Patent Number: 060410914
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a control rod for a nuclear reactor, and more particularly, to a control rod for a nuclear reactor of the long-life type having an improved mechanical soundness in a boiling water reactor. A control rod for a boiling water reactor (BWR) has usually four wings formed by housing neutron absorber plates in a plurality of long sheaths having a deep U-shaped cross-section. A leading end structural member is provided at an insertion leading end portion of each of the wings, or a terminal end structural member, at an insertion terminal end portion thereof, and the U-shaped openings of the sheaths in each of four wings are secured to an integral type center structural member (known also as a tie rod) having a cruciform cross-section to provide a cruciform sectional configuration or arrangement. In a conventional control rod, the sheath is made of stainless steel (S.S such as SUS, hereinafter abbreviated as "SUS"), and a SUS tube having a diameter of 5 mm filled with boron carbide (B.sub.4 C) powder has been employed as a neutron absorbing rod. Boron (B) has however a short nuclear life because it reacts with neutron to generate helium (He) and lithium (Li), resulting in a degraded neutron absorbing ability, and helium causes an increase in internal pressure, leading to a decrease in soundness of the SUS tube and hence to a shorter mechanical and physical service life. In order to provide a control rod having a long service life, there has been used a long-life type control rod manufactured by replacing a conventional neutron absorbing rod partially or totally by hafnium (hereinafter abbreviated as "Hf") which is a long-life type neutron absorber. Since Hf has a large specific gravity (density) as about 13, an Hf rod having the same cross-section as a conventional neutron absorbing rod using boron carbide results in a weight about 1.5 times as large as the control rod as a whole, although the neutron absorbing ability (reactivity value) is substantially the same, making it impossible to back-fit it into a nuclear reactor in operation. As a counter-measure, Japanese Patent Laid-open (KOKAI) Publication No. HEI 1-34358 "Control Rod for Nuclear Reactor" proposes an Hf control rod of the type known as a trap type in which Hf is formed into a plate shape, and two Hf plates are arranged opposite to each other with a gap for introduction of water. Further, in view of the fact that in about the terminal end side half of a control rod for the BWR, upon insertion thereof into the reactor core, a decreased neutron absorbing ability would cause no inconvenience in the control of the BWR, a control rod configured so as to use a smaller Hf content in the portion on the insertion terminal end side than in the portion on the insertion leading end side is proposed in Japanese Patent Laid-open Publication No. HEI 7-3468 "Control Rod for Nuclear Reactor". Regarding the long-life control rod having the trap structure using the Hf plate, excellent results have already been achieved in many BWRs, and it is the usual practice to set a short service life for maintenance purposes. When setting a longer service life, it becomes now clearer than ever that it is effective to improve mechanical strength of SUS structural members such as a sheath in the control rod. FIGS. 19 to 21 illustrate an outline of an Hf trap type control rod, in which FIG. 19A is a partially cutaway perspective view, FIG. 19B is a sectional view of a wing and FIG. 19C is a perspective view of a load supporting member (also referred to as a "load supporting spacer" of "top spacer"). FIG. 20A is a partially cutaway front view of a sheath shown in FIG. 19A, and FIG. 20B illustrates an example of thickness of an Hf plate which is a neutron absorber plate of a neutron absorbing material attached in the interior of a sheath, as illustrated in a distribution diagram in the control rod insertion/withdrawal direction which is the sheath longitudinal direction. FIG. 21A is a partially enlarged front view of FIG. 20A, FIG. 21B is an enlarged front view of a pair of Hf plates shown in FIG. 21A, and FIG. 21C is a sectional view of FIG. 21C taken along the line XXIC--XXIC of FIG. 21B. Referring to these figures, a long-life type control rod 1 has a cruciform section with four wings 2, and a leading end structural member 4 integral with a handle 3 is secured to the insertion leading end portion into the reactor core, and a terminal end structural member 5 is secured to an insertion terminal end portion. Further, a cruciform integral type center structural member 6 made of SUS is provided at an axial center of the control rod 1 (central tie rod), and an opening portion of a sheath 7 made of SUS having a deep U-shaped cross-section forming an outer periphery of the wing 2 is secured by welding to each projection of this integral type center structural member 6. A plurality of sheath holes 8 and water holes 9 are pierced in the sheath 7, in which two Hf plates 10 which are neutron absorber plates are supported by a load supporting member 12 also serving as a gap (interval) maintaining space, and a water gap 11 (gap through which cooling water flows during use in the reactor) is formed between the two Hf plates 10. The load supporting member 12 has a top-like shape, and the thickness of a gap maintaining portion 12a at the center thereof has a function of spacer. The Hf plates 10 are supported by attaching the Hf plates 10 from both the sides to a support shaft 12b through an attachment hole 13 and causing engagement of the support shaft 12b with a sheath hole (bore) 8, which are secured together by means of welding. When inserting or withdrawing the control rod 1 into or from the reactor core, a percussive force is applied to the sheath 7 upon intermittent driving, or particularly, upon starting driving or decelerating during scram of the reactor. In a long-life type control rod 1, the sheath 7 and the load supporting member 12 made of SUS forming the wing 2 have a thermal expansion coefficient about three times as high as that of the Hf plate 10, which is the neutron absorber formed of a material different from those of the sheath 7 and the load supporting member 12. For example, while SUS has a thermal expansion coefficient of 17.8.times.10.sup.