Patent Number: 052767183
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates the shape of a first embodiment of a control blade for a nuclear reactor according to the present invention, the overall body of the control blade for a nuclear reactor being represented by reference numeral 10. The control blade 10 for use in a nuclear reactor is arranged to be inserted into or withdrawn form the nuclear reactor core by a control-blade drive mechanism (control rod drive) (omitted from illustration). Each of fuel assemblies is a set composed of four fuel assemblies, the fuel assemblies being disposed in the reactor core portion of a boiling water nuclear reactor serving as a light water reactor. As a result of the insertion/withdrawal of the control blade to and from the reactor core portion, the operation of the nuclear reactor can be shut down and as well as the power of the reactor can be adjusted and controlled. The control blade 10 for a nuclear reactor comprises an upper structure member 11 serving as an upper structure means, a lower structure member 12 serving as a lower structure means, a central tie member 13 disposed between the upper structure member 11 and the lower structure member 12 and serving as a central tie means and four rectangular wings 14 connected to the central tie member 13 to form a cross-shaped lateral cross section and serving as a wing means. A handle 15 for handling the control blade 10 for a nuclear reactor is integrally provided for the upper structure member 11. On the other hand, a speed limiter 16 is fastened to the lower portion of the lower structure member 12. A coupling socket 17 is disposed below the speed limiter 16, the coupling socket 17 being detachably fastened to the control blade drive mechanism (omitted from the illustration). Reference numerals 18 and 19 respectively represent guide rollers for smoothly guiding the control blade 10 for a nuclear reactor. The central tie member 13 for connecting the four wings 14 of the control blade 10 may be continuously formed from the upper structure member 11 toward the lower structure member 12. As an alternative to this, a plurality of central tie members 13 are disposed at intervals of 20 to 30 cm. The top end portion 14a of the wing 14 integrally fastened to the central tie member 13 by welding or the like is secured to the upper structure member 11, while the lower portion of the same is secured to the lower structure member 12. Each of the wings 14 is, as shown in FIGS. 2 and 3, formed by a rectangular hafnium metal plate, a metal plate containing hafnium mainly, a plate made of an alloy of hafnium and zirconium (Zr) or an alloy of hafnium and titanium (Ti) or a plate made of an alloy the main component of which is zirconium (Zr) and titanium (Ti). The wing 14 has a multiplicity of accommodating holes 20 disposed in the widthwise direction of the wing 14 for length L which is equivalent to the overall axial length of the core of the nuclear reactor the length L being equivalent to the height of the effective core portion of the nuclear reactor, that is, for example, 3.6 m (144 inches) or 3.7 m (146 inches). Each of the accommodating holes 20 formed in the wing 14 are sectioned into regions (A) to (E) or more regions when viewed from the front end in the direction of the insertion. Hollow tubular members (sleeves) 21 each of which is made of Zr or pure Zr are inserted into the accommodating holes 20 in the region (A), the Zr or pure Zr hollow tubular members 21 each of which is filled with Zr or pure Zr particles (grain) 22 are inserted into those in the region (B) and Zr or pure Zr tubular members 2, into each of which an Hf metal rod (a hafnium alloy rod or Ag-In-Cd alloy rod) 23 which is a long-lived absorbing material member is inserted, are inserted into those in the region (C). Furthermore, B.sub.4 C particles (powder) 27a and Zr or pure Zr particles 27b are, after being mixed, inserted into the accommodating holes 20 in the region (D) and B.sub.4 C particles are inserted into the accommodating holes 20 in the region (E). Since hafnium (Hf) serving as the long-lived type neutron absorber is resonance neutron absorbing material, it is necessary to form into a rod shape having a large surface area with respect to the volume. Hafnium is a metallic element which is extremely chemically stable. A multiple kinds of isotopes Hf-176, Hf-177, Hf-178, Hf-179, Hf-180 and the like exist each of which is able to satisfactorily absorb neutrons and which relatively considerably absorb neutrons of resonance energy. In particular, Hf-177 and Hf-178 exhibit an excellent neutron-absorption effect. For example, Hf-176 absorbs a neutron to change into Hf-177 ,And sequentially absorbs neutrons to change into Hf-180 through Hf-179 via Hf-178. As a result, one Hf atomic nucleus is able to absorb a plurality of neutrons and continues to absorb neutrons for a long time. Therefore, it can be said that hafnium is a long-lived type neutron absorber. In the above-described regions (A) to (D), a Hf metal rod (or an Ag-In-Cd alloy rod) 24 is disposed in the outer side end portion of the wing 14. Furthermore, a sheet (strip) 25 made of Zr or pure Zr is disposed between the Hf metal rod 24 and the end surface of