Patent Number: 
Section: description

The present invention will be described in detail in conjunction with what is presently considered as preferred or typical embodiments thereof by reference to the drawings. In the following description, like reference characters designate like or corresponding parts throughout the several views. Further, as is apparent from the description described below, the present invention is not limited to those embodiments but various modifications and equivalents can be resorted to. FIG. 1 shows a control rod according to a first embodiment of the present invention. At this juncture, it should be mentioned that the structure of the control rod cluster itself which is constituted by the control rods according to the invention as well as that of the fuel assembly into or from which the control rod cluster is inserted/withdrawn may be same as those known heretofore. Accordingly, for the detail of these structures, reference may have to be made to FIGS. 3 to 5 as occasion requires. Referring to FIG. 1, the control rod according to the first embodiment of the invention includes a cladding tube 11 formed of stainless steel and hermetically closed at both sides thereof by a top end plug 12 and a bottom end plug 13. Accommodated within the cladding tube 11 is a rod-like neutron absorber 14 according to the instant embodiment of the invention. The neutron absorber 14 is formed of a neutron absorbing material such as an Agxe2x80x94Inxe2x80x94Cd (silver-indium-cadmium) alloy or boron carbide or the like and pushed or pressed downwardly onto the bottom plug 13 at a bottom end surface by means of a hold-down spring 15 which is disposed on atop end face within the cladding tube 11. Further, the neutron absorber 14 includes a reduced-diameter portion 14a located on the side to the bottom end plug and an ordinary diameter portion 14b located above the reduced-diameter portion, wherein a sleeve 16 is disposed within an annular space defined between an outer peripheral surface of the reduced-diameter portion 14a and an inner peripheral surface of the cladding tube 11. The sleeve 16 may be formed of a same material as that of the cladding tube 11 or a material with a thermal expansion coefficient (or rate of thermal expansion) smaller than that of the cladding tube 11 and which has strength that is high enough to withstand a force which may be applied to the sleeve 16 due to expansion of the reduced-diameter portion 14a in the radial direction. Dimensional relations among the sleeve 16, the neutron absorber 14 and the reduced-diameter portion 14a are selectively determined to satisfy the conditions that LA greater than LB and that dA less than dB, where LA represents the axial length (length in the axial direction) of the sleeve 16, LB represents the axial length of the reduced-diameter portion 14a, dA represents the outer diameter of the reduced-diameter portion 14a, and dB represents the outer diameter of the other ordinary portion 14b of the neutron absorber 14. Parenthetically, the axial length LB of the reduced-diameter portion 14a may be selected to be equal to the axial length L of the reduced-diameter portion 54a of the conventional neutron absorber heretofore so long as the control rod according to the instant embodiment of the invention is of same type as the conventional one. As mentioned above, the sleeve 16 may be formed of a same material as that of the cladding tube 11 or a material having a smaller thermal expansion coefficient than the cladding tube 11. In this conjunction, it is preferred to select the material for forming the cladding tube 11 from of austenite type stainless steel (e.g. SUS 304, SUS 316, SUS 347, SUS 348 and so forth which are employed for forming the cladding tube) and anti-corrosion/heat-resistant nickel-based alloys such as Inconel 718 (registered trade name) and the like. Unless the sleeve 16 is formed of the same material as that of the cladding tube 11, material for the sleeve 16 should be so selected that the conditions mentioned below can be satisfied. 1) In respect to the thermal expansion, the sleeve 16 should not be brought into contact with the inner peripheral surface of the cladding tube 11 nor exert internal pressure load to the cladding tube 11 due to excessively large thermal expansion of the sleeve 16 in the high temperature operating state. 2) With regard to the yield strength, the sleeve should have a strength equivalent to or greater than that of the cladding tube 11 so as to be capable of withstanding a load of radial direction (internal pressure) as applied. Additionally, in respect to the load applied in the axial direction, the sleeve should exhibit a buckling strength which can withstand the load applied upon stepwise driving of the control rod cluster. 3) Concerning the thermal conduction, the material for the sleeve should have a thermal conductivity which allows the temperature of a center portion of the neutron absorber to remain lower than the melting point of the neutron absorber even with the temperature rise due to xcex3-induced heat generation in the reduced-diameter portion 14a of the absorber upon irradiation. Parenthetically, in the case of a low melting point Agxe2x80x94Inxe2x80x94Cd (silver-indium-cadmium) alloy, the temperature of the center portion of the neutron absorber should not exceed ca. 800xc2x0 C.). 4) Concerning the crack yield strength, the cracking strain of the sleeve after the irradiation should be equivalent to or more than the cladding tube 11. With the phrase xe2x80x9ccrack strainxe2x80x9d, it is intended to mean such a strain at which initiation of fracture can be observed in a cylindrical vessel subjected to an internal pressure. In practice, crack strain is conventionally used on the basis of experimentally obtained knowledges for indicating a strain of magnitude smaller than the fracture strain (elongation) and the uniform strain in conventional tensile tests. Concerning the dimensions of the sleeve 16, the diameter (thickness) thereof is determined in combination with the selection of the material for satisfying the condition imposed in respect to the strength as mentioned in paragraph 2 above. In this conjunction, the sleeve is so designed as to meet the conditions mentioned below. 