-6 /deg-C., that of Hf is 5.9.times.10.sup.-6 /deg-C. ("Nuclear Reactor Materials Handbook" published by Nikkan Kogyo Shinbun-sha). To avoid inconveniences resulting therefrom, the attachment hole 13 of the Hf plate 10 to be attached to the support shaft 12b of the load supporting member 12 has a diameter larger than that of the support shaft 12b to provide a margin, thereby permitting avoidance of their mutual interference through expansion and contraction in heat cycles during operation of the reactor. In the example shown in FIG. 20, the Hf plate 10 of the control rod 1 having a length L in the inserting direction into the reactor core or the sheath longitudinal direction is longitudinally and equally divided into eight sections. The length 1 of a single Hf plate 10 is therefore about L/8. FIGS. 20 and 21A are described with a scale compressed in the axial direction for convenience of illustration, and FIG. 21B represents the Hf plate substantially similarly to the actual one. Within the wing 2, two Hf plates 10 which are neutron absorber plates arranged opposite to each other form an Hf plate pair 14 which is held by the sheath 7 through four (or three, five or six) load supporting members 12. The attachment hole 13 of the Hf plate and the sheath hole 8 of the sheath 7 have the same pitch 15 size in the sheath longitudinal direction. During inserting or withdrawing operation of the control rod 1, the sheath 7 is subjected, not only to a static load caused by the weight of the Hf plate pair 14 applied through the load supporting member 12 in the stationary state, but also to a dynamic load caused by the relative displacement to the Hf plate pair 14. This load caused by relative displacement becomes an impact load particularly upon intermittent operation or driving starting operation or decelerating operation during a rapid driving in the scram of the reactor. These loads tend to be believed to be shared by the four load supporting members 12 in general and transmitted to the sheath 7. Actually, however, even when a margin is provided taking account of a difference in thermal expansion caused by the difference in thermal expansion coefficient between different materials such as between the attachment hole 13 of the Hf plate and the support shaft 12b of the load supporting member 12, it is conceivable that a single load supporting member bears all the load because of, for example, the manufacturing tolerance. In the worst case, a specific unknown load supporting member 12 receives a large stress, thus causing a local stress to concentrate on the sheath hole 8 at the position where that load supporting member is secured. This is therefore undesirable from the point of view of ensuring soundness of the sheath 7. In the Hf plate 10 of the Hf plate pair 14, as shown in FIG. 20B, the neutron irradiation is larger at a position closer to the insertion leading end and the reactivity value must be increased. The thickness becomes therefore thicker toward the insertion leading end and thinner toward the insertion terminal end. The length in the control rod insertion/withdrawal direction, which is the sheath longitudinal direction of each Hf plate 10, is usually constant, and the thickness of the sheath 7 is also uniform in the control rod insertion/withdrawal direction. A wider water gap corresponds to a higher reactivity value. The sheath 7 should therefore be preferably the thinnest possible. However, since the thickness of the sheath 7 is associated with strength of the sheath 7, an excessive reduction of thickness causes deterioration of mechanical soundness, thus preventing improvement of service life of the control rod 1. More specifically, the distribution in the sheath longitudinal direction of the load acting on the sheath 7 is such that the load is larger toward the insertion leading end because of the thicker Hf plate 10. When designing the thickness of the sheath 7, therefore, it is necessary to sufficiently take account of the weight of the Hf plates 10 serving as the neutron absorber plates and the impact load received upon operation of the control rod 1, as well as mechanical strength with due regards to the manufacturing tolerance. When a horizontal impact is caused by an earthquake or the like, a relatively large stress occurs near the central portion in the longitudinal direction of a long control rod 1. Ensuring a satisfactory mechanical strength for the proximity of the middle is an important task. Therefore, using the control rod 1 for the longest possible period of time contributes to improvement of reliability of the control rod and economic merits of reactor operation. In order to further extend the service life of the long-life type control rod 1, it is necessary to increase mechanical strength having so far formed a restriction as compared with the nuclear life in neutron absorption. SUMMARY OF THE INVENTION A primary object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art and to provide a control rod for a nuclear reactor, which has a mechanical strength improved through supporting manner of neutron absorbers in a long-life type control rod and has a further longer service life in balance with the nuclear life. Another object of the present invention is to provide a control rod for a nuclear reactor which is improved in mechanical and physical strength, making it possible to use for a long time and achieving reliability and economical merits in the reactor operation. A further object of the present invention is to provide a control rod for a