Patent Number: 050341855
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to reactor control blades for controlling the power of a light-water nuclear reactor such as a boiling water reactor and relates more particularly to a high-reactivity-worth, long-lived nuclear reactor control blade designed to increase the reactor shut-down margin and extend the lifetime. In general, conventional boiling water reactor control blades have a construction such that a multiplicity of neutron absorbing rods are inserted in a plurality of wings formed of elongated U-shaped sheaths fixed to a central tie rod. Each of the neutron absorbing rods is constituted by, for example, a stainless steel cover tube filled with boron carbide (B.sub.4 C) grains provided as a neutron absorber. While this reactor control blade is inserted in a core section of a nuclear reactor such as a boiling water reactor, the neutron absorber which fills the sheaths is irradiated with neutrons and gradually loses neutron absorbing ability. The nuclear reactor control blade is therefore changed after being used for a predetermined operation period. Each wing of the control blade used in the core section of a nuclear reactor is not irradiated with neutrons uniformly over the entire area. An insertion end region and an outer edge region of each wing, for example, are irradiated with neutrons more intensely. Part of the neutron absorber which fills each of those region therefore absorbs neutrons at a higher rate, becomes worn faster and reaches the nuclear lifetime faster. Consequently, the whole of the reactor control blade must be scrapped even though the nuclear lifetime of the rest of the neutron absorber in the other region is sufficient. The conventional control blade is thus disadvantageous in terms of economy. In addition, an increase in the frequency of replacement of the reactor control blades means an increase in the total period of time taken for replacement operations, resulting in a reduction in the plant factor and, hence, a considerable economical demerit. There is also a risk of increasing the rate at which operators are exposed to radiation. To prevent this problem and risk, the inventors of the present invention have already proposed a type of nuclear reactor control blade in which a neutron absorber having a comparatively long lifetime such as hafnium is provided in some sections of the control blade where the intensity of neutron irradiation is high. This reactor control blade has, as disclosed in Japanese Patent Laid-Open No. 53-74697, a hybrid structure in which a long-lived neutron absorber is provided in top end portions and blade edge portions of the wings. This hybrid type of reactor control blade has a lifetime twice as long as that of an ordinary type of control blade. In the conventional reactor control blades, the wing is filled with a neutron absorber with a density distribution uniform over the entire region of the wing, and sections of the wing divided in the axial direction are equalized with respect to neutron absorbing ability or reactivity. This arrangement, however, allows a certain dispersion of reactivity with passage of time owing to non-uniformity of the neutron irradiation rate such as mentioned above. There is therefore a possibility of a local deterioration in terms of reactor shut-down margin at the last stage of the operating cycle of the reactor. The reactor shut-down margin distribution (or subcriticality) in the axial direction in the case of operation of the reactor for a predetermined period of time using the above-described type of reactor control blade varies slightly depending upon the design specification of the fuel assembly or the method of operating the reactor, but this distribution is always substantially the same. That is, the reactor shut-down margin is high with respect to the upper and lower ends of the core and is minimum with respect to a position slightly lower than the upper end. This phenomenon can be explained by the following reason. If the effective axial length of the reactor core is L, the void coefficient during operation is particularly high in a section close to the upper end of the core ranging between this upper end and a position at a distance of 3/4.L from the lower end of the core. In this section, the power density of the reactor is relatively low and the amount of remaining uranium of a mass number of 235 (U-235) which is a fissile material is comparatively large Neutron spectrum hardening takes place by the effect of generating voids. As a result, the plutonium generation reaction (neutron absorption reaction) is promoted. For this reason, the enrichment of fissile materials in an upper section of the core becomes relatively high after the operation of the reactor, so that the reactor shut-down margin becomes relatively reduced with respect that region. In the present circumstances, the nuclear fuel burn-up extension and the operating cycle extension will inevitably be promoted because of demands for improvements in the reactor in terms of operation economy. To satisfy such demands, fuels of high enrichment factors are increasingly adopted and, correspondingly, reactor control blades having a long nuclear lifetime and improved in the reactor shut-down margin are urgently required. If the conventional reactor control blades are applied to a reactor