JPS60220893A - 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 configuration of control rods used for power distribution control and reactivity control in boiling water nuclear reactors and the like.

[Background of the invention]

Does a nuclear reactor have surplus reactivity, taking into account the reduction in reactivity due to fuel combustion? I have it. Reactor? Control rods are provided for well-controlled operation and reliable stopping. Figure 1 shows a cruciform control rod used in a boiling water reactor. The cruciform control rod 1 is composed of a control rod sheath 3 and a neutron absorption rod 2 in which boron carbide (B4C) powder is oscillated in a stainless steel tube at a density of about 70% of the theoretical density.

1O at the same position of boron, which is contained in about 20% of boron carbide
Does B have a large absorption cross section of about 3800 burns for thermal neutrons? and totally absorbs neutrons through the (n, α) reaction. Therefore, a control rod using boron carbide has the advantage of having a large reactivity value, and since boron carbide is relatively cheap and light, the control rod is cheap and light in weight.

However, the lithium and helium produced by the neutron absorption reaction of 10B are weak neutron absorbers, so their reactivity value rapidly decreases as the IOB burns, and the helium gas increases the internal pressure of the neutron absorption rod. Because of its high performance, it has a relatively short nuclear and mechanical lifespan.

Hafnium, europium, etc. are used as alternative absorbing materials for 84C. These have long lifetimes because the nuclide produced by the neutron absorption reaction is again a strong neutron absorber and is not accompanied by gaseous products. However, these are I
It has a smaller neutron absorption cross section than OB, and is expensive and heavy.

Control rods between fuel assemblies in boiling water reactors? A gap is provided for insertion. When the control rod is withdrawn, this gap (hereinafter referred to as the water gag) is filled with light water, which actively slows down neutrons. As a result, fuel rods close to the water gap, that is, fuel rods located on the outer periphery of the fuel assembly, tend to have higher output, so the enrichment of the fuel rods on the outer periphery of the fuel assembly is lowered to flatten the power distribution. Ke used to be there. In addition, the water gap is a non-boiling region and has a high hydrogen density, so hydrogen absorbs many neutrons.

In this manner, the water gap serves as a device that increases non-homogeneity within the reactor and obstructs equalization of the enrichment of the fuel rods and improvement of fuel economy. Therefore, it is desirable to make the water gap width as narrow as possible, but is this? What is the thickness of the control rod blades to achieve this? It is necessary to make it thinner. In this case, sufficient reactivity value? A control rod structure that can be maintained for a long period of time? It is important to have

The neutron absorption rods conventionally used in cruciform control rods are not suitable for thinning. That is, compared to arranging equal amounts of neutron absorbing material in the form of round rods, the thickness can be reduced by forming the same amount of neutron absorbing material into a flat plate.

Hafnium is a metal with good corrosion resistance, so it can be used in thin plate form, making it a necessary material for thin control rods. We are prepared. However, as mentioned above, since the absorption cross section is smaller than that of IOB, it has the disadvantage that sufficient reactivity value cannot be obtained when the plate is made thinner.

On the other hand, although boron can provide a high reactivity, when used in the form of a thin plate, it requires more measures for life than a cruciform control rod.

[Purpose of the invention]

The purpose of the present invention is to solve the above-mentioned drawbacks of the prior art. Is there a control rod configuration that is effective for making control rod blades thinner? It is about providing.

[Summary of the invention]

In order to achieve the above object, the control rod of the present invention is characterized in that a neutron absorbing plate stacked on all the layers of flat neutron absorbing material is used for all or part of the control rod blade. On this occasion,
The neutron absorbing material in the outermost layer is preferably a completely isomeric neutron absorbing material such as hafnium. Also, does the neutron absorbing material in the inner layer have sufficient reactivity value, which is approximately equal to that of a conventional cruciform control rod? To obtain a neutron absorber with a larger neutron absorption cross section (Σa)k than boron carbide? Must be included.

Neutron absorbers with large Σal of boron carbide have
'. Examples include boron carbide, GdzOa, EuB6, and Cd in which the abundance ratio of B is 19.8% or more (natural abundance ratio).

