to lower a water-uranium fuel ratio and to shift neutron spectrum on a high energy side (spectrum is hardened). As a result, the damage ratio of a member constituting a fuel assembly due to neutrons is increased. Accordingly, to significantly increase the burn-up of a light water reactor and to realize a high conversion type future reactor, it becomes important to improve the neutron damage resistance and the corrosion resistance of a member constituting a fuel assembly and to reduce the capture amount of neutrons of the member. An object of the present invention, therefore, is to provide a Zr alloy for use with a fuel assembly element, which has a high neutron damage resistance and a high corrosion resistance, and further has a small resonance neutron capture cross-section. Another object of the present invention is to provide a method of manufacturing a member such as a fuel sheath tube constituting a fuel assembly usable for a high conversion type future reactor which is capable of keeping an excellent performance for a long period of time. SUMMARY OF THE INVENTION For improving a neutron damage resistance, reduction in crystal gains has been found to be very effective. This is because a pair of an interstitial atom and a vacancy produced by neutron irradiation rapidly disappear at crystal grain boundaries, thus preventing the generation of irradiation defect in the crystal grains. Even if the irradiation defect is generated, the density thereof is significantly lowered. According to one embodiment of the invention, therefore, crystal grain size of 1000 nm or less gives the most reduction in irradiation defect. According to a further embodiment of the invention, significant reductions in the radiation defect occur with crystal grain sizes below 100 nm, as explained with regard to other embodiments, below. According to one more specific embodiment of the invention, there is provided a fuel assembly for a nuclear reactor comprising fuel assembly elements, said fuel assembly elements comprising: a fuel pellet made of uranium containing plutonium; a fuel sheath tube for sheathing said pellet; a spacer for holding said fuel sheath tube; and a channel box for containing a plurality of said sheath tubes, wherein at least one fuel assembly element comprises a Zr-containing metal, and an average crystal grain size of said Zr-containing metal is in the range of 1000 nm or less. According to further embodiments, said average crystal grain size is in the range of 100 nm or less; at least one of said fuel assembly elements comprises a Zr alloy having a random crystal orientation; and at least one fuel assembly element comprises a Zr alloy which comprises at least about 0.02 wt % of Fe. In some embodiments, at least one fuel assembly element comprises a Zr alloy comprising at least about 0.05 to 30 wt % of Fe, and an average crystal grain size of said Zr alloy is in the range of 100 nm or less. According to still further embodiments, at least one fuel assembly element comprises a ZrFe.sub.2 intermetallic compound containing at least about 33 atomic percent Zr. According to another embodiment, at least one fuel assembly element comprises a ZrFe.sub.2 intermetallic compound containing at least about 66 atomic percent Fe. Alternatively, there are embodiments in which at least one fuel assembly element comprises a Zr(Fe, Ni, Cr, Sn).sub.2 intermetallic compound containing a range of Zr between about 30 and about 35 atomic percent, and in other embodiments, at least one fuel assembly element comprises a Zr(Fe, Ni, Cr, Sn).sub.2 intermetallic compound containing a range of (Fe, Ni, Cr, Sn) of between about 65 and about 70 atomic percent. According to still another embodiment, there is provided a fuel assembly element for a nuclear reactor comprising a Zr-containing metal having an average crystal grain size of 1000 nm or less. Still further, in one embodiment of the invention, there is provided a fuel assembly element manufacturing method of a Zr alloy or compound, said fuel assembly element being chosen from a group consisting of: a fuel sheath tube for sheathing a fuel pellet made of uranium containing plutonium, a spacer for holding said sheath tube, or a channel box for containing a plurality of said sheath tubes, which constitute a fuel assembly used for a core of a nuclear reactor, said method comprising: mechanically mixing a Zr-containing metal and an alloying element, the alloying element being chosen from a group consisting of: Fe, Cr, Ni, Nb, Mo, Te, Bi, and Sn, whereby a Zr alloy is produced; crystallizing the pressure-treated Zr alloy in a temperature range of between the crystallization temperature of the pressure-treated Zr alloy and a maximum crystallization temperature, said maximum crystallization temperature being 200 degrees C. above the crystallization temperature of the pressure-treated Zr alloy; subjecting the Zr alloy to an isostatic pressure, whereby a pressure-treated Zr alloy is produced; and forming the pressure-treated alloy into a shape of the fuel assembly element. Further, in some embodiments, said crystallizing occurs during said subjecting, wherein said subjecting comprises subjecting the Zr alloy to an isostatic pressure at a temperature lower than a crystallization temperature of the Zr alloy, and in other embodiments, said crystallizing comprises working the pressure-treated Zr alloy at a temperature range between about 100 degrees C. and about 200 degrees C. According to still further embodiments, said subjecting occurs at a temperature above the crystallizing temperature for said Zr alloy, while according to other embodiments, said mechanically mixing comprises: hydrogenation of the Zr-containing metal; crushing of the Zr-containing metal into a powder, and; dehydrogenation of the powder. According to even further embodiments, said