nuclear reactor capable of reducing a load applied per one load supporting member acting on a U-shaped sheath and reducing a local stress applied to the U-shaped sheath to thereby enhance the soundness of the control rod. A still further object of the present invention is to provide a control rod for a nuclear reactor capable of improving a reactivity value as well as improving the soundness of the U-shaped sheath. A still further object of the present invention is to provide a control rod for a nuclear reactor capable of further improving a stress resisting property and the mechanical strength of the U-shaped sheath by separately carrying an integral type neutron absorbing element. A still further object of the present invention is to provide a control rod for a nuclear reactor capable of maintaining the soundness of the load supporting member and the integral type neutron absorbing element to thereby improve the life time and the reliability of the control rod. A still further object of the present invention is to provide a control rod for a nuclear reactor having an improved mechanical and physical strength by increasing and ensuring reliability of a weld portion through abut-welding of the U-shaped sheath against a recessed projection of a central structural member. These and other objects can be achieved according to the present invention, which provides in a general aspect, a control rod for a nuclear reactor comprising a center structural member, a plurality of wings each composed of a sheath member of long plate structure having a U-shaped cross section having an opening which is secured to the center structural member, a front end structural member secured to a front end side of the wing viewed from a wing inserting direction in a reactor core, a terminal end structural member secured to a terminal end side of the wing viewed from the wing inserting direction in the reactor core, a plurality of integral type neutron absorbing elements each having a plate structure accommodated in each of the sheaths in a row in a longitudinal direction thereof and each being formed in plate shape by integrating one or more neutron absorbing plates, and a plurality of load supporting members for supporting weights of the integral type neutron absorbing elements. According to the present invention, the following characteristic features will be provided to the control rod for a nuclear reactor of the structure mentioned above. The length in the sheath longitudinal direction of at least one set of the integral type neutron absorbing elements accommodated in the wing is reduced, the reduced integral type neutron absorbing elements are supported to the U-shaped sheath by the load supporting members to thereby reduce a local load applied to the U-shaped sheath. At least a set of integral type neutron absorbing elements having a relatively large weight are divided into a plurality of sections in a wing width direction, weight of the divided integral type neutron absorbing element sections are supported to the U-shaped sheath respectively by a plurality of load supporting members so that total supporting ability of the plurality of load supporting members which support the divided integral type neutron absorbing element sections surpasses supporting ability of the load supporting members of the integral type neutron absorbing elements not yet divided. The integral type neutron absorbing elements divided into the plurality of sections in the wing width direction is provided with a stiffener containing a neutron absorber to at least one portion of the divided neutron absorbing element and the stiffener is secured to the U-shaped sheath. In the integral type neutron absorbing elements divided into the plurality of sections in the wing width direction, a neutron absorbing ability per unit length in the wing width direction of the integral type neutron absorbing elements located on an outside edge of the wing is increased as compared with that of portions other than the wing outside edge. The integral type neutron absorbing element is formed with an attachment hole into which a support shaft of the load supporting member is inserted with a margin, at least two attachment holes are formed with space in the sheath longitudinal direction respectively on both inside and outside portions in the wing width direction, and a distance between a pair of two load supporting members in the sheath longitudinal direction is made different in accordance with a pitch of the attachment hole in the sheath longitudinal direction being in a range less than a margin of a diameter of the attachment hole exceeding manufacturing tolerance. Two load supporting members paired in the sheath longitudinal direction having the attachment holes of different pitches in the sheath longitudinal direction are disposed inside in the wing width direction. The integral type neutron absorbing element is formed with an attachment hole into which a support shaft of the load supporting member is inserted with a margin, at least two attachment holes are formed with space in the sheath longitudinal direction respectively on both inside and outside portions in the wing width direction, and a pitch in the sheath longitudinal direction of the attachment hole of the integral type neutron absorbing element is different in a range less than a margin of a diameter of the attachment hole exceeding a manufacturing tolerance compared with a pitch in the sheath longitudinal direction between the load supporting members. The attachment hole having a pitch in the sheath longitudinal direction different from a pitch in the sheath longitudinal direction between the load supporting members are formed inside in the wing width direction of the integral type neutron absorbing element. At least a part of the plurality of load supporting members supporting the integral type neutron absorbing element is arranged in a vicinity of a central