the above-described accommodating holes 20. The Hf metal rod (or the Ag-In-Cd alloy rod) 24 is, as shown in FIGS. 4 to 7, surrounded by a sheet 25 made of Zr or pure Zr. In the above-described region (E), an Hf-Zr alloy rod (or an Hf-Ti alloy rod) 26 is disposed in the outer side end portion (peripheral) of the wing 14. The above-described regions (A) to (D) are exposed to a large amount of neutrons. In particular, the regions (A) to (C) are considerably exposed to the same. Therefore, a boron compound (exemplified by B.sub.4 C and EuB.sub.6), the life of which is relatively short, is not enclosed, but the Hf metal rod (or Ag-In-Cd alloy rod) 23 exhibiting a long neutron-absorbing life is enclosed in the form of the long-lived type neutron absorber. The region (D) is exposed to a relatively large amount of neutrons and is a region the neutron absorption characteristic of which must be improved. Therefore, the boron compound must be used in the region (D) in such a manner that a mixture 27 formed by mixing the Zr or pure Zr particles 27b, which are hydrogen absorbers, and B.sub.4 C 27a is enclosed in order to absorb tritium (.sup.3 T) which is produced as a result of a reaction between boron and the neutrons and hydrogen (H) which can be produced due to the radiolysis of water. The above-described region (A) possesses a function as a gas plenum serving as a portion for absorbing (.sup.3 T) and (H) . It is preferable that the axial length of the region (A) be restricted to a length (about 1 to 3 cm) equivalent to the length of one to three accommodating holes 20. A typical example of the designed control blade according to this embodiment is arranged in such a manner that the thickness of the wing 14 is about 8 mm, the diameter of the accommodating hole 20 is about 6 mm and the distance between the central axes of the two accommodating holes 20 is about 8 mm. Therefore, according to this design example, the length for one hole is about 8 mm (to 1 cm) in a lengthwise direction of the wing 14 and that for three holes is 28 mm (=3.times.8+2.times.2) to 3 cm. In the above-described region (B), absorptions of .sup.3 T and H are mainly performed. In a case where the Zr or pure Zr particles 22 are enclosed, it is preferable that the length (the number) of the holes in a lengthwise direction of the wing in the portion which is the sum of the regions (A) and (B) be restricted to be shorter than 2 to 3 cm (about 2 to 3 holes) for the purpose of preventing the deterioration in the reactivity worth of the control blade. Assuming that the region (A) includes one hole and the region (B) includes two holes, three holes are disposed in the region (A)+(B), causing the length to be about 3 cm similarly to the above-described structure. In any case, since the regions (A) and (B) are not filled with the neutron absorber, the reactivity worth will be deteriorated if the regions (A) and (B) are arranged to be long. The region (C) is a portion in which means to prevent swelling of the long-lived type neutron absorber in such a manner that the tubular member 21 made of Zr or pure Zr absorbs .sup.3 T ahd H and as well as absorbs the generation of stress due to the swelling of the long-lived type neutron absorber. The Zr or pure Zr serving as the hydrogen absorber is a soft material exhibiting excellent hydrogen absorbing performance. A gap is necessarily formed between the accommodating hole 20 and the tubular member 21 made of Zr or pure Zr, the gap as well as serving as the swelling absorbing space. Furthermore, the structure is arranged in such a manner that the Zr particles 27b mixed with B.sub.4 C 27a absorb .sup.3 T and H in the region (D) so as to prevent the diffusion to the other portions. Since the above-described region (E) is exposed to neutrons by a small amount, the ratio of production of .sup.3 T is low and as well as the radiolysis speed is low. Therefore, there is no special means in this region. The length of the regions (A) to (D) of the control blade of a type which is inserted during the operation is usually arranged to be 1/4 to 3/4 of the overall length L of the reactor core and about 1/2.multidot.L of the same. However, it is varied depending upon how the control blade is used. For example, in a case where it is completely removed during the operation, it might be considered feasible to make the total length of (A) to (D) to be shorter than 1/4.multidot.L of the same, for example, about 30 cm or shorter. Then, the reason why the above-described range (1/4 to 3/4 of the overall length L of the reactor core) is employed will now be described. That is, evaluating the actual and average neutron exposure amount distribution in the control blade of a type which is inserted during the operation, the amount of exposure is plane and high in a region of a length of about 1/2 of the overall length L from the front end portion of the insertion of the control blade 10. The amount of neutron exposure increases in a region of about 15 cm (in a region of about 35 cm or shorter) from the leading end portion of the insertion. In particular, an extremely high amount of the exposure is shown in a region of about 5 cm in the front end portion. On the other hand, the amount of the neutron exposure