dehydrogenation comprises heating in a vacuum atmosphere. According two to alternate embodiments of the method, said Zr-containing metal comprises a powder of pure Zr or a Zr alloy. According to still a further embodiment of the method, the temperature is never allowed above about 650 degrees C., and in another embodiment, there is further provided hot-working, performed below about 650 degrees C. According to some embodiments of the invention, annealing is performed at a temperature higher than about 530 degrees C. According to still a further embodiment, for improving the corrosion resistance, it is effective to dissolve in solid a corrosion resistance improving element such as Fe, Ni, or Cr in a matrix. The super-saturated solid-solution with ultra-fine crystals can be obtained by a means of realizing a non-equilibrium crystal structure, for example, mechanical alloying, molten metal quenching, or splat cooling. The neutron capture cross-section of Fe is about 1/3 that of Zr in an energy range (of resonance neutron) of 10.sup.0 to 10.sup.4 eV. The reduction in the capture mount of resonance neutrons of a member constituting a fuel assembly is achieved by the methods of: (a) reducing the resonance neutron capture cross-section by increasing the added amount of Fe, and (b) thinning the member by increasing the strength of the Zr alloy. By increasing the added mount of Fe in a zirconium alloy, the above-described precipitations are coarsened, thereby leading to the embrittlement of the material. In particular, the precipitations produced upon melting are significantly coarsened, so that the zirconium alloy cannot be manufactured by a conventional manufacturing process. Accordingly, even in this case, the above-described means of realizing a non-equilibrium crystal structure is effective. According to even further embodiments of the present invention, there is provided a fuel assembly for a nuclear reactor comprising: a fuel pellet made of uranium containing plutonium; a fuel sheath tube for sheathing the pellet; a spacer for holding the fuel sheath tube; and a channel box for containing a plurality of the sheath tubes, wherein at least one member of the fuel sheath tube, the spacer and the channel box is made of a Zr alloy containing 0.05 to 30 wt % of Fe, and an average crystal grain size of the Zr alloy is in the range of 1000 nm or less. In the above fuel assembly, at least one member of the fuel sheath tube, the spacer and the channel box may be made of a Zr alloy, and an average crystal grain size of the Zr alloy may be in the range of 1000 nm or less. Also in the above fuel assembly, at least one member of the fuel sheath tube, the spacer and the channel box may be made of a Zr alloy containing an alloy element forcibly dissolved in solid in an amount of 2 wt % or more. Further in the above fuel assembly, at least one member of the fuel sheath tube, the spacer and the channel box may be made of a Zr alloy containing 0.5-30 wt % of Fe, 0-5 wt % of Ni, 0-5 wt % of Cr, 0-5 wt % of Nb, 0-1 wt % of Mo, 0-1 wt % of Te, 0-5 wt % of Sn, 0-2 wt % of Bi, 0-1 wt % of O, and 0-0.5 wt % of Si. According to still a further embodiment of the present invention, there is provided a Zr alloy containing 0.5-30 wt % of Fe, 0-5 wt % of Ni, 0-5 wt % of Cr, 0-5 wt % of Nb, 0-1 wt % of Mo, 0-1 wt % of Te, 0-5 wt % of Sn, 0-2 wt % of Bi, and 0-0.5 wt % of Si. And, according to one aspect of such an embodiment, there is provided a Zr alloy containing an alloy element forcibly dissolved in an mount of 2 wt % or more. According to an even further embodiment of the present invention, there is provided a Zr alloy powder made of an amorphous Zr alloy containing crystal grains having a crystal grain size of 1000 nm or less In still a further embodiment, there is provided a Zr alloy powder made of a Zr alloy containing 0.5-30 wt % of Fe, 0-5 wt % of Ni, 0-5 wt % of Cr, 0-5 wt % of Nb, 0-1 wt % of Mo, 0-1 wt % of Te, 0-5 wt % of Sn, 0-2 wt % of Bi, and 0-0.5 wt % of Si, and the Zr alloy powder made of a Zr alloy containing an alloy element forcibly dissolved in solid in an amount of 2 wt % or more. Moreover, according yet another embodiment of the present invention, there is provided a fuel assembly manufacturing method of manufacturing either of a fuel sheath tube for sheathing a fuel pellet made of uranium containing plutonium, a spacer for holding the sheath tube, and a channel box for containing a plurality of the sheath tubes, which constitute a fuel assembly used for a core of a nuclear reactor, the method comprising: a) a process of mechanically mixing pure metal powders including Zr powder or a crystalline Zr alloy powder for alloying, thereby manufacturing an amorphous alloy powder made of a Zr alloy being mostly amorphous; b) a process of solidifying the amorphous alloy powder under an isostatic pressure at a temperature lower than a re-crystallization temperature of the amorphous alloy powder; c) a process of forming the solidified block into the shape of either of the member by hot-working or cold-working; and d) a process of crystallizing the metal structure of the formed product by heat-treatment. In the above-described process of manufacturing a pure Zr powder or a crystalline Zr alloy powder, preferably, sponge-like pure Zr or an ingot of a Zr alloy is hydrogenated and crushed into a powder having a specified particle size, and the powder is dehydrogenated by heating in a vacuum atmosphere. In the above-described process of forming the solidified block into a specified shape by hot-working or cold-working, preferably, the hot-working is performed at 650 degrees C., followed by cold-working. In the above-described process of crystallizing the metal structure by heat-treatment, preferably, the final crystallization annealing is performed at a temperature higher than 530 degrees C.