JPS63753B2 - - Google Patents

Description

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

A control rod of a conventional boiling water nuclear reactor is constructed as shown in FIGS. 1 to 3. That is, an elongated sheath with a U-shaped cross section is attached to each protruding leg of a tie rod 1 with a cross-shaped cross section at its opening to form wings 2, and a neutron absorption rod 3 shown in FIG. 3 is loaded into each wing. It is used as a control rod.

Note that 4 in FIG. 1 indicates a through hole for coolant circulation provided in the wing. The neutron absorption rod 3 is placed inside a stainless steel pipe 5 as shown in Fig. 3.
Filled with B ₄ C powder 6, end plugs 7 are provided at both open ends of the pipe, and a plurality of partitions are provided in the middle of the pipe 5 by a partition ball 8 and dimples 9 above and below it, The inside of the pipe 5 is divided into a plurality of sections.

In the neutron absorption rod with the above configuration, B ₄ C is
¹⁰ B absorbs neutrons and generates gases such as He, so in order to form a storage plenum for the generated gas,
The packing density of B ₄ C should be approximately 70% of the theoretical density. From the standpoint of nuclear lifetime, it is better to have a higher B ₄ C packing density, but if the packing density is higher than about 70% of the conventional one, the pressure of the produced gas will rise significantly faster than the nuclear lifetime. It will reach the end of its lifespan. Furthermore, if the packing density is less than about 70% of the above, vibrations, shocks, etc. acting on the control rods will create areas where no B ₄ C powder exists, which will deteriorate the core properties of the control rods and shorten the mechanical life. Reaching nuclear lifespan.

In any case, it is not possible to satisfy the condition for a long-life control rod, which requires that the mechanical life and nuclear life be approximately the same and long.

In addition, B ₄ C exhibits extremely high hardness, as is known from its use as an abrasive, and is harder than the pipes that make up the neutron absorption rod. Therefore, the behavior of the B ₄ C powder may damage the pipe from the inside.

The present invention was made based on the above-mentioned circumstances, and aims to obtain a long-life control rod for a nuclear reactor that has a similar mechanical life and a long nuclear life.

In the present invention, a space is provided in the neutron absorption rod for B ₄ C to escape to prevent the generation of local stress in the metal pipe, and a space free of B ₄ C is provided to store gases such as He generated by neutron absorption. By concentrating ¹⁰ B in B 4 C or making B ₄ C into pellets to improve the packing density, _{the decrease in B 4 C} filling amount due to the provision of the space can be compensated for. The above objectives have been achieved.

The invention will be explained in detail below with reference to the drawings. FIG. 4 shows a neutron absorption rod used in the control rod of the present invention. The cladding tube 11 shown _in FIG.
4, and the inner tube 13 is provided with a ventilation hole 15. In addition, B ₄ C pellets 1
B ₄ C in 4 represents the internal cross-sectional area of the outer tube 12 as A ₀ ,
When the inner pipe 13 is A, at least A ₀ /A
It is twice as concentrated in ¹⁰ B than B ₄ C using natural boron.

Such a neutron absorbing rod can have a long mechanical life because the space 16 between the inner and outer tubes forms a large plenum. In addition, since the B ₄ C pellet 14 is enriched with ¹⁰ B as described above,
It exhibits a nuclear life equivalent to that of a case in which the entire cross section of the outer tube 12 is filled with B ₄ C. Furthermore, since the B ₄ C pellets 14 are contained within the inner tube, there is no risk that the outer tube 12 will be damaged by the pellets.

Figure 5A shows the neutron absorption rod 11 shown in Figure 4A.
Shows a control rod constructed using . That is, the control rod 16 of the present invention has the length of the neutron absorption rod 11 shown in FIG. A conventional neutron absorption rod 17 is arranged in the other part.

