Patent Number: 052767183
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a control blade for use in nuclear reactors for adjusting and controlling the power of a boiling water reactor or the like, and, more particularly, to an excellent reactivity and long-lived type control blade for nuclear reactors capable of controlling undesirable swelling of a neutron absorber or capable of preventing deterioration in the mechanical and physical life if swelling takes place. Since the neutron absorption power (capacity) of a control blade for use in boiling water nuclear reactors is gradually deteriorated when it absorbs neutrons, the control blade must be taken out from the core of the nuclear reactor after it has been used for a predetermined period so as to be replaced by a new control blade. However, the above-described replacement work must be performed while shutting down the operation of the nuclear reactor, causing a large scale work to be performed which takes a too long time. Therefore, the time in which the operation of the nuclear reactor must be shut down takes a too long time, causing the availability to be deteriorated. What is even worse, there is a risk for operators to be exposed to radiation. Furthermore, since the used control blade is a large and strong radioactive waste disposal, there recently arises a desire to lengthen the life of the control blade. As a result, a variety of novel long-lived type control blades for nuclear reactors have been developed, resulting a kind of them to be put into practical use. The inventor of the present invention has promptly recognized the necessity of realizing a long-lived type control blade and thereby has disclosed a control blade for nuclear reactors for the purpose of lengthening the life thereof in Japanese Patent Laid-Open No. 53-74697 (Japanese Patent Publication No. 59-138987). The control blade for nuclear reactors according to the above-made disclosure is basically arranged in such a manner that a long-lived type neutron absorber exemplified by Hf metal or an Ag-In-Cd alloy is, in place of a boron compound, disposed in a portion which is exposed to a relatively large amount of neutrons. A control blade for nuclear reactors is detachably inserted from a lower portion of the core into a gap having a cross-shaped lateral cross section and formed between fuel assemblies each of which is a set composed of four fuel assemblies loaded in the core portion of a nuclear reactor. The front portion of insertion and the outer end portion (the outer side end portion of the wing) of the control blade for nuclear reactors are exposed to a particularly large amount of neutrons. Therefore, hafnium (Hf), the neutron absorption power (capacity) of which cannot be largely deteriorated even if it is exposed to a large amount of neutrons, is disposed in the above-described portions so as to lengthen the life of the control blade for nuclear reactors. Furthermore, cheap and light boron carbide (B.sub.4 C) is disposed in the other portion of the control blade. A conventional control blade for nuclear reactors is basically arranged as shown in FIGS. 34 to 36 in such a manner that a plurality of tie members 1 each of which has a cross-shaped connecting portion are disposed in the axial direction at predetermined intervals. Furthermore, four wings 2 each of which is formed into an elongated rectangular plate are secured to the above-described connecting member 1 in such a manner that the four wings 2 form a cross-shape. Reference numeral 3 represents a handle connected to the front portions of the four wings 2. Each of the wings 2 has-a plurality of accommodating holes 2a formed in its widthwise direction, each of the accommodating holes 2a being formed in line in the lengthwise direction of the wing 2. In the accommodating hole 2a formed in the front insertion portion of the wing 2, a long-lived type neutron absorber 3a composed of Hf is disposed. Furthermore, a neutron absorber 4 made of B.sub.4 C powder is enclosed in the residual accommodating holes 2a. In the outer side end portion (periphery) of the wing 2 which corresponds to the outer periphery of the control blade A for nuclear reactors, a space 2b is formed in the lengthwise direction of the wing 2 in such a manner that it is communicated with each of the accommodating holes 2a. In the above-described space 2b, a long-lived type neutron absorber 3b composed of Hf is disposed to close an end portion of each of the accommodating holes 2a. The neutron absorber 3b acts to prevent the B.sub.4 C powder drop from the accommodating holes 2a. A stainless steel member 5 for use