The nuclear life of a control rod in a boiling water reactor is defined as the period during which control rod value deterioration reaches 10% of its value at the initial stage of combustion. Generally, when the amount of neutron irradiation is constant, the burning rate of a substance with a larger neutron absorption cross section is faster. therefore,
If the control rod of the present invention uses a neutron absorbing material with a large neutron absorption cross section in the inner layer, the neutron absorbing material in the outer layer can reduce the amount of neutron irradiation. can be reduced and combustion can be delayed. For example, in Figure 2, is there hafnium in the outer layer? The graph shows the change in the neutron absorption rate of the neutron absorbing material in the inner layer when the hafnium thickness (gold) is changed when the hafnium thickness is approximately 0.
In the case of 8 ym, the neutron absorption rate of the inner layer neutron absorber decreases to about 50%. By delaying the combustion of the neutron absorbing material in the inner layer in this way, the deterioration of the control rod reactivity value is also delayed, leading to the nuclear lifespan? It can be extended. Furthermore, does it affect the nuclear lifetime? The effective reduction in control rod blade length can be seen as a contributing factor.

As shown in Figure 3, the outer end of the blade of the cruciform control rod has a neutron absorption rate approximately 1.7 times higher than the average. As a result, combustion at the outer end of the blade progresses rapidly, effectively shortening the blade length. Especially for materials with relatively short lifetimes like boron carbide? This becomes more noticeable in the case of the thin control rods used. Therefore, when using a short-lived neutron absorbing material, at least the outer end of the material should be a mixture or compound with a long-lived neutron absorbing material such as tin hafnium. It is necessary to use it.

When using a neutron absorbing material such as boron carbide that generates helium gas due to the (n+α) reaction, it is important to lengthen the period until local damage occurs due to helium gas, that is, the overall mechanical life.

Does the length of mechanical life depend on the amount of helium gas generated within the blade? This can be achieved by equalization. As shown in Figure 4, this is done by changing the mixing ratio of B and C powders and powders such as No-funium, which do not produce gaseous products, depending on the distribution of the neutron absorption rate of B4C. It can be easily realized. For example, in Figure 4, the IOB at the center of the control rod wing
When the combustion rate reaches 40 inches, the combustion rate of the IOB will be about 40% even at the very tip, so the amount of helium generated will be equal.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail with reference to all embodiments.

FIG. 5 shows a first embodiment of a control rod for a nuclear reactor according to the present invention. FIG. 6 is a sectional view of the control rod blade of this embodiment. The control rod 1 is configured by arranging a plurality of neutron absorption plates 6 in the length direction. The control rod sheath 3 has a reinforcing material 7 welded to it, and is strong enough to withstand stress caused by deformation of the neutron absorption plate. have.

The neutron absorption plate is inserted into a space surrounded by the control rod tie rod 4, the control rod sheath and reinforcing material, and the side edge material 8, and the side edge material is welded or screwed to the nondle portion 5 and the control rod sheath. ing. The neutron absorption plate of this example is
It has a two-layer structure consisting of a box-shaped no-funium filled with boron carbide powder. Since boron carbide generates helium gas and increases the internal pressure, as shown in FIG. 7, partition members 11 are placed on the hafnium plate 9 at intervals of about 2 cm in the direction of the control rod blade. are provided, thereby changing the mixing ratio of boron carbide and hafnium powder in each separated area.

At this time, the diameter of the hafnium powder was made larger than that of the boron powder, so that the sedimentation of the hafnium powder, which has a specific gravity about 7 times larger, was reduced. The height of the neutron absorption plate was approximately 2 Crn at the upper end of the control rod, and the height was approximately equal to the partition interval. By doing so, it becomes possible to increase the strength of the upper end of the control rod and create a structure that can withstand a larger amount of helium generation. For example, nome funium plate thickness 0
．． In the case of 8 turns, the maximum strain at a 10B combustion rate of 42 is less than IH, and the irradiation time required to reach the above combustion rate is about twice that of the conventional metal, so the mechanical life is extended.

FIG. 8 shows a second embodiment of a control rod for a nuclear reactor according to the present invention. FIG. 9 is a cross-sectional view of the control rod of this embodiment. In this example, the outer layer is made of feronium, and the inner layer is made of gadolinium (Gd).
The GdO neutron absorption reaction is (
n, r) reaction, and does not involve gaseous radioactive substances, so there is no need for a structure that takes strength into account. Therefore, the structure of the neutron absorption plate is also simple, as shown in FIG. 1O, in which a box-shaped hafnium 9 is filled with Gd20312. However, although Gd has a large neutron absorption cross section and has sufficient control rod value, the control rod life is shortened, so the control rod of this embodiment is mainly used for the purpose of reactor shutdown.