The neutron absorption rod 18 _shown in FIG.
is filled. ¹⁰ B in this pellet is also concentrated as in the previous example. This neutron absorption rod 18 also has a large plenum and has a long mechanical life and a long nuclear life. Further, since the pellet 19 has a smaller diameter than the inner diameter of the cladding tube 19 and has a relief, there is no fear that the cladding tube 19 will be damaged by the pellets. The neutron absorbing rod shown in FIG. 4C is a modification of the rod shown in FIG. 4B, in which the pellet 20 is coated 21 with a material less hard than the cladding tube 19, such as carbon. In this modification, damage to the cladding tube 19 is more completely prevented. The neutron absorption rod shown in FIG. 4D is constructed by pressing one side of a pellet 20 with a pellet presser 22 in the form of a leaf spring made of the same material as the cladding tube 19 or softer than it. The pellet shown in FIG. 4E is formed by pressing pellets 20 on both sides with pellet pressers 22. In these neutron-absorbing rods, the movement of the pellets is suppressed and their breakage can be prevented, so that local stress on the cladding tube due to broken pellet fragments is not generated, and damage to the cladding tube is prevented.

Moreover, what is shown in FIG. 4F is one in which the cladding tube 19 has an elliptical cross section. In addition, Figure 4 D to F
In the neutron absorbing rod shown in Figure 4C, the pellets may be coated as shown in Figure 4C.

FIG. 5B shows a control rod of the present invention constructed using the neutron absorption rods shown in FIGS. 4B-F. Fourth
The control rods shown in Figs. A conventional neutron absorption rod 17 is placed in the section.

The control rod of the present invention having the above configuration can be a long-life control rod since a neutron absorption rod with a mechanically and nuclearly long life is disposed at the tip of the control rod, which is most important from a nuclear point of view.

Note that the present invention is not limited to the above embodiments. For example, depending on the usage conditions of the control rods, it may be more effective to use long-life neutron absorbing rods over their entire length. In addition, as a neutron absorption rod used in the control rod shown in Fig. 5B, B ₄ C powder of uniform particle size is added with carbides, nitrides, etc., if necessary.
It is possible to use a tube coated with an oxide or the like and filled in a cladding tube. In this case, the packing density is reduced compared to filling B ₄ C powder blended with particles of different particle sizes, but this reduction can be compensated for by concentrating ¹⁰ B.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a conventional control rod, FIG. 2 is a cross-sectional view thereof, FIG. 3 is a longitudinal cross-sectional view of the neutron absorption rod, and FIGS. 4 A to F are cross sections of the neutron absorption rod of the present invention. 5A and 5B are longitudinal cross-sectional views of the main parts of each embodiment of the present invention. 11, 18...Neutron absorption rod, 12...Outer tube, 13
...inner tube, 14, 20... pellet, 19... cladding tube,
21...Coating, 22...Pellet presser.

Claims (1)

[Claims] 1. In a neutron absorbing rod in which the cladding tube is filled with B ₄ C, when the filling area of the B ₄ C inside the cladding tube is A and the cross-sectional area inside the cladding tube is A ₀ , A<A ₀ and in B ₄ C
A control rod for a nuclear reactor, characterized in that a wing is loaded with ¹⁰ B concentrated at least A ₀ /A times more than ¹⁰ B in B ₄ C made from natural boron. 2. The control rod for a nuclear reactor according to claim 1, wherein the neutron absorption rod is configured to have a short length and is loaded in a wing with its axis perpendicular to the control rod axis. 3. A control rod for a nuclear reactor according to claims 1 and 2, characterized in that the B ₄ C is in the form of pellets. 4. The control rod for a nuclear reactor according to claim 2, wherein the B ₄ C is made into a powder with uniform particle size, and each particle is coated with a material whose hardness is smaller than that of the cladding tube. The control rod for a nuclear reactor according to claim 2, characterized in that 5 B ₄ C is formed into pellets and coated with a material having a harderness than that of the cladding tube.