at the time of welding work is disposed between the neutron absorber 3b and the outer periphery of the wing 2. When the wing 2 thus-arranged is assembled, the neutron absorbers 3a and 4 are inserted (injected) into the accommodating hole 2a through an opening portion 2c (FIG. 35) formed at a position confronting the outer periphery of the wing 2. Then, the neutron absorber 3b and the stainless steel member 5 are placed in the space 2b before a pair of plate portions 2d confronting each other in the opening portion 2c are bent inward so as to be closed by welding. Since the melting point of the neutron absorber 3b made of Hf is 2200.degree. C. which is considerably higher than the melting point 1400.degree. C. of the stainless steel member 5, the neutron absorber 3b is not melted at the time of the welding work but is held in the outer periphery of the wing 2 made of stainless steel. Therefore, the undesirable mixture of Hf atoms of the neutron absorber 3b with the metal present in the portion to be welded can be prevented. As a result, sound weld portion can be obtained. It is preferable that range 1.sub.1 (see FIG. 34) in which the long-lived type neutron absorbers 3a are disposed 3 cm or longer and as well as 35 cm or shorter from the front insertion portion of the wing which constitutes the control blade for nuclear reactor. If it is shorter than 3 cm, the B.sub.4 C powder which constitutes the neutron absorber 4 is undesirably placed in the region which is exposed to a large amount of neutrons. On the contrary, if it is longer than 35 cm, the reactivity worth will be deteriorated at the time of shutting down the nuclear reactor because Hf displays insufficient neutron absorption effect in comparison to B.sub.4 C. Furthermore, the expensive Hf is excessively used and its weight is heavy, causing an economical disadvantage to be taken place and the overall weight of the control blade for nuclear reactors to be increased undesirably in comparison to the conventional control blade for nuclear reactors. It is preferable that the length of range 1.sub.2 of the neutron absorber 3b disposed in the outer periphery of the wing 2 be about 0.5 to, 2 cm. If it is shorter than 0.5 cm, the B.sub.4 C powder which constitutes the neutron absorber 4 is undesirably placed in the region which is exposed to a large amount of neutrons. If it exceeds 2 cm, Hf which constitute the neutron absorber 3b is undesirably placed in the region (the region adjacent to the central portion of the wing) in which the value of the neutron flux has been reduced, causing the problem similar to the above-made description to arise. The inventor of the present invention has, in Japanese Patent Laid-Open No. 1-202691, disclosed a novel control blade for nuclear reactors arranged in such a manner its structure is similar to that shown in FIG. 34 and an Hf diluted alloy which is an alloy of Hf and zirconium (Zr) or an alloy of Hf and titanium (Ti) is used in place of stainless steel. Hafnium forms an all-solid solution type alloy in association with Zr or Ti at an arbitrary ratio. Although a fact is well known that Hf, Zr and Ti are able to maintain satisfactory soundness in nuclear reactors, another fact is known that they absorbs hydrogen, thereby forms a hydride and will generate swelling, which is the cubical expansion, in a state where the quantity of hydrogen is excessively large but oxygen is insufficient. Hf is an excellent neutron absorber possessing multiple types of isotopes and it is known as a typical long-lived type neutron absorber because it is able to prevent the deterioration in the neutron absorbing performance even if it is exposed to neutrons. Furthermore, Hf has a characteristic of particularly absorbing resonance neutrons and its neutron absorption power (capacity) is not rapidly deteriorated even if it is diluted by a diluent to a certain degree. As described above, although Hf possesses the excellent nuclear neutron-absorbing characteristic, it suffers from an excessively large density (about 13.1 g/cm.sup.3). Therefore, there arises a problem in that it cannot be easily employed as a material for the control blade for the conventional nuclear reactors. On the other hand, Zr has a small density (6.5 g/cm.sup.3) and it is able to form an excellent alloy in association with Hf as described above. Also Ti has a small density (4.5 g/cm.sup.3) and it is able to form an excellent alloy with Hf. Therefore, an alloy of Hf and Zr or an alloy of Hf and Ti (since Zr or Ti serves as a diluent in view from Hf, they are called diluted alloy in this specification) is able to maintain excellent