JP2017167109A - Control bar and boiling water reactor having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control bar which can suppress corrosion in a gap and a boiling water reactor having the same.SOLUTION: A control bar 1 of the present invention comprises: a tie rod 3 having a cross-shaped horizontal section; a handle 2 which is jointed to one end of the tie rod 3 and has a cross-shaped horizontal section; and a sheath 5 jointed to the tie rod 3 and the handle 2 and having a U-shaped horizontal section. The sheath 5 has a coating part 9 coating a part of the handle 2. The coating part 9 has one or more exposed parts 8 in which the handle 2 is exposed, and a total L+ +Lof dimensions LLin a horizontal direction of the exposed part 8 as it is used is designed to be 30% or less of a dimension La of the coating part 9 in the horizontal direction. A boiling water reactor having the control bar according to the invention is also provided.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a control rod and a boiling water reactor including the control rod.

  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 is inserted into the core or pulled out from the core by the drive operation of the control rod drive mechanism. The control rod is an important device used for reactivity control and output distribution adjustment.

  With reference to FIG. 9, the structure of a conventional hafnium rod-type control rod used in a boiling water reactor will be briefly described. In the control rod 101, a handle 102 is joined to the upper end portion of the tie rod 103, and a drop speed limiter 111 is joined to the lower end portion of the tie rod 103. The control rod 101 has four blades 112 extending from the tie rod 103 in all directions. Each blade 112 has a sheath 105 attached to the tie rod 103 and having a U-shaped cross section. The sheath 105 has a rod-shaped hafnium member 104 that is a neutron absorber. The hafnium member 104 may be tubular or plate-shaped.

  In recent years, it has been reported that a micro crack is generated on the surface of a sheath 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 generated in the sheath, for example, in Patent Document 1, a flow path of cooling water inside the sheath is formed by providing a recess in the end surface of the pin that fixes the handle and the neutron absorber (hafnium member). The technology is described.

JP2011-208959A

  In the case of a hafnium rod type control rod, the cooling water flows from the lower side to the upper side of the control rod. If the gap between the sheath and the hafnium rod is controlled at an appropriate interval, the cooling water can flow through the gap. For this reason, accumulation of impurities on the inner surface of the sheath adjacent to the hafnium rod is suppressed. As a result, the sheath inner surface maintains the same oxygen concentration as the sheath outer surface, so that crevice corrosion can be suppressed.

  However, since the gap between the handle and the sheath at the upper part of the control rod is narrow and close in a wide range, there is a possibility that the cooling water cannot flow uniformly and stagnates. When stagnant water is generated in this way, it is difficult for the corrosion products to be discharged, and the accumulation and fixation of substances proceed, so that it is easy to physically obstruct the flow of cooling water. Furthermore, it becomes difficult to supply oxygen contained in the cooling water to the region where the handle and the sheath overlap. The dissolution reaction proceeds in the region where oxygen depletion occurs (the region where the handle and the sheath overlap), and the corrosion reaction proceeds due to the reduction reaction proceeding in a region where oxygen is sufficiently present (such as the outer surface of the sheath). Therefore, there is a possibility that dissolution of the inner surface of the sheath and cracking through the sheath may also occur.

  Although the technique described in Patent Document 1 can suppress IASCC, since stress is applied because the pin is applied, and the gap width is narrow and close in a wide range, gap corrosion is sufficiently suppressed. There is a risk that it will not be possible.

  This invention is made | formed in view of the said situation, and makes it a subject to provide the control rod which can suppress crevice corrosion, and a boiling water reactor provided with the same.

  The control rod according to the present invention that has solved the above problems is a tie rod having a cross-shaped horizontal section, a handle having a cross-section joined to one end of the tie rod, and a horizontal section joined to the tie rod and the handle. A U-shaped sheath, and the sheath has a covering portion that covers a part of the handle, and the covering portion has one or more exposed portions from which the handle is exposed, The total of the horizontal dimension of the said exposed part in the aspect used is set as the structure which is 30% or less of the horizontal dimension of the said coating | coated part.

