JP6013896B2 - Control rod - Google Patents

Description

  The present invention relates to a control rod, and more particularly to a control rod suitable for application to a boiling water reactor.

  The structure of a conventional control rod used in a boiling water reactor and the environment in which it is installed will be described. A boiling water reactor has a reactor core loaded with a plurality of fuel assemblies in a reactor pressure vessel. Uranium 235 contained in the nuclear fuel material present in these fuel assemblies absorbs neutrons, causes nuclear fission, and generates heat. Reactor water (cooling water) supplied to the core is heated and boiled by the heat, and a part thereof becomes steam. In the core, a chain reaction occurs in which neutrons newly generated by the above fission split other uranium 235.

  In order to control the amount of chain reaction of fission, a control rod that houses a neutron absorber is used. Among these, the control rods normally used in boiling water reactors have a cross-shaped cross section, and are in the gap (saturated water region) formed between the channel boxes of the four fuel assemblies. Inserted into. One control rod is provided for each cell constituted by four fuel assemblies. A control rod guide tube is disposed below the four fuel assemblies for each cell in the reactor pressure vessel. The control rod uses each channel box of the four fuel assemblies in the cell and the control rod guide tube as a guide member. The lower end of the control rod is connected to the control rod drive mechanism, and the control rod is inserted into the core by a drive operation of the control rod drive mechanism and pulled out from the core. The control rod is an important device used for reactivity control and output distribution adjustment.

  The structure of a conventional control rod used in a boiling water reactor will be briefly described. This control rod has four blades extending in four directions from a tie rod located on the central axis of the tie rod, with a handle joined to the upper end of the tie rod and a drop speed limiter joined to the lower end of the tie rod. Each blade has a sheath having a U-shaped cross section attached to a tie rod, and a plurality of hafnium members, which are neutron absorbers, are arranged in the sheath (for example, Japanese Patent Laid-Open No. 9-61576). No. and JP 2009-41994).

  In the control rod using a hafnium member, four hafnium elliptic tubes having an elliptical cross section and an axial length that is approximately half the axial length of the tie rod are disposed in the sheath. Of the four hafnium elliptic tubes, the upper ends of the two hafnium elliptic tubes are respectively attached to two tongues provided at the lower end of the handle by fixing pins. The lower ends of the remaining two hafnium elliptic tubes are respectively attached to the two tongues provided at the upper end of the lower support member (or the drop speed limiter) by fixing pins. The two hafnium elliptic tubes attached to the lower support member (or the drop speed limiter) are arranged below the two hafnium elliptic tubes attached to the handle.

  In recent years, it has been reported that micro cracks are generated on the sheath surface in a control rod used in a boiling water reactor. This crack is considered to be radiation induced stress corrosion cracking (IASCC) that occurs when three environmental factors of stress, corrosion, and radiation irradiation overlap.

  In order to suppress IASCC in the sheath of the control rod, a gap formed between a neutron absorbing member formed by inserting a plurality of neutron absorbing rods into a diluted alloy plate containing hafnium and the sheath is maintained at a predetermined width. A control rod is described in JP-A-1-284966. The gap is maintained by bringing the inner surfaces of the plurality of depressions formed in the sheath into contact with the surface of the neutron absorbing member. Techniques for maintaining a gap between a neutron absorbing member made of a hafnium plate and a sheath at a predetermined width are also described in Japanese Patent Application Laid-Open Nos. 60-60585 and 4-289490.

  Japanese Patent Application Laid-Open No. 2011-208959 discloses that in the control rod, impurities are accumulated in gaps formed between both ends of the fixing pin for attaching the hafnium elliptic tube to the tongue-like portion and the sheath facing each of them. In addition, it is described that a crevice corrosion environment is formed in the gap and irradiation-induced stress corrosion cracking may occur in the sheath near the fixing pin. In order to suppress the occurrence of this radiation-induced stress corrosion cracking, in the control rod described in Japanese Patent Application Laid-Open No. 2011-208959, a cooling water passage having an inner surface open to the sheath and crossing the end of the fixing pin Are formed at both ends of the fixing pin, and this coolant passage is arranged in the axial direction of the tie rod.

