Patent Number: 061378542
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Various embodiments of the present invention will be explained in detail with reference to the accompanying drawings hereinafter. First Embodiment A reactor control rod according to a first embodiment of the present invention will be explained with reference to FIGS. 1A and 1B and FIGS. 2A and 2B hereinbelow. FIG. 1A is a perspective view, partially cut away, showing the reactor control rod according to the first embodiment. FIG. 1B is a front view showing the reactor control rod according to the first embodiment, with partially cutting away wings of the reactor control rod. As shown in FIGS. 1A and 1B, in the reactor control rod 20 according to the first embodiment, a top end structure 4 which is formed integrally with a handle 3 is secured to a longitudinal top end of a long sheath 7 which has a deep U-shaped cross sectional shape, and a bottom end structure 5 is secured to a longitudinal bottom end of the long sheath 7. The sheath 7 is formed of stainless steel. A plurality of water feed holes 9 are formed in the sheath 7. A plurality of neutron absorber elements 21, each being made of longlife neutron absorbing material, e.g., hafnium (Hf), Hf alloy, or the like are aligned like a column in the sheath 7 along the sheath longitudinal direction. As a consequence, a plurality (four sheets) of wings 2 are formed. An opening portion of the sheath 7 constituting the wing 2 is fitted to each projected portion of a tie rod (central structure) 6 and then secured thereto by welding. The tie rod 6 acts as a central assembling material and is made of cruciform stainless steel. As a result, the reactor control rod 20 which has a cruciform cross sectional shape can be assembled by using a plurality of wings 2 in combination. FIGS. 2A and 2B are views showing a pertinent portion of the wing 2 of the reactor control rod according to the first embodiment in an enlarged manner respectively. FIG. 2A is a sectional view showing the pertinent portion of the wing of the reactor control rod taken along a line A--A in FIG. 2B. FIG. 2B is a cross sectional view showing the pertinent portion of the wing of the reactor control rod taken along a line B--B in FIG. 2A. As shown in FIGS. 2A and 2B, the neutron absorber element 21 is constructed by opposing a pair of neutron absorber plates (Hf plates) 22 made of hafnium (Hf) or hafnium alloy to each other. In the middle portion of the neutron absorber plates 22 along the sheath longitudinal direction, a pair of supporting rod through holes 23 are formed at different height levels so as to penetrate the neutron absorber plates 22 in their thicknesswise direction. Then, load supporting rods 24 which support the weight of the neutron absorber plates 22 via the sheath 7 are inserted into the supporting rod through holes 23. As shown in FIG. 2B, the load supporting rod 24 comprises a pair of top end portions 26 which are inserted into supporting rod fitting holes 25 formed in the sheath 7 and then secured thereto by welding, and a main body portion 27 which is inserted into the supporting rod through hole 23 and has a diameter larger than those of the top end portions 26. In addition, because of difference in diameter between the top end portions 26 and the main body portion 27, steps 28 are formed on the load supporting rod 24. A minute clearance can be created by the step 28 between an inner surface 29 of the sheath 7 and an outer surface 30 of the neutron absorber plate 22. Because of the presence of small steps 28, the sheath 7 and the neutron absorber plates 22 can avoid the situation that they are subjected to strong pressure mutually. The load supporting rods 24 and the sheath 7 are formed of weldable metal, and normally they are formed of stainless steel. Both the Hf plate and the stainless steel constituting the neutron absorber plate 22 are extremely excellent in corrosion resistance. However, since the Hf plate and the stainless steel are metals of a different kind, it cannot be assured that there is no possibility of a battery action being caused electrochemically. Therefore, in order not to generate a long-term stagnation of the core water being interposed between the neutron absorber plates 22, a plurality of longitudinal grooves 31 are formed on a surface of the main body portion 27 of the load supporting rod 24 in almost parallel with the longitudinal direction (axis direction) of the load supporting rod 24. A diameter of the top end portion 26 which is inserted into the supporting rod fitting hole 25 of the sheath 7 is reduced by scraping off to such an extent that the longitudinal grooves 31 are eliminated or more, so that the step 28 is formed on the top end portion 26 as described above. Because of the presence of the step 28, the sheath 7 and the top end portion 26 of the load supporting rod 24 can be correctly positioned when they are to be welded, and also heat leakage into the neutron absorber plate (Hf plate) 22 in welding can be suppressed. Since the longitudinal grooves 31 are formed on the main body portion 27 of the load supporting rod 24, the core water can be passed between the load supporting rod 24 and the neutron absorber plate 22 to thus prevent the stagnation of the core water. As a result, clevis corrosion can be suppressed. In this case, a clearance holding mechanism for holding the clearance between the sheath 7 and the neutron absorber plate (Hf plate) 22 is not depicted in vicinity of the load supporting rod 24 in FIGS. 2A and 2B. However, various mechanisms, e.g., formation of dimpling on the sheath 7 toward the inside from the outside, placement of an interposition such as a washer (including formation of projections on the Hf plate), provision of pins, etc. which protrude slightly from the inner surface of the sheath 7 toward the inside from the outside, employment of a top axis structure, or the like may be employed. Great difference in thermal expansion coefficient resides between the Hf and the stainless steel. Hence, if mutual distances between a plurality of load supporting rods 24 are set long, a diameter of the supporting rod through hole 23 formed in the neutron absorber plate (Hf plate) 22 must be enlarged in order to avoid the problem of difference in thermal expansion coefficient. In this case, since the impact load applied to the load supporting rod 24 upon driving the reactor control rod 20 is increased, it is desired that mutual intervals between the load supporting rods 24 should not be set long. For example, preferably such intervals should be set to about 3 to 5 cm. As shown in FIGS. 2A and 2B, a predetermined clearance (water gap) is held between a pair of opposing neutron absorber plates 22 by using a plurality of local spacers (Hf spacers) 32. Each of the local spacers 32 comprises a body portion 33 provided between a pair of neutron absorber plates 22 to hold a predetermined clearance between the neutron absorber plates 22, and axis portions 34 which are protruded from both ends of the body portion 33 to be inserted into spacer through holes 35 formed in the neutron absorber plates 22. Thus, the core water can be flown through via the clearance between the neutron absorber plates 22. Like the neutron absorber plates 22, the local spacer 32 is also formed of hafnium or hafnium alloy. The axis portions 34 of the local spacer 32 are welded to the neutron absorber plates (Hf plates) 22 respectively. Unlike the case of the load supporting rod 24, the local spacers 32 are not secured to the sheath 7. Top ends of the axis portions 34 of the local spacer 32 are projected outward slightly (e.g., 0.2 to 0.5 mm) from the outer surface 30 of the neutron absorber plates 22 respectively. Therefore, minute clearances can be created by such projected portions between the inner surface 29 of the sheath 7 and the outer surfaces 30 of the neutron absorber plates 22 so as to enable flow of the core water. Since the minute clearances are created by the projected portions of the axis portions 34 of the local spacers 32 as described above, full contact between the inner surface 29 of the sheath 7 and the outer surfaces 30 of the neutron absorber plates 22 can be prevented and also excessive generation of the oxide film on the outer surfaces 30 of the neutron absorber plates 22 can be suppressed. It is preferable that top ends of the axis portions 34 of the local spacer 32 should be shaped so as to reduce contact areas between the inner surface 29 of the sheath 7 and the top ends. For instance, it is preferable that the axis portions 34 should be formed to have a tapering shape respectively, or local convex portions should be provided on end surfaces of the axis portions 34 respectively. In order to prevent the long-term stagnation of the core water, grooves 36 are formed on surfaces of the local spacer (Hf spacer) 32. However, in the event that the corrosion problem due to the stagnation of the core water does not particularly become an issue, such grooves 36 are not always provided. The local spacer 32 is shown in FIGS. 2A and 2B by way of example. The shape of the local spacer 32 is not limited to the illustrated shape. In other words, if a portion of the local spacer, which is placed in the clearance formed between a pair of the neutron absorber plates (Hf plates) 22, can have a diameter slightly larger than that of the spacer through hole 35, the local spacer can be positioned upon welding. Therefore, any local spacer to satisfy the above may be employed. A left end shown in FIG. 2B is a side end portion of the cruciform tie rod 6. A convex cut 37 is formed on both side surfaces so as to leave a pair of outer thickness 38 which correspond to the thickness of the sheath 7. The top end portion of the sheath 7 is welded to top ends of a pair of convex portions 38 on both side surfaces of the cruciform tie rod 6 by using butt welding. In addition, in the side end portions of the neutron absorber plates 22 which are located on the tie rod 6 side, respective surfaces 39 facing to the sheath 7 side are formed thin. According to these structures, excessive heat application at welding of the sheath 7 can be prevented to thus improve the soundness of welding, the reactivity effect can be improved since a wing width of the neutron absorber plate (Hf plate) 22 is expanded, and the electrochemical corrosion problem such as clearance corrosion can be considerably relaxed by creating the clearance at the corner portions. As described above, according to the reactor control rod of the first embodiment, contact portions of the sheath 7 made of stainless steel to the members made of hafnium or hafnium alloy can be limited to the top end portions of the axis portions 34 of the local spacers 32. As a result, contact areas between metals of a different kind can be remarkably reduced and also the possibility of the electrochemical