loaded with a nuclear fuel of a high enrichment factor, the reactor shut-down margin becomes relatively reduced and it is necessary to periodically replace the reactor control blades with a short operating cycle. To replace the reactor control blades, it is necessary to perform complicated operations of shutting down the reactor and preliminarily removing from the core a multiplicity of fuel assemblies disposed around the control blades which are to be replaced. The period of time during which the reactor is shut down is thereby extended, resulting in a considerable reduction in the operation efficiency of the reactor and, hence, a deterioration in terms of economy. There is also a possibility of a considerable increase in the amount of working for management. To satisfy demands for extension of the lifetime of control blades, the applicant of the present invention has developed a long-lived reactor control blade greatly improved. As disclosed in Japanese Patent Laid-Open No. 58-55887, this reactor control blade is constituted by inserting neutron absorbing plates formed from a long-lived neutron absorbing material, e.g., hafnium in wings formed from stainless steel. As a result of the use of long-lived neutron absorbing plates formed from hafnium or the like, the lifetime of the control blade has been increased to a large extent. This reactor control blade, however, is considerably heavy and expensive as a whole since it makes use of hafnium in the form of a plate which is more expensive than ordinary neutron absorbers and which also has a high density. This control blade cannot be applied to units using conventional control blade driving mechanisms without condition since the design of the mechanism for driving this control blade must be changed to enable the mechanism to withstand the heavy load. SUMMARY OF THE INVENTION In view of these problems of the conventional art, an object of the present invention is to provide a high-reactivity-worth, long-lived type of control blade for a nuclear reactor designed to increase the reactor shut-down margin as well as the lifetime by providing an optimum amount of a long-lived neutron absorber in a region where the reactor shut-down margin tends to become smaller to specially increase the reactivity worth thereof. Another object of the present invention to provide a control blade for a nuclear reactor designed to be improved in the total reactivity worth as well as to extend the lifetime by devising means to cope with swelling of a neutron absorber. Still another object of the present invention is to provide a control blade for a nuclear reactor designed to be reduced in the total weight by forming a neutron absorber from a special light-weight alloy. A further object of the present invention is to provide a control blade for a nuclear reactor designed to extend the nuclear, mechanical lifetime and to become backfittable to existing reactor units as well as to attain the above objects. To attain these objects, the present invention provides one of its aspects a control blade for a nuclear reactor, having: a plurality of wings each in the form of a generally rectangular plate having an longitudinal axis extending in the longitudinal direction of the control blade, the wings being closed at their widthwise ends to each other so as to form a cross-shaped cross section of the control blade; an upper end structural member fixed to an upper end of each of the wings inserted into a core of the reactor; a lower end structural member fixed to a lower end of the wing inserted into the reactor core; a central connection member connecting the upper end structural member and the lower end structural member so as to support the wing; and a packing means formed in the wing, a neutron absorber being packed in the packing means; wherein the wing is formed of a diluted alloy obtained by diluting a long-lived neutron absorber such as hafnium with a diluent. More specifically, the neutron absorber packing means is formed as a multiplicity of aligned neutron absorber housing holes extending in the widthwise direction of the wing, and some of these housing holes formed in a region where the subcriticality becomes smaller or shallower during shut-down of the reactor are enlarged so as to have an elongated cross section while the structure including these elongated holes is formed with means to cope with swelling. The present invention also provides in another of its aspects a control blade for a nuclear reactor based on the above construction wherein the neutron absorber packing means is formed as a neutron absorber packing space section which is divided into a first region on the side of the inserted upper end and a second region on the side of the inserted lower end and adjacent to the first region. The first region includes a high-reactivity-worth region in which a diluted alloy obtained by diluting a long-lived neutron absorber with a diluent is packed. A plurality of lateral holes extending in the widthwise direction of the wing are arranged in a row over the region where the long-lived neutron absorber is contained, and a neutron absorber different from the long-lived neutron absorber is packed in these lateral holes. The present invention also provides in still another of its aspects a control blade for a nuclear reactor wherein the neutron absorber packing