JP-A-9-61576 JP 2009-41994 Japanese Patent Laid-Open No. 1-28496 Japanese Patent Laid-Open No. 60-60585 JP-A-4-289490 JP2011-208959A

  In Japanese Patent Application Laid-Open No. 2011-208959, a coolant passage arranged in the axial direction of the tie rod is formed at an end portion of a fixing pin facing the sheath, and cooling water flows in the cooling water passage in the sheath. Therefore, accumulation of impurities between the end portion of the fixing pin and the sheath inner surface is suppressed. For this reason, it is possible to suppress the occurrence of irradiation-induced stress corrosion cracking in the sheath near the fixing pin.

  However, since the fixing pin is not fixed to the hafnium elliptic tube and the tongue, the fixing pin may rotate due to the flow of the cooling water flowing in the sheath during the operation of the boiling water reactor. . For example, when the cooling water passage formed at the end of the fixing pin faces the direction perpendicular to the axial direction of the tie rod by the rotation of the fixing pin, the lower side is present on both sides of the cooling water passage. Since the convex portion of the fixing pin hinders the flow of the cooling water trying to rise between the fixing pin and the sheath, the stagnation of the cooling water is generated below the convex portion. This stagnation may cause irradiation-induced stress corrosion cracking in the sheath near the end of the fixing pin.

  The objective of this invention is providing the control rod which can suppress generation | occurrence | production of the stagnation of a cooling water, even when the fixing pin which supports a hafnium member rotates.

A feature of the present invention that achieves the above object is that the hafnium member in the sheath is attached to a holding portion provided at the lower end portion of the handle by a pin member whose cross section perpendicular to the axis is a circle, and is directed toward the inner surface of the sheath. projections projecting Te are formed on both ends of the pin member, and the inner surface of the protrusion and the sheath has a point contact, towards the root of the protruding portion from the position of the contact this point, the cross-sectional of projections The area is increased, and a gap surrounding the point contact position is formed between the surface of the protrusion and the inner surface of the sheath.

Since the respective protrusions formed at both ends of the pin member and the inner surface of the sheath are in point contact, even if the pin member rotates due to the flow of cooling water in the sheath during operation of the nuclear reactor, Cooling water flows through a gap between the surface of the protrusion and the inner surface of the sheath formed so as to surround the point contact position . Therefore, within the sheath, stagnation of the cooling water can be suppressed from being generated by the projection portion, of the inner surface of the sheath, it is possible to suppress the occurrence of irradiation assisted stress corrosion cracking in the vicinity of the protrusion.

  According to the present invention, even when the fixing pin that supports the hafnium member rotates in the control rod of the boiling water reactor, the generation of stagnation of cooling water can be suppressed, and the boiling irradiation induced stress generated in the sheath Corrosion cracking can be suppressed.

It is a side view of the control rod of Example 1 which is one suitable Example of this invention. It is II-II sectional drawing of FIG. FIG. 3 is a sectional view taken along line III-III in FIG. 1. It is explanatory drawing which shows the flow state of the cooling water in the edge part of the fixing pin shown in FIG. It is a characteristic view which shows the neutron irradiation amount distribution of the axial direction in the sheath of a control rod. It is explanatory drawing which shows the presence or absence of the generation | occurrence | production of the crack in the sheath simulation test piece obtained in the crevice corrosion test using a sheath simulation test piece. It is explanatory drawing which shows an example of the crevice corrosion test using the test body which simulated the sheath and the pin for fixing. It is a characteristic view which shows the relationship between the depth of the clearance gap used as the corrosion suppression range, and a clearance width based on the result of the clearance corrosion test using a sheath simulation test piece. It is an enlarged view of the protrusion part formed in the edge part of the pin for fixation created reflecting the result of the crevice corrosion test. It is a longitudinal cross-sectional view of the fixing pin vicinity in the control rod of Example 2 which is another suitable Example of this invention. It is an enlarged view of the projection part formed in the edge part of the fixing pin shown in FIG.

  Inventors examined the control rod described in Unexamined-Japanese-Patent No. 2011-208959. As a result, as described above, the cooling pin flowing through the sheath during the operation of the boiling water reactor rotates the fixing pin that supports the hafnium member and is formed at the end of the fixing pin. There is a possibility that the water passage is oriented in a direction perpendicular to the axial direction of the tie rod, and there is a risk of stagnation of the cooling water below the convex portion of the fixing pin that exists on both sides of the cooling water passage and becomes the lower side. It has been found. Cooling water stagnation may cause irradiation-induced stress corrosion cracking in the sheath near the end of the fixing pin.