is rapidly reduced at a position of about 1/2.multidot.L from the front end portion of the insertion of the control blade 10 toward the end portion of the insertion. The amount of the neutron exposure is considerably reduced in a region of 1/4.multidot.L from the end portion of the insertion toward the front portion of the insertion. The control blade is changed depending upon how it is used and the above-described value 1/2.multidot.L is changed in a range between 1/4 and 3/4.multidot.L. In the above-described range from (A) to (D), the sheet (strip) 25 made of Zr or pure Zr and disposed on the outer surface of the accommodating hole 20 and the Zr or pure Zr material placed to surround the outer Hf metal rod (or the Ag-In-Cd alloy rod) 24 act similarly to the tubular member 21 disposed in the above-described region (C) and made of pure Zr. The total length of the region (A) to (C) is arranged to be about 15 to 35 cm. If it is longer than the above-described length, it is not preferable because the weight of the control blade will be increased and the reactivity worth can be deteriorated. A second embodiment of a control blade 10A for a nuclear reactor according to the present invention will now be described with reference to FIGS. 9 and 10. Referring to FIGS. 9 and 10, the same or equivalent elements to those according to the above-described first embodiment are given the same reference numerals at the time of making the descriptions about them. Referring to FIG. 9, a range of the front end portion of the insertion designated by symbol (a) is constituted by integrally forming, by welding, metal the main component of which is Hf (for example, Hf metal containing Zr by 2 to 3 wt. %) with an Hf-Zr alloy or an Hf-Ti alloy member accommodating the accommodating holes 20. The length of this range is arranged to be about 3 to 35 cm, usually about 10 to 15 cm. Since this range is exposed to a large amount of neutrons, the Hf material, which is the long-lived absorber, is used as it is (approximating 100%, usually 97%). A range (b) is constituted similarly to that shown in a cross sectional view taken along line II--II of FIG. 2. It is preferable that the number of the accommodating holes be about two or less. In a range (c), the tubular member 21 made of Zr or pure Zr is inserted into the accommodating hole 20. Furthermore, the Hf metal rod (or the Ag-In-Cd alloy rod) 23 the diameter of which is reduced and the length of the same is shortened and the Zr or pure Zr particles 22 which are hydrogen absorbers are alternately inserted into it. The tubular member 21 made of Zr or pure Zr forms a small gap in association with the accommodating hole 20, the small gap serving as a space into which swelling can be received. In addition, the tubular member 21 acts to absorb swelling of the Hf metal rod (or the Ag-In-Cd alloy rod) 23, serves as a hydrogen getter for getting .sup.3 T and H and forms a space for absorbing the lengthwise swelling of the Hf metal rod (or the Ag-In-Cd alloy rod) 23. The total length of (b) and (c) is usually arranged to be about 15 cm. It is preferable that the range (c) has about two or three accommodating holes 20 bored therein. However, since the material for the wing forming the accommodating hole 20 contains the neutron absorber of Hf, it can be eliminated. In the region (d), the tubular memer 21 made of Zr or pure Zr is inserted into the accommodating hole 20. Furthermore, a mixture of the boron compound and the hydrogen absorber made of Zr or pure Zr particles are inserted into the above-described tubular member 21 made of Zr or pure Zr. The tubular member 21 made of Zr or pure Zr forms the swelling space from the accommodating hole 20. Furthermore, the tubular body serves as a hydrogen getter for getting .sup.3 T and H and forms a region for absorbing (relieving) the swelling of boron. The Zr or pure Zr particles mainly serve as the hydrogen getter for getting .sup.3 T and H. A range (e) is constituted similarly to that shown in a cross sectional view taken along line V--V of FIG. 2. Although the structures shown in FIGS. 2 and 9 are described to show the embodiments, the control blade for a nuclear reactor is in actual constituted in a considerably complicated manner because the above-described embodiments are combined. They can, of course, be simplified to meet a desire. For example, the portions shown in the cross sectional views respectively taken along lines II--II and III--III of FIG. 2 can be replaced by the portion shown in the cross sectional view taken along lines IV--IV. Furthermore, the regions (c) and (d) of FIG. 9 may be replaced by structures to be described later referring to FIGS. 11(B) to 11(E) and 11(G). FIGS. 11(A) to 11(I) respectively illustrate structures of the accommodating hole portions of the control blade for a nuclear reactor according to the present invention. Referring to FIG. 11(A), the neutral absorber made of the boron compound (exemplified by B.sub.4 C and EuB.sub.6) 27 is enclosed in the tubular member (sleeve) 21 made of Zr or pure Zr. The Hf metal rod (or the Ag-In-Cd alloy rod) 23 is disposed at the outer side end portion of the wing. Furthermore, the sheet (strip) 25 made of Zr is disposed in the portion from the