In the above examples, a powdered substance was used as the neutron absorbing material in the inner layer, but the metal may be boron, gadolinia, etc. Dispersed neutron absorber? When used, the structure of the neutron absorbing plate is further simplified. For example, if hafnium is used in the outer layer and t(fB) is used in the inner layer, it is possible to weld and stack the respective neutron absorption plates.Also, the outer layer t hafnium, the middle layer care amorphous boron, and the inner layer IGd. Thus, a configuration of 3R or more in which all the inner layers have larger neutron absorption cross sections can be easily realized. Such a multilayer structure can maintain control rod reactivity value for a longer period of time.

〔Effect of the invention〕

As explained above, according to the present invention, the disadvantage that the reactivity value of a neutron absorbing material with a small loss of neutron absorption rate, such as hafnium, is compensated for by the neutron absorbing material in the inner layer, and conversely, the neutron absorbing material in the inner layer While compensating for the decrease in absorption rate with the outer layer neutron absorbing material, it becomes possible to construct a thin control rod with a long nuclear life by stacking them in a flat plate shape. Furthermore, is the main component gaseous like boron carbide? As for the accompanying absorbing material, the mechanical life can be extended by using a mixture of a gaseous product such as neutronium, which has a small loss in neutron absorption rate, and a substance that does not. In this way, the present invention makes it possible to make the control rod thinner, making it possible to reduce the width of the water gear. It can be made narrower. As a result, the non-homogeneity of the core is reduced, making it possible to simplify the fuel and improve fuel economy.

[Brief explanation of drawings]

Figure 1 is a perspective view showing a cruciform control rod used in boiling water reactors, Figure 2 is a diagram of changes in neutron absorption rate of the inner layer neutron absorber when the hafnium thickness of the outer layer is changed, and 3
The figure is a neutron absorption rate distribution diagram in the direction of the cruciform control rod blade, Figure 4 is a distribution diagram of boron carbide and hafnium mixed so that the amount of helium gas generated by IOB combustion is constant, and Figure 5 is the present invention. A diagram showing the first embodiment of a control rod for a nuclear reactor, FIG. 6 is a cross-sectional view of the control rod blade of the embodiment shown in factor 5, and FIG. 7 is a diagram showing neutrons in the neutron absorption plate of the first embodiment. Is the absorption material distribution diagram, Figure 8, an example of M2 of the nuclear reactor control rod according to the present invention? Figure 9 is a sectional view of the control rod blade of the embodiment shown in factor 8, and Figure 1O is the configuration of the neutron absorption plate of the second embodiment. FIG. l... Control rod, 2... Neutron absorption rod, 3... Control rod sheath, 4... Control rod tie rod, 5... Handle portion, 6... Neutron absorption plate, 7... Control rod sheath reinforcing material, 8... Side edge material, 9... Hafnium plate (outer layer neutron absorbing material), 10... Inner layer neutron absorbing material, 11...
・Partition material, 12...rate 2 figure octium spring (mT11), 3 figure rate, 5 figure, 6 figure, 1 figure area

Claims (1)

[Claims] 1. Part or all of the neutron absorption part in the control rod of a nuclear reactor? At least two or more types of neutron absorbing material
A nuclear reactor control rod characterized by a multi-layered neutron absorption plate. 2. In claim 1, the inner layer of the neutron absorbing plate contains at least one kind of neutron absorbing material having a larger neutron absorption cross section than boron carbide, and the neutron absorbing material has a smaller neutron absorption loss toward the outer layer. Material? For use in nuclear reactors, which is characterized by the following: control rod. 3. In claim 1, if all substances with large neutron absorption loss are used as the neutron absorbing material in the inner layer, at least at the control rod blade tip, a mixture with a neutron absorbing material with small neutron absorption loss or A nuclear reactor control rod characterized by using a compound. 4. In claim 3, in the mixture of boron carbide and an absorbing material with a small loss of neutron absorption rate, the particle diameter of the neutron absorbing material is an atom characterized in that the particle diameter of the boron carbide is also large. Furnace control rod.