  For this reason, the inventors examined the structure of a control rod that does not cause stagnation of cooling water below the fixing pin even if the fixing pin rotates during operation of the nuclear reactor. The result of this examination will be described below.

  Irradiation-induced stress corrosion cracking can be prevented by removing any one factor of radiation exposure, stress and gap. However, since the control rod is inserted into the core, it is difficult to remove the radiation irradiation environment. In particular, as shown in FIG. 5, a sheath of a control rod that is inserted into the core during operation of the reactor to control the reactor output is a hafnium member (for example, a hafnium elliptic tube) that is a neutron absorbing member disposed in the sheath. The cumulative amount of neutron irradiation during the operation of the reactor is increased at the upper end of (). This is because the period in which the range is located in the central portion in the axial direction of the core, which is a region where the neutron flux density is high, is long, and the time when the core is pulled out from the core is the latest. Most of the many control rods inserted into the core when starting up the nuclear reactor have been fully extracted from the core at the time when the temperature raising and pressurizing process is completed.

  The crevice corrosion environment is a position where the neutron flux density of the control rod is high, and impurities and corrosion products contained in the cooling water in the reactor accumulate at specific positions on the inner surface of the sheath, forming a gap in the sheath. The inventors believe that this causes corrosion in the sheath. In particular, the width of the gap formed between the sheath inner surface and the end face of the fixing pin is narrower than the width of the gap formed between the sheath inner surface and the hafnium elliptic tube outer surface. Is inserted into the core, the corrosion products contained in the cooling water flowing in the sheath easily accumulate in the latter gap.

  Therefore, the inventors considered that it is desirable to ensure a gap with a predetermined width between the sheath and the sheath on both end faces of the fixing pin even during the operation of the nuclear reactor. By forming a gap with a predetermined width, the corrosive environment between the end face of the fixing pin and the sheath can be improved, and the occurrence of cracks in the sheath can be suppressed. Details will be described below.

  As shown in FIG. 6 (a), the inventors use a simulated test specimen in which a gap is formed between the two stainless steel plates 13 by simulating the gap formed between the hafnium elliptic tube and the sheath. A crevice corrosion test was conducted. In the crevice corrosion test, each simulated specimen was prepared by changing the width of the gap between the opposing stainless steel plates 13 into four types of 0 mm, 0.1 mm, 0.2 mm, and 0.3 mm. It was performed using. Two stainless steel plates 13 were connected by bolts that are two fasteners 14, and the width of the gap in each simulated test specimen was adjusted by adjusting the fasteners 14. The crevice corrosion test was performed under the conditions of both irradiating and not irradiating each simulated specimen having two stainless steel plates 13 each having a gap width with radiation γ rays. Furthermore, in the crevice corrosion test, the immersion time in the water to which the corrosion product simulation material is added is 500 hours for each of the simulated test body irradiated with γ rays and the simulated test body not irradiated with γ rays having the stainless steel plate 13. Each case was changed to 1000 hours and 1500 hours, and each case was also changed in which the immersion time in water without adding the corrosion product simulation material was changed to 500 hours, 1000 hours and 1500 hours. The temperature of water is 288 ° C.

  The result of this crevice corrosion test is shown in FIG. In FIG. 6B, the case where no crack is generated on the stainless steel plate 13 of the simulated test specimen is indicated by “x”, and the case where the stainless steel plate 13 of the simulated test specimen is cracked is indicated by •, ■ and □. ● represents a case in which a corrosion product is added to water in which the simulated specimen is immersed, and the simulated specimen is irradiated with γ rays at 20000 Gy / hr. (2) is a case where a corrosion product is added to the water in which the simulated specimen is immersed, and the simulated specimen is not irradiated with γ rays. □ is a case where no corrosion products are added to the water in which the simulated specimen is immersed and no γ rays are irradiated to the simulated specimen. Based on the result of the crevice corrosion test shown in FIG. 6 (b), the inventors do not generate cracks in the sheath when the width G of the gap formed between the fixing pin and the sheath is wider than 0.2 mm. It was confirmed. Considering the insertability of the control rod between the fuel assemblies loaded in the core, the width G of the gap must be 1.0 mm or less. Therefore, the width G of the gap formed between the fixing pin and the sheath needs to satisfy 0.2 mm <G ≦ 1.0 mm.

