JP4095983B2 - Control element - Google Patents

Description

  The present invention relates to a control element of a nuclear reactor applied to control rods such as a fast breeder reactor and a light water reactor.

  An example of the structure of control elements loaded in the nuclear reactor is shown in FIG.

  When the absorber pellet 102 that suppresses the nuclear reaction is irradiated with neutrons, it causes volume expansion called swelling. In order to prevent the soundness of the cladding tube 103 from being impaired by swelling of the absorbent pellet 102, the control element 101 is provided with a gap 104 that allows for the swelling expansion amount of the absorbent pellet 102.

  Further, the absorber pellets 102 are cracked due to embrittlement due to neutron irradiation and the influence of the internal temperature distribution, and the broken pieces 102a are deposited in the gaps 104. This is called relocation. The debris 102a accumulated in the gap 104 due to the relocation gives a forced displacement to the cladding tube 103 by the subsequent swelling expansion, which may cause damage to the cladding tube 103.

  Therefore, the side surface of the absorber pellet 102 is held as shown in FIGS. 5 and 6 to prevent relocation so that the soundness of the cladding tube 103 is not impaired by the absorber pellet 102 or the fragment 102a of the absorber pellet. A shroud tube 106 is used in the conventional control element 101. The shroud tube 106 has a tube shape containing the absorbent pellets 102 aligned in a row, and an end plug 105 for closing the bottom is inserted at the lower end. Here, the end plug 105 is fitted only inside the shroud tube 106.

  Incidentally, several solutions have been devised so far for the problem that the shroud tube 106 is damaged due to forced displacement due to swelling of the absorbent pellet 102 (Patent Document 1, Patent Document 2, Patent Document). 3). Such a shroud tube 106 has a structure that can be expanded in the circumferential direction in accordance with the expansion of the absorbent pellet 102, and there are some that can cope with an expansion of about 10% of the outer diameter.

JP-A-11-287885 JP-A-11-287886 JP 2000-284085 A

  FIG. 7 shows a state where swelling of the absorbent pellet 102 occurs in the conventional control element 101 described above. Even when the end plug 105 is configured to be fitted only to the inner periphery of the lower end of the shroud pipe 106 and the fitting portion between the end plug 105 and the shroud pipe 106 is welded, the absorber pellet 102 is generated during swelling. When the shroud tube 106 expands due to the swelling of the absorbent pellet 102 due to the possibility that the welded portion may break due to stress or the like, a gap may be generated in the fitting portion with the end plug 105. If a gap is generated and the broken piece 102a flows out of the gap, the cladding 103 is forcibly displaced due to swelling of the absorbent pellet 102 and the leaked broken piece 102a, which may cause damage to the coated tube 103.

  For this reason, the present invention aims to provide a control element 101 that satisfies the retention effect of the absorber pellet 102 and the absorber pellet fragments 102a even when the shroud tube 106 expands due to swelling of the absorber pellet 102. To do.

  A control element according to claim 1 of the present invention for solving the above-described problem has a side wall portion surrounding at least a lower end of a shroud pipe containing at least an absorber pellet, and the shroud pipe is inserted inside the side wall section. It has an end plug provided with a portion to be inserted.

  A control element according to a second aspect of the present invention is the control element according to the first aspect, wherein a spacer is installed between the end plug and the absorbent pellet contained in the shroud tube, and the spacer is higher than the side wall portion. It is characterized by that.

  According to the control element of the present invention, the end plug that closes the lower end of the shroud tube is structured so as to wrap the lower end of the shroud tube, so that even if the shroud tube expands due to swelling expansion of the absorbent pellet, the end plug is closed. Since the lower end of the shroud tube is always closed by the stopper, it is possible to prevent the fragments of the absorber pellets from flowing out.

  Further, even when the absorber pellets swell and expand, the shroud tube swells and a forced displacement occurs in the fitting portion between the shroud tube and the end plug, the fitting portion between the shroud tube and the end plug is swollen. Since the distance from the lower end absorbent pellet can be sufficiently maintained by the spacer, the stress generated in the fitting portion can be relaxed and the soundness of the shroud tube and the end plug can be maintained.

  Hereinafter, the details of the present invention will be described with reference to the drawings of one embodiment.

  As shown in FIG. 1, the control element 1 is configured such that the absorbent pellets 2 stacked in a row are placed on a spacer 8 and wrapped with a shroud tube 6, and both ends of the shroud tube 6 are closed with end plugs 5 and spring retainers 11. The structure is included in the cladding tube 3. A lower end plug 7 and an upper end plug 9 are inserted into both ends of the cladding tube 3 and are joined by welding or the like, and the shroud tube 6 is covered between the shroud tube 6 and the upper end plug 9. A spring 10 and a spring retainer 11 are installed for fixing in the tube 3.

