JPH0145598B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a neutron absorption rod used as a control rod for a nuclear reactor.

Control rods used in nuclear reactors are constructed by attaching elongated sheaths with a U-shaped cross section to each protruding leg of a tie rod with a cross-shaped cross section at their openings to form cross-shaped wings. It is equipped with a neutron absorption rod.

The neutron absorbing rod is constructed by filling a cladding tube with a neutron absorbing substance, such as B ₄ C powder or its pellets. When a control rod is inserted into a reactor core and is irradiated with neutrons, the neutron-absorbing substance powder in the neutron-absorbing rod absorbs neutrons and is worn out, and also generates gas. Therefore, as the neutron irradiation time passes, the neutron absorption capacity decreases and the gas pressure within the cladding tube increases. The nuclear lifetime is the neutron irradiation time required for the neutron absorption capacity to decrease to a predetermined value.
The neutron irradiation time required for the gas pressure to rise to an allowable value is called the mechanical life.

By the way, when looking at one control rod, not all of the many neutron absorption rods within the control rod are equally irradiated with neutrons. For example, when the above-mentioned cross-shaped control rod is partially inserted, the neutron absorption rods arranged near the side edges of the wing and the upper end portions of all the neutron absorption rods receive particularly strong neutron irradiation. Therefore, in a conventional control rod in which all neutron rods are made uniformly, the upper end of the neutron absorbing rod is also the neutron absorbing rod located on the side edge of the wing. loss of absorption capacity. Also, the neutron absorbing rods near the side edges of the wings lose their mechanical life before the others. As a result, conventional control rods had to be replaced even though most of the neutron absorbing rods were still usable.

The present applicant is a patent application filed on December 13, 1975.
-148720 (Japanese Patent Application Laid-Open No. 53-74697) discloses a long-life control rod that eliminates the above-mentioned conventional drawbacks. In this control rod, long-life neutron absorbing rods made of, for example, H _f metal are placed in areas that receive strong neutron irradiation. It is clear that by doing this, the life of the part that is strongly irradiated with neutrons will be extended, and the life of the control rod will also be extended.

However, since the control rod described above uses a regular neutron absorption rod filled with B ₄ C and a short long-life neutron absorption rod, assembly is troublesome.

In addition, especially in Figure 9D of JP-A-55-70792, a short long-life neutron absorbing rod made of _Hf metal is placed in the part of one cladding tube that receives strong neutron irradiation, and a B A neutron absorbing rod is disclosed that is filled with a _4C ordinary neutron absorbing material and eliminates the trouble of assembly.

However, in this neutron absorption rod, the short long-life neutron absorption rod and the B ₄ C neutron absorption material are in direct contact within the cladding tube, so when neutrons are irradiated, the B ₄ C neutron absorption material is exposed to the upper part of the neutron absorption rod. When a void is created where no neutron absorbing material exists, the long-life neutron absorption rod falls and the B ₄ C powder moves, causing the B ₄ C powder to form a gap between the cladding tube and the long-life neutron absorption rod. Once the powder enters, the hard B ₄ C powder expands when irradiated with neutrons, so the powder remaining between the cladding tube and the neutron absorption rod may damage the inner surface of the cladding tube. In addition, the difference in thermal expansion between the cladding tube and the long-life neutron absorption rod could not be absorbed, which could have an adverse effect on the neutron absorption rod.

The present invention was made in consideration of the above-mentioned circumstances, and it not only effectively prevents damage to the inner surface of the cladding tube, but also reliably absorbs the difference in thermal expansion between the cladding tube and a long-life neutron absorption rod, etc. The purpose is to provide a neutron absorption rod that can absorb neutrons.

In order to achieve the above-mentioned object, the present invention fills a cladding tube with powder of a B ₄ C neutron absorbing material from one end to the vicinity of the other end, and above the powder is made of H _f metal or Ag-In-Cd alloy. In neutron absorption rods filled with long-life neutron absorption material rods,
Metal wool is interposed between the powder of the neutron absorbing substance and the rod.

This neutron absorbing rod has metal wool interposed between a long-life neutron absorbing material such as H _f metal and a powdered neutron absorbing material of B ₄ C to separate the two, so the metal wool absorbs neutrons. It is possible to prevent the movement of substance powder, and the elasticity of metal wool can prevent the long-life neutron absorber from rattling. This prevents the highly hard B ₄ C powder from penetrating between the cladding tube and the rod and damaging the inner surface of the cladding tube. Furthermore, this neutron absorbing rod can absorb the difference in thermal expansion between the cladding tube, the rod material, and the neutron absorbing material due to the elasticity of the metal wool.