  The configuration satisfying 0.2 mm <G ≦ 1.0 mm is a configuration in which a protrusion toward the inner surface of the sheath is formed on the end surface of the fixing pin, and a part of the sheath is recessed inward toward the end surface of the fixing pin. Either a configuration or a configuration in which a member having a width G that satisfies the above conditions is inserted between the sheath and the end face of the fixing pin can be employed. However, even if any of the above-described configurations is employed, the sheath 4 or a member inserted between the fixing pin 6 and the sheath 4 comes into contact with the end surface of the fixing pin 6. When such contact occurs, there is a possibility that the condition of 0.2 mm <G may not be satisfied in the vicinity of the contact point, and there is a concern about the occurrence of crevice corrosion.

  Therefore, the inventors further conducted a crevice corrosion test using a simulated specimen that simulated a configuration in which the sheath and the circular member are in contact with each other by sandwiching a round bar between two stainless steel plates. In the crevice corrosion test, as shown in FIG. 7A, a round bar 15 (simulating a fixing pin) is sandwiched between opposing stainless steel plates 13 (simulating a sheath) and brought into contact with each stainless steel plate 13; A simulated test body formed by fastening the stainless steel plate 13 with the fastener 14 was used. In this crevice corrosion test, a round bar 15 having a different radius is used, and a simulated test body in which a round bar 15 having a different radius and a stainless steel plate 13 are brought into contact with each other is immersed in high-temperature water at 280 ° C. and irradiated with γ rays. Carried out. As a result of this crevice corrosion test, it was found that crevice corrosion does not occur when the radius r of the round bar 15 is 2.5 mm. That is, it was experimentally confirmed that even when the end face of the fixing pin is in contact with the sheath, there is a condition that the crevice corrosion does not occur in the sheath. Such experimental results show that the distance d of the depth of the gap from the contact point between the stainless steel plate 13 and the side surface of the round bar 15 (the contact point between the sheath and the end face of the fixing pin) (see FIG. 7B), It is shown that crevice corrosion can be prevented even near the contact point by optimizing the gap width h (see FIG. 7B) at the distance d of the gap depth. The depth d of the gap is a perpendicular to the plate surface of the stainless steel plate 13 and the position of the intersection of the perpendicular passing through the axis of the round bar 15 and the plate surface of the stainless steel plate 13 (in FIG. 7B, the round bar 15). And the contact point between the stainless steel plate and the stainless steel plate 13 along the plate surface. The gap width h means the distance between the side surface of the round bar 15 and the plate surface of the stainless steel plate 13 in the direction perpendicular to the plate surface of the stainless steel plate 13.

  The inventors have a problem with the control rod described in Japanese Patent Application Laid-Open No. 2011-208959, that is, a fixing pin having a cooling water passage formed at an end thereof rotates during operation of the nuclear reactor and moves in one direction. The structure which can eliminate the stagnation of the cooling water generated below the protrusion formed at the end of the fixing pin that defines the extending cooling water passage was further examined.

  Therefore, the inventors changed the radius r of the round bar 15 simulating the protrusion formed at the end of the fixing pin into three test bodies (1.0 mm, 2.5 mm, and 3.0 mm) ( (See FIG. 7), and crevice corrosion tests were conducted using the respective specimens. The curved surface on the side surface of the round bar 15 represents the shape of the protrusion formed on the end surface of the fixing pin. The inventors organize the results obtained in the crevice corrosion test by the relationship between the gap depth d and the gap width h in the protrusion and the inner surface of the sheath, and are shown in FIG. In the crevice corrosion test, when a round bar having a radius r of 2.5 mm was used, no crevice corrosion was observed in the stainless steel plate 13. From this result, crevice corrosion can be suppressed in a range where the gap width is larger than the correlation between the gap width h and the gap depth d in a round bar having a radius r of 2.5 mm. That is, in the range where the gap depth d is larger than 1.0 mm, the gap width h needs to be larger than 0.2 mm, and in the range where the gap depth d is smaller than 1.0 mm, the gap larger than the circular arc having the radius r of 2.5 mm. It has been found that the width h is necessary.