portion of the integral type neutron absorbing element, and a margin between a support shaft of the load supporting member disposed near the central portion thereof and an attachment hole of the integral type neutron absorbing element into which the support shaft is inserted is made less than a margin between another support shaft of another load supporting member other than that disposed near the central portion of the neutron absorbing element and an attachment hole formed thereto into which the another support shaft is inserted to thereby increase a load supporting ability. The integral type neutron absorbing element is composed of a long-life type neutron absorbing plate by forming a hafnium alloy into a plate shape diluted by diluting hafnium with a diluting agent such as zirconium or titanium, and the integral type neuron absorbing element has a trap structure based on a combination of a plurality of the neutron absorber plates with a water gap serving as a reactor water channel, and the load supporting member is provided with a gap maintaining function between the neutron absorber plates. The integral type neutron absorbing element is formed with an attachment hole into which a support shaft of the load supporting member is inserted with a predetermined margin, the load supporting members are disposed on the control rod inserting front end side and inserting terminal end side of the integral type neutron absorbing element in a manner separated in a wing width direction, and a margin in engagement between either one of support shafts in the wing width direction of the load supporting members and the attachment hole of the integral type neutron absorbing element is made smaller than a margin in engagement between another one support shaft of the load supporting member and the attachment hole of the integral type neutron absorbing element. The integral type neutron absorbing element is provided with an attachment hole to which the load supporting member secured to the sheath is inserted with a predetermined margin, the load supporting members secured to the sheath are attached to the attachment holes of the integral type neutron absorbing elements and also provided with a frictional load supporting member having a gap maintaining function and a frictional resistance function between the neutron absorber plate constituting the integral type neutron absorbing element and the U-shaped sheath. The frictional load supporting member is composed of a projection projecting from an inner surface of the U-shaped sheath, and a recessed portion formed to a surface of the neutron absorber plate and engaged with the projection to thereby imparting the frictional resistance function. The support shafts of the load supporting members secured to the U-shaped sheath are fitted to attachment holes formed to the integral type neutron absorbing element with a predetermined margin, and the support shaft of a specific member of the load supporting members and a corresponding attachment hole of the integral type neutron absorbing element have large diameters in comparison with those of the other load supporting members to improve a load supporting ability. The specific load supporting member is arranged inside in a wing width direction. The plurality of integral type neutron absorbing elements include the neutron absorbers each having a thickness gradually reduced from the leading end side of control rod insertion toward the terminal end side thereof and the length of the neutron absorber is increased toward the insertion terminal end side. The plurality of integral type neutron absorbing elements are reduced in lengths thereof at portions near a central portion of an entire length of the control rod and a load supporting member is arranged also at central portions in a wing width direction and in a sheath longitudinal direction of at least part of the integral type neutron absorbing elements on the control rod insertion end side. The plurality of integral type neutron absorbing elements have substantially uniform lengths throughout an entire length of the control rod and a load supporting member is provided at a central portion in a wing width direction and in a sheath longitudinal direction of the integral type neutron absorbing element up to substantially 2/3 length from the leading end in the control rod inserting direction. The integral type neutron absorbing element is composed of a neutron absorbing plate by forming a hafnium alloy into a plate shape diluted by diluting hafnium with a diluting agent such as zirconium or titanium, the integral type neuron absorbing element has a trap structure based on a combination of a plurality of the neutron absorber plates with a water gap being interposed therebetween, and the opposing neutron absorber plates with the water gap being interposed therebetween are arranged stepwise in a control rod insertion/withdrawal direction. The integral type neutron absorbing element is composed of a neutron absorbing plate by forming a hafnium alloy into a plate shape diluted by diluting hafnium with a diluting agent such as zirconium or titanium, the integral type neuron absorbing element has a trap structure based on a combination of a plurality of said neutron absorber plates with a water gap being interposed therebetween, the neutron absorber plate has a thickness gradually reduced from the leading end side of the control rod insertion toward the terminal end side thereof and the load supporting member to be secured to the U-shaped sheath is fitted to the attachment hole with a predetermined margin, two such load supporting members are each provided at an interval in the sheath longitudinal direction, and an interval maintaining member secured to the U-shaped sheath at central portions in a wing width direction and a sheath longitudinal direction between the load supporting members is mounted to at least one set of the integral type neutron