  Furthermore, as shown in FIG. 2, the end plug 5 that closes the lower end of the shroud tube 6 has an insertion hole 5 a formed by a side wall portion 5 b that surrounds the lower end of the shroud tube 6. The lower end is closed in a state of being wrapped by the stopper 5. Moreover, the absorber pellet 2 is moved upward by the spacer 8 which is higher than the side wall 5b and has a lower diameter smaller than the upper diameter, and is spaced from the end plug 5.

  Next, the operation of the control element 1 configured as described above will be described.

  FIG. 3 shows the control element 1 loaded in the nuclear reactor and irradiated with neutrons. The shroud tube 6 also expands in the circumferential direction along with swelling expansion of the absorber pellet 2 that has been irradiated with neutrons. However, since the lower end of the shroud tube 6 is suppressed by the end plug 5, the shroud tube 6 has a lower end. The opening is always kept closed, and the absorbent pellet debris 2 a does not flow out of the shroud tube 6. For this reason, damage to the cladding tube 3 due to relocation of the fragments 2a of the absorber pellets can be prevented.

  Further, when the absorbent pellet 2 is placed directly on the end plug 5, the shroud pipe 6 expands due to swelling of the absorbent pellet 2, and a forced displacement occurs in the fitting portion between the end plug 5 and the shroud pipe 6. The end plug 5 and the shroud pipe 6 may be damaged, but a spacer 8 is installed between the end plug 5 and the absorbent pellet 2 that swells and expands, and the spacer 8 is higher than the side wall portion 5b. The end plug 5 is moved away from the portion where the tube 6 expands, the forced displacement generated in the fitting portion is reduced, and the end plug 5 and the shroud tube 6 can be prevented from being damaged.

In the embodiment, the spacer 8 has a stepped shape, but is not limited thereto. Moreover, although the end plug 5 is fitted only on the outer side of the shroud tube 6, it may be fitted on the inner side. Furthermore, the spacer 8 may have the same outer diameter up and down, and the end plug 5 and the spacer 8 may be integrated.
In addition, since the control element 1 is sufficiently long with respect to the spacer 8 and the absorber pellet 2 has sufficient space, the problem is that the space of the absorber pellet 2 is limited by installing the spacer 8. Not seen.

An example of control element design is shown below.
End plug material: SUS316
Spacer material: SUS316
Shroud tube material: SUS316
Structure of shroud tube: Double C-type shroud tube Inner and outer diameter of shroud tube: 12.3 mm / 13.6 mm
Inner and outer diameter of end plug: 13.7mm / 15.0mm
Inner and outer diameter of cladding tube: 15.5mm / 16.9mm
End plug insertion hole depth: 10mm
Spacer height: 50mm

  According to the present invention, even when the absorbent pellet is swollen, the fragments of the absorbent pellet are prevented from flowing out from the fitting portion between the shroud tube and the end plug, and the damage of the cladding tube can be prevented. The control element can be used in the furnace over a period in which the holding function can be maintained, and the life of the control element can be extended, and the cost of replacing the control element and radioactive waste can be reduced.

It is sectional drawing which shows the structure of the control element of this invention. It is sectional drawing which shows the lower structure of the control element of this invention. It is sectional drawing which shows the state in which relocation had arisen with the control element of this invention. It is sectional drawing which shows the state which the relocation produced with the conventional control element without a shroud pipe | tube. It is a perspective view which partially fractures and shows the lower structure of the conventional control element. It is sectional drawing which shows the lower structure of the conventional control element. It is sectional drawing which shows the state in which relocation had arisen with the conventional control element with a shroud pipe | tube.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,101 Control element 2,102 Absorber pellet 2a, 102a Absorber pellet fragment 3,103 Cladding tube 4,104 Crevice 5,105 End plug 5a Insertion hole 5b End plug side wall 6,106 Shroud tube 7, 107 Lower end plug 8 Spacer 9 Upper end plug 10 Spring 11 Spring retainer

Claims (2)

  A control element comprising a side wall portion that surrounds at least a lower end of a shroud tube containing an absorbent pellet, and an end plug provided with a portion into which the shroud tube is inserted inside the side wall portion. 2. The control element according to claim 1, wherein a spacer is installed between the end plug and the absorbent pellet contained in the shroud tube, and the spacer is higher than the side wall portion.

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