corrosion can be reduced. Furthermore, the minute clearance is created between the inner surface 29 of the sheath 7 and the outer surface 30 of the neutron absorber plate 22. As a result, the flow of the core water can be accelerated via the clearance, and exfoliation of the passive state oxide film which is formed on the surface of the neutron absorber plate 22 hardly occurs, so that the inside of the neutron absorber plate 22 can be protected over a long term. Since the corrosion problem can be relaxed extremely in this manner, the long-term reactor control rod, in which the nuclear lifetime and the electrochemical lifetime can be well balanced, can be derived. As a result, economical efficiency and safety of the nuclear power generation can be improved and also an amount of radioactive waste can be reduced. Second Embodiment Next, a reactor control rod according to a second embodiment of the present invention will be explained with reference to FIGS. 3A to 3E hereinbelow. A feature of the reactor control rod according to the second embodiment reside in that the load supporting rod used in the above first embodiment are formed as spacer/load supporting rods which can also be used as spacers. In the second embodiment, description of configurations common to those in the first embodiment will be omitted, but configurations of different constituent portions will be explained in detail in the following. FIG. 3A is a cross sectional view showing a pertinent portion of the reactor control rod according to the second embodiment. FIG. 3B is a view showing only a load supporting rod shown in FIG. 3A. FIG. 3C is a sectional view showing the load supporting rod taken along a line C--C in FIG. 3B. FIG. 3D is a sectional view showing the load supporting rod taken along a line D--D in FIG. 3B. FIG. 3E is a sectional view showing the pertinent portion of the reactor control rod taken along a line E--E in FIG. 3A. A difference of the second embodiment from the first embodiment is that, as shown in FIG. 3E, a spacer/load supporting rod 41 is constructed by inserting a load supporting rod 24 into an annular grooved spacer 40. In the second embodiment, the grooved spacer 40 is made of hafnium (Hf), and thus generation of the electrochemical corrosion between the grooved spacer 40 and the neutron absorber plate (Hf plate) 22 becomes difficult. But, in order to make perfection more perfect, grooves 42 are provided on the spacer/load supporting rod 41. Material for the grooved spacer 40 is not always limited to hafnium (Hf). The load supporting rod 24 is formed of stainless steel like the sheath 7 and thus the electrochemical corrosion will be anticipated between the load supporting rod 24 and the grooved spacer 40. Hence, as in the load supporting rod in the above first embodiment, a plurality of longitudinal grooves 31 are provided on a surface of a main body portion 27 of the load supporting rod 24 in substantially parallel with the longitudinal direction (axial direction). Thus, a consideration not to generate the long-term stagnation of the core water is taken. As shown in FIG. 3C, like the first embodiment, top end portions 26 of the load supporting rod 24 are scraped off so as to reduce its diameter to such extent that the longitudinal grooves 31 can be eliminated or more. As described above, according to the reactor control rod of the second embodiment, the spacer/load supporting rod 41 can be constructed by inserting the load supporting rod 24 into the annular grooved spacer 40. As a result, an interval between two opposing neutron absorber plates (Hf plates) 22 can be properly held still in the neighborhood of the load supporting rod 24. Because flow of the core water can be formed between constituent materials by the longitudinal grooves 31 of the load supporting rod 24 and the grooves 42 of the annular grooved spacer 40, the progress of corrosion can be suppressed. The spacer/load supporting rods 41 may be employed in place of the local spacers (Hf spacers) 32 (see FIGS. 2A and 2B) in the above first embodiment. Otherwise, four local spacers 32 are arranged as shown in FIGS. 2A and 2B and then the spacer/load supporting rods 41 may be provided separately from these local spacers 32. Third Embodiment A method of manufacturing a reactor control rod according to a third embodiment of the present invention will be explained with reference to FIG. 4 hereinbelow. FIG. 4 is a sectional view showing the method of manufacturing the reactor control rod according to the third embodiment. The method of manufacturing the reactor control rod according to the third embodiment is employed to manufacture the reactor control rod having the structure shown in FIGS. 1A and 1B. More particularly, this method is employed to manufacture the reactor control rod by securing the top end structure 4 and the bottom end structure 5 respectively to the longitudinal top end and the longitudinal bottom end of the long sheath 7 which has a deep U-shaped cross sectional shape, then aligning a plurality of plate-like neutron absorber elements 21 made of a long-life neutron absorbing material in the sheath 7 along the sheath longitudinal direction like the column to thus form the wing 2, and then fitting and securing the opening portions of the wings 2 to the tie rod (central structure) 6 to assemble a plurality of wings 2 in combination. As can be seen from FIG. 4, the reactor control rod, which is manufactured by the