absorbing elements. The neutron absorber plates of at least one set of integral type neutron absorbing elements are made of a hafnium alloy diluted by a diluting agent such as zirconium or titanium and having a hafnium content equal to that in neutron absorber plates of another integral type neutron absorbing element, and the neutron absorber plates made of the hafnium alloy each has an increased thickness to thereby improve mechanical and physical strengths. The integral type neutron absorbing element is composed of a neutron absorbing plate by forming a hafnium alloy into a plate shape diluted by diluting hafnium with a diluting agent such as zirconium or titanium, the integral type neuron absorbing element is formed with a water gap in a control rod insertion/withdrawal direction in the U-shaped sheath and has substantially a box-shaped sectional shape in a direction perpendicular to the control rod insertion/withdrawal direction. The center structural member is formed with a recessed projection having a thickness equal to that of the wing and extending in a longitudinal direction thereof on a side to be secured to the U-shaped sheath. According to the present invention of the various aspects mentioned above, there are achieved the following functions and advantageous effects. In one aspect, there is provided a control rod for a nuclear reactor constructed by securing a leading end structural member to a leading end, and a terminal structural member to a terminal end in the core inserting direction by means of a long sheath having a U-shaped cross-section, securing or integrating flat with a tolerance of slight sliding one or more sheet-shaped neutron absorbing plates divided into a plurality of sections in the sheath longitudinal direction, housing the integral type neutron absorbing elements into the sheath, forming a plurality of wings so as to hold the weight of the individual integral type neutron absorbing elements through a plurality of load supporting members by the sheath, and securing a U-shaped opening of each of the plurality of wings to an opening side structural member formed by an integral type center structural member of an independent type structural member, and the length in the sheath longitudinal direction of at least one set of integral type neutron absorbing elements, of which the weight becomes relatively large, is reduced to alleviate a local load exerted on the sheath. According to the above aspect of the present invention, by reducing the weight by reducing the length of the integral type neutron absorbing elements which are relatively heavy at the insertion leading end into the reactor core, the load per load supporting member is reduced, thereby securing the load supporting member and reducing local stress at the position where the load supporting member is secured in the sheath holding the load of the integral type neutron absorbing elements. At least a set of integral type neutron absorbing elements having a relatively large weight are divided into a plurality of sections in a direction at right angles to the longitudinal direction of the sheath, so that the total supporting ability of the plurality of load supporting members which support the divided integral type neutron absorbing elements surpasses the supporting ability of the load supporting members of the integral type neutron absorbing elements not as yet divided. Furthermore, because the load per load supporting member is reduced by the reduction of weight of the integral type neutron absorbing elements, local stress on the sheath holding the load of the integral type neutron absorbing elements by securing the load supporting member is reduced. According to the present invention, there is provided a control rod for a nuclear reactor, in the integral type neutron absorbing elements divided into the plurality of sections in a direction normal to the longitudinal direction of the sheath, a stiffener containing a neutron absorber is arranged at least at a place in the integral type neutron absorbing element sections resulting from division of the integral type neutron absorbing elements into the plurality of sections, and the stiffener is secured to the sheath. Accordingly, mechanical and physical strength of the sheath is improved because the load applied per load supporting member is reduced, and further, since a long-life type neutron absorber is housed in the stiffener, there occurs almost no decrease in the reactivity value of the control rod. According to the present invention, in the integral type neutron absorbing elements divided into the plurality of sections in a wing width direction, the neutron absorbing ability per unit length in the wing width direction of the integral type neutron absorbing elements located on the outer edge of the wing is increased as compared with that of portions other than the wing outer edge. Therefore, the reduction of the load acting on the load supporting member leads to an improved soundness of the sheath. When using integral type neutron absorbing elements of the same weight, the reactivity value of the control rod can be improved with an extended nuclear life. When it is not necessary to improve the reactivity value or the nuclear life, the weight is reduced by reducing the quantity of integral type nuclear absorbing elements, thereby further improving soundness of the sheath supporting the same. According to the present invention, the integral type neutron absorbing elements are attached to the support shaft and the attachment hole of the load supporting member secured to the sheath in the attachment hole with a predetermined margin, at least two such load supporting members each are provided at an interval in the sheath longitudinal direction, and the distance between predetermined two load supporting members forming a pair in the sheath longitudinal direction varies within a range in which the pitch between a pair of