The invention will be explained in detail below with reference to the drawings. In FIG. 1, the cladding tube 1 is filled with a neutron-absorbing material powder 2 such as B ₄ C, except for the upper end of the cladding tube 1 which is exposed to strong neutron irradiation, and the said portion is filled with H _f metal. It is filled with rods 3 made of a long-life absorbent material. Further, metal wool 5 such as SUS wool is disposed at the boundary between the two and between the bar 3 and the upper end plug 4, respectively. Note that the bar 3 is provided with a plurality of circumferential constrictions 6 distributed in the axial direction. In the figure, 7 indicates a partition ball that prevents the powder from clogging at the bottom.

If the neutron absorption rod of the present invention having the above configuration is used,
Just like normal control rod assembly, the neutron absorption rods are simply loaded into the wings.
The control rod disclosed in No. 148720 can be assembled.
Furthermore, in the neutron absorption rod of the present invention, since the rod 3 is provided with the constricted portion 6, even the rod made of a material with high rigidity can be easily bent, and the rigidity of the control rod will not become excessive. Even if it comes into contact with the fuel assembly during insertion, there is no risk of the fuel assembly being pushed up. In addition, since the metal wool 5 prevents the movement of the powder 2 and the shaking of the bar 3, there is no risk of the powder 2 moving toward the bar 3, and the hard powder may enter the bar 3 and the cladding tube. 1, there is no risk of damage to the inner surface of the cladding tube 1 due to local stress. In addition, if the metal wool 5 is pushed in by the lower end of the bar 3 when filling the bar 3, the powder adhering to the inner surface of the cladding tube 1 will be pushed down by the metal wool, so that the bar 3 and the cladding tube No powder remains between the two inner surfaces, and the coated inner surface is not damaged thereby. Furthermore, the metal wool 5 can absorb the difference in thermal expansion between the cladding tube 1 and the bar material 3.

FIG. 2 shows another embodiment of the invention. In this embodiment, the bar 3 is made up of short sections 3a. According to this embodiment, the rigidity of the control rod tip can be further reduced. High hardness
B ₄ C neutron absorbing substance powder can be prevented from entering the gap between the cladding tube and the long-life neutron absorption rod, and since there is no intrusion of the hard B ₄ C powder, it is possible to prevent the powder from entering the gap between the cladding tube and the long-life neutron absorption rod. Damage to the inner surface of the cladding tube can be effectively prevented without causing significant stress.

FIG. 3 shows yet another embodiment. In this embodiment, the bar 3 is composed of short sections 3a, and a dimple 8 is provided on the cladding tube 1 between each section 3a.
Metal wool 9 is interposed between each section.
In this way, the cladding tube 1 can be
The difference in thermal expansion between the bar 3 and the bar 3 is absorbed and movement of the section 3a is prevented by the dimples 8.

In addition, in the present invention having the above-mentioned structure, since only necessary portions are filled with rods of a long-life neutron absorbing material, the rods may be short. If the bar is long, it must be made thin to increase the gap with the cladding tube, but it is not necessary to make the bar so thin. Although long-life neutron absorption rods are used only for a part of the neutron absorption rod, their reactivity value is lower than that of ordinary neutron absorption materials, so it is not preferable to make them too thin. This is convenient because there is no need to do so.

In addition to the exemplified H _f metal, an Ag-In-Cd alloy may be used as the long-life neutron absorbing material.

As described above, the neutron absorbing rod according to the present invention uses a long-life neutron absorbing material rod made of H _f metal or Ag-In-Cd alloy and powder of a B ₄ C neutron absorbing material. Since the metal wool is interposed in between, this metal wool clearly separates the neutron absorbing substance and the rod material, prevents the B ₄ C powder from moving to the rod material side, and prevents the B ₄ C powder, which has a large hardness, from moving to the rod material side. It is possible to prevent powder from entering the gap between the cladding tube and the bar material of the long-life neutron absorption rod, and effectively prevent damage to the inner surface of the cladding tube. Also,
Due to the elasticity of metal wool, it is possible to absorb the difference in thermal expansion between the cladding tube and the rod of the long-life neutron absorber, and it also has the effect of preventing the long-life neutron absorber from wobbling.

[Brief explanation of drawings]

1 to 3 are cross-sectional views of first to third embodiments of the present invention. 1... Coating tube, 2... Neutron absorbing substance powder, 3
... Bar material, 3a ... Division, 4 ... Upper end plug, 5,
9...metal wool, 6...neck part, 7...partition ball, 8...dimple.

Claims (1)

[Claims] 1. A cladding tube is filled with powder of a B ₄ C neutron absorbing substance from one end to the vicinity of the other end, and above it is H _f
A neutron absorption rod filled with a rod of a long-life neutron absorption material made of metal or Ag-In-Cd alloy, characterized in that metal wool is interposed between the powder of the neutron absorption material and the rod. A neutron absorption rod. 2. The neutron absorbing rod according to claim 1, wherein the rod of the long-life neutron absorbing material has a bendable portion formed by constrictions provided at intervals in the axial direction. 3. The neutron absorbing rod according to claim 1, wherein the rod of the long-life neutron absorbing material has short sections arranged in series to form a bendable portion.