method of manufacturing the reactor control rod according to the third embodiment, is constructed by employing local spacers, of which body portions 33 are thiner than those of the local spacers (Hf spacers) 32 shown in FIG. 2B, and the spacer/load supporting rods 41 shown in FIG. 3A in combination. In the method of manufacturing the reactor control rod according to the third embodiment, first the supporting rod through holes 23 are formed so as to penetrate through the neutron absorber plate 22, acting as the neutron absorber element, along its thicknesswise direction, and then the load supporting rods 24 which are used to support the weight of the neutron absorber plate 22 by the sheath 7 are inserted into the supporting rod through holes 23. The spacer/load supporting rod 41 can be constructed by inserting the load supporting rod 24 having the longitudinal grooves 31 thereon into the grooved spacer 40. Then, the load supporting rods 24 are fitted into the supporting rod fitting holes 25 of the sheath 7. Then, while keeping minute clearances between the sheath 7 and the neutron absorber plates 22 by using a plurality of thin manufacturing spacers 43 which are interposed in the neighborhood of the load supporting rods 24, the top end portions 26 of the load supporting rods 24 are secured to the sheath 7 by welding. The manufacturing spacers 43 are interposed between the sheath 7 and the neutron absorber plates (Hf plates) 22 along the direction at a right angle relative to the longitudinal direction. In this manner, since a predetermined interval is assured between the sheath 7 and the neutron absorber plates 22 by the manufacturing spacers 43, relative positional relationships between the inner surface 29 of the sheath 7 and the outer surface 30 of the neutron absorber plate 22 can be precisely positioned when the top end portions 26 of the load supporting rods 24 are secured to the sheath 7 by welding. As shown in FIG. 4, the steps 28 of the load supporting rods 24 are slightly separated from the inner surfaces 29 of the sheath 7 in the state that the manufacturing spacers 43 are interposed between the sheath 7 and the neutron absorber plates 22. In this state, after the top end portions 26 of the load supporting rods 24 are secured to the sheath 7 by welding, the plurality of manufacturing spacers 43 are removed. Play between the neutron absorber plates 22 and the inner surfaces 29 of the sheath 7 can be prevented by the sheath dimpling, the local projection of the neutron absorber plate (Hf plate) 22, employment of the washer, the top axis structure, etc. if the case may be. As described above, according to the method of manufacturing the reactor control rod of the third embodiment, it is possible to manufacture the reactor control rod in which predetermined clearances are created between the inner surface 29 of the sheath 7 and the outer surfaces 30 of the neutron absorber plates 22. The reactor control rod being manufactured as above can achieve the same advantages as those in the above first and second embodiments. Also, according to the method of manufacturing the reactor control rod of the third embodiment, since the slight clearance can be created between the steps 28 of the load supporting rods 24 and the inner surface 29 of the sheath 7, a stagnation time of the core water in this area can be shortened. As has been explained in the above first and second embodiments, the mechanism utilizing the local spacers 32, etc. may be employed to hold the clearances between the inner surface 29 of the sheath 7 and the outer surfaces 30 of the neutron absorber plates (Hf plates) 22. If such clearances are held, the stagnation time of the core water can be shortened and in addition generation of a corrosion product can be suppressed. Fourth Embodiment Next, a reactor control rod according to a fourth embodiment of the present invention will be explained with reference to FIGS. 5A to 5B hereinbelow. In the reactor control rod according to the fourth embodiment, the local spacers in the above first embodiment are modified. In the fourth embodiment, description of configurations common to those in the first embodiment will be omitted, but configurations of different constituent portions will be explained in detail in the following. FIGS. 5A and 5B are views showing a pertinent portion of the wing 2 (see FIGS. 1A and 1B) of the reactor control rod according to the fourth embodiment of the present invention in an expanded manner. FIG. 5A is a sectional view showing the pertinent portion of the wing 2 of the reactor control rod taken along a line A--A in FIG. 5B. FIG. 5B is a cross sectional view showing the pertinent portion of the wing 2 of the reactor control rod taken along a line B--B in FIG. 5A. At first, a difference between the fourth embodiment and the first embodiment is the configuration and the arrangement location of the local spacers. More particularly, in the reactor control rod according to the fourth embodiment, a plurality of (e.g., four) local spacers 44 are positioned at end portions of the neutron absorber plates 22, acting as the neutron absorbing elements 21, along the sheath widthwise direction. Each of the local spacers 44 has a convex portion 45 which is interposed between a pair of neutron absorber plates 22 to hold a predetermined clearance therebetween. There are many modifications of the fourth embodiment. For example, as shown in FIG. 6A, the neutron