space section is divided into a first region on the side of the inserted upper end where the neutron irradiation rate is particularly high, a second region next to the first region, where the subcriticality becomes smaller during shut-down of the reactor, and a third region bordering the second region on the side of the inserted lower end, a long-lived neutron absorber being packed in housing holes formed in the first region, a neutron absorber such as boron carbide being packed in housing holes formed in the second and third regions, at least one of the housing holes formed in the third region being formed as a gas plenum. In these control blades also, some of the housing holes formed in a region where the subcriticality becomes smaller during shut-down of the reactor are enlarged so as to have an elongated cross section while the structure including these elongated holes is formed with means to cope with swelling. In the thus-constructed control blades for nuclear reactors, each wing is formed from a diluted alloy containing an optimum amount of hafnium having a long lifetime and a high density, and this diluted alloy is formed of a solid solution containing zirconium or titanium having a low density. It is therefore possible to manufacture a light-weight control blade having a smaller weight and stable physically and chemically. This control blade can therefore be adopted for use in conventional reactors without changing design specifications relating to load withstanding performance of the existing control blade driving mechanisms. The reactivity worth of the reactor control blade is increased by the complementary neutron absorption effects of hafnium contained as a long-lived neutron absorber in the diluted alloy forming each wing and of the neutron absorber packed in the housing holes in each region, thereby improving the reactor shut-down margin and increasing the nuclear lifetime to a large extent. In the control blades of the above constructions, a larger amount of the neutron absorber is provided in a portion where the subcriticality becomes smaller during shut-down of the reactor while a long-lived neutron absorber is provided in a portion where the neutron irradiation rate is particularly high, gas plenums are disposed in an optimized fashion in the other portions to receive gasses such as helium generated by the reaction between the neutron absorber and neutrons, thereby limiting the increase in the gas pressure and improving the mechanical strength. In the control blades for nuclear reactors in accordance with the present invention, housing holes are formed in each wing in such a manner that they extend in the widthwise direction of the wing and that they are arranged in a row in the longitudinal direction of the wing. A long-lived neutron absorber is disposed at least in accommodation holes which are formed in the inserted upper end section of the wing and which contribute to the reactivity worth. Therefore the neutron absorbing ability of the inserted upper end section which is exposed to neutron irradiation when inserted in the core during operation of the reactor or even when drawn out is maintained for a long period of time, thus improving the nuclear lifetime. When the control blade is used by being fully inserted into the core, nuclear reaction of the interior of the fuel assembly is restricted by the void phenomenon with respect to the second region which is formed next to the first region of the control blade in the longitudinal direction and where the subcriticality becomes smaller. In this event, therefore, the amount of remaining nuclear fuel with respect to the second region is large. Besides, the density of fissile materials becomes relatively increased by the plutonium generation reaction. However, the present invention has optimized the amount of the neutron absorber packed in the second region by changing the pitch, shape and dimensions of the housing holes formed in the second region so as to increase the hole capacity per unit length in the lengthwise direction of the wing compared with the other regions. The desired reactivity worth of the neutron absorber in the second region can be thereby maintained even during a long-term operation of the reactor. In consequence, it is possible to ensure a sufficient overall reactor shut-down margin while the control blade is fully inserted in the reactor core. The portion of the wing in which the elongated holes are formed is constructed to cope with swelling in such a manner that a neutron absorber which does not swell by neutron absorption reaction is provided in the extreme end portions of each housing hole closer to the adjacent holes, or that the thicknesses of the wing wall portions on opposite sides of the hole are increased at the extreme end portions to increase the mechanical strength. Instead, inner tubes filled with a neutron absorber may be fitted in the elongated holes, an inner sleeve is fitted in each elongated hole while a neutron absorber containing boron is packed in this inner sleeve, or dimples are formed in the outer surfaces of the wing wall portions. The control blade is thus designed to reduce the degree of stress around the elongated hole caused by swelling, delay the time at which the stress starts generating, or prevent the generation of the stress, thus extending the mechanical lifetime.