JPH022557B2 - - Google Patents

Description

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

A nuclear reactor control rod is constructed by loading a large number of neutron absorption rods into a plurality of wings formed by attaching an elongated U-shaped sheath to a central structural member. The above-mentioned neutron absorption rod is generally constructed by filling a cladding tube made of stainless steel (SUS) with B ₄ C powder as a neutron absorbing material, and arranging partition balls at regular intervals within the cladding tube to prevent powder movement. There is.

B ₄ C in the neutron absorption rod absorbs neutrons and gradually loses its neutron absorption ability, and during this time ¹⁰ B reacts with neutrons to generate He gas and increase the pressure inside the cladding tube. The life determined by the neutron absorption capacity is called the nuclear life, and the life determined by the gas pressure inside the tube is called the mechanical life.

Therefore, the control rod is not uniformly irradiated with neutrons; for example, the side edges and upper end of each wing are exposed to intense neutron irradiation, and the neutron absorption rods near these side edges and in the upper end area are exposed to strong neutron irradiation. Because the neutron absorbing material absorbs a large amount of neutrons, it reaches its nuclear lifetime earlier than other neutron absorbing rods. As a result, the control rods had to be disposed of as radioactive waste, even though the other neutron absorption rods had sufficient nuclear lifespan left.

The present invention was made based on the above circumstances, and
The objective is to obtain a control rod for a nuclear reactor in which the entire control rod reaches the end of its nuclear life almost uniformly, and there are almost no parts with nuclear life remaining at the time of disposal.

The present invention is based on the following findings regarding the nuclear lifetime of control rods, specifically neutron absorbing rods within control rods. That is, the arrival of the nuclear lifetime is based on the depletion and imbalance of ¹⁰ B, which is a neutron absorbing material. Among the above, depletion includes depletion due to reaction with neutrons, and loss of B ₄ C powder caused by a change in the properties of the powder as a result of reaction with neutrons, which has a detrimental effect on the cladding tube. The bias is as follows. In other words, even if there is no change in the amount of B ₄ C powder,
If voids are created in the neutron absorption rod due to powder bias, the decrease in reactivity value in the voids will exceed the increase in reactivity value due to densification at the destination of the powder, so the bias will eventually reduce the nuclear lifetime. It will be shortened. Therefore, the neutron absorption capacity of the area of the control rod that is particularly highly irradiated is made larger than other areas, and the harmful effects on the cladding tube due to the reaction between the neutron absorbing material and the neutrons are prevented, The above objective can be achieved by preventing the neutron absorbing material from being biased.

The invention will be explained in detail below with reference to the drawings. FIG. 1 shows the sheath of one wing 2 of the cruciform control rod 1 removed, and the side edge 2 of the wing 2 is shown with the sheath removed.
The neutron absorption rod 3 arranged in the wing side edge region 1 cm to 2 cm from a is constructed as shown in FIG. That is, a plurality of pellets 4 made of sintered powders of B ₄ C, Eu ₂ O ₃ , etc. are inserted into the Hf thin-walled tube 5,
The diameter of both ends of the Hf thin-walled tube 5 is reduced to hold the pellets 4 inside the tube, and the unit absorbent body 7 is made of SUS over the entire effective length by applying wool-like filling 6 to both ends of the Hf thin-walled tube 5. A plurality of rods are filled in the cladding tube 8 to form the neutron absorption rod 3. However, the boundaries of the unit absorbers 7 within the cladding tube 8 of the nuclear neutron absorbing rod 3 should not be at the same level. To do this, the lengths of the unit absorbers 7 inserted and filled into the lowest and highest positions of the cladding tube 8 of each neutron absorption rod 3 are varied,
All of the intermediate unit absorbent bodies 7 may have the same length.

Further, a unit absorber 10 having the same structure as the unit absorber 7 is also loaded into the cladding tube 8 in the upper end region of about 15 cm from the insertion tip of the neutron absorbing rod 3 disposed outside the wing side edge region.

Further, B ₄ C powder 11 as a neutron absorbing material powder is filled in the cladding tube 8 in areas other than the wing side edge area and the upper end area.

Further, a wool-like inclusion 12 is arranged at the boundary between the part filled with B ₄ C powder 11 and the part filled with unit absorbent body 10. In addition, in these neutron absorption rods 9,
The lengths of the filled parts of the unit absorbent bodies 10 are alternately long and short so that the boundaries between the filled parts of the B ₄ C powder 11 and the unit absorbent bodies 10 are not all at the same level.