absorber plate (Hf plate) 22 without the local spacer 44 may be curved or bent at the location of the local spacer 44 and then secured to the opposing neutron absorber plate (Hf plate) 22 by welding. As shown in FIG. 6B, the neutron absorber plate (Hf plate) 22 without the local spacer 44 may curved or bent at the end portions in the sheath widthwise direction and then secured by welding over the total length of the plate 22. The same advantages as those of the above first embodiment can be achieved by the reactor control rod according to the fourth embodiment. No strong friction can be generated between the inner surface 29 of the sheath 7 and the outer surfaces 30 of the neutron absorber plates 22, so that the passive state oxide film can be protected. Fifth Embodiment Next, a reactor control rod according to a fifth embodiment of the present invention will be explained with reference to FIG. 7 hereinbelow. In the reactor control rod according to the fifth embodiment, the configuration of the above first embodiment shown in FIG. 2 is partially modified. In the fifth embodiment, description of configurations common to those of the first embodiment will be omitted, but configurations of different constituent portions will be explained in detail in the following. FIG. 7 is a front view showing a part of a wing 2 of a reactor control rod according to the fifth embodiment in the state that the sheath 7 is removed therefrom. As shown in FIG. 7, in the reactor control rod according to the fifth embodiment, two spacer/load supporting rods 41 shown in FIG. 3 are provided in upper and lower areas of the neutron absorber plates 22, serving as the neutron absorbing elements 21, respectively. More particularly, in the configuration of the first embodiment shown in FIG. 2A, two load supporting rods 24 positioned in the middle area shown in FIG. 2A can be omitted by replacing the local spacers 32 positioned in upper and lower areas with the spacer/load supporting rods 41 shown in FIG. 3. In addition, in the reactor control rod according to the fifth embodiment, supporting rod through holes 23a formed in the upper area (the top end side along the sheath longitudinal direction) of the neutron absorber plates 22 and supporting rod through holes 23b formed in the lower area (the bottom end side along the sheath longitudinal direction) of the neutron absorber plates 22 are formed to have different shapes. In other words, an inner diameter of the upper supporting rod through hole 23a is set such that a clearance between a hole wall surface of the supporting rod through hole 23a and a peripheral surface of the main portion 27 of the load supporting rod 24 along the sheath longitudinal direction is reduced small. More specifically, the upper supporting rod through hole 23a is formed as a slightly longitudinal hole in the sheath widthwise direction (the lateral direction of FIG. 7), and is formed such that a clearance is scarcely provided in the sheath longitudinal direction (control rod inserting/withdrawing direction). The longitudinal hole in the sheath widthwise direction is provided to absorb the problem of difference in thermal expansion. The reason that the clearance is scarcely formed in the inserting/withdrawing direction is to prevent increase in the impact load which is applied to the load supporting rods 24 from the neutron absorber plates (Hf plates) 22 when the control rods are inserted and withdrawn. In contrast, an inner diameter of the lower supporting rod through hole 23b is set large so as to allow movement of the neutron absorber plates (Hf plates) 22 in the sheath longitudinal direction due to thermal expansion. Since there exists difference in thermal expansion between the neutron absorber plates (Hf plates) 22 and the sheath 7, the neutron absorber plates (Hf plates) 22, whose position along the sheath longitudinal direction is fixed by the upper load supporting rod 24, expands and contracts downward. Therefore, a sufficient clearance is provided in the lower supporting rod through hole 23b along the sheath longitudinal direction so as to respond to such expansion/ contraction. As one modification, two upper spacer/load supporting rods 41 or two lower spacer/load supporting rods 41 of four spacer/load supporting rods 41 may be replaced with the local spacers 32 shown in FIG. 2. The upper spacer/load supporting rods 41 or the lower spacer/ load supporting rods 41 are not bound by the load supporting rods 24 if the structure is set as above. As a result, such structure can correspond to the case where the neutron absorber plates (Hf plates) 22 is moved relative to the sheath 7 because of difference in thermal expansion. Sixth Embodiment Next, a reactor control rod according to a sixth embodiment of the present invention will be explained with reference to FIGS. 8A to 8B hereinbelow. In the reactor control rod according to the sixth embodiment, the configuration of the above first embodiment shown in FIG. 2 is partially modified. In the sixth embodiment, description of configurations common to those of the second embodiment will be omitted, but configurations of different constituent portions will be explained in detail in the following. FIG. 8A is a front view showing a reactor control rod according to a sixth embodiment of the present invention in the situation that a sheath is partially cut away. FIG. 8B is a front view showing a neutron absorber plate of the reactor control rod