  Thus, for example, as shown in FIG. 9, if a configuration in which a hafnium member is supported by a fixing pin 9U having a cone-shaped projection 10 having an elevation angle θ at the end is applied to the control rod, the elevation angle θ is 11. In the case of forming the cone-shaped protrusion 10 having an angle of 3 ° or more, the gap width h can satisfy 0 mm <h ≦ 0.2 mm in a region where the depth d of the gap is 1.0 mm or less. The elevation angle θ is an angle formed by the inner surface of the sheath 3 with which the cone-shaped protrusion 10 contacts and the surface of the cone-shaped protrusion 10. Although the elevation angle θ can be less than 90 °, it is considered that 45 ° or less is appropriate in consideration of size and workability. The cone-shaped protrusion 10 is in contact with the sheath 3 at only one point at the tip. The maximum value of the gap width h is the width G of the gap formed between the fixing pin and the sheath.

Based on the above examination, the inventors were able to obtain the following findings (1) to (3).
(1) An axial center 18 (see FIGS. 9 and 11) of a projection formed at the end of the fixing pin, and the axial center 18 existing at a position where the projection contacts the inner surface of the sheath In a region outside 1.0 mm along the inner surface of the sheath from the position of the intersection C (see FIGS. 9 and 11) intersecting the inner surface, the distance h satisfies 0.2 mm <h ≦ 1.0 mm. Generation | occurrence | production of crevice corrosion in the inner surface of can be suppressed.
(2) The formation of the crevice corrosion can be suppressed by forming on the end face of the fixing pin a cone-shaped protrusion having an elevation angle θ of 11.3 degrees or more that contacts the sheath.
(3) By forming a hemispherical projection having a radius of 2.5 mm or less in contact with the sheath on the end face of the fixing pin, the occurrence of crevice corrosion can be suppressed.

  Each configuration of (2) and (3) is a specific example of the configuration of (1). By adopting the configurations of (2) and (3), a gap is formed between the surface of the protrusion and the inner surface of the sheath even in a region within 1.0 mm along the inner surface of the sheath from the position of the intersection C. And since a cooling water flows in this clearance gap, possibility that a crevice corrosion will arise in a sheath inner surface can be reduced rather than the structure of (1).

  Examples of the present invention based on the above examination results will be described below.

  A control rod according to Embodiment 1, which is a preferred embodiment of the present invention, will be described with reference to FIGS. The control rod 1 of the present embodiment is a control rod used in a boiling water reactor (BWR).

  The control rod 1 has a cross shape in cross section, and has four blades 2 extending in four directions from a tie rod 4 arranged at the axial center. The handle 5 is attached to the upper end portion of the tie rod 4, and the lower support member 6 is attached to the lower end portion of the tie rod 4. The lower support member 6 is a lower support plate or a drop speed limiter.

  Each blade 2 includes a sheath 3 (see FIG. 2) having a U-shaped cross section, and hafnium members 7U and 7L which are flat tubes, for example, elliptical tubes (see FIG. 1). Each side surface of the hafnium members 7U and 7L facing the inner surface of the sheath 3 is a flat surface. The sheath 3 is made of stainless steel (SUS304, SUS316L, etc.). The upper end of the sheath 3 is welded to the handle 5, and the lower end of the sheath 3 is welded to the lower support member 6. A plurality of tabs (projections) 16 are formed at both ends of the U-shape of the sheath 3 at predetermined intervals in the axial direction. These tabs 16 are portions of the sheath 3 but are portions protruding toward the tie rod 4 side. These tabs 16 are joined to the tie rod 4 by welding. The above-described joining of the sheath 3, the tie rod 4, the handle 5, and the lower support member 6 is performed by, for example, laser welding.

  Two hafnium members (first hafnium member) 7U and two hafnium members (second hafnium member) 7L are arranged in a space formed in the sheath 3 of one blade 2. The hafnium member 7U is located above the hafnium member 7L, and the axial lengths thereof are the same. The hafnium member 7U is attached to a tongue-like portion (first holding portion) 8U formed at the lower end portion of the handle 5 with a fixing pin (pin member) 9U. The hafnium member 7L is attached to a tongue-like portion (second holding portion) 8L formed at the upper end portion of the lower support member 6 with a fixing pin 9L. That is, the hafnium member 7U has an upper end attached to the handle 5 and a hafnium member 7L attached to the lower support member 6. These hafnium members are neutron absorbing members. A gap (not shown) is formed between the lower end of the hafnium member 7U and the upper end of the hafnium member 7L so that even if the hafnium members 7U and 7L are thermally expanded during the operation of the BWR, the hafnium members do not contact each other. Has been.