1) In respect to the temperature at a center portion of the neutron absorber 14, the sleeve is so designed that this temperature can remain lower than the melting point of the absorber through thermal conduction even with the heat generation in the reduced-diameter portion 14a, the sleeve 16 and the cladding tube 11 due to the xcex3-radiation. By way of example, in the case of the Agxe2x80x94Inxe2x80x94Cd alloy which has a relatively low melting point, the temperature at the center portion of the absorber must not exceed ca. 8OOxe2x80x2C. 2) In view of realization of an extended service life of the cladding tube in the wholesome state, the time taken for the internal pressure applied to the cladding tube 11 to make appearance after the start of the irradiation is at least longer than the corresponding time in the conventional control rods. More specifically, so long as the sum of clearance between the outer diameter of the reduced-diameter portion 14a and the inner diameter of the sleeve 16 and clearance between the outer diameter of the sleeve 16 and the inner diameter of the cladding tube 11 is same as the clearance between the outer diameter of the reduced-diameter portion 54a and the inner diameter of the cladding tube 51 in the conventional cladding tube shown in FIG. 6, it is expected that the service life of the control rod can be extended for a time period which corresponds to the time taken for the reduced-diameter portion 54a to expand in the axial direction under the irradiation. Thus, in the control rod according to the instant embodiment of the invention, the outer diameter dA of the reduced-diameter portion 14a of the neutron absorber is further reduced when compared with the diameter d, of the reduced-diameter portion 54a of the neutron absorber 54 in the conventional control rod on the condition that the neutron absorbing capability can be sustained within a tolerance range, while the thickness of the sleeve 16 is increased by an amount corresponding to the difference between the diameters dA and (d1 mentioned above (i.e., d1xe2x88x92dA). 3) The clearance between the outer diameter of the reduced-diameter portion 14a and the inner diameter of the sleeve 16 as well as the clearance between the outer diameter of the sleeve 16 and the inner diameter of the cladding tube 11 can be set to appropriate values, respectively, which may be determined by taking into consideration the assemblability and manufacturability of the control rod. In that case, these clearances should be so determined that smooth insertion can be ensured without incurring interference even when tolerances imposed on the above-mentioned outer diameters and inner diameters in combination are most severe. In practical applications, the clearances may be set to values obtained by adding ca. 0.05 mm to the differences between the aforementioned outer diameters and the inner diameters, respectively, for the most severe tolerances while taking into account bend of the sleeve 16. Accordingly, when the control rod according to the present invention and a conventional one are of the same size, at least the condition that dB=d0 (see FIGS. 1 and 6) holds true. However, because the diameter of the reduced-diameter portion 14a of the neutron absorber in the control rod according to the invention is further reduced down to the limit at which the neutron absorbing capability can be sustained, the relation between the diameter dA of the reduced-diameter portion 14a of the neutron absorber in the control rod according to the invention and the corresponding diameter d1 of the reduced-diameter portion in the conventional control rod can naturally be represented by dA less than d1 (see FIGS. 1 and 6). Thus, the diameter reduction of the neutron absorber in the control rod according to the invention should preferably exceed the diameter reduction of the neutron absorber in the conventional control rod by a value falling within a range of about 0 to 0.7 mm. Furthermore, the axial length LA of the sleeve 16 should preferably be so selected as to be substantially equal to the axial height of the reduced-diameter portion 54a of the conventional control rod, while the axial length LB of the reduced-diameter portion 14a of the neutron absorber may be selected to a value obtained by subtracting height of a tapered portion (ca. 20 mm) from the axial length LA of the sleeve 16. Further, in the control rod according to the instant embodiment of the invention, the axial length of the bottom end plug 13 is lengthened by xcex94L when compared with that of the conventional control rod having the same overall length as the control rod according to the instant embodiment of the invention. However, because the axial length of the top end plug is shortened by xcex94L in the control rod according to the instant embodiment of the invention, the axial lengths of the cladding tube 11 and the neutron absorber 14, respectively, of the control rod according to the invention are substantially the same as those of the conventional control rod. When the axial length of the bottom end plug is increased by xcex94L, as described above, the relative positional relation between the neutron absorber and the fuel will naturally deviate in the state where the control rod is fully inserted into the guide tube of the fuel assembly, as a result of which in the region where neutrons are emitted from the fuel, the region where the neutrons cannot be covered by the neutron absorber (a region in the vicinity of the bottom end of the fuel rod) will increase. In this conjunction, increase of the region