JPH0149915B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to 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 absorption rods are made uniformly, the upper end of the neutron absorption rod and the neutron absorption rod on the side edge of the wing are located earlier than other parts or other neutron absorption rods. loses its ability to absorb neutrons. 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 No. 53-74697) discloses a long-life control rod that eliminates the above-mentioned conventional drawbacks. In this control rod, a long-life neutron absorption rod made of, for example, _Hf metal, which does not generate gas by absorbing neutrons, is placed in a region that is exposed to strong neutron irradiation. By doing this, it is obvious that the life of the control rod will be extended because the part that is strongly irradiated with neutrons will have a longer life.

However, for example, neutron absorption rods filled with B ₄ C powder may become clogged with powder during use of the control rod.
A portion where no powder exists, ie, a void, is generated near the upper end. It is undesirable for these voids to line up on the same cross section of the control rod because it lowers the reactivity value of the control rod.

On the other hand, subsequent experimental research has revealed that the neutron absorption rate is not only high at the side edges of the wing, but is also considerably high at locations far down from the tip. If all the voids are generated in such a region on the same cross section, the performance of the control rod will deteriorate, which is not preferable.

The present invention has been made based on the above-mentioned circumstances, and consists of loading a normal neutron absorption rod filled with powder of a neutron absorbing material such as B ₄ C into a portion other than the control rod tip, and a H _f metal or other such material at the tip. The purpose of the present invention is to obtain a control rod loaded with long-life neutron absorption rods in which even if voids occur, they will not all line up on the same cross section of the control rod.

In the present invention, there are two lengths of normal neutron absorption rods, long and short, and they are arranged in alternating lengths, and long-life neutron absorption rods are loaded in the space formed above them in the wing. By doing so, the above objective is achieved.

The invention will now be explained in detail with reference to the drawings. In the drawing, each protruding leg of the tie rod 1, which is cruciform in cross-section, has an elongated sheath 2, which has a U-shaped cross-section, attached at its opening, respectively, forming a cruciform wing, with an elongated sheath 2 having a U-shaped cross-section, respectively. is B ₄ C
A plurality of neutron absorption rods (referred to as powder type absorption rods) 5 of two types, long and short, are made by filling powder 3 into a cladding tube 4.
are loaded in such a way that their lower ends coincide with the lower end of the sheath and they are alternately long and short, and in the space formed in the sheath above them, there is a material such as _Hf metal that does not generate gas by absorbing neutrons. A long-life type absorption rod 6 and a short type long-life type absorption rod 6 consisting of a powder type absorption rod 5 are respectively loaded in contact with the upper part of the powder type absorption rod 5. Note that one or two short powder-type absorption rods are arranged on the side edge of the wing.

In the control rod of the present invention having the above structure, the two types of powder type absorption rods 5, long and short, are arranged so that the long and short ones are alternately arranged, and the long-life type absorption rod 6 is loaded in the space above them. The boundaries do not lie on the same cross section of the control rod. Therefore, the vibration of the control rod,
Even if voids are formed at the upper end of the powder absorber rod 5 due to shocks when starting or stopping the nuclear reactor, they will not all be lined up on the same cross section, and reactivity loss will be minimal.

As described above, in the present invention, a long-life neutron absorption rod is disposed at the upper end portion of the control rod that is exposed to strong neutron irradiation, so that the control rod can have a long life. Furthermore, the powder-type absorption rods are available in two types, long and short, and they are arranged alternately, so even if voids occur at their upper ends, all the voids will not line up on the same cross section of the control rod. The reactivity loss can be reduced to a minimum. Furthermore, in the present invention, it is only necessary to prepare powder-type and long-life type absorption rods in two types, long and short, and they are loaded in the wing so that they are arranged alternately. Since the determination can be made accurately and clearly based on the length, manufacturing and management of neutron absorption rods is facilitated, and mistakes in alignment of neutron absorption rods can be effectively prevented, and special care must be taken when assembling control rods. This simplifies the production of control rods. Also, even if there is a mistake in assembly and the lengths of two adjacent absorption rods are equal,
If it occurs at about one location, the decrease in reactivity is slight and can be ignored.

Further, although the above embodiment is an example in which the present invention is applied to a control rod having a cross-shaped wing, the present invention is not limited thereto.

[Brief explanation of drawings]

FIG. 1 is a partially cutaway front view of an embodiment of the present invention, and FIG. 2 is a cross-sectional view taken along the front line. 1... tie rod, 2... sheath, 3... B ₄ C powder,
4...Coated tube, 5...Powder type absorption rod, 6...Long life type absorption rod.

Claims (1)

[Claims] 1. In a tie rod, an elongated sheath with a U-shaped cross section is attached to each protruding leg to form a plurality of wings, and a large number of neutron absorption rods are loaded in each wing in parallel to the tie rod, in which the neutron absorption rods are A neutron absorber that does not generate gas by absorbing neutrons is arranged in two types of long and short powder type absorption rods arranged alternately, long and short, and is arranged in the wing space above each of the powder type absorption rods in contact with the upper part of each of the powder type absorption rods. A control rod for a nuclear reactor, characterized in that it is comprised of a long-life absorption rod consisting of: