Patent Number: 052971775
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fuel assembly for a light water reactor, and more particularly to a fuel assembly to be charged in a boiling water reactor. Recent increasing interests are in longer operating cycles of light water reactors and higher burnup levels of uranium fuel, because of an increase in economical merits such as reduction in discharge of spent fuel, power generation cost, etc. Particularly in Japan, the atomic power generation is based on reprocessing of spent fuel as its premise, with keen requirements for higher burnup level, including reutilization of plutonium extracted by the reprocessing. The nuclear fuel is now discharged from the reaction at a burnup level (discharge burnup level) of about 30 GWd/t, and when a nuclear fuel can have a discharge burnup level of 60 GWd/t, the economical merit will be much improved. In order to attain a higher burnup level in light water reactors, it has been so far tried to improve the corrosion resistance of materials of members for a fuel assembly, prevent deformation of members of a fuel assembly in a neutron irradiation circumstance, optimize the enrichment and arrangement of uranium fuel, and improve the thermohydraulic characteristics of a fuel assembly. A higher corrosion resistance is required for materials for a fuel assembly of high burnup level than for the conventional materials. As materials of members for the fuel assembly, a zircaloy (Zry: Zn--Sn--Fe--Cr--Ni alloy having the following composition: Sn: 1.2-1.7 wt. %, Fe: 0.07-0.24 wt. %, Cr: 0.05-0.15 wt. %, Ni: &lt;0.08 wt. %, the balance being Zr and impurities) is now used. On the zircaloy members of a fuel assembly, local corrosion called "nodular corrosion" develops in the prevailing circumstance of the boiling water type, light water reactor (BWR). To prevent such a corrosion, processes of improving the corrosion resistance of zircaloy by heat treatment, for example, by heating it to an (.alpha.+.beta.) phase or .beta.-phase temperature region for a short time, followed by quenching, have been proposed (Japanese Patent Publications Nos. 61-45699 and 63-58223). Furthermore, a technique of improving the corrosion resistance by changing the alloy composition is known. For example, a zircaloy having higher Fe and Ni content is known [Japanese Patent Applications Kokai (Laid-open) Nos. 60-43450 and 62-228442]. The Zr alloy material is used in locations subjected t neutron irradiation and thus undergoes irradiation growth and deformation. Particularly, when a curving deformation or expansion deformation takes place at the channel box (FCB), clearances between FCB and control rod are decreased (e.g. to zero), resulting in nuclear reactor operation troubles. To prevent the deformations, a process for suppressing the irradiation growth by making the crystallographic orientation parameter, in the FCB longitudinal direction of (0002) face of hexagonal Zr crystal of 0.15 to 0.5, has been proposed in Japanese Patent Application Kokai (Laid-open) No. 59-229475. In a boiling water type nuclear reactor, cooling water flows into clearances among fuel rods from the lower tie plate of a fuel assembly and is heated and boils, while passing through the clearances among the fuel rods from the bottom position upwards, to form a two-phase stream of steam voids and liquid water which flows out through through-holes of the upper tie plate. The void ratio is 0% at the bottom position of the fuel assembly and reaches about 70% at the top position. That is, a ratio of hydrogen atoms (H) to heavy metal atoms (U) (H/U ratio) differs between the bottom and the top of the fuel rods. At the bottom position of a fuel assembly, where the H/U ratio is high, the average neutron energy is lowered and the fission reaction of thermal neutrons with nuclear fuel substance is promoted, whereas at the top position of the fuel assembly where the H/U ratio is low, the fission reaction of neutrons with the nuclear fuel substance is suppressed. As a result, the linear heat rating is higher at the bottom position of the fuel assembly than at the top position of the fuel assembly, resulting in uneven power distribution in the axial direction of the fuel rods. Uneven power distribution occurs even in the radial direction of the fuel assembly. The outermost periphery of a square lattice arrangement of 8.times.8, 9.times.9 or 10.times.10 fuel rods is surrounded by an FCB to form a water gap between the outermost periphery of the fuel rods and the adjacent FCB. That is, the H/U ratio is higher at the outermost peripheral region of a fuel assembly than at the inner region thereof, and thus the linear heat rating will be higher. To attain a longer operating cycle and a higher burnup level of nuclear fuel, it is necessary to increase the uranium enrichment. In a fuel assembly having a higher uranium enrichment, such an uneven power distribution is more pronounced. In order to flatten the power distribution in the axial direction and the radial direction, optimization of shape and arrangement of water rods, optimization of uranium enrichment distribution, partial change of fuel rod length, prevention of local power peaking at the initial burnup period with burnable poisons such as Gd, B, etc., and the like have been carried out. All the above-mentioned techniques relate to the so-called element techniques. Even if some element technique is distinguished, a fuel assembly of higher burnup level cannot be obtained when the fuel assembly partially has some inconvenience. For example, Japanese Patent Application Kokai (Laid-open) No. 59-229475 discloses that irradiation growth and curving deformation can be prevented by controlling an Fl value as a crystallographic orientation parameter of a channel box to 0.15-0.5; but among the crystallographic orientation parameters a crystallographic orientation parameter in the normal-to-plate direction (Fr value) is most important. Furthermore, the fuel rods undergo irradiation growth and are elongated more than the initial length. As a result, the following inconveniences appear. Since the bottom ends of the fuel rods are fixed to the lower tie plate, the elongated fuel rods push the upper tie plate upwards. Since the top end of the channel box is fixed to the upper tie plate and the bottom end of the channel box is inserted into the lower tie plate, the channel box is pushed upwards due to the irradiation growth of the fuel rods, and at the final burnup stage the length of fitting allowance between the lower tie plate and the channel box is considerably decreased. In the nuclear fuel for high burnup level, the irradiation growth of the fuel rods is so large that the channel box is pushed upwards beyond the length of fitting allowance between the lower tie plate and the channel box. This has been a problem. SUMMARY OF THE INVENTION An object of the present invention is to provide a fuel assembly for a higher burnup level. In order to load the restricted space in a channel box with more than the predetermined amount of uranium, it is necessary to decrease the thickness of zirconium alloy members (such as fuel cladding tubes, spacers, etc.), because the space for loading uranium is reduced in case of the fuel cladding tubes and spacers having the conventional thickness due to an increase in the number of fuel rods arranged in the tetragonal lattice pattern and due to various shapes of water rods. When the thickness of zirconium alloy members such as fuel cladding tubes, spacers, etc. is reduced, particularly hydrogen embrittlement due to the reduction in the thickness is expected to take place. Hydrogen is generated by corrosion reaction with reactor core water and partly absorbed (picked up) into the members of the fuel assembly; but even if the amount of absorbed hydrogen is the same, the hydrogen content of the members will be higher with decreasing thickness. Since corrosion of members of a fuel assembly destined to a higher burnup level proceeds much more than those for the conventional burnup level, a higher corrosion resistance and a lower hydrogen pickup are required for the fuel cladding tubes and spacers for a higher burnup level. Another object of the present invention is to provide a fuel assembly for a higher burnup level, optimized for corrosion resistance and for hydrogen pickup resistance of members of the fuel assembly, and optimized against curving deformation of a channel box due to irradiation growth. In most general terms, the present invention provides a fuel assembly, and components thereof, and method of making and using such components and such fuel assembly. The components include (illustratively) fuel rods, each comprising a cladding tube and a nuclear fuel loaded therein, a spacer for providing the fuel rods at desired positions and integrating the fuel rods and water rods, tie plates, the water rods provided, e.g., at the center of the spacer, and a channel box. The cladding tubes, spacer, water rods and channel box, for example, are made of zirconium-based alloys used in nuclear reactors. The cladding tubes have a higher concentration of solid solution-state iron and nickel (or of iron, nickel and tin), which form components of the zirconium-based alloys, at the outer surfaces thereof than at the inner surfaces. The channel box has a crystallographic orientation parameter in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50. The channel box can also have a crystallographic orientation parameter in the longitudinal (rolling) direction (Fl value) of 0.25 to 0.36 and a crystallographic orientation parameter in the lateral direction (normal-to-the-rolling direction) (Ft value) of 0.25 to 0.36. The water rod has a crystallographic orientation parameter in the tube thickness direction of &lt;0001&gt; crystallographic orientation of 0.25-0.50, a crystallographic orientation parameter in the tube longitudinal (rolling) direction of 0.25-0.36, and a crystallographic orientation parameter in the tube circumferential direction of 0.25-0.36. More specifically, the present invention provides a fuel assembly, which comprises a plurality of fuel rods, each comprising a cladding tube made from a zirconium-based alloy and a nuclear fuel pellets loaded therein, a spacer for providing the fuel rods at desired positions, an upper tie plate and a lower tie plate for supporting the thus provided fuel rods at their upper ends and the lower ends, a water rod provided at the center of the spacer, and a channel box made from a zirconium-based alloy for integrating the fuel rods and the water rod into one assembly and encasing the assembly of the fuel rods and the water rod; the cladding tubes, the spacer and the channel box each contain 1 to 2% by weight of tin, 0.20 to 0.35% by weight of iron, 0.03 to 0.16% by weight of nickel, the balance being substantially zirconium, the cladding tubes have a higher concentration of solid solution-state iron and nickel at the outer surfaces of the cladding tubes than at the inner surfaces, and the channel box has a crystallographic orientation parameter in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50. The present invention further provides a fuel assembly, which comprises a plurality of fuel rods, each comprising a cladding tube made from a zirconium-based alloy and nuclear fuel pellets loaded therein, a spacer for providing the fuel rods at desired positions, an upper tie plate and a lower tie plate for supporting the thus provided fuel rods at their upper ends and the lower ends, a water rod provided at the center of the spacer, and a channel box made from a zirconium-based alloy for integrating the fuel rods and the water rod into one assembly and encasing the assembly of the fuel rods and the water rod; the cladding tubes are hardened and have a higher content of solid solution-state iron, chromium and nickel on the outer surfaces of the cladding tubes than that on the inner surfaces, and the channel box has a crystallographic orientation parameter in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50. In the above-mentioned fuel assembly of the present invention, the channel box has a crystallographic orientation parameter in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50 and is fixed to the upper tie plate, and the water rod is fixed to the upper tie plate and the lower tie plate at its both ends, respectively, and has a crystallographic orientation parameter in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50. In the above-mentioned fuel assembly, the cladding tubes are hardened and have a higher content of solid solution-state iron and nickel on the outer surfaces of the cladding tubes than that on the inner surfaces, and the channel box is thicker at the corners than on the surface sides and has a crystallographic orientation parameter in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50. In the above-mentioned fuel assembly, the cladding tubes are hardened to a depth not more than a half of the thickness and have a higher content of solid solution-state iron, chromium and nickel on the outer surfaces of the cladding tubes than that on the inner surfaces, the channel box is thicker at the corners than on the surface sides and thicker at the lower level in the longitudinal direction than at the upper level and has a crystallographic orientation parameter in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50, and the cladding tubes, the spacer and the channel box each contain 1 to 2% by weight of tin, 0.20 to 0.35% by weight of iron, 0.03 to 0.16% by weight of nickel, the balance being substantially zirconium. The water rod contains 1 to 2% by weight of tin, 0.05 to 0.20% by weight of iron, 0.05 to 0.15% by weight of chromium, and 0.03 to 0.1% by weight of nickel, the balance being substantially zirconium, or 1 to 2% by weight of tin, 0.18 to 0.24% by weight of iron and not more than 0.01% by weight of nickel, the balance being substantially zirconium. The present channel box is made from a welded straight square cylinder having a substantially uniform thickness at the corners and on the surface sides and has a crystallographic orientation parameter in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50, and at least one of the cladding tubes, the spacer, the channel box and the water rod contains 1 to 2% by weight of tin, 0.05 to 0.02% by weight of iron, 0.05 to 0.15% by weight of chromium, and 0.03 to 0.1% by weight of nickel, the balance being substantially zirconium, or 1 to 2% by weight of tin, 0.18 to 0.24% by weight of iron and not more than 0.01% by weight of nickel, the balance being substantially zirconium. The spacer is subjected to a hardening treatment by quenching from an (.alpha.+.beta.) phase region after a final plastic hot working. The present invention provides a channel box for a fuel assembly, which is made from a zirconium-based alloy plate containing 1 to 2% by weight of tin, 0.20 to 0.35% by weight of iron and 0.03 to 0.16% by weight of nickel, the balance being substantially zirconium, a crystallographic orientation in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50, a crystallographic orientation parameter in the longitudinal (rolling) direction (Fl value) of 0.25 to 0.36 and a crystallographic orientation parameter in the lateral direction (normal-to-the rolling direction) (Ft value) of 0.25 to 0.36. Furthermore, 0.05 to 0.15% by weight of chromium can be contained in the alloy state. The present invention provides a channel box for a fuel assembly, which is made from a zirconium-based alloy plate containing 1 to 2% by weight of tin, 0.05 to 0.20% by weight of iron, 0.05 to 0.15% by weight of chromium and 0.03 to 0.10% by weight of nickel, the balance being substantially zirconium, a crystallographic orientation in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50, a crystallographic orientation parameter in the longitudinal (rolling) direction (Fl value) of 0.25 to 0.36 and a crystallographic orientation parameter in the lateral direction (normal-to-the rolling direction) (Ft value) of 0.25 to 0.36. Preferably, the zirconium-based alloy plate has an average grain size of 50 to 300 .mu.m which is formed during the .beta.