  The control rod 1 is disposed in the reactor pressure vessel of the BWR, and is inserted into and removed from a core loaded with a plurality of fuel assemblies by a control rod drive mechanism (not shown) in order to control the reactor power. The control rod 1 is connected to a control rod drive mechanism provided at the bottom of the reactor pressure vessel by a connector 17 provided at the lower end of the lower support member 6. The control rod drive mechanism performs an operation of inserting the control rod 1 into the core and an operation of extracting the control rod 1 from the core. Cooling water (coolant) flowing in the reactor pressure vessel flows into the sheath 3 from a part of the openings 8 formed in the sheath 3 and a plurality of openings 11L formed in the lowermost end portion of the sheath 3, and hafnium. The members 7U and 7L are cooled and flow out of the sheath 6 from the other openings 8 (particularly, the opening 8 located at the upper end) and the plurality of openings 11U formed at the uppermost end of the sheath 3. The cooling water flowing into the sheath 3 flows into the hafnium member 7U through the small-diameter opening 9 provided in the hafnium member 7U, and passes through the small-diameter opening 9 formed in the hafnium member 7L. It flows into 7L. As described above, the cooling water flows into the hafnium members 7U and 7L, whereby the cooling effect of these hafnium members is increased.

  A plurality of, for example, four pyramidal projections (for example, quadrangular pyramidal projections) 10 are formed at the respective ends of the fixing pins 9U and 9L used in the control rod 1 (FIG. 4). (See (a)). Each of the cone-shaped protrusions 10 is formed on a plane 19 that is formed at the end of each of the fixing pins 9U and 9L and is perpendicular to the axis of the pins. Each cone-shaped protrusion 10 has a point at the tip that contacts the inner surface of the sheath 3 and an elevation angle θ of, for example, 30 °. In the present embodiment, the gap G (maximum value of the gap width h) between the flat surface 19 and the inner surface of the sheath 3 is, for example, 1.0 mm. Each cone-shaped protrusion 10 has a cross-sectional area that increases from the contact point between the cone-shaped protrusion 10 and the inner surface of the sheath 3 toward the root of the cone-shaped protrusion 10, that is, the plane 19.

The fixing pin 9U is a support member for suspending the hafnium member 7U from the tongue portion 8U, and supports the entire weight (about 15 kg) of the hafnium member 7U. When the control rod 1 is shaken in the horizontal direction due to an earthquake or the like, the entire weight of the hafnium member 7U is added to the tip of the cone-shaped protrusion 10 of the fixing pin 9U that is pressed against the inner surface of the sheath 3. Since the tensile strength of the austenitic stainless steel used for the fixing pin 9U including the cone-shaped protrusion 10 is about 590 N / mm ² , the hafnium member 7U is formed on the cone-shaped protrusion 10 as described above. When a total weight of about 15 kg is applied, if two or more cone-shaped projections 10 are formed at the end of the fixing pin 9U, there is a margin in the mechanical strength of the cone-shaped projections 10.

  According to the present embodiment, the cone-shaped protrusion 10 protrudes from the flat surfaces 19 of the fixing pins 9U and 9L toward the inner surface of the sheath 3, and the tip of the cone-shaped protrusion 10 contacts the inner surface of the sheath 3. Therefore, a gap of 1.0 mm is formed between the inner surface of the sheath 3 and the flat surface 19. For this reason, the cooling water flowing in the sheath 3 flows in a 1.0 mm gap formed between the inner surface of the sheath 3 and the flat surface 19, and impurities contained in the cooling water flow between the inner surface of the sheath 3 and the flat surface 19. Accumulation between them is prevented. Thereby, it can suppress that an irradiation induction type stress corrosion crack arises in the part of the sheath 3 facing each of the fixing pins 9U and 9L.

  In the present embodiment, the angle (elevation angle θ) formed by the inner surface of the sheath 3 and the surface of each cone-shaped projection 10 is 30 °, so that the angle near the inner surface of the sheath 3 facing the cone-shaped projection 10 is 30. Generation of crevice corrosion is suppressed.