characteristics in nuclear reactors. Furthermore, the neutron absorption power is not deteriorated in comparison to the structure in which Hf is employed. Furthermore, the density of the diluted alloy can be reduced so that the density of the level possessed by stainless steel (about 8 g/cm.sup.3) can easily be realized. Therefore, a control blade for nuclear reactors which can be adapted to conventional nuclear reactors and which is constituted similarly to that shown in FIG. 34 can be constituted. In this case, the stainless steel member for welding and represented by reference numeral 5 can be omitted from the structure shown in FIGS. 34 to 36. Therefore, it might be feasible to employ a member made of the above-described diluted alloy as the member for performing the welding work. However, as described above, Hf or the Hf alloy will absorb hydrogen and generate swelling when an excessively large quantity of hydrogen is present and the quantity of oxygen is insufficient. The above-described novel control blade for nuclear reactors (see Japanese Patent Laid-Open No. 1-202691) is arranged in such a manner that accommodating holes are formed in the Hf diluted alloy sheet and the B.sub.4 C (boron carbide) powder is enclosed in the accommodating holes. .sup.10 B of B.sub.4 C reacts with neutrons to generate .sup.4 He and .sup.7 Li and as well as .sup.3 T (tritium). If the amount of neutrons exposed to the control blade increases, the amount of .sup.3 T produced cannot be neglected. Although the major portion of .sup.3 T is left in B.sub.4 C, a portion of it is removed from B.sub.4 C. Since tritium is hydrogen, it can be absorbed by Hf, Zr or Ti. Therefore, there arises a problem in that the soundness of Hf, the Hf-Zr alloy or the Hf-Ti alloy which also serves as the structure member of the Control blade will be deteriorated. The control blade for nuclear reactors of the type described above is arranged in such a manner that a multiplicity of the accommodating holes are formed in a sheet made of a diluted alloy prepared by diluting Hf by Zr or Ti serving as a diluent and the boron compound is enclosed in a portion or the major portion of the accommodating holes. There is a fear that tritium (.sup.3 T) produced when boron reacts with neutrons and hydrogen produced due to a radiolysis of water introduced at the manufacturing process move between accommodating holes and thereby hydrides HfH.sub.2 (Hf.sup.3 T.sub.2) , ZrH.sub.2 (Zr.sup.3 T.sub.2) ,TiH.sub.2 (Ti.sup.3 T.sub.2) or the like is produced with Hf, Zr or Ti. It leads to cubical expansion (swelling) due to the increase in the volume from the original Hf alloy. Therefore, there is a possibility that the stress will be generated in the Hf-Zr alloy or the Hf-Ti alloy from the inner surface of the accommodating hole, the Hf-Zr alloy or Hf-Ti alloy serving as the structure material. Another long-lived type control blade has been disclosed by the inventor of the present invention in Japanese Patent Publication No. 1-45598 (Japanese Patent Laid-Open No. 57-171291). The control blade for nuclear reactors according to the above-made disclosure employs a neutron absorbing rod. The neutron absorbing rod is arranged in such a manner that boron carbide (B.sub.4 C) powder and a Hf metal rod or an Ag-In-Cd alloy rod are enclosed in an elongated covered pipe made of stainless steel. Furthermore, metal wool is disposed between the above-described two elements. In the front end portion which is exposed to a large amount of neutrons when viewed in the direction into which the control blade is inserted, the Hf metal rod or the Ag-In-Cd alloy rod is placed, while the B.sub.4 C powder is enclosed in the end portion opposing the front insertion portion because the amount of the neutron exposure is relatively small in this portion. However, a fact has been found from the ensuing research that there is a room for improvement in the neutron absorbing rod, which is arranged in such a manner that both the boron compound and the Hf metal or the Ag-In-Cd alloy rod are enclosed. That is, a portion of tritium (.sup.3 T) produced as a result of boron-neutron reaction in the boron compound is absorbed into the surface layer of the Hf metal or the Ag-In-Cd alloy, which is sealed together, causing swelling to be generated in the neutron absorber. Therefore, the soundness of the covered pipe can be deteriorated. Furthermore, a portion of water left in the covered tube at the time of the manufacturing process is changed into hydrogen due to radiolysis. Furthermore, since hydrogen is able to transmit the stainless pipe, hydrogen generated due to the