incapable of neutrons up to ca. 15 mm at maximum is considered to be permissible from the nuclear standpoint. Accordingly, the upper limit of the increase xcex94L in the axial length of the bottom end plug 13 should be ca. 15 mm. As is apparent from the foregoing, in the control rod according to the first embodiment of the invention, the sleeve 16 disposed within the annular space defined between the outer peripheral surface of the reduced-diameter portion 14a and the inner peripheral surface of the cladding tube 11 has a sufficient strength against the expansion of the reduced-diameter portion 14a in the radial direction. Thus, the tendency of the reduced-diameter portion 14a to expand in the radial direction can be suppressed by the sleeve 16. In this way, not only the expansion of the neutron absorber 14 in the radial direction under irradiation with neutrons but also radial expansion thereof due to shock applied upon stepwise driving of the control rod cluster can be effectively suppressed, whereby the integrity of the cladding tube 11 can be maintained over an extended period. Furthermore, because the sleeve 16 is formed of the same material as the cladding tube 11 or a metal material having a lower thermal expansion coefficient than the cladding tube 11, the integrity of the cladding tube 11 can be protected against damage due to thermal expansion of the sleeve 16. In addition, because the lower peripheral edge of the ordinary diameter portion 14b, exclusive of the reduced-diameter portion 14a of the neutron absorber 14, is chamfered with the top end portion of the sleeve 16 being also chamfered complementarily, the axial length LA of the sleeve 16 is slightly increased beyond the axial length LB of the reduced-diameter portion 14a, so the ordinary diameter portion 14b of the neutron absorber 14 above the reduced-diameter portion 14a thereof can be placed in a state supported from the underside. Thus, it is difficult for the shock applied upon stepwise driving of the control rod cluster to be transmitted to the reduced-diameter portion 14a, whereby the tendency of the reduced-diameter portion 14a to expand radially can be more positively suppressed. Besides, by increasing the length of the bottom end plug 13 by xcex94L, possible interference of the control rod with the control rod guide tube 34 is limited to the bottom end plug 13 of the control rod. Thus, the cladding tube 11 can be protected against abrasion due to such interference. The control rod according to a second embodiment of the present invention will be described by reference to FIG. 2. As can be seen in the figure, the control rods according to the second embodiment are implemented in such a structure that a cladding tube 11 formed of a stainless steel is hermetically closed at both ends by a top plug 12 and a bottom plug 13, respectively, wherein a rod-like neutron absorber is accommodated within the cladding tube 11. The neutron absorber 14 is formed of a neutron absorbing material such as an Agxe2x80x94Inxe2x80x94Cd (silver-indium-cadmium) alloy or boron carbide or the like and pressed downwardly against a bottom plug 13 by means of a hold-down spring 15 disposed within the cladding tube 11 at a top end portion thereof. Further, the neutron absorber 14 includes a reduced-diameter portion 14a which is located at the side of the bottom end plug and which has a smaller diameter than the other portion of the neutron absorber 14 having an ordinary diameter, wherein a sleeve 16 is disposed within an annular space defined between the outer peripheral surface of the reduced-diameter portion 14a and the inner peripheral surface of the cladding tube 11. The sleeve 16 is formed of the same material as that of the cladding tube 11 or a material of a smaller thermal expansion coefficient (or rate of thermal expansion) than the cladding tube 11 and has a sufficient strength for withstanding expansion of the reduced-diameter portion 14a in the radial direction. Further, the sleeve 16 has a cover head 17 at a top end thereof, and the neutron absorber 14 is divided into the reduced diameter portion 14a and the other portion 14b of the ordinary diameter by the cover head 17. In the control rod according to the second embodiment of the invention, the sleeve 16 disposed within the annular space defined between the outer peripheral surface of the reduced-diameter portion 14a of the neutron absorber 14 and the inner peripheral surface of the cladding tube 11 has sufficient strength to withstand the expansion of the reduced-diameter portion 14a of the neutron absorber 14 in the radial direction. Thus, there can be obtained advantageous effects similar to those of the control rod according to the first embodiment of the invention described hereinbefore. Besides, owing to the structure in which the neutron absorber 14 is separated into the reduced-diameter portion 14a and the portion 14b of the ordinary diameter by the cover head 17 of the sleeve 16, it is difficult for shock generated when the control rod cluster is driven stepwisely to be transmitted to the reduced-diameter portion 14a of the neutron absorber 14. As a result, expansion of the reduced-diameter portion 14a of the neutron absorber 14 in the radial direction can be suppressed more positively. As will now be understood from the foregoing description, according to the teachings of the present invention, expansion of the reduced-diameter portion of the neutron absorber in the radial direction can be suppressed notwithstanding the shocks applied during each stepwise driving of the control rod cluster, whereby the integrity of the cladding tube can be sustained over a remarkably extended period. Many modifications and variations of the present invention are possible in light of the above techniques. It is therefore to be understood that the invention may be practiced otherwise than as specifically described, within the scope of the appended claims.