In the reactor control rod having the above configuration, pellets 4 of B ₄ C and Eu ₂ O ₃ are inserted into the Hf thin-walled tube 5 in the wing side edge region and upper end region which are exposed to high-intensity neutron irradiation. Neutron absorption rods 3 and 9 formed by filling unit absorbers 7 and 10 into a cladding tube 8
are arranged, the excellent resonance absorption ability of Hf increases the neutron absorption ability of those parts, and prevents them from reaching their nuclear lifetimes earlier than other parts. In other words, even if Hf absorbs neutrons multiple times, it still forms an isotope with a large neutron absorption cross section, so it has a lower neutron absorption capacity than B ₄ C, which loses its absorption capacity after one neutron absorption. Ability lasts for a long time.

In addition, since B ₄ C, Eu ₂ O ₃ , etc. are packed as solid pellets 4, they do not settle over time and become uneven like powders, so the shortening of the nuclear life due to unevenness is avoided. It will never occur. moreover,
Between the pellet 4 and the inner periphery of the cladding tube 8 is an Hf thin-walled tube 5.
Since the pellets 4 and the cladding tube 8 are interposed, there is no risk of direct contact between the pellets ₄ and the cladding tube 8, and B ₄ C does not cause swelling during high irradiation and has no harmful effect on the cladding tube. Since there is no loss of C, shortening of the nuclear lifetime is prevented.

Further, the wool-like filling 6 has a function of preventing minute fragments from falling out of the Hf cladding tube 5 even if the pellet 4 is broken. Therefore, the fragments do not enter between the cladding tube 8 and the Hf thin-walled tube 5 and generate local stress.

In addition, by using pellets 4 as the neutron absorbing material, small voids are created in the cladding tube, but due to the high density of the pellets 4 and the large resonance absorption capacity of the thin-walled Hf tube 5, even if the gas plenum is used, the cladding is slightly smaller than the conventional one. It is possible to achieve a high degree of reactivity. Note that when Eu ₂ O ₃ pellets are used, the reactivity value can be adjusted by adjusting their concentration.

Furthermore, B ₄ C produced by neutron irradiation,
Swelling caused by changes in the properties of the Eu ₂ O ₃ pellets 4 can be sufficiently absorbed by the small gap between the Hf thin-walled tube 5 and the cladding tube 8.

In addition, Hf is heavy and expensive, but in the present invention, a thin-walled Hf tube is used, and Hf neutron absorption is mainly done on its surface, so Hf
The amount of Hf used for thin-walled pipes is extremely small;
The increase in weight and price of control rods is not so significant. Therefore, a control rod drive device designed based on the weight of conventional control rods can be used as is and can be supplied at a price almost the same as that of conventional control rods.

Note that the present invention is not limited to the above embodiments. For example, even if a neutron absorption rod is placed in the wing side edge area, only the upper quarter of the effective length receives high-intensity neutron irradiation, so the filling of the unit absorber is in the upper edge area of the wing side edge. may be limited to.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a main part showing an embodiment of a control rod for a nuclear reactor according to the present invention, and FIG. 2 is an enlarged partial sectional view of the neutron absorption rod shown in FIG. 1. 2... Wing, 3, 9... Neutron absorption rod, 4
... Pellet, 5 ... Hf thin-walled tube, 6 ... wool-like filling, 7, 10 ... unit absorber, 8 ... cladding tube, 11 ... B ₄ C powder, 12 ... wool-like inclusion.

Claims (1)

[Claims] 1 A nuclear reactor in which a plurality of sheaths with elongated U-shaped cross sections are attached to a central structural member to form wings, and a large number of neutron absorption rods formed by filling the cladding tube with a neutron absorption material are loaded into the wings. In control rods for control rods, at least the upper 1/4 of the neutron absorption rods placed in the wing side edge area are inside the cladding tube, and approximately 15 cm from the insertion tip of the neutron absorption rods placed outside the wing side edge area. The cladding tube in the upper end region is filled with a unit absorber made by inserting neutron absorbing material pellets into the Hf thin-walled tube, while the cladding tube in regions other than the upper side edge region and the upper end region is filled with neutron absorbing material powder. A nuclear reactor control rod characterized by being filled with each.