  Further, in this embodiment, the fixing pins 9U and 9L form the cone-shaped projections 10 at both ends, respectively, so that the contact point (intersection C) between the cone-shaped projection 10 and the inner surface of the sheath 3 Is formed between the inner surface of the sheath 3 and the surface of the cone-shaped projection 10. Even when each of the fixing pins 9U and 9L is rotated by the flow of the cooling water in the sheath 3 during the operation of the nuclear reactor, the cooling water always flows between the inner surface of the sheath 3 and the surface of the cone-shaped protrusion 10. It flows in the gap formed between them. For this reason, in this embodiment, the problem that occurs in Japanese Patent Application Laid-Open No. 2011-208959, that is, when the cooling water passage formed at the end of the fixing pin faces in the direction perpendicular to the axial direction of the tie rod, The occurrence of stagnation of the cooling water under the convex portion located on the lower side of the convex portions defining the cooling water passage can be eliminated. In this embodiment, the cooling water always flows in the gap formed between the inner surface of the sheath 3 and the surface of the cone-shaped projection 10, and the cones are rotated even if each of the fixing pins 9U and 9L rotates. The stagnation of the cooling water does not occur below the body-shaped protrusion 10. For this reason, in this embodiment, irradiation-induced stress corrosion cracking that occurs in the sheath near the end of the fixing pin due to the stagnation of the cooling water in JP 2011-208959A does not occur.

  The flow state of the cooling water between the fixing pin 9U (or 9L) and the inner surface of the sheath 3 in the vicinity of the cone-shaped protrusion 10 will be described with reference to FIGS. 4B and 4C. FIG. 4B and FIG. 4C show a state in which the cone-shaped protrusion 10 formed on the flat surface 19 at the end of the fixing pin 9U is viewed from the inner surface side of the sheath 3. As shown in FIG. 4A, four cone-shaped projections (square pyramidal projections) 10 are formed on the flat surface 19 formed at the end of the fixing pin 9U. Assume that the body-like protrusions 10 are arranged as shown in FIG. The cooling water flow indicated by the arrow rises between the adjacent cone-shaped projections 10 between the flat surface 19 and the inner surface of the sheath 3 and between the inner surface of the sheath 3 and the surfaces of the respective cone-shaped projections 10. It is assumed that the fixing pin 9U is rotated by 90 ° from the state of FIG. 4B due to the flow of cooling water in the sheath 3 during the operation of the nuclear reactor (see FIG. 4C). As in FIG. 4B, the cooling water flow indicated by the solid line arrows is between the adjacent cone-shaped projections 10 between the plane 19 and the inner surface of the sheath 3, and between the inner surface of the sheath 3 and each cone-shaped projection. It rises between the surfaces of the part 10. The cooling water flow that hits the cone-shaped protrusion 10 flows around the cone-shaped protrusion 10 as indicated by a broken line. In a state before the fixing pin 9U is rotated by 90 ° (FIG. 4B) and a state after the fixing pin 9U is rotated by 90 ° (FIG. 4C), the cooling water flow is changed between the inner surface of the sheath 3 and each of the inner surfaces of the sheath 3. Since it flows in the gap formed between the surfaces of the cone-shaped projections 10, the stagnation of the cooling water does not occur below the cone-shaped projections 10. Such a flow of the cooling water also occurs at the end of the fixing pin 9L.

  According to the present embodiment, the accumulation of impurities between the flat surface 19 formed at the end of each of the fixing pins 9U and 9L and the sheath 3, and the cone-shaped protruding portion when the fixing pin rotates. By preventing stagnation of the cooling water below 10, the occurrence of irradiation-induced stress corrosion cracking in the sheath 3 can be further suppressed.

  A control rod according to embodiment 2 which is another preferred embodiment of the present invention will be described with reference to FIGS. The control rod 1A of the present embodiment is a control rod used in a boiling water reactor (BWR).

  The control rod 1A has a configuration in which the cone-shaped protrusion 10 formed on the flat surfaces 19 of the fixing pins 9U and 9L in the control rod 1 is replaced with a hemispherical protrusion 10A. Other configurations of the control rod 1A are the same as those of the control rod 1.

  The fixing pins 9U and 9L used in the control rod 1A form a plane 19 perpendicular to the axis of the fixing pin at each end, and from each of these planes 19 toward the inner surface of the sheath 3. A plurality of hemispherical protrusions 10A are protruded. The radius r of each hemispherical protrusion 10A is 2.5 mm or less, for example, 2.0 mm. Each hemispherical protrusion 10A has a cross-sectional area that increases from the contact point between the hemispherical protrusion 10A and the inner surface of the sheath 3 toward the root of the hemispherical protrusion 10A, that is, the plane 19. The fixing pins 9U and 9L are made of austenitic stainless steel. Also in the present embodiment, a plurality of, for example, four are formed on the respective flat surfaces 19 at both ends of the fixing pins 9U and 9L. The intersection C between the perpendicular 18 (corresponding to the axis 18 of the cone-shaped projection 10 in Example 1) and the inner surface of the sheath 3 passing through the center of each hemispherical projection 10A and the inner surface of the sheath 3 is a base point. When the depth d of the gap is 1.0 mm or less, the gap width h between the inner surface of the sheath 3 and the surface of the hemispherical protrusion 10A in the direction perpendicular to the inner surface of the sheath 3 is 0 mm <h ≦ 0. In a region where 2 mm is satisfied and the gap depth d is greater than 1.0 mm, 0.2 mm <h ≦ 1.0 mm is satisfied. For this reason, also in the present embodiment, the occurrence of crevice corrosion near the inner surface of the sheath 3 facing the hemispherical protrusion 10A is suppressed.