accommodating hole 20. Referring to FIG. 11(B), the diameter of the Hf metal rod (or the Ag-In-Cd alloy rod) 23 to be inserted is reduced for the purpose of maintaining the swelling space. The Hf metal rod 23 disposed at the outer end portion of the wing is surrounded by the tubular member 21 made of Zr or pure Zr and the sheet 25 made of Zr or pure Zr. Referring to FIG. 11(C), the diameter of the Hf metal rod (or the Ag-In-Cd alloy rod) 23 to be inserted is reduced. Furthermore, a multiplicity of projecting portions 23a which can be easily deformed at the time of swelling are formed in the circumferential direction of the Hf metal rod 23. Referring to FIG. 11(D), the Hf metal rod (or the Ag-In-Cd metal rod) 23 to be inserted is constituted by forming projection 23b around a bolt so that portions to be crushed are formed. Referring to FIG. 11(E), the length of the Hf metal rod (or the Ag-In-Cd metal rod) 23 to be inserted is made to be shorter than the depth of the accommodating hole 20 so that the pure Zr particles hydrogen absorber 22 are inserted in the space created as a result of shortening the above-described length. Referring to FIG. 11(F), the Hf metal rod (or the Ag-In-Cd metal rod) 23 to be inserted are sectioned into a plurality of short pieces (elements) so that the Zr or pure Zr particles 22 are inserted between the short pieces. Referring to FIG. 11(G), the Hf metal rod (or the Ag-In-Cd metal rod) 23 to be inserted is vertically divided in the lengthwise direction. The portions between the vertically divided sections have projections which can be crushed by the swelling or the strips made of pure Zr are placed between the vertically divided sections. Therefore, the former structure is employed to relief the swelling, while the latter structure is able to relief the swelling and as well as is able to serve as a hydrogen getter because the low hardness (soft) pure Zr strip 25 is interposed. Referring to FIG. 11(H), the boron compound 27, for example, B.sub.4 C particles 27a mixed with the Zr or pure Zr particles 27b are enclosed in the accommodating hole 20. Furthermore, Hf particles (grain) 28 are enclosed in the outer side end portion of the wing 14. The Hf particles 28 placed in the outer portion of the wing 14 possesses a function as the long-lived t e neutron absorber and as a hydrogen getter. Since Hf displays restricted hydrogen absorbing performance in comparison to that possessed by the Zr or pure Zr, the particle size of it is made to be smaller, that is powder (small particles) in comparison to the Zr or pure Zr particles 22 to enlarge the surface area. Referring to FIG. 11(I), the Hf particles 28 are enclosed in the portion adjacent to the end portion of the accommodating hole 20. Furthermore, the boron compound 27 are enclosed in the accommodating hole 20. In addition, a Zr string is disposed between the end surface of the accommodating hole 20 and the Hf metal rod 24 surrounded by Zr or pure Zr on the outer side end portion of the wing 14. In this case, the Hf particles 28 possesses the function of the hydrogen getter, which is the hydrogen absorber, and the function of the long-lived type neutron absorber. If an oxide is formed on the surface of the Hf metal rod 23 to be inserted into the accommodating hole 20, the hydrogen absorption of the Hf metal rod 23 can be restricted. Therefore, further swelling can be restricted. The description "further swelling can be restricted" means that additional swelling due to the hydrogen absorption is substantially prevented because the swelling has been taken place in such a manner that the Hf metal rod 23, in which swelling has taken place due to the oxidation, is inserted into the accommodating hole 23 while keeping a certain gap. In an oxide film is formed on the inner surface of the accommodating hole 20, the oxide film thus-formed will prevent the generation of the swelling from the inner surface of the accommodating hole 20 made of the Hf, Hf-Zr, or Hf-Ti alloy and constituting the wing 14 can be prevented or reduced. FIGS. 12 and 13 respectively illustrates a state when a selected neutron absorber or a hydrogen getter is enclosed in the accommodating hole 20 so as to hermetically weld there. The hollow tubular member 21 made of Zr or pure Zr is inserted into the accommodating hole 20, the hollow tubular member 21 being filled with the neutron absorber 27 composed of the B.sub.4 C powder. Furthermore, the Hf metal rod 24 surrounded by the sheet 25 made of Zr or pure Zr is placed in the vicinity of the outer side end portion of the wing. Since the Hf alloy member forming the wing 14, that is, the Hf-Zr member (or the Hf-Ti member), is formed into a shape having an end opening, this portion is turned inside so as to close the outer portion by welding as shown in FIG. 13. Since the Hf alloy which constitutes the wing and the Zr or pure Zr are changed to an alloy in association with each other, a portion of the sheet 25 made of the Zr or pure Zr which surrounds the Hf at the time of the hermetical welding is melted and is moved to the welded portion. However, no problem arises because the above-described alloy is an extremely safety alloy. At the time of welding, an Hf alloy member composed substantially similarly