JPS6057288A - 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 the structure of a control rod used for power distribution control and reactivity control in a boiling water nuclear reactor or the like.

[Background of the invention]

Figure 1 shows a cruciform control rod used in a boiling water reactor. The control rod 1 is connected to the control rod sheath 2 and inside the sheath.
It is composed of about 20 neutron absorption rods 3 arranged in two blades, and the neutron absorption rods are made of boron carbide (B4C) powder 7, which is a neutron absorbing material, and a stainless steel cladding tube 6, as shown in Fig. 2. It consists of p.

The boron isotope "B" contained in B4C has a large absorption cross section of about 3800 burns for thermal neutrons,
The neutron absorption reaction '0B+n + 7Li +'I(e) changes into dilithium and helium.However, both lithium and helium have very weak neutron absorption ability, so
The neutron absorption capacity of the control rod, that is, the control rod value is IOB
decreases rapidly as .

In the core operation of a boiling water reactor, the life of a control rod is defined as the period during which the average control rod value deterioration of 1 part of the control rod effective length reaches 10 inches of the initial control rod value. That is, a control rod whose value deterioration reaches the above value is replaced with a new control rod. Therefore, if the control rod life is short, the number of control rods to be replaced will increase, which will contribute to an increase in the amount of waste.
Control rods with long lifespans are desired.

FIG. 3 shows changes in the neutron absorption rate of the B4C rods A and B in FIG. As shown in the figure, in the neutron absorption rod A at the tip of the control rod blade, the neutron absorption rate decreases rapidly because 10B burns quickly. As a result, the average control rod value deterioration of the control rods is also faster. In consideration of this point, a control rod in which the neutron absorbing rod at the tip of the control rod blade is replaced with a neutron absorbing material with a slow deterioration rate, such as hafnium or europium, is proposed in Japanese Patent No. 53-74697. However, the specific gravity of behafnium and europium is about 7 times and about 4 times heavier than B4C, respectively, and they are also very expensive. For this reason,
From the viewpoint of reducing the weight and cost of control rods, it is necessary to limit the use of these rods and to use them in combination with 134C, which is effective.

In the above-mentioned Japanese patent, hafnium, etc. is used in the form of a rod, so when the control rod reaches its end of life, burnt remains are formed in the center of the rod, which has been burned in a low neutron flux state.
Not being used effectively.

[Purpose of the invention]

The object of the present invention is to eliminate the drawbacks of the prior art mentioned above, to prolong the life of the control rod, and to provide a control rod that is light and inexpensive.

[Summary of the invention]

In order to achieve the above object, in the control rod of the present invention, in addition to the main neutron absorbing material forming the neutron absorbing rod, a second neutron absorbing material is arranged to reduce the neutron absorbing material of the neutron absorbing rod. It is characterized by controlling the amount of this neutron absorbing material at the tip of the rod blade compared to the center. Second
As neutron absorbing materials, in addition to those whose neutron absorption rate decreases slowly such as hafnium and europium, cadmium and
A-g-I n -Cd alloy, boron-containing stainless steel, etc. can be used.

Neutrons are generated by fission of fissile material in the fuel rods, and are decelerated by deceleration before reaching the control rods.

The second neutron absorbing material absorbs a portion of the neutrons that reach the control rod, and serves to reduce the amount of neutron irradiation of the neutron absorbing rod. FIG. 4 shows the change in neutron absorption rate of the neutron absorption rod when the second neutron absorption material is provided between the hafnium sheetra control rod sheath and the neutron absorption rod and the sheet thickness is increased. From the figure, by increasing the sheet thickness, p,
It can be seen that the neutron absorption amount of the neutron absorption rod decreases.

FIG. 5 shows the neutron absorption rate distribution of the neutron absorption rod along the control rod blade. The neutron absorption rod at the control rod blade tip has a neutron absorption rate of 1.2 to 1.8 times the average neutron absorption rate of the control rod. From the above results, in order to minimize the amount of hafnium used and extend the life of the control rod, it is sufficient to increase the thickness of the hafnium sheet at the tip of the control rod and reduce the thickness at the center. That is, this measure flattens the neutron absorption rate in the direction of the control rod blade, and the neutron absorbing material burns uniformly. Furthermore, by using hafnium in sheet form, the surface area to volume ratio increases and there are no residual burn marks, making it possible to use hafnium effectively.

As explained above, in order to extend the life of control rods using cheap and light 84C, it is effective to provide a second neutron absorber to suppress the combustion of 10B.
Furthermore, it is effective to use the second neutron absorber as follows.

(1) The amount of the second neutron absorbing material in the control rod blade/end portion is greater than that in the central portion to make the combustion of the absorbing material in the neutron absorbing rod uniform.

(2) Increase the surface area to volume ratio of the second neutron absorber to eliminate unburnt residue.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in more detail with reference to Examples.

FIG. 6 shows a configuration diagram of a control rod for a nuclear reactor according to the present invention.