  The boiling water reactor according to the present invention includes the control rod according to the present invention.

  The control rod according to the present invention and the boiling water reactor equipped with the control rod can suppress crevice corrosion.

It is a perspective view which shows a part of control rod concerning one Embodiment of this invention. It is an enlarged view of the II part of FIG. It is the IIIa-IIIa line | wire cut part end view of FIG. It is an enlarged view of the IIIb part of Drawing 3A. FIG. 4 is an end view taken along line IV-IV in FIG. 2. It is the VV line | wire cutting part end surface figure of FIG. It is a partial enlarged view of the control rod for explaining an example of the shape of the exposed portion. It is a partial enlarged view of the control rod for explaining an example of the shape of the exposed portion. It is a partial enlarged view of the control rod for explaining an example of the shape of the exposed portion. It is a partial enlarged view of the control rod for explaining an example of the shape of the exposed portion. It is a partial enlarged view of the control rod for explaining an example of the shape of the exposed portion. It is a partial enlarged view of the control rod for explaining an example of the shape of the exposed portion. It is the enlarged view to which a part of control rod concerning other embodiments of the present invention was expanded. It is a schematic explanatory drawing explaining the outline of a structure of the boiling water reactor which concerns on one Embodiment of this invention. It is a perspective view which shows the structure of the conventional hafnium rod type | mold control rod used for a boiling water reactor.

Hereinafter, a control rod according to the present invention and a boiling water reactor equipped with the control rod will be described in detail with reference to the drawings as appropriate.
(Control rod)
(One embodiment)
FIG. 1 is a perspective view showing a part of a control rod according to an embodiment of the present invention. FIG. 2 is an enlarged view of a portion II in FIG. 3A is an end view taken along the line IIIa-IIIa in FIG. FIG. 3B is an enlarged view of a portion IIIb in FIG. 3A. FIG. 4 is an end view taken along line IV-IV in FIG. FIG. 5 is an end view taken along the line VV in FIG. 6A to 6F are partially enlarged views of the control rod for explaining an example of the shape of the exposed portion.
As shown in FIGS. 1 and 2, the control rod 1 according to the present embodiment includes a tie rod 3 having a cross section (horizontal section) cut in a horizontal direction with respect to the ground plane. The control rod 1 includes a handle 2 having a cross-shaped horizontal cross section joined to one end of the tie rod 3. The cross of the tie rod 3 and the cross of the handle 2 are provided so as to overlap in a plan view.

The control rod 1 is provided with a sheath 5 so as to extend from each cross-shaped end of the tie rod 3 and to extend below the handle 2. In other words, the four sheaths 5 are joined to the tie rod 3 so as to extend in four directions along the shape of the cross-shaped tie rod 3 arranged at the shaft center, and the sheath extends so as to extend below the cross-shaped handle 2. 5 is joined.
The hafnium member 4 has a rod shape and is suspended below the handle 2 as shown in FIG. 3A. The sheath 5 has a U-shaped horizontal cross section (see FIG. 5 described later), and is installed so as to cover the hafnium member 4. The sheath 5 is joined in the longitudinal direction of the control rod 1 at an arbitrary portion of the portion overlapping with the tie rod 3, and is joined in the lateral direction of the control rod 1 at an arbitrary portion of the portion overlapping with the handle 2. These joining is performed by welding, for example. These weldings can be performed by general equipment and conditions which are usually performed.