to the wing structural view may be placed on the reverse (inner) side of the welding portion similarly to the conventional structure shown in FIGS. 35 and 36. FIGS. 14 and 15 illustrates a third embodiment of the control blade for a nuclear reactor according to the present invention. The same or equivalent elements to those according to the above-described first embodiment are given the same reference numerals. Although this embodiment is basically arranged in the same manner as the above-described first and second embodiments, the Hf metal rod 23 in which a portion of the above-described embodiments is employed is inserted into the front portion 1.sub.3 of the insertion of the control blade. In a region of length 1.sub.4 (about 1/4 to 1/2 of the overall length L of the effective portion) from the above-described front portion toward the end portion of the insertion of the control blade, the accommodating hole 20 is formed into an elongated hole so that a larger quantity of the neutron absorber composed of the boron compound 27a such as B.sub.4 C is inserted. Furthermore, the pure Zr particle 27b are mixed so as to serve as hydrogen getter (the hydrogen absorber). In this case, the reactivity worth can be improved since a larger quantity of boron is enclosed. Since the ratio of .sup.3 T generation is low in the portion 1.sub.2 and as well as there is no necessity of particularly improving the reactivity worth, usual circular holes are simply arranged in which the neutron absorber such as B.sub.4 C is enclosed. FIG. 16 illustrates a fourth embodiment of a control blade 10C for a nuclear reactor according to the present invention, where the same or equivalent elements to those according to the above-described first embodiment are given the same reference numerals. The control blade according to this embodiment is arranged in such a manner that the front portion 1.sub.1 (about 1/4 to 1/2.multidot.L) of the insertion thereof is structured such that the accommodating hole is formed in the Hf-Zr or Hf-Ti alloy plate and the end portion 1.sub.2 of the insertion is structured such that two Hf plates are disposed to confront each other while holding a gap therebetween. Water is enclosed in the portion between the two Hf plates so as to serve as the neutron moderator and to perform the cooling operation. The prevention of the absorption of .sup.3 T and H from the inside of the accommodating hole and the like are arranged similarly to the above-described first to third embodiments. FIG. 17 illustrates a fifth embodiment of a control blade 10D for a nuclear reactor according to the present invention, where the same or equivalent elements to those according to the above-described first embodiment are given the same reference numerals. The control blade 10(D) according to this embodiment is arranged in such a manner that the wing 2 is constituted by placing the neutron absorber in a U-shaped sheath. In the region 1.sub.1 (about 1/4 to 1/2.multidot.L) of the front portion of the insertion of the control blade, the accommodating hole is formed in the Hf metal plate, Hf-Zr or Hf-Ti alloy of an ordinary composition containing 2 to 3 wt. % Zr. Furthermore, the neutron absorber is enclosed. The end portion 1.sub.2 of the insertion is arranged in such a manner that conventional neutron absorbing rods structured such that the B.sub.4 C powder is enclosed in a stainless steel pipe are arranged. If the U-shape sheath is made of the Hf-Zr or Hf-Ti alloy, the reactivity can be improved and the life can be lengthened. The sheath member may be made of stainless steel in a case where the above-described requirements are not made. The neutron absorbing element to be inserted into the portion 1.sub.1 is sectioned into a portion 1.sub.11 and a portion 1.sub.12. If the density of Hf is made to be high in the portion 1.sub.11, it is effective to improve the reactivity and to lengthen the life. However, since the weight of the control blade is undesirably increased, the density of Hf is lowered in the portion 1.sub.12 so as to reduce the weight and the overall cost. The portion 1.sub.3 acts to restrict a local peaking of the neutron flux due to the reduction in the quantity of the neutron absorber in the boundary portion. Accordingly, the Hf metal is placed. Since the control blade for a nuclear reactor according to this embodiment is arranged in such a manner that a mixture of a material containing boron and at least either the Zr or pure Zr particles or hafnium particles is enclosed in the accommodating hole, hydrogen and tritium can be absorbed and the hydrogen absorption on the inner surface of the accommodating hole can be prevented. Furthermore, since the boron compound surrounded by the zirconium or pure zirconium sheet is enclosed in the accommodating hole formed in the wing, swelling taken place due to the neutron reaction of the boron compound can be absorbed and as well as hydrogen and tritium can be absorbed. As a result, generation of stress in the accommodating hole and hydrogen absorption can be prevented. In addition, since hafnium metal member or Ag-In-Cd alloy member is longitudinally sectioned so as to hold the Zr or pure Zr strip between two confront sides, the zirconium strip acts to a