Twenty-one neutron absorbing rods 3 are arranged in a row in a control rod sheath 2 that is bath-welded to a control rod central support 5 made of stainless steel. In this example, a second neutron absorbing material is installed in the gap between the neutron absorbing rod at the tip of the control rod blade and the control rod sheath.
A funium sheet 8 is arranged 9, and there is no funium sheet in the central part of the body. FIG. 7 shows the neutron absorption rate distribution in the direction of the control rod blade when the thickness of the neutronium sheet is 0.3 sm. At the tip of the control rod blade, the neutron absorption rate is reduced by about 20 degrees due to the hafnium sheet, and an attempt is made to flatten the neutron absorption rate along the blade. The hafnium sheets used in this example weigh about 280 g each even when inserted over the entire length of the control rod, which only increases the weight of the control rod by about 2y4. In addition, the control rod of this example has approximately 15% less weight than a control rod without a hafnium sheet.
Longer lifespan.

FIG. 8 shows a control rod shown in the above embodiment in which a sheet 9 made of a light material is placed in the center of the rod to prevent the neutron absorption rod from shifting. FIG. 9 shows an embodiment of a control rod in which the diameter of the neutron absorption rod at the center of the control rod is increased to prevent misalignment. In these embodiments, the neutron absorption rod with the highest neutron absorption rate, that is, the absorption rod at the very tip of the control rod, shown in FIG.
and j7.

FIG. 10 is a diagram showing a second embodiment of the control rod for a nuclear reactor according to the present invention. In this example, the hafnium sheet thickness was changed stepwise, so that the control rod blade tip was thicker and the center portion was thinner. By changing the thickness of the 7-frame sheet in the cumulative direction in this manner, the neutron absorption rate of the neutron absorption rod is flattened as shown in FIG. By using such a Fnium sheet and using Fnium rods as the absorption rods at the tip of the blade, the life of the control rods is as long as the total neutron absorption rods are B.
This is approximately 1.3 times the number of control rods with 10 rods. Also, the 11th
Example 7 in the figure: The amount of Hfnium used is a constant thickness of about 0.2 W@... It is the same as the control rod with Hfnium sheets arranged, but it is controlled by distributing the hafnium sheet thickness as in the example. The rod life will be 5 inches longer. Therefore, by varying the amount of hafnium, which is the second absorbing material, along the control rod blade, it is possible to effectively extend the life of the control rod. In this embodiment, the thickness of the no-funium sheet was changed stepwise, but the same effect can be obtained even if the thickness is changed continuously.

〔Effect of the invention〕

As explained above, according to the present invention, the amount of neutron irradiation of the neutron absorbing rod can be reduced by using a small amount of the second neutron absorbing material, and as a result, the combustion of 10B can be suppressed, thereby extending the life span and reducing the weight of the control rod.・Cost reduction can be achieved. Furthermore, as the lifespan of control rods becomes longer, the number of control rods to be replaced during periodic reactor inspections will decrease, reducing the amount of waste, reducing exposure during replacement work, and shortening the period of periodic inspections. This makes it possible to improve the rate.

[Brief explanation of the drawing]

Figure 1 is a perspective view of a cruciform control rod used in a boiling water reactor, Figure 2 is a sectional view of the control rod blade in Figure 1, and Figure 2(a) is a cross-sectional view of the control rod blade in Figure 2. Figure 3 is a graph of changes in the neutron absorption rate of the neutron absorption rod at the tip and center of the control rod, Figure 4 is a graph of changes in neutron absorption rate of the neutron absorption rod as the hafnium sheet thickness increases, Figure 5 6 is a graph of the neutron absorption rate of a neutron absorption rod along a conventional control rod blade, FIG. ) is a view taken along the line xx' in FIG. 6, FIG. 7 is a graph of the neutron absorption rate of the neutron absorption rod along the control rod blade of the first embodiment shown in FIG. 6, and FIG.・FIG. 9 is a sectional view showing the configuration of the control rod blade of the first embodiment shown in FIG. 6, and FIG. 10 is a control rod blade according to the second embodiment of the control rod for a nuclear reactor according to the present invention. Sectional view of the configuration, 11th
The figure is a graph showing the neutron absorption rate of the neutron absorption rod along the control rod blade of the second embodiment shown in FIG. DESCRIPTION OF SYMBOLS 1... Control rod, 2... Control rod sheath, 3... Neutron absorption rod, 4... Handle, 5... Central support member, 6
... Cladding tube, 7 ... B4C powder, 8 ... Absorbent sheet, 9 ... Sheet made of lightweight material, 10 ...
No-funium stick. Mushrooms,, 2CI-) 3 diagrams and calculations 5 diagrams

Claims (1)

[Claims] 1. In a control rod formed by arranging a large number of neutron absorbing rods to form a control rod blade, a second neutron absorbing material surrounding the neutron absorbing material is provided, and the amount of the neutron absorbing material is A control rod for a nuclear reactor characterized by a change along the control rod blade. 2. A control rod for a nuclear reactor according to claim 1, characterized in that a second neutron absorbing material is provided only at the blade tip of the control rod. 3. A control rod for a nuclear reactor according to claim 1, characterized in that the amount of the second neutron absorbing material at the control rod blade end is greater than that at the center.