Since it is such a structure, the sheath 5 has the coating | coated part 9 which coat | covers a part of handle 2 as shown in FIG. FIG. 4 illustrates one sheath 5 and the handle 2 out of the four sheaths 5. Although not shown in FIG. 4, a sheath 5 and a handle 2 having the same structure are provided at the other three ends of the tie rod 3.
As shown in FIGS. 3B and 4, the gap width d between the handle 2 and the sheath 5 in the covering portion 9 is very narrow, being larger than 0.2 mm and not larger than 1.0 mm. Here, for example, Japanese Patent Application Laid-Open No. 2009-128349 describes that the gap width between the hafnium member and the sheath is set to be larger than 0.2 mm and not larger than 1.0 mm. The gap width d is equivalent to this. Since the hafnium member 4 is rod-shaped, a substantially triangular gap wider than the gap width d (greater than 1 mm) is provided between the sheath 5 and the two hafnium members 4 as shown in FIG. Arise.
Accordingly, since the cooling water can flow through the gap between the sheath 5 and the hafnium member 4 more easily than the gap between the handle 2 and the sheath 5 shown in FIGS. 3A and 3B, the supply of oxygen from the cooling water is also facilitated. . Therefore, there is no great difference in the oxygen concentration between the gap between the sheath 5 and the hafnium member 4 and the outside of the sheath 5, and crevice corrosion is less likely to occur.
On the other hand, the gap width d between the handle 2 and the sheath 5 in the covering portion 9 is narrow as described above, and it is difficult for the cooling water to flow therethrough, so that it is difficult to supply oxygen from the cooling water. Therefore, a large difference in oxygen concentration may occur between the handle 2 and the sheath 5 and outside the sheath 5, and crevice corrosion is likely to occur.

As shown in FIG. 2, in the present embodiment, the covering portion 9 that covers the handle 2 has one or more exposed portions 8 from which the handle 2 is exposed. Furthermore, as shown in FIG. 2, the coating unit 9, in an embodiment used, the total _{L 1 + ... + L n-} 1 in the horizontal dimension _{L 1} ... _{L n-1} of the exposed portion 8 of the cover portion 9 The horizontal dimension La is 30% or less (where n indicates the number of exposed portions 8 and is a number of 1 or more). As shown in FIG. 2, when a part of the covering portion 9 is cut off (the upper left portion of the sheath 5 is cut off in FIG. 2), this dimension La includes the upper end of the covering portion 9. The size of the remaining part. If it does in this way, since the exposed part 8 which the handle | steering-wheel 2 exposes in the coating | coated part 9 is provided in the predetermined ratio, the handle | steering-wheel 2 can contact a cooling water through the exposed part 8. FIG. Therefore, oxygen is supplied between the handle 2 and the sheath 5 by the cooling water, the difference in oxygen concentration between the handle 2 and the sheath 5 and the outside of the sheath 5 is reduced, and crevice corrosion is suppressed. If the total L ₁ +... + L _n-1 of the horizontal dimension L ₁ ... L _n-1 of the exposed part 8 exceeds 30% of the horizontal dimension La of the covering part 9, the strength of the sheath 5 may be reduced. There is. Incidentally, while suppressing more crevice corrosion, from the viewpoint of ensuring the strength of the sheath 5, the sum _{L 1 + ... + L n-} 1 in the horizontal dimension L ₁ ... L _n-1 of the exposed portion 8 covering part 9 The horizontal dimension La is preferably 10 to 30%.

  Here, according to the reference (Yoichi WADA, et al., 14th Int. Conf. On Environmental Degradation of Materials in Nuclear Power Systems, P.598 (2009)), the outside of the sheath has a high oxygen concentration (1000 ppb). In this case, the corrosion potential is 100 to 200 mV. Further, when the oxygen concentration between the handle and the sheath is set to 1/20 oxygen concentration (50 ppb) outside the sheath, the corrosion potential is −200 to −300 mV. From this, when the oxygen concentration between the handle 2 and the sheath 5 becomes 1/20 of the oxygen concentration outside the sheath 5, a large difference occurs in the corrosion potential, and therefore there is a high possibility that crevice corrosion will occur. It can be said.