attachment holes in the integral type neutron absorbing elements is over a manufacturing tolerance and under the margin of the attachment hole. When the integral type neutron absorbing elements move relative to the sheath, any one of the two specified load supporting members forming a pair in the sheath longitudinal direction bears the load when inserting or when withdrawing. The two load supporting members are specified depending upon the direction of insertion or withdrawal, thus certainly sharing the load. The load per each load supporting member is therefore reduced to a half, and this improves, together with the load supporting members, stress resistance and mechanical strength of the sheath. According to the present invention, the two load supporting members forming a pair in the sheath longitudinal direction, different from the pitch between a pair of the attachment holes are closer to the structural member for securing the wing. In the sheath of the wing, the side secured to the structural member has a higher mechanical strength than that on the opposite side. By causing the load supporting member closest to this structural member to bear the load of the integral type neutron absorbing elements, the stress resistance and the mechanical strength of the sheath securing this load supporting member can be improved. According to the present invention, the integral type neutron absorbing elements are attached to the support shaft and the attachment hole of the load supporting member secured to the sheath in the attachment hole with a predetermined margin, at least two such load supporting members each are provided at an interval in the sheath longitudinal direction, and as compared with the pitch between a pair of the load supporting members, the distance between the two prescribed load supporting members forming a pair in the sheath longitudinal direction varies within a range in which the pitch between a pair of attachment holes in the integral type neutron absorbing elements is over a manufacturing tolerance and under the margin of the attachment hole. When the integral type neutron absorbing elements move relative to the sheath, any one of the two prescribed load supporting members forming a pair in the sheath longitudinal direction bears the load when inserting or when withdrawing. The two load supporting members are specified depending upon the direction of insertion or withdrawal, thus certainly sharing the load. The load per each member is therefore reduced to a half, and this improves, together with the load supporting members, the stress resistance and the mechanical strength of the sheath. According to the present invention, the attachment holes of the two integral type neutron absorbing elements forming a pair in the sheath longitudinal direction are closer to the structural member securing the wing. In the sheath of the wing, the side secured to the center structural member has a higher mechanical strength than that on the opposite side. By Causing the load supporting member closest to this structural member to bear the load of the integral type neutron absorbing elements, the stress resistance and the mechanical strength of the sheath securing this load supporting member can be improved. According to the present invention, a part of the plurality of load supporting members are provided at least in the proximity of the center of the integral type neutron absorbing elements, and the margin between the support shaft of the load supporting member near the central portion and the attachment hole of the integral type neutron absorbing element is reduced to under the margin between the other load supporting members and the attachment hole, thereby increasing the load supporting ability. The plurality of load supporting members provided closer to the central portion of heavy integral type neutron absorbing element shares the impact load and the like by a long engagement with the integral type neutron absorbing element, thereby improving the load resistance of the sheath and ensuring intervals between the integral type neutron absorbing elements. According to the present invention, the integral type neutron absorbing element has a trap structure based on a combination of a plurality of neutron absorber plates prepared from hafnium metal or by forming a hafnium alloy made by diluting hafnium with zirconium or titanium into plates with a water gap serving as a reactor water channel, and an interval (gap) maintaining function between the neutron absorber plates to the load supporting members. By realizing the trap structure by using neutron absorber plates made of hafnium metal or a hafnium alloy as integral type neutron absorbing elements and arranging them opposite to each other with a water gap therebetween, while maintaining the interval near the central portion, it is possible to eliminate a deflection toward inside of the neutron absorber plates, prevent a decrease in reactivity value and improve mechanical strength of the sheath. According to the present invention, the load supporting members secured to the sheath are attached to the attachment hole of the integral type neutron absorbing element with a predetermined margin, the load supporting members are provided at an interval in a direction normal to the sheath longitudinal direction on the leading end side and on the terminal end side of insertion of the control rod in the integral type neutron absorbing element, and a margin between the support shaft of any of the load supporting members and the attachment hole of the integral type neutron absorbing element is reduced to under the margin between the other load supporting member and the attachment hole. The load on the integral type neutron absorbing elements is certainly borne by sharing by the plurality of load supporting members in any of the inserting and withdrawing directions of the control rod, thus alleviating a local impact load of the