according to the sixth embodiment of the present invention. As shown in FIGS. 8A and 8B, in the reactor control rod 50 according to the six embodiment, a plurality of (three) supporting rod through holes 23 are formed linearly along the sheath longitudinal direction in the almost center area of the neutron absorber plate 22, acting as the neutron absorbing element 21, in the sheath longitudinal direction and the sheath widthwise direction. The load supporting rods 24 shown in FIG. 2 are inserted into these supporting rod through holes 23. In addition, in the reactor control rod according to the sixth embodiment, a pair of spacer through holes 35 are formed respectively in an upper area (top end side in the sheath longitudinal direction) and a lower area (bottom end side in the sheath longitudinal direction) of the neutron absorber plate 22. The local spacers 32 shown in FIG. 2 are fitted into the spacer through holes 35. Respective diameters and shapes of the supporting rod through holes 23 and the main body portions 27 (see FIG. 2B) of the load supporting rods 24 are set such that a clearance between hole wall surfaces of the supporting rod through holes 23 and peripheral surfaces of the main body portions 27 of the load supporting rods 24 in the sheath longitudinal direction can be set minutely. Accordingly, the weight of the neutron absorber plate 22 can be supported by the load supporting rods 24. In addition, if the neutron absorber plate 22 is moved relative to the sheath 7 relatively due to thermal expansion, relative movement is caused from the center portion in the vertical direction since the center portion of the neutron absorber plate 22 is fixed. As a result, even if the neutron absorber plate 22 and the sheath 7 are rubbed together because of their relative movement, a rubbing distance can be shortened, so that damage of the passive state oxide film formed on the surface of the neutron absorber plates (Hf plates) 22 can be suppressed. Besides, it is preferable that, like the upper supporting rod through hole 23a shown in FIG. 7, the supporting rod through holes 23 be formed as a longitudinal hole along the sheath widthwise direction. If the supporting rod through holes 23 are so formed, they can respond to the movement of the neutron absorber plate 22 in the sheath widthwise direction due to thermal expansion. Seventh Embodiment Next, a reactor control rod according to a seventh embodiment of the present invention will be explained with reference to FIG. 9 hereinbelow. The reactor control rod according to the seventh embodiment is common in basic structure to the reactor control rod having the structure shown in FIGS. 1A and 1B. More particularly, this reactor control rod according to the seventh embodiment is constructed by securing the top end structure 4 and the bottom end structure 5 respectively to the longitudinal top end and the longitudinal bottom end of the long sheath 7 which has a deep U-shaped cross sectional shape, then aligning a plurality of plate-like neutron absorber elements 21 made of long-life neutron absorbing material in the sheath 7 along the sheath longitudinal direction like the column to thus form the wing 2, and then fitting and securing the opening portions of the wings 2 to the tie rod (central structure) 6 to assemble a plurality of wings 2 in combination. More specifically, the reactor control rod according to the seventh embodiment is formed by modifying partially the configuration of the reactor control rod shown in FIG. 4. As shown in FIG. 9, in the reactor control rod according to the seventh embodiment, edge portions 60 and edge portions 61 of water feed holes 9 formed in the sheath 7, both are positioned on the neutron absorber plate 22 side (inner side) and the outer side respectively, are chamfered. Edge portions 60 of the water feed holes 9 are separated from the neutron absorber plate 22. Both edge portions of water feed holes 62 formed in the neutron absorber plate 22 are chamfered. In the reactor control rod according to the seventh embodiment, the edge portions 60 of the water feed holes 9 formed in the sheath 7, which are positioned on the neutron absorber plate 22 side, are chamfered. Therefore, even if corrosion product 63 is generated on surfaces of the neutron absorber plate (Hf plate) 22, such a situation that corrosion product is scraped off by flashes formed on the edge portions 60 of the water feed holes 9 in the sheath 7 upon movement by the thermal expansion can be eliminated. In order to supplement the description, normally the corrosion product is soft rather than original metal and its density is low, and therefore it can become inflated spatially. Unless the edge portions 60 of the water feed holes 9 are chamfered, such a phenomenon occurs that the corrosion product is scraped off in a thermal expansion cycle caused at start/stop, etc. of the reactor. The corrosion product being scraped off is discharged into the core water to stray therein. In the case of hafnium (Hf), the problem of radioactivity storage does not occur since a half life is relatively short like about 43 days, but a possibility that a radioactivity level of the core water, which has been reduced remarkably up to this day, is made worse even slightly can be supposed since Hf-181 (which emits gamma rays such as 482 keV, 346 keV, etc.) is contained. For this reason, the above problem can be overcome by the configuration of the reactor control