  The present inventors have made various studies on the self-diffusion of oxygen in a state where the cooling water is stagnated in a predetermined gap by numerical analysis (using COMSOL ver. 2 manufactured by COMSOL). As a result, when the gap width d is greater than 0.2 mm and 1.0 mm or less, the range within 9 mm from the end of the covering portion 9 in the horizontal direction and the range within 9 mm from the exposed portion 8 are handled by cooling water. It was found that oxygen could be supplied between 2 and the sheath 5 and the oxygen concentration could be increased. Therefore, in this embodiment, as shown in FIG. 2, it is preferable that the dimension α from the end of the covering portion 9 to the end of the exposed portion 8 in the horizontal direction is 18 mm or less. In this way, as shown in FIGS. 2 and 4, when an intermediate point y from the end of the covering part 9 to the end of the exposed part 8 in the horizontal direction is set, the intermediate point from the end of the covering part 9 The dimension α1 up to y is 9 mm or less, and the oxygen concentration between the handle 2 and the sheath 5 is higher than 1/20 of the oxygen concentration outside the sheath 5. Further, the dimension α2 from the exposed portion 8 to the middle point y in the horizontal direction is 9 mm or less, and the oxygen concentration between the handle 2 and the sheath 5 is higher than 1/20 of the oxygen concentration outside the sheath 5. That is, the oxygen concentration between the handle 2 and the sheath 5 between the end portion of the covering portion 9 and the end portion of the exposed portion 8 in the horizontal direction (the portion indicated by the dimension α) is the oxygen concentration outside the sheath 5. It becomes higher than 1/20. Therefore, if it is set as such an aspect, crevice corrosion can be suppressed more.

  Moreover, in this embodiment, it is preferable that the number of the exposed portions 8 is two or more, and the dimension β between the one exposed portion 8 and the other exposed portion 8 in the horizontal direction is 18 mm or less. In this way, similarly to the above, assuming an intermediate point z between one exposed portion 8 and the other exposed portion 8, the dimension β1 from one exposed portion 8 to the intermediate point z in the horizontal direction is 9 mm or less, The oxygen concentration between the handle 2 and the sheath 5 becomes higher than 1/20 of the oxygen concentration outside the sheath 5. In addition, the dimension β2 from the other exposed portion 8 to the intermediate point z in the horizontal direction is 9 mm or less, and the oxygen concentration between the handle 2 and the sheath 5 is higher than 1/20 of the oxygen concentration outside the sheath 5. That is, the oxygen concentration between the handle 2 and the sheath 5 between one exposed portion 8 and the other exposed portion 8 in the horizontal direction (portion indicated by the dimension β) is 1 / of the oxygen concentration outside the sheath 5. Higher than 20. Therefore, if it is set as such an aspect, crevice corrosion can be suppressed more.

In the present embodiment, the shape of the exposed portion 8 is at least one of a rounded rectangle (see FIGS. 6A and 6B), a rectangle (see FIGS. 6C and 6D), and a circle (see FIGS. 6E and 6F). And the end of the exposed portion 8 is preferably connected to or not connected to the end of the sheath. If it does in this way, crevice corrosion can be controlled more.
Here, “the end portion of the exposed portion 8 is connected to the end portion of the sheath 5” means that, as shown in FIG. 2, the upper end portion 8 a of the exposed portion 8 is the sheath of the sheath in a mode in which the control rod 1 is normally used. The state connected with the upper end part 5a is said. Specifically, as shown in FIG. 2, it refers to a state in which the predetermined shape is formed from an arbitrary position of the covering portion 9 and the upper end of the covering portion 9 (up to the upper end portion 5a of the sheath) is cut out.
Further, “the end portion of the exposed portion 8 is not connected to the end portion of the sheath 5” means that, as shown in FIGS. 6A to 6F, in the mode in which the control rod 1 is normally used, the upper end portion 8 a of the exposed portion 8. Is not connected to the upper end portion 5 a of the sheath 5. Specifically, as shown in FIGS. 6A to 6F, the opening is formed in the predetermined shape at an arbitrary position in the covering portion 9.

