JPS6060585A - Control rod for nuclear reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Field of application of the invention] The present invention relates to a control rod for a nuclear reactor.

[Background of the invention]

FIG. 1 is a perspective view of a so-called cruciform control rod (whole number 1) commonly used in boiling water reactors, and shows boron carbide B4 containing poron isotopic element B10f.
A large number of absorber rods 2 each made of a neutron absorbing material made of carbon sealed in a stainless steel tube are arranged in a cross shape, and a stainless steel sheath 3 having a cooling water inflow hole extends outside the absorber rods 2. The tie rod 4 connects four sheaths 3 forming a cross shape at the center. A handle 5 is attached to the upper part of the sheath 3, so that the control rod can be grasped and handled from the upper part. A roller 6 is attached to the handle 5, and the roller 6 is attached to the handle 5, and when the control rod moves in the reactor core (insertion into and withdrawal from the reactor core), a roller 6 is installed between the handle 5 and the side wall of the fuel channel (see FIG. 2) that forms the passage for the control rod. A fall limiter 7 is attached to the lower part of the sheath 3 to limit the falling speed of the control rod. The lower part of the drop limit 7 is connected to the drive system of the control rod.

FIG. 2 is a cross-sectional view showing the mutual positional relationship with fuel assemblies when the control rods shown in FIG. 1 are inserted into the reactor core, and is a unit formed by four fuel assemblies 8. One control rod 1 is arranged for each cell.

The control rods 1 are inserted into passages formed between the side walls of the fuel channels of the fuel assembly 8 at the bottom of the reactor core.

The lower half of FIG. 2 shows a typical example of the neutron flux distribution when the control rods described above absorb neutrons within the reactor core and control the output of the reactor.

That is, the neutron flux is low in the center of the unit cell and high in the outer part of the cell. Also,
Normally, the tip of the control rod receives a large amount of neutron irradiation. As a natural result of these, the amount of neutron absorption by the control rod tends to increase from the side to the top of the cross-shaped blade.

By the way, the life of a control rod is determined by the following factors. In other words, (1) the neutron absorbing material absorbs neutrons, resulting in nuclear deterioration and no longer having sufficient neutron absorption capacity; (2) the neutron absorbing material and control rod structural members absorb neutrons, causing the material to deteriorate. This is due to physical deterioration and the inability to maintain the necessary strength and performance.

In the control rod mentioned above, the column of neutron absorption amount? Ron is
When it absorbs neutrons, it transmutes into lithium and helium, which have no ability to absorb neutrons. In addition, the product helium increases the internal pressure of the absorber rod 2 as a gas, or increases the internal pressure of the B4C
Since it stays inside and expands, a mechanical load is applied to the material of the absorber rod 2. Due to these nuclear and material changes, the conventional control rods suffer from relatively short lifetimes.

On the other hand, control rods using hafnium or cadmium that do not lose their neutron absorption ability or generate helium are also known, but such known control rods generally have the disadvantage of being heavy.

[Purpose of the invention]

An object of the present invention is to provide a nuclear reactor control rod that has a long mechanical and nuclear life and is as light as a conventional control rod, and therefore does not require changes to the specifications of a conventional drop limiter. .

[Summary of the invention]

A feature of the present invention is that in a nuclear reactor control rod equipped with a sheath surrounding a neutron absorbing material, the neutron absorbing material is composed of an (n, γ) reaction type interwoven plate-shaped neutron absorbing material; When the material is housed in the sheath, the material has irregularities that ensure a space for the moderator to enter between the inner surface of the sheath and the plate-shaped neutron absorbing material and between the plate-shaped neutron absorbing materials. This, coupled with the presence of a moderator, allows the plate-shaped neutron absorbing material such as hafnium to exhibit sufficient absorption capacity despite its small volume fraction, and to reduce the weight of the entire control rod. I can do it.

[Embodiments of the invention]

An embodiment of the control rod of the present invention is shown in FIG. This figure is a cross section of one wing of the cruciform control rod, where 3 is the sheath and 4 is the tie rod. Inside the sheath 3, two hafnium plates 9 with dimple-like irregularities 10 and 11 are arranged facing each other along the sheath 3! has been done. The plan view and sectional view of the dimple (l hafnium plate 9) are as shown in FIG. A moderator (
Dimple-like unevenness 10 and 11 are alternately formed and arranged to ensure a space into which water (for example, water) can enter.

The hafnium plate 9 is provided with an opening 13 for allowing the moderator to flow into the space.

Incidentally, hafnium is an (n+γ) reaction type neutron absorbing substance, does not generate helium even if it absorbs neutrons, and its neutron absorption ability decreases relatively slowly.

In this embodiment, the amount of hafnium as a neutron absorbing material is determined without changing the current specifications of the conventional drop limiter 7. In other words, as the weight of the control rod increases, the falling speed of the control rod increases, and when the weight of the control rod exceeds a certain amount, the falling speed of the control rod exceeds the current specification value, which adversely affects the health of the fuel. It turns out. For this reason, an upper limit is set for the weight of hafnium. Furthermore, in order to obtain the necessary control rod value as a control rod, a lower limit value is set for the weight of hafnium. That is, the hafnium weight is required to be greater than or equal to the above lower limit and less than or equal to the upper limit. This concept will be explained with reference to FIG.