relief portion for stress which acts on the accommodating hole due to swelling because the pure zirconium strip has low hardness. Furthermore, since it exhibits excellent hydrogen absorbing performance, it is able to absorb hydrogen if hydrogen exists prior to other materials. As a result, the hydrogen absorption into the inner surface of the accommodating hole, the Hf metal and Ag-In-Cd alloy can be prevented and as well as swelling taken place caused from the hydrogen absorption can be prevented. Therefore, the generation of stress acting on the accommodating hole can be prevented. Furthermore, the control blade for a nuclear reactor according to this embodiment is arranged in such a manner that a tubular member made of zirconium or pure zirconium exhibiting satisfactory hydrogen absorbing performance is inserted into at least a portion of the accommodating holes disposed in a region of at least 3 cm and as well as 35 cm or shorter from the front portion of the insertion. Furthermore, the portion inside and outside of the tubular member is formed into a non-hermeical shape so as to serve as a gas plenum. As a result, the hydrogen absorption of the hafnium and Ag-In-Cd alloy to be inserted can be prevented. In addition, the generation of stress due to the swelling can be prevented and to restrict the gas pressure rise in the control blade. Consequently, the soundness of the control blade can be satisfactorily maintained. Then, a sixth embodiment of a control blade for a nuclear reactor will now be described. A control blade 30 for a nuclear reactor is a conventional typed control blade as shown in FIG. 18, the control blade 30 having four wings 31 which accommodating an improved neutron absorbing rod 32. Each of the wings 31 comprises sheath plate 33 having a deep U-shape cross section and serving as a sheath plate means in such a manner that the opening sides of the sheath plate 33 is secured to a central tie rod 34 serving as a central tie means in such a manner that the opening sides of the wings 31 are disposed to form a cross-shape lateral cross section. A multiplicity of neutron absorbing rods 32 are disposed in line in the sheath plate 33, the neutron absorbing rod 32 serving as a neutron absorber rod means. Reference numeral 35 represents an upper structure member, 36 represents a lower structure member, 37 represents a handle, 38 represents a speed limiter and 39 represents a coupling socket. As shown in FIG. 19, the neutron absorbing rod 32 has an elongated cladding or covering pipe 40 serving as a poison tube, the covering pipe 40 accommodating neutron absorbing members and material 41 and 42. The two end portions of the covering pipe 40 are closed by plugs 43 serving as plug means. Although the covering pipe 40 is usually made of stainless steel, it may be made of Hf metal, an Hf alloy the main component of which is Hf and Zr or another Hf alloy the main component of which is Hf and Ti. The neutron absorbing rod 32 is mainly sectioned into three portions X, Y and Z. The portion X is a portion exposed to neutron by a large amount when it is accommodated in the control blade 30. Therefore, a long-lived type neutron absorbing member 41 made of Hf metal, an Hf alloy composed of Hf and Zr or Ti, or an Ag-In-Cd alloy is disposed in the portion X. According to this embodiment, the diameter of the long-lived type neutron absorbing member 41 is arranged to be smaller, by a certain quantity, than the inner diameter of the covering pipe 40 as unsealed type inner pipe. Furthermore, a thin sleeve 44 made of pure Zr, Hf, Ti or stainless steel is formed around the long-lived neutron absorbing member 41 is inserted into the covering pipe 40. In the portions Y and Z, the boron compound 42 formed into powder and serving as the neutron absorbing member is disposed. Since the portion Y is exposed to neutron by a relatively large amount, pure Zr particles and/or Hf powder 45 serving as the hydrogen getter (the hydrogen absorber) is mixed with the boron compound 42. Since the portion Z is exposed to neutron by a reduced quantity and thereby the ratio ofgeneration of tritium (.sup.3 T) is low, the hydrogen getter is omitted from the structure. Although the boron compound 42 is exemplified by B.sub.4 C, EuB.sub.6 or BN may be employed. Since the boron compound 42 encounters swelling because He gas is produced as a result of a reaction with neutron, the charging density of the boron compound 42 must be determined after the quantity of swelling which can be generated due to the neutron irradiation has been estimated. Although the length of the portion X can be made to be 3/4 of the overall length L if necessary, it can be set to about 3 to 5 cm if a short length is required. The length must be determined depending upon how to use the control blade 30 which accommodates the neutron absorbing rod 32. In a case where the control blade 30 is considerably inserted into the nuclear core during the operation of the nuclear reactor, it is arranged to be about 1/2 of the overall length L. In case where the control blade 30 is withdrawn from the nuclear core during the operation of the nuclear reactor, it is usually arranged to be about 15 cm. Although the length of the portion Z is usually arranged