The rounded rectangle may be one in which the corners of the rectangle are formed with a predetermined roundness, and two roundnesses are formed continuously to form a semicircle. The rectangle includes a square and a rectangle. The circular shape includes a circular shape whose center is one and an elliptical shape having two focal points. In the present embodiment, the shape of the exposed portion 8 is not limited to these, and can be an arbitrary shape.
For example, as illustrated in FIGS. 6C, 6E, and 6F, in the case where a plurality of exposed portions 8 are scattered in the covering portion 9, the horizontal dimension of the exposed portions 8 in the used manner The total is 30% or less of the sum of the dimensions of the exposed portions 8 having the same horizontal height.
Further, in the case of a mode in which a plurality of exposed portions 8 are scattered in the covering portion 9, the exposed portions 8 can be arranged in the vertical direction as shown in FIG. 6F, and as shown in FIG. It is also possible to dispose them in stages.

  Moreover, in this embodiment, as shown to FIG. 6A-FIG. 6F, when the exposed part 8 is an opening part which does not connect with the edge part of the sheath 5, it is the edge part of the exposed part 8 from the edge part of the coating | coated part 9 in a perpendicular direction. The dimension γ and the dimension δ up to the end are preferably 9 mm or less. The end portion of the covering portion 9 and the end portion of the exposed portion 8 are both upper end portions or lower end portions. For example, as shown in FIGS. 6B and 6C, when the upper end portion 9 a of the covering portion 9 and the upper end portion 8 a of the exposed portion 8 are viewed, the upper end portion 9 a of the covering portion 9 to the upper end portion 8 a of the exposed portion 8 The dimension γ is preferably 9 mm or less. For example, as shown in FIGS. 6B and 6C, when the lower end portion 9 b of the covering portion 9 and the lower end portion 8 b of the exposed portion 8 are viewed, the lower end portion 8 b of the exposed portion 8 is changed from the lower end portion 9 b of the exposed portion 8. The dimension δ up to is preferably 9 mm or less.

As described above, the cooling water flows from the lower side to the upper side of the control rod 1. Since the covering portion 9 is often located near the surface of the cooling water, the flow of the cooling water is disturbed by vigorous boiling and may be generated from the upper side to the lower side, but it is not always obtained in the same manner. In some cases, the control rod is operated to reach a height at which the covering portion is not sufficiently immersed in the cooling water. Therefore, crevice corrosion may occur.
Accordingly, by setting the dimensions γ and δ from the end of the covering portion 9 to the end of the exposed portion 8 in the vertical direction to be 9 mm or less as described above, the cooling water flowing from the lower side to the upper side is allowed to flow from the handle 2 and the sheath. Oxygen can be supplied during 5 and the oxygen concentration can be increased. Therefore, crevice corrosion can be suppressed more.

  If the concentration of oxygen contained in the cooling water is high, oxygen can be easily supplied to the covering portion 9, so that the dimensions α and β can be made larger than 18 mm, and the dimensions γ and δ can be made larger than 9 mm. Can be bigger. That is, these dimensions can be changed according to the oxygen concentration contained in the cooling water. The relationship between the oxygen concentration and the dimensions can be easily grasped by experiments or the like.

The handle 2, the tie rod 3, and the sheath 5 described above can be obtained by molding into a predetermined shape using, for example, stainless steel (SUS304, SUS316L, etc.).
The exposed portion 8 can be obtained by opening a part of the sheath 5 by machining or cutting it out.

(Other embodiments)
FIG. 7 is an enlarged view of a part of a control rod according to another embodiment of the present invention.
As shown in FIG. 7, the control rod 1 according to the present embodiment is obtained by eliminating many of the covering portions 9 described above. Specifically, the height dimension ε of the portion where the covering portion 9 covers the handle 2 is set to 9 mm or less. Also in this case, the cooling water flowing from below to above can supply oxygen between the handle 2 and the sheath 5, and the oxygen concentration becomes high. Therefore, crevice corrosion can be suppressed more. Also in this aspect, if the oxygen concentration contained in the cooling water is high, it becomes easy to supply oxygen to the covering portion 9, so that the above-described dimension ε can be made larger than 9 mm.