-heat treatment, i.e. while said plate is heated to the .beta. phase temperature (.beta.-phase Zn crystal grain size of 50 to 300 .mu.m). The present invention provides a spacer for a fuel assembly, which is made from a zirconium-based alloy containing 1 to 2% by weight of tin, 0.20 to 0.35% by weight of iron and 0.03 to 0.16% by weight of nickel, the balance being substantially zirconium, where fine grains of intermetallic compound of tin and nickel are precipitated in the .alpha.-phase zirconium crystal grains. Furthermore, 0.05 to 0.15% by weight of chromium can be contained in the alloy state. The present invention provides a water rod for a fuel assembly, which is made from a zirconium-based alloy tube containing 1 to 2% by weight of tin, 0.05 to 0.15% by weight of chromium and 0.03 to 0.10% by weight of nickel, the balance being substantially zirconium, a crystallographic orientation parameter in the tube thickness direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50, a crystallographic orientation parameter in the tube longitudinal (rolling) direction (Fl value) of 0.25 to 0.36, and a crystallographic orientation parameter in the tube circumferential direction (Ft value) of 0.25 to 0.36. Furthermore, 0.05 to 0.15% by weight of Cr can be contained in the alloy state. Preferably, the zirconium-based alloy has an average grain size of 50 to 300 .mu.m which is formed during the .beta.-heat treatment, i.e. while said plate is heated to the .beta. phase temperature (.beta.-phase Zr crystal grain size of 50 to 300 .mu.m). The present channel box is prepared by bending the zirconium-based alloy plate into a channel-type member, welding the channel-type member to another channel-type member, thereby obtaining a long square cylindrical member, locally heating the long square cylindrical member in a .beta.-phase temperature region and maintaining the member in the heated state for a short time while continuously moving the member, and forcedly cooling the heated part of the member with a cooling medium, thereby making the forcedly cooled part have a crystallographic orientation parameter in the normal-to-plate direction of &lt;0001&gt; crystallographic orientation of the zirconium-based alloy (Fr value) of 0.25 to 0.50. The present cladding tubes are each prepared by continuously moving either a thick tube shell of the zirconium-based alloy after a final hot plastic working or a thin tube shell in the course between the final hot plastic working and a final cold plastic working in the longitudinal direction, locally heating the outer surface of the tube shell in an (.alpha.+.beta.) phase or .beta.-phase temperature region and maintaining the outer surface in the heated state for a short time, while cooling the inner surface of the tube shell, and forcedly cooling the outer surface of the tube shell in the heated parts with a cooling medium. The present spacer comprises spacer cells each prepared by continuously moving either a thick tube shell of the zirconium-based alloy after a final hot plastic working or a thin tube shell in the course between the final hot plastic working and an final cold plastic working in the longitudinal direction, locally heating the tube shell in an (.alpha.+.beta.) phase or .beta.-phase temperature region and maintaining the tube shell in the heated state for a short time, and forcedly cooling the heated part of the tube shell with a cooling medium. The present spacer can comprise the cells integrated with a frame member from a plate-shaped material being subjected to the same heat treatment as for the tube shell in place of the tube shells. The present spacer comprises lattice cells integrated with a frame member made from a plate-shaped material being subjected to the same hardening as for the tube shell, in place of the tube shell. The present water rod is prepared by continuously moving a long tube made from the zirconium-based alloy and subjected to a final cold plastic working in the longitudinal direction, locally heating the tube in a .beta.-phase temperature region and maintaining the tube in the heated state for a short time, and forcedly cooling the heated part with a cooling medium, thereby making the tube have a crystallographic orientation parameter in the tube thickness direction of &lt;0001&gt; crystallographic orientation (Fr value) of 0.25 to 0.50, a crystallographic orientation parameter in the tube longitudinal (rolling) direction (Fl value) of 0.25 to 0.36, and a crystallographic orientation parameter in the tube circumferential direction (Ft value) of 0.25 to 0.36 .