sheath securing the load supporting members and improving the soundness of the sheath. According to the present invention, the load supporting members secured to the sheath are attached to the attachment holes of the integral type neutron absorbing elements, and there is provided a frictional load supporting member having an interval maintaining function and a frictional resistance function between the neutron absorber plate of the integral type neutron absorbing element and the sheath. Relative motion of the sheath and the neutron absorber plates of the integral type neutron absorbing elements along with operation of the control rod inhibits an impact load through frictional resistance between the sheath and the neutron absorber plates given by the frictional load supporting members. As a result, the burden on the load supporting members is alleviated and the soundness of the sheath is improved. According to the present invention, the frictional resistance function of the frictional load supporting member causes engagement of a dimpling projecting into the inner surface of the sheath and a recess formed on the surface of the neutron absorber plate. By realizing the trap structure by using neutron absorber plates made of hafnium metal or a hafnium alloy as integral type neutron absorbing elements and arranging them opposite to each other with a water gap therebetween, while maintaining an interval near the central portion, it is possible to eliminate a deflection toward inside of the neutron absorber plates, prevent a decrease in the reactivity value and improve the mechanical strength of the sheath. According to the present invention, the load supporting members secured to the sheath are attached to the attachment hole of the integral type neutron absorbing element with a prescribed margin, the load supporting members are provided at an interval in a direction normal to the sheath longitudinal direction on the leading end side and on the terminal end side of insertion of the control rod in the integral type neutron absorbing element, and a margin between the support shaft of any of the load supporting members and the attachment hole of the integral type neutron absorbing element is reduced under the margin between the other load supporting member and the attachment hole. The load on the integral type neutron absorbing elements is certainly borne by sharing by the plurality of load supporting members in any of the inserting and withdrawing directions of the control rod,thus alleviating a local impact load of the sheath securing the load supporting members and improving the soundness of the sheath. According to the present invention, the load supporting members secured to the sheath are attached to the attachment holes of the integral type neutron absorbing elements, and there is provided a frictional load supporting member having an interval maintaining function and a frictional resistance function between the neutron absorber plate of the integral type neutron absorbing element and the sheath. The relative motion of the sheath and the neutron absorber plates of the integral type neutron absorbing elements along with operation of the control rod inhibits an impact load through frictional resistance between the sheath and the neutron absorber plates given by the frictional load supporting members. As a result, the burden on the load supporting members is alleviated and the soundness of the sheath is improved. According to the present invention, the frictional resistance function of the frictional load supporting member causes engagement of a dimpling projecting into the inner surface of the sheath and a recess formed on the surface of the neutron absorber plate. By achieving engagement of the dimpling of the t sheath and the recess of the neutron absorber plate and holding this state with the frictional load supporting member, frictional resistance occurs between them. According to the present invention, the support shaft of the load supporting member secured to the sheath is attached to the attachment hole of the integral type neutron absorbing element with a predetermined margin, and the support shaft of the load supporting member and the attachment hole of the integral type neutron absorbing element have large diameters at a specific load supporting member position from among the load supporting members, thereby improving the load supporting ability. Because of the large diameters of the attachment hole, the support shaft of the load supporting member and the sheath hole which support the impact load and the weight of the integral type neutron absorbing elements, the load bearing ability is increased and the soundness of the sheath and the like is improved. According to the present invention, the specific load supporting member at the position of which the supporting shaft of the load supporting member and the attachment hole of the integral type neutron absorbing element have larger diameters than the others is closer to the center structural member securing the wing. In the sheath of the wing, the side secured to the center structural member has a higher mechanical strength than that on the opposite side. By causing the load supporting member closest to the center structural member to bear the load of the integral type neutron absorbing elements, the stress resistance and the mechanical strength of the sheath securing this load supporting member can be improved. According to the present invention, the thickness of the neutron absorber is gradually reduced from the leading end side of control rod insertion toward the terminal end side, and the length of the neutron absorber is increased toward the terminal end side. Since the weight is substantially uniform for all the neutron absorber plates, the impact load borne by the individual load supporting members are substantially equalized to the load stress in the sheath, thereby improving the soundness