The upper μmj diagram in Figure 5 shows the relationship between the weight fTi of a single thick sheet of hafnium and the control rod value.If the current specification value is not met, the lower limit of the hafnium weight is set at aK9. be done. The lower part of Figure 5 shows the relationship between hafnium weight and control rod falling speed when the plate thickness of sheath 3 is not changed. To meet the current specification value, the upper limit of hafnium weight is bK9. Set. - For example, if the upper limit of hafnium weight is 35 kg, if the sheath plate thickness is made 0.7 mm thinner than the conventional one, the sheath weight will decrease by 2
0 becomes 9. Since the control rod falling speed depends on the weight of the entire control rod, in this case, if the falling speed remains the same as before, the upper limit value of the hafnium weight can be raised by the decrease in the north weight. That is, the upper limit value in this case is 55 to 2. From the above, the hafnium sheet 9 is made to have a thickness of 0.75 mm or more and 1.5 mm or less.

In addition, in this embodiment, the dimple-like unevenness described above is provided on the hafnium plate in order to secure the moderator between the two hafnium plates and increase the value of the control rod.However, on the other hand, since there is no moderator in the dimple part Control rod value decreases.

Therefore, there are constraints on the area of the dimples, and when calculated from the above-mentioned nuclear and weight constraints, it is necessary to make the dimple area so that it is about 10 factor or less out of the total area of the neutron absorbing portion of the control rod.

The above-mentioned embodiment shown in FIG. 3 has a structure in which several dimple-like uneven plates 9 of a constant length are arranged in the horizontal direction and the effective length in the axial direction as shown in FIG. 4. But the 6th
As shown in the figure, the dimpled uneven plate 9
An embodiment having a structure in which one sheet is arranged in the lateral direction and the other is arranged in the axial direction by the effective length is also possible.

This is to prevent deformation of the rays.

The embodiment shown in FIG. 7 is a modification of the one shown in FIG. 3, in which the thickness of the hafnium sheet on the wing tip side is made thicker than the thickness of the hafnium sheet on the tie rod side. This example takes into account the fact that the wing tip receives a higher amount of neutron irradiation than the center of the wing, as shown in the lower part of Figure 2. In other words, the value of the control rod is increased compared to the one using the same weight as the embodiment shown in FIG. In other words, when obtaining the same control rod value, the weight can be reduced and the sheath plate thickness can be reduced by a small amount, which improves the strength of the sheath.

Embodiment of Fig. 8 (at the wing tip) - Funium metal lump 12
This embodiment has the same effect as the embodiment shown in FIG. 7. In Fig. 8, a square bar is shown as the shape of the metal lump [2], but other shapes such as a connected round bar shape or a corrugated plate shape may also be used; the latter gives a rather preferable result in terms of increasing the surface area. It will be done.

FIG. 9 shows the relationship between hafnium weight and control rod life. The life of a control rod is approximately proportional to the Fnium Ichikawa, but in the embodiment of the present invention, the minimum required Fnium weight is approximately 30 to 9. The life of a control rod is about 4/3 times that of a conventional control rod. It is clear that it has a long lifespan.

〔Effect of the invention〕

The control rod of the present invention uses an (n, γ)-reactive neutron absorbing material such as hafnium, which has a relatively slow decline in neutron absorption capacity even when it absorbs neutrons, and does not generate helium, so it has a long life. In addition, since the neutron absorbing material is a plate material with irregularities and has a large surface area, and the contact area with the moderator is large, sufficient control ability can be obtained even if the amount thereof is reduced, and the weight can be reduced. Do not use the entire weight of the control rod as a sheath.

[Brief explanation of drawings]

Fig. 1 is an external view of a conventional control rod, Fig. 2 is a schematic diagram showing the arrangement of control rods and neutron flux distribution, Fig. 3 is a partial sectional view showing an embodiment of the control rod of the present invention, Fig. 4 The figures are a plan view and a cross-sectional view of a hafnium plate with dimples used in the control rod of Fig. 3, and Fig. 5 is a diagram showing the relationship between the weight increase of the control rod of the present invention, the control rod falling speed, and the control rod value. , FIG. 6 is a partial elevational view of another embodiment of the invention, FIG. 7 is a partial sectional view of still another embodiment of the invention, and FIG.
The figure is a partial sectional view of still another embodiment of the present invention, and FIG. 9 is a diagram showing the relationship between hafnium weight and control rod life Explanation of symbols 1... Control rod 2... Absorber rod 3...
Sheath 4... Tie rod 7... Drop limiter 8... Fuel channel 9...
・Hafnium plate 10.11... Dimples (unevenness) 12... Neutron absorbing material mass 13... Opening Figure 1 Figure 2 Figure 3 Figure 7 Figure 4 Figure 5 Control amount Continuation of Figure 6, page 1 Inventor Kurihara Kokuryoku Hitachi Engineering Co., Ltd., 3-2-1 Saiwai-cho, Hitachi City

Claims (1)

[Claims] 1. In a nuclear reactor control rod equipped with a sheath surrounding a neutron absorbing material, the neutron absorbing material is composed of stacked plate-shaped neutron absorbing materials of the (n+γ) reaction type, and the above-mentioned plate-shaped neutron When the absorbing material is housed in the sheath, it has irregularities that ensure a space for the moderator to enter between the inner surface of the sheath and the plate-shaped neutron absorbing material and between the plate-shaped neutron absorbing materials. A nuclear reactor control rod featuring: 2. The nuclear reactor control rod according to claim 1, wherein the plate-shaped neutron absorbing material is made of a plate-shaped material of hafnium or cadmium. 3. The nuclear reactor control rod according to claim 2, wherein the plate-shaped neutron absorber is provided with an opening for moderator inflow. 4. Claim 2 in which the plate-shaped neutron absorbing material at the lateral end of the control rod is thicker than that at other parts
Nuclear reactor control rods as described in Section. 5. Placing a mass of neutron absorbing material at the lateral end of the control rod)
3. The reactor control rod according to claim 2, wherein the plate-shaped neutron absorbing material is arranged in other parts.