to be about 1/4 to 3/4 of the overall length L, the length of portions (Y+Z) may be arranged to be a length obtained by subtracting 15 cm from the overall length in a case where the control blade 30 is fully withdrawn from the operation. Furthermore, the length of the portion Y may be made to be zero. A metal wool 46 made of Hf, Zr, stainless steel or iron is enclosed in a portion between the plug 43 and the long-lived type neutron absorbing member 41. Also a metal wool 47 is interposed between the long-lived type neutron absorbing member 41 and the boron compound mixture layer Y for the purpose of preventing the mixture of the powder boron compound with the portion around the long-lived type neutron absorbing member 41. The above-described metal wool 47 may be basically the same as the metal wool 46 placed between the plug 43 and the long life type neutron absorbing member 41. However, it is preferable that wool made of the neutron absorbing material, for example, Hf wool, be employed in a case where the length into which the metal wool 47 is enclosed is longer than about 5 mm. The reason for this lies in that, if non-absorbing material is employed, the neutron flux peak will take place in the above-described portion, causing the adjacent boron compound to be locally exposed to neutron by a large amount. As a result, the soundness for the neutron absorbing element will be deteriorated. On the other hand, the length of the layer in which the metal wool 46 is enclosed is usually made to be about 5 to 10 mm. With the neutron absorbing rod thus-constituted, tritium produced by the boron compound can be absorbed by the Zr particles or the Hf powder mixed with the boron compound so as to prevent the diffusion of it into the long-lived type neutron absorbing member. Furthermore, since the long-lived type neutron absorbing member is surrounded by the sleeve made of pure Zr, Hf or Ti serving as a hydrogen getter or a stress absorber or a sleeve made of stainless steel serving as a stress relaxer, the stress generation in the covering pipe due to swelling can be satisfactorily prevented. FIGS. 24 to 29 respectively illustrates modifications of the portion X shown in FIG. 20, where the equivalent elements to those shown in FIG. 20 are given the same reference numerals. According to a first modification shown in FIG. 24, the diameter of the long-lived type neutron absorbing member 41 is, by a certain quantity, made smaller than the inner diameter of the covering pipe 40 so as to create a gap 50 between the long-lived type neutron absorbing member 41 and the covering pipe 40. Furthermore, the undesirable looseness generated due to the reduction of the diameter of the long-lived type neutron absorbing member 41 is prevented by forming a plurality of small projecting portions 51 in the portions of the long-lived type neutron absorbing member 41. As a result of the structure thus-constituted, the projecting portions 51 can be easily crushed and the generation of large stress in the covering pipe 40 can be prevented even if the long-lived type neutron absorbing member 41 generates swelling. According to a second modification shown in FIG. 25, the local projecting portions 51 according to the first modification are replaced by thread type projecting portions 52 formed on the surface of the long-lived type neutron absorbing member 41. According to this modification, an effect similar to that obtainable according to the first modification can be obtained. FIG. 26 illustrates a third modification which is arranged in such a manner that dimplings 53 are formed for the purpose of making the covering pipe 40 locally projecting and coming contact with the long-lived type neutron absorbing member 41. In a case where the long-lived type neutron absorbing member 41 encounters swelling, the dimplings 53 can easily restore their original shapes. Therefore, the generation of large stress cannot be generated in the covering pipe 40. FIG. 27 illustrates a fourth modification in which the long-lived type neutron absorbing member 41 is sectioned into a multiplicity of short pieces. Furthermore, at least either the Zr or pure Zr particles or the Hf powder 45 is disposed between the pieces thus-formed. According to this modification, the Zr or pure Zr particles or the Hf powder 6 absorbs hydrogen and tritium. Therefore, the prevention of swelling in the long-lived , type neutron absorbing member 41 can be substantially prevented. Although the Zr or pure zr particles and the Hf powder 45 generates swelling, it can be present as it is in the same gap even if the swelling takes place because it is in a low density state. Therefore, the generation of large stress in the covering pipe 40 can be prevented. FIG. 28 illustrates a fifth modification which is arranged in such a manner that the long-lived type neutron absorbing member 41 is longitudinally divided into elongated pieces. Furthermore, small projecting portions 54 which can be easily crushed by swelling are locally formed between the elongated pieces. If swelling takes place, the small projecting portions 54 are sequentially crushed so that the generation of large stress in the covering pipe 40 can be prevented during the above-described effect. Therefore, the