radiolysis of the reactor core water can be introduced into the covered pipe. The above-described introduction of hydrogen will cause swelling since it can be absorbed into the surface layer of the Hf metal or the Ag-In-Cd alloy similarly to the above-described case about tritium. SUMMARY OF THE INVENTION A major object of the present invention is to provide a large reactivity and long-lived type control blade for nuclear reactors capable of effectively preventing generation of a swelling phenomenon in a long-lived type neutron absorber thereof to improve soundness and thereby lengthening the life while maintaining the mechanical life. Another object of the present invention is to provide a long-lived type control blade for nuclear reactors capable of, even if hydrogen or tritium, which easily reacts with Hf, Zr or Ti, is present in accommodating holes or the like formed in the wing, restricting production of hydride or preventing generation of stress in accommodating holes if hydride is produced, capable of improving soundness and lengthening the life. Another object of the present invention is to provide a neutron absorbing rod for use in the long-lived type control blade for nuclear reactors. Another object of the present invention is to provide a neutron absorbing rod capable of preventing generation of a swelling phenomenon in a neutron absorber, reducing stress generated in a covered pipe due to the swelling if takes place and maintaining soundness. In order to achieve the above-described objects, a control blade for use in nuclear reactors according to the present invention comprises: an upper structure means; a lower structure means; a central tie means disposed between the upper structure means and the-lower structure means; a wing means having a plurality of wings connected to each other by the central tie means in such a manner that a plurality of the wings are disposed to form a cross-shape lateral cross section; and neutron absorber means enclosed in at least a major portion of a multiplicity of accommodating holes formed in a widthwise direction of each wing and in line disposed in a lengthwise direction of the wing, wherein the wing means is arranged in such a manner that each of the wings is constituted by a plate member made of hafnium metal, a hafnium alloy composed of hafnium and zirconium or titanium, or an alloy the main component of which is zirconium or titanium, the neutron absorber means comprises a long-lived type neutron absorber which is enclosed in the accommodating holes formed in the front insertion portion of the wing which is exposed to a large amount of neutrons and made of hafnium, metal the main component of which is hafnium, a silver-indium-cadmium Ag-In-Cd alloy and a neutron absorber which is inserted into at least the major portion of the residual accommodating holes and which contains boron, and a mixture of a material containing boron and at least one hydrogen absorber composed of at least either zirconium particles or hafnium powder is enclosed in the accommodating holes among the accommodating holes for accommodating the neutron absorber containing boron disposed in a range from a front insertion portion, which is exposed to a large amount of neutrons, to 1/4.multidot.L of the height L of an effective heating portion of a reactor core. In order to achieve the above-described objects, a control blade for use in nuclear reactors according to the present invention comprises: an upper structure means; a lower structure means; a central tie means for establishing a connection between the upper structure means and the lower structure means; a sheath plate means connected to the central tie means and having a U-shaped lateral cross section to constitute wings disposed to form a cross-shaped lateral cross section; and a neutron absorber rod means accommodated in the sheath plate means in line, wherein the neutron absorber rod means is constituted by inserting the long-lived type neutron absorber made of hafnium metal, metal the main component of which is hafnium or a silver-indium-cadmium (Ag-In-Cd) alloy or the like into a covered pipe while forming a gap or sleeve around the neutron absorber or while forming an oxide film on the surface of the neutron absorber. In order to achieve the above-described objects, a neutron absorbing rod according to the present invention comprises: an tubular covering(cladding) pipe; a plug means for sealing two end portions of the covering pipe; and a neutron absorber means accommodated in the covering pipe, wherein said neutron absorber means comprises a long-lived type neutron absorber enclosed in