rod according to the seventh embodiment. The seventh embodiment may be combined arbitrarily with any of the above first to sixth embodiment or eighth and ninth embodiments to be described later. Eighth Embodiment Next, a reactor control rod according to an eighth embodiment of the present invention will be explained hereinbelow. The eighth embodiment may be combined arbitrarily with any of the above first to seventh embodiment or a ninth embodiment to be described later. The reactor control rod according to the eighth embodiment is common in basic structure to the reactor control rod having the structure shown in FIGS. 1A and 1B. More particularly, this reactor control rod according to the eighth embodiment is constructed by securing the top end structure 4 and the bottom end structure 5 respectively to the longitudinal top end and the longitudinal bottom end of the long sheath 7 which has a deep U-shaped cross sectional shape, then aligning a plurality of plate-like neutron absorber elements 21 made of a long-life neutron absorbing material in the sheath 7 along the sheath longitudinal direction like the column to thus form the wing 2, and then fitting and securing the opening portions of the wings 2 to the tie rod (central structure) 6 to assemble a plurality of wings 2 in combination. However, the shape of the neutron absorbing element is not limited to a plate shape. In addition to the so-called trap type control rod employing the Hf plate shown in FIG. 1, the eighth embodiment may be applied to the control rod of a type in which a plurality of rod-like neutron absorbing material (e.g., Hf rods) are immersed directly in the core water. In the reactor control rod according to the eighth embodiment, the neutron absorber element is constructed by forming the neutron absorbing material containing at least hafnium into a plate, a rod, etc. Further, a hafnium density on the surface area of the neutron absorber element is set lower than an internal hafnium density. More particularly, the neutron absorber element of the reactor control rod according to the eighth embodiment is constructed by covering the surface of the Hf member formed a plate, a rod, etc. with an alloy containing the low Hf density (e.g., zircaloy-2, zircaloy-4, Hf--Zr alloy containing the low Hf density, etc.). Hafnium (Hf) and zirconium (Zr) are materials which can be employed to form an alloy at any rates, i.e., to form a full composition solid-solution type alloy. Such alloy having a different composition ratio is formed weldably. The passive state film having the low Hf composition ratio is formed on the surface of the neutron absorber element. Assume the case where this film is released once because of generation of a strong friction force, the reactivity value is not affected at all since the Hf density is low and also the Hf-Zr alloy, if employed, can less contribute to the increase of the radioactivity density of the core water since Zr is different to radioactivate rather than Hf. That is, the reactivity and radioactivity problems are scarcely caused. Since in principle the corrosion advances from the surface of the neutron absorber element, the corrosion of Zr first occurs in the reactor control rod according to the eighth embodiment and therefore start of the corrosion of Hf can be considerably deferred. Since Zr has the induced radioactivity density extremely lower than Hf and also Zr is used widely as the fuel rod covering tube, etc., the problem of increase of the radioactivity level due to the reactor control rod can be completely eliminated. As in the case where the zircaloy having excellent corrosion resistance has been invented by adding iron, chromium, nickel, tin, etc. into zirconium, the possibility that the corrosion resistance of Hf can be improved by adding these elements into the Hf member itself may be thought of, nevertheless no necessity of such improvement has arisen in the related art. It has already become evident that such improvement is effective for the Hf--Zr alloy. However, in case it is possible to aim at the longer lifetime of the control rod employing Hf, the advantage can be achieved by improving the corrosion resistance of the Hf itself much more and also coating the control rod with the zircaloy, etc. As described above, according to the reactor control rod of the eighth embodiment, the neutron absorber element is formed of the neutron absorbing material containing at least hafnium, and also the hafnium density on the surface portion of the neutron absorber element is set lower than its inner density. As a result, discharge of the hafnium into the core water because of corrosion of the neutron absorber element can be suppressed, so that the radioactivity level in the periodical inspection, for example, can be suppressed low. Ninth Embodiment Next, a reactor control rod according to an ninth embodiment of the present invention will be explained hereinbelow. The ninth embodiment may be combined arbitrarily with any of the above first to eighth embodiments. The reactor control rod according to the ninth embodiment is common in basic structure to the reactor control rod having the structure shown in FIGS. 1A and 1B. More particularly, this reactor control rod according to the ninth embodiment is constructed by securing the top end structure 4 and the bottom end structure 5 respectively to the longitudinal top end and the longitudinal bottom end of the long sheath 7 which