(Boiling water reactor)
FIG. 8 is a schematic explanatory diagram for explaining the outline of the configuration of a boiling water reactor according to an embodiment of the present invention.
As shown in FIG. 8, the boiling water reactor 80 according to the present embodiment includes a reactor containment vessel 81 and a pressure vessel 82.
The reactor containment vessel 81 is a facility for forming a barrier against the release of radioactive materials as well as a pressure barrier when cooling water is lost, and is important for a pressure vessel (reactor) containing a fuel assembly. Covering various devices. In order to achieve its purpose, the reactor containment vessel 81 has water inside, and has a pressure suppression chamber pool 83 that condenses steam released from the reactor with water to prevent an increase in pressure.
The pressure vessel 82 is a vessel that holds the internal pressure in a state where the reactor core is housed. Cooling water W is stored in the pressure vessel 82. The cooling water W is circulated in the pressure vessel 82 by the recirculation pump 84.
And the boiling water reactor 80 which concerns on this embodiment is provided with the control rod 1 which concerns on the above-mentioned this invention in the lower part of the pressure vessel 82. FIG. In the present embodiment, one control rod 1 is provided per cell constituted by four fuel assemblies 85 (simply shown in FIG. 8). In the control rod 1, a drop speed limiter 11 (see FIG. 1) is joined to the lower end portion of the tie rod 3. The lower part of the drop speed limiter 11 is connected to a control rod drive mechanism (not shown), and the control rod 1 is inserted into the pressure vessel 82 (core) or pulled out from the core by the drive operation of the control rod drive mechanism. Thus, reactivity control and output distribution adjustment are performed.

  Since the boiling water reactor 80 according to this embodiment includes the control rod 1 according to the present invention, oxygen can be supplied between the handle 2 and the sheath 5 by cooling water, and the oxygen concentration can be increased. Therefore, the oxygen concentration between the handle 2 and the sheath 5 becomes higher than 1/20 of the oxygen concentration outside the sheath 5, and crevice corrosion is suppressed. Therefore, the boiling water reactor 80 according to the present embodiment can reduce labor and cost for maintenance.

  As mentioned above, although the control rod which concerns on one Embodiment of this invention, and the boiling water reactor provided with this were demonstrated in detail, the main point of this invention is not limited to this, Various modifications are included. The above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to the one having all the configurations described. Further, a part of the configuration of an embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of an embodiment. In addition, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

1 control rod 2 handle 3 tie rod 4 hafnium member 5 sheath 8 exposed portion 9 covering portion d gap width

Claims (6)

A tie rod with a cross-shaped horizontal cross section,
A horizontal section joined to one end of the tie rod has a cross-shaped handle,
A sheath having a U-shaped horizontal section joined to the tie rod and the handle,
The sheath has a covering portion that covers a part of the handle,
The covering portion has one or more exposed portions from which the handle is exposed,
The control rod according to the aspect of the invention, wherein a total of the horizontal dimensions of the exposed portion is 30% or less of the horizontal dimension of the covering portion. In claim 1,
A control rod characterized in that a dimension from an end portion of the covering portion to an end portion of the exposed portion in the horizontal direction is 18 mm or less. In claim 1,
A control rod having two or more exposed portions, wherein a dimension of one exposed portion and the other exposed portion in the horizontal direction is 18 mm or less. In claim 1,
The shape of the exposed portion is at least one of a rounded rectangle, a rectangle, and a circle, and the end of the exposed portion is connected to the end of the sheath or is not connected. Control rod to play. In claim 4,
When the exposed portion is an opening not connected to the end of the sheath,
A control rod characterized in that a dimension from an end portion of the covering portion to an end portion of the exposed portion in the vertical direction is 9 mm or less.   A boiling water reactor comprising the control rod according to claim 1.

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