one side of the covering pipe, which is exposed to a large amount of neutrons, and made of hafnium metal, alloy the main component of which is hafnium or a silver-indium-cadmium alloy and a neutron absorber enclosed in the residual region and composed of a boron compound, and the long-lived type neutron absorber is enclosed in the covering pipe while forming a gap or a sleeve around the long-lived type neutron absorber or while forming an oxide film on the surface of the long-lived type neutron absorber. According to the present invention constituted as described above, a boron compound such as boron carbide (B.sub.4 C) and europium hexaboride (EuB.sub.6) and a hydrogen absorber composed of at least either pure zirconium (Zr) particle exhibiting an excellent hydrogen absorbing efficiency or hafnium (Hf) powder (since Hf displays lower activity than that of Zr, it is preferable that the particle size of it be small). Therefore, tritium (.sup.3 T) atoms which are produced as a direct or indirect reaction between boron with neutrons and active hydrogen (H) atoms which are produced as a radiolysis of water introduced when the control blade has been manufactured can be effectively absorbed. Therefore, the hydrogenative reaction at the inner surface of the accommodating hole formed in the wing can be effectively prevented. Furthermore, another hydrogenative reaction of the Hf metal or the Ag-In-Cd alloy to be inserted into a portion of the accommodating holes formed in the wing can also be effectively prevented. Since the volume of the Zr particles and the Hf powder increases their volume due to the hydrogen absorption, the enclosing mixture density of the boron compound and at least either the Zr particles or the Hf powder must be adjusted while previously estimating the volume increase. Furthermore, a zirconium (Zr) sheet exhibiting satisfactory hydrogen absorbing efficiency and reduced hardness is, in a manner to form a sleeve, placed on the inner surface of the accommodating hole formed in the wing. In addition, the boron compound such as B.sub.4 C and EuB.sub.6 is enclosed in the sleeve. In this case, a manufacturing process may be employed which is arranged in such a manner that a tubular member made of Zr is used to surround it before they are inserted into the accommodating hole. As a result, a gap having a certain size is necessarily formed between the tubular member made of Zr and the inner surface of the accommodating hole. Therefore, if swelling takes place because the tubular member made of Zr absorbed hydrogen, the above- described gap absorbs the cubic expansion, causing the stress induction start moment in the accommodating hole can be significantly delayed. In a case where no hydrogen or little hydrogen has been absorbed since the density of hydrogen in the tubular member made of Zr is low, the tubular member made of Zr or the above-described gap absorbs the cubic expansion due to the swelling of the boron compound. Therefore, the moment at Wee which the stress acts on the accommodating hole can be significantly delayed. Furthermore, the Hf metal rod or the Ag-In-Cd alloy rod to be inserted into the accommodating hole formed in the wing is longitudinally divided into pieces. A Zr strip is interposed between the above-described pieces. Therefore, if stress acts on the inner surface of the accommodating hole due to some reason, the strip can be crushed, and the generation of excessively large stress can be prevented. Since Zr exhibits satisfactory hydrogen absorbing performance, it absorbs hydrogen if the density of hydrogen has been raised so that the density can be reduced. As a result, swelling generated due to the hydrogen absorption by the insertion member such as the Hf metal rod or the Ag-In-Cd alloy rod to be inserted into the accommodating hole can be significantly prevented. Furthermore, swelling which will take place due to the hydrogen absorption by Hf, the Hf-Zr alloy or the Hf-Ti alloy which forms the accommodating hole can be significantly delayed. Furthermore, the tubular members made of Zr are inserted into a portion of the accommodating holes formed in the vicinity of front insertion portion. The above-described tubular member is formed into a non-sealed type and a hollow shape to form a gas plenum. If hydrogen or tritium is contained in the gas, it can be absorbed by the Zr tubular member. Therefore, the absorption of hydrogen or tritium into the inner surface of the accommodating hole can be prevented. As a result, the soundness of the accommodating hole can be maintained. That is, it possesses a function as a hydrogen getter and a