Patent Number: 048760609
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a control blade for use in a nuclear reactor which is adapted to be inserted into and extracted from a nuclear reactor core for the purpose of controlling the power of the nuclear reactor. More particularly, the invention is concerned with a long-life flux-trap type control blade suitable for use in a boiling water reactor (BWR). In general, a control blade for use in a boiling water reactor has a central tie rod and a plurality of wings formed by U-shaped sheath plates attached to the tie rod, each wing containing a multiplicity of neutron absorber rods. Each neutron absorber rod is composed of a clad tube made of a steel such as stainless steel and grain of boron carbide (B.sub.4 C) charged in the clad tube. In order to prevent the grain of boron carbides from moving freely within the clad tube, partition balls are placed at a predetermined interval within the clad tube. The boron carbides in the form of grain charged in the neutron absorber rod progressively decreases its neutron absorption power (capacity) due to absorption of neutrons, and generates He gas as a result of reaction between boron-10 (.sup.10 B) and neutrons resulting in a rise of the pressure within the clad tube. The lifetime of the control blade determined by the neutron absorption power is referred to as "nuclear lifetime", while the lifetime determined by the internal gas pressure of the clad tube is referred to as "mechanical lifetime". The control blade, which is adapted to be inserted into and extracted from the nuclear reactor core, is not uniformly exposed to neutrons. For instance, the rate of neutron exposure rate is high at the side edges and upper end of each wing. This means that these portions of the control blade absorb greater amounts of neutron than other portions of the control blade and, therefore, the nuclear lifetime is reached earlier in these portions than in other portions of the control blade. In consequence, the control blade has to be disposed of as a radioactive waste, even though sufficient lifetime is left in other portions thereof. In order to obviate this problem, the present inventors have developed an improved control blade in which long-life neutron absorbers are disposed in the vicinity of side edges of wings where the degree of neutron exposure is high, as disclosed in Japanese Patent Laid-Open No. 74697/1978. This improved control blade, however, is still unsatisfactory from the view point of prolongation of lifetime of control blades, because it exhibits a lifetime which is only twice as long as that of ordinary control blades containing B.sub.4 C. In order to cope with the demand for prolongation of lifetime of control blades, the present inventors have developed a long-life control blade capable of operating much longer than the above-mentioned improved control blade. This long-life control blade has, as disclosed in Japanese Patent Laid-Open No. 55887/1983, solid neutron absorption plates made of a long-life neutron absorber and disposed in each wing thereof. The neutron absorption plate has apertures or recesses whose sizes and distribution are so determined that the amount of material removed by the presence of such apertures or recesses is comparatively small in the portion where the axial distribution of the shut down margin is small and is comparatively large in the portion where the axial distribution of the shut down margin is large. This long-life control blade, however, suffers from the following disadvantage, due to the use of hafnium (Hf) sheet as the neutron absorber. Namely, hafnium is expensive and has a large specific gravity (13.3 g/cm.sup.3) so that the cost and the weight of the control blade are increased undesirably. The increased weight of the control blade in turn requires a design of a control rod drive mechanism which can safely operate such heavy control blades because conventional control rod drive mechanism cannot withstand such heavy weight of the control blades. The inventors, however, have confirmed that there still is a margin for the removal of material in the hafnium sheet which is used as long-life neutron absorber for the purpose of reducing the weight of the hafnium sheet, and that ordinary control blade drives are still usable provided that the weight of the control blade is reduced by the removal of the material. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a long-life control blade for use in a nuclear reactor such as BWR in which the weight of the long-life neutron absorber and, hence, the overall weight of the control blade are effectively reduced so as to enable conventional control rod drive mechanism to safely drive such a long-life control blade, thereby overcoming the above-described problems of the prior art. Another object of the present invention is to provide a long-life control blade having almost the same size, shape and total weight as those of ordinary B.sub.4 C control blades and, hence, usable in existing boiling water reactors. Still another object of the present invention is to provide a long-life control blade for nuclear reactors, suitable for use in operation of the reactor at a high burn-up and in long-term operation of the reactor. A further object of the present invention is to provide a long-life control blade for nuclear reactors, which is improved in such a way as to effectively avoid damaging due to electrochemical corrosion. A still further object of the present invention is to provide a hybrid-type long-life control blade for nuclear reactors, which is improved in such a way as to avoid damaging due to electrochemical corrosion and to increase mechanical strength so as to exhibit greater resistance to deformation by any external force. A still further object of the present invention is to provide a long-life control blade for nuclear reactors, which is improved such as to exhibit a greater resistance to buckling while reducing the weight of sheaths, thus reducing the total weight of the control blade. According to the present invention, these and other objects can be achieved in one aspect by providing a control blade for nuclear reactors comprising: an upper structure; a lower structure; a central tie rod having radial projections and interconnecting the upper and lower structures together; and wings composed of sheath plates each having a substantially U-shaped cross-section and secured to the end of each projection of the central tie rod, and long-life neutron absorber charged in each of the sheath plate; wherein the neutron absorber in each sheath is divided into a plurality of neutron absorber elements (sections) along the axis of the central tie rod, each neutron absorber element (section) being composed of neutron absorber plates spaced from and opposing each other such that a water gap for guiding the flow of a moderator is defined between these neutron absorber plates. In this control blade, the reactivity worth is increased by virtue of the water gap for guiding the flow of a moderator (coolant) between the opposing neutron absorber plates. The provision of the water gap also enables the thickness of the neutron absorber plates to be reduced in accordance with the amount of the neutron exposure. For these reasons, the control blade can have almost the same size, shape and weight as those of ordinary B.sub.4 C type control blades, even though a heavy long-life neutron absorber such as hafnium sheets is used. Therefore, the control blade can be used in existing nuclear reactors without requiring modification of the control rod drive mechanism, and can operate for a period which is much longer than those offered by known control blades. In another aspect, the present invention provides, in order to avoid any electrochemical corrosion due to contact between different metals, a control blade for nuclear reactors comprising: an upper structure; a lower structure; a central tie rod having radial projections and interconnecting the upper and lower structures together; and wings composed of sheath plates each having a substantially U-shaped cross-section and secured to the end of each projection of the central tie rod, and long-life neutron absorber charged in each of the sheath plate; wherein the neutron absorber in each wing is composed of a plurality of neutron absorber plates such as of hafnium which are spaced from each other in the thicknesswise direction of the wing by means of supporting spacers, such that a water gap for guiding the flow of a moderator is defined between opposing neutron absorber plates, and wherein a water passage is formed between the external surface of each neutron absorber plate and the adjacent inner surface of the sheath. In still another aspect, the present invention provides, in order to prevent buckling through enhancement of strength to lateral bending force, a control blade for nuclear reactors comprising: an upper structure; a lower structure; a central tie rod having radial projections and interconnecting the upper and lower structures together; and wings composed of sheath plates each having a substantially U-shaped cross-section and secured to the end of each projection of the central tie rod, and long-life neutron absorber charged in each of the sheath plate; wherein the neutron absorber in each wing is divided into a plurality of neutron absorber elements along the axis of the tie rod, each the element being composed of a plurality of neutron absorber plates spaced from and opposing each other; and wherein a plurality of spacers are disposed between the opposing neutron absorber plates such that a linear flow passage for a moderator is defined so as to extend in the axial direction of the tie rod, the spacers being arranged at a substantially constant interval along the axis of the tie rod but the interval is slightly reduced in the regions between adjacent neutron absorber plates. These and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments when the same is read in conjunction with the accompanying drawings.