has a deep U-shaped cross sectional shape, then aligning a plurality of plate-like neutron absorber elements 21 made of long-life neutron absorbing material in the sheath 7 along the sheath longitudinal direction like the column to thus form the wing 2, and then fitting and securing the opening portions of the wings 2 to the tie rod (central structure) 6 to assemble a plurality of wings 2 in combination. However, the shape of the neutron absorbing element is not limited to a plate shape. In addition to the so-called trap type control rod employing the Hf plate shown in FIG. 1, the eighth embodiment may be applied to the control rod of a type in which a plurality of rod-like neutron absorbing material (e.g., Hf rods) are immersed directly in the core water. The reactor control rod according to the ninth embodiment is characterized in that an effective surface area can be reduced by processing the surface of the neutron absorber element to improve smoothness. That is, the effective surface area of the surface of the Hf member such as the plate, the rod, etc. to thus suppress an amount of surface corrosion. In order to supplement the description, since there is a minute unevenness on the actual surface of the neutron absorber element and such uneven surfaces contact with the core water, the actual surface area (reaction area) is increased remarkably rather than the apparent surface area. In addition, unevenness on the surface of the neutron absorber element causes the stagnation of the core water and also causes the corrosion. Therefore, if the unevenness on the surface like the reactor control rod according to the ninth embodiment is suppressed, an amount of the corrosion product can be suppressed considerably. There are many known methods as the method of suppressing the unevenness on the surface. For example, there are mechanical polishing, chemical polishing (chemical processing), electrochemical polishing (electro- chemical processing), composite polishing employing above polishing in combination, or the like. In the above first to ninth embodiments of the present invention, the description of the example of "the trap type configuration in which two sheets of Hf plates are opposed to sandwich the water gap inside the sheath made of stainless steel" has been made mainly. But most of the embodiments of the present invention may be applied to the control rod of a type in which Hf is exposed directly to the core water. In the above embodiments of the present invention, the description of the control rod employing the integral type center assembling material (tie rod) has been made. But the present invention may be applied to the "control rod of the type in which Hf is exposed to the core water", i.e., in which independent structural materials which have been developed in Europe and in which center axes of the control rods are not perfectly integrated with each other is employed. As described above, according to the reactor control rod of the present invention, since the minute clearance can be formed without fail between the sheath and the neutron absorber elements, contact areas between metals of a different kind can be remarkably reduced and also the possibility of the electrochemical corrosion can be reduced. Also, damage of the passive state oxide film formed on the surface of the neutron absorber element can be prevented. Since the corrosion problem can be relaxed extremely in this manner, the long-term reactor control rod in which the nuclear lifetime and the electrochemical lifetime can be well balanced can be obtained and economical efficiency and safety of the nuclear power generation can be improved. Also, an amount of radioactive waste can be reduced. According to the reactor control rod of the present invention, the neutron absorber element is formed of the neutron absorbing material containing at least hafnium, and also the hafnium density on the surface portion of the neutron absorber element is set lower than its inner density. As a result, discharge of the hafnium into the core water because of corrosion of the neutron absorber element can be suppressed. According to the reactor control rod of the present invention, since the effective surface area can be reduced by processing the surface of the neutron absorber element to improve smoothness, an amount of surface corrosion of the neutron absorber element can be suppressed. According to the reactor control rod of the present invention, since the edge portions of the water feed holes formed in the sheath, which are positioned on the neutron absorber plate side, are chamfered, discharge of the corrosion product generated on the surface of the neutron absorber element into the core water can be suppressed. Moreover, according to the method of manufacturing the reactor control rod of the present invention, it is possible to manufacture the reactor control rod in which the minute clearance can be formed without fail between the sheath and the neutron absorber elements. The reactor control rod being manufactured in this manner can suppress generation of the corrosion product. Since the corrosion problem can be relaxed extremely in this manner, the long-term reactor control rod in which the nuclear lifetime and the electrochemical lifetime can be well balanced can be obtained. As a result, economical efficiency and safety of the nuclear power generation can be improved and also an amount of radioactive waste can be reduced.