function as a gas plenum. The Hf or the Hf alloy or the Ag-In-Cd alloy constituting the long-lived type neutron absorber to be enclosed in the neutron absorbing rod for use in the control blade for nuclear reactors will absorb, to the surface thereof, hydrogen generated due to the radiolysis of water left when the neutron absorbing rod was manufactured, hydrogen supplied from the nuclear core water and introduced after it has transmitted the covering pipe and tritium generated and discharged from the reaction taken place between boron and neutrons, in a case where the boron compound is enclosed together. However, according to the present invention, a gap is formed between the long-lived type neutron absorber and the covering pipe, the long-lived type neutron absorber is surrounded by the thin sleeve made of zirconium, hafnium, titanium or stainless steel, or an oxide film is formed on the surface of the long-lived type neutron absorber before it is accommodated in the covering pipe. Therefore, the generation of stress in the covering pipe due to the swelling of the long-lived type neutron absorber can be significantly prevented. That is, in a case where the gap is formed between the long-lived type neutron absorber and the covering pipe, the influence of the swelling, if taken place, upon the covering pipe can be significantly reduced. In a case where the thin sleeve is formed around the long-lived type neutron absorber and the sleeve is made of Zr, Hf or Ti, the sleeve serves as the hydrogen getter. Therefore, the swelling of the long-lived type neutron absorber due to the hydrogen absorption can be prevented. Although the sleeve member encounters swelling, the influence upon the covering pipe can be reduced because the gap of a certain size, which serves as the space capable of absorbing swelling, is necessarily formed between the inner surface of the tubular pipe and the outer surface of the sleeve and between the inner surface of the sleeve and the surface of the long-lived type neutron absorber. In the case where the sleeve is made of stainless steel, the long-lived type neutron absorber will generate swelling. However, the formed gap serves as the swelling absorbing space, causing the generation of stress in the covering pipe can be significantly reduced. In the case where the oxide film is formed on the surface of the long-lived type neutron absorber, the oxide film serves as a barrier against the hydrogen absorption. Therefore, the generation of swelling in the long-lived type neutron absorber can be prevented. In the case where both the above-described long-lived type neutron absorber and the boron compound are enclosed in the covering pipe, the Zr particles or the Hf powder serving as the hydrogen getter is mixed with the boron compound powder placed in a region which is exposed to a relatively large amount of neutrons. As a result, tritium generated when the boron compound is exposed to neutrons is caused to be absorbed by the Zr particles or the Hf powder. Therefore, the discharge of tritium can be prevented, causing swelling of the long-lived type neutron absorber to be prevented. Although the boron compound generates swelling due to He gas generated due to the reaction of the boron compound with neutrons, the generation of stress acting on the covering pipe can be prevented by adjusting the density of enclosing the boron compound. Another method can be employed in which the boron compound present in a region which is exposed to a relatively large amount of neutrons is enclosed in the inner pipe made of Zr, Hf or stainless steel. In a case where the inner pipe is made of Zr, it is able to serve as the hydrogen getter or a stress relaxer. In the case where it is made of Hf, it possesses a functions as the hydrogen getter and the stress relaxer and as well as a function as the long-lived type neutron absorber. The inner pipe made of stainless steel serves as the stress relaxer realized by forming the gap. However, it is preferable in this case that Zr particles or Hf powder be mixed with the boron compound. Also in this case, the diffusion of tritium discharged from the boron compound into the long-lived type neutron absorber can be substantially prevented. The swelling of the boron compound generated due to the He gas can be absorbed by the inner pipe so that stress generated in the covered pipe is significantly reduced. Therefore, according to the present invention, the soundness of the covered pipe can significantly be improved. Other and further objects, features and advantages of the invention will be appear more fully from the following description.