of the sheath and the like. According to the present invention, the length of the plurality of integral type neutron absorbing elements is reduced near the central portion of the entire length of the control rod, and a load supporting section is provided also at the central portion in the wing width direction and in the longitudinal direction of at least a part of the integral type neutron absorbing elements. Therefore, seismic resistance is improved by increasing the attachment density of the load supporting members through reduction of the length of the integral type neutron absorbing elements at the central portion of the entire length of the control rod receiving a large stress in an earthquake. Addition of the load supporting members to the relatively heavy integral type neutron absorbing elements reduces the supported load per load supporting member, thereby improving the soundness of the sheath. According to the present invention, the plurality of integral type neutron absorbing elements have substantially uniform lengths throughout the entire length of the control rod, and a load supporting member is provided also at the central portion in the width direction and in the longitudinal direction of the integral type neutron absorbing element up to about 2/3 length from the leading end in the control rod inserting direction. The addition of load supporting members to the relatively heavy integral type neutron absorbing elements and to the portions receiving a large stress in an earthquake reduces the supported load per load supporting member, thereby improving the soundness of the sheath. According to the present invention, the integral type neutron absorbing element has a trap structure based on a combination of a plurality of neutron absorber plates prepared from hafnium metal or by forming a hafnium alloy made by diluting hafnium with zirconium or titanium into plates with a water gap therebetween, and neutron absorber plates having the water gap is arranged stepwise in the control rod inserting direction. The integral type neutron absorbing elements in the sheath comprise neutron absorber plates made of hafnium metal or a hafnium alloy, arranged stepwise with the water gap. The strength in the horizontal direction is therefore reinforced and the strength of the sheath is improved. In the integral type neutron absorbing elements, there is no longitudinal gap between the neutron absorber plates, with no crossing neutrons, thus permitting improvement of reactivity value of the control rod. According to the present invention, the integral type neutron absorbing element has a trap structure based on a combination of a plurality of neutron absorber plates prepared from hafnium metal or by forming a hafnium alloy made by diluting hafnium with zirconium or titanium into plates with a water gap and the thickness of the neutron absorber plate is gradually reduced from the leading end side of the control rod insertion toward the terminal end side thereof, the load supporting member secured to the sheath is attached to the attachment hole with a predetermined margin, two such load supporting members are each provided at an interval in the sheath longitudinal direction, and an interval maintaining member secured between the sheaths of both the surfaces at the central portion in the longitudinal direction and in the width direction between the load supporting members is provided on each of at least on set of integral type neutron absorbing elements. Since the interval maintaining member is secured between two oppositely arranged neutron absorber plates at the central portion from the load supporting member, the neutron absorbing plates become difficult to be bent, and the sheath strength is reinforced, thus improving the soundness of the sheath. An appropriate gap is kept between the neutron absorber plates arranged opposite to each other, thereby preventing a decrease in reactivity value of the control rod. According to the present invention, at least one set of integral type neutron absorbing elements, the neutron absorber plates are made of a hafnium alloy comprising hafnium and a metal having a specific gravity smaller than that of hafnium which forms an alloy with hafnium such as zirconium or titanium, at a content of hafnium equal to that in hafnium metal, and therefore, the mechanical strength is improved by increasing thickness. A neutron absorber plate in an integral type neutron absorbing element made of a hafnium alloy has an increased thickness as compared with hafnium metal having the same hafnium content. This increases strength and reinforces the sheath strength, resulting in the improved soundness of the sheath. According to the present invention, the integral type neutron absorbing elements comprise neutron absorber plates formed from hafnium metal or a hafnium alloy prepared by diluting hafnium with zirconium or titanium into a plate shape, the integral type neutron absorbing elements form water passages in the control rod inserting direction in the sheath and the cross-section in a direction at an angle normal to the insertion/withdrawal direction forms substantially a box shape. Since the integral type neutron absorbing element has a substantially box-shaped cross-section, there is an increase in toughness, leading to a higher mechanical strength, and an increased sheath strength improves the soundness of the sheath. The water passages are provided on both the sides of the integral type neutron absorbing element in the sheath. According to the present invention, a projection having the same thickness as that of the sheath is provided on a center structural member (tie rod) and the projection having a thickness equal to the wing thickness is welded together with the sheath. Welding of plates having the same thickness leads to a uniform heat input to both the plates in the welding, thus reducing welding defects such as insufficient penetration.