  In the present embodiment, the fixing pins 9U and 9L form hemispherical projections 10A at both ends, respectively, and therefore surround the contact point (intersection C) between the hemispherical projection 10A and the inner surface of the sheath 3. A gap is formed between the inner surface of the sheath 3 and the surface of the hemispherical protrusion 10A. Even if each of the fixing pins 9U and 9L is rotated by the flow of the cooling water in the sheath 3 during the operation of the nuclear reactor, the cooling water always flows between the inner surface of the sheath 3 and the surface of the hemispherical projection 10A. It flows in the gap formed between them.

  In the present embodiment, each effect produced in the first embodiment can be obtained. Further, according to the present embodiment, the hemispherical protrusions 10A formed on the fixing pins 9U and 9L are less workable than the cone-shaped protrusions 10 in the first embodiment, but a large load is applied in the lateral direction. Even if it is a case, it is considered that there is little risk of penetrating the sheath.

  DESCRIPTION OF SYMBOLS 1,1A ... Control rod, 2 ... Blade, 3 ... Sheath, 4 ... Tie rod, 5 ... Handle, 6 ... Lower support member, 7U, 7L ... Hafnium member, 9U, 9L ... Fixing pin, 10 ... Conical projection Part, 10A ... hemispherical projection, 19 ... plane.

Claims (5)

A tie rod, a handle attached to the upper end of the tie rod, a lower support member attached to the lower end of the tie rod, an upper end attached to the handle, a lower end attached to the lower support member, and Four sheaths attached to the tie rod and having a U-shaped cross section and extending in four directions from the tie rod, and a hafnium member disposed in each sheath,
The hafnium member, the first holding portion provided at the lower end of the handle includes a first hafnium member cross-section perpendicular to the axis is attached by a first pin member is circular,
First protrusions projecting toward the inner surface of the sheath are formed on both ends of the first pin member,
The first protrusion and the inner surface of the sheath are in point contact, and the cross-sectional area of the first protrusion increases from the position of the first point contact toward the root of the first protrusion. And
A control rod characterized in that a first gap surrounding the position of the first point contact is formed between the surface of the first protrusion and the inner surface of the sheath. The first protrusion in a direction perpendicular to the inner surface of the sheath in a region outside 1.0 mm along the inner surface of the sheath from the position of the first point contact between the inner surface of the sheath and the first pin member. The control rod according to claim 1, wherein a distance h between the surface of the portion and the inner surface of the sheath satisfies 0.2 mm <h ≦ 1.0 mm.   The first projection portion is a cone-shaped projection portion in which an angle θ formed by an inner surface of the sheath and a surface of the first projection portion satisfies 11.3 ° ≦ θ <90 °. Control rod as described. Wherein at both end portions of the pin member is formed a plane perpendicular respectively to the axis of said pin member, said first protrusion protrudes toward the inner surface of the sheath from the plane, the said plane control rod according width G of the second gap to claim 1 or 2 which satisfies the 0.2 mm <G ≦ 1.0 mm formed between the inner surface of the sheath. The hafnium member is attached to a second holding portion provided at an upper end portion of the lower support member by a second pin member having a circular cross section perpendicular to the axis, and is disposed below the first hafnium member. A second hafnium member,
Second projection portion projecting toward the inner surface of the sheath are formed on both ends of the second pin member,
The second protrusion and the inner surface of the sheath are in point contact, and the cross-sectional area of the second protrusion increases from the position of the second point contact toward the base of the second protrusion. And
The control rod according to claim 1, wherein a third gap surrounding the position of the second point contact is formed between the surface of the second protrusion and the inner surface of the sheath.

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