time at which stress is generated can be delayed considerably. FIG. 29 illustrates a sixth modification in which the long-lived type neutron absorbing member 41 is, similarly to the fifth modification, longitudinally sectioned. Furthermore, strips 55 made of Zr or pure Zr are interposed in at least a portion of gaps between the elongated pieces. Since strip 56 made of Zr or pure Zr absorbs hydrogen, the generation of swelling in the long-lived type neutron absorbing member 41 can be substantially prevented. Although the strip 55 made of Zr or pure Zr encounters swelling, the generation of large stress in the covering pipe 40 can be satisfactorily prevented because the hardness is small and the gap will absorb the swelling. Although omitted from illustration, a seventh modification is arranged in such a manner that an oxide film is formed on the surface of the long-lived type neutron absorbing member 41. In this case, since the oxide film prevents the hydrogen absorption, the generation of swelling in the long-lived type neutron absorbing member 41 can be prevented. Eighth and ninth modifications are modofications about the portion Y shown in FIG. 20. FIGS. 30 and 31 respectively are vertical cross sectional views of the same. The eighth modification shown in FIG. 30 is arranged in such a manner that the boron compound 42 enclosed in a non-sealed type inner pipe 56 made of pure Zr, Hf or stainless is accommodated in the covering pipe 40. In a case where the inner pipe 56 is made of pure Zr, the inner pipe 56 serves as a hydrogen getter and a stress relaxer, and as well as serves as a neutron absorber. The inner pipe 56 made of stainless steel simply serves as a stress relaxer by forming gaps to prevent the swelling generated due to the He gas of the boron compound. Therefore, it is preferable in this case that a mixture composed by mixing pure Zr particles (grain) or Hf powder (grain) serving as a hydrogen getter with the boron compound 42 be inserted into the inner pipe 56. As a result, diffusion of tritium generated by the boron compound 42 to the long-lived type neutron absorbing member 41 can be prevented. A control blade shown in FIG. 32 has been developed recently and disclosed in Japanese Patent Laid-Open No. 2-254895. FIG. 33 is an enlarged view of a portion L shown in FIG. 32. A neutron absorbing member charge portion 58 is formed into a circular cross section, while the outer portion of the same is formed into a substantially square shape by padding material in units of 90.degree.. By alternately welding the pad portions 59 (to form welded portions 60), the control blade shown in FIG. 32 and having an outer shape which is substantially the same as the shape shown in FIG. 14 can be obtained. However, an apparent difference between the structure shown in FIG. 32 and that shown in FIG. 19 lies in that the sheath 33 is omitted from the structure shown in FIG. 32. Since the thickness of the wing 31 must be the same so as to be mounted on the same nuclear reactor, the diameter of the neutron absorbing member charge portion 58 can be enlarged by a quantity which corresponds to the sheath 33 omitted from the structure. Therefore, a larger amount of neutron absorber absorbers can be enclosed. As a result, the reactivity worth of the control blade for a nuclear reactor can be improved and as well as the nuclear life can be lengthened. As shown in FIGS. 19 and 20, the structure of the neutron absorbing rod 32 can be similarly adapted to a neutron absorbing rod 61 shown in FIGS. 32 and 33 and formed into a substantially square shape. The inventor of the present invention has disclosed the structure of an inner pipe for use as a control blade in Japanese Patent Laid-Open No. 2-2983. Although a structure for preventing the deterioration in the reactivity worth by using a neutron absorber in the plug for the inner pipe has been disclosed in Japanese Patent Laid-Open No. 2-2983, the present invention is arranged in such a manner that the pure Zr sleeve (inner pipe) serving as a hydrogen getter is used. Although a similar structure of an inner pipe has been disclosed in Japanese Patent Laid-Open No. 2-13888, the concept of the hydrogen getter is not included. As can be understood from the above-made description, the neutron absorbing rod is constituted in such a manner that the covering pipe can be protected from excessively large stress. Therefore, a long-lived type neutron absorbing element exhibiting an extremely improved soundness can be provided. Although the embodiments of the neutron absorbing rod according to the present invention is adapted to a control blade for use in a water boiling nuclear reactor, the present invention is not limited to this. The present invention can be adapted to a control blade for use in a pressurized water reactor. Furthermore, the structure of the neutron absorbing rod can be adapted to a control blade for use in a light water reactor, a heavy water reactor, a converter reactor or a fast breeder reactor. Although the invention has been described in its preferred form with a certain degree of particularly, it is understood that the present disclosure of the preferred form has been changed in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.