JPS6239912B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel element containing tag gas.

In a reactor core in which a large number of nuclear fuel assemblies are arranged in a honeycomb shape, early detection of where fission products are being released from the nuclear fuel assemblies is important for reactor safety. This is important for monitoring operating conditions and preventing radioactive contamination of the surrounding area. The release of fission products is mainly caused by damage to the cladding tubes of nuclear fuel elements within a nuclear fuel assembly, and methods for early detection of this damage include sampling methods and methods of inspecting coolant, cover gas, etc. ing. In particular, tag gas is sealed in the nuclear fuel element incorporated in the nuclear fuel assembly, and the composition or components of the tag gas that is released when the cladding tube is ruptured is detected to determine from which position in the nuclear fuel assembly the fission products are released. A tag gas method for detecting fuel damage has been proposed. The present invention consists in improving the nuclear fuel elements used in this tag gas process.

The tag gas may be a rare gas such as argon, xenon, or neon, which is inert to reactor structural materials, nuclear fuel materials, coolants, moderators, etc.
Used as seeds or in mixtures.

Conventionally, a window is provided in a part of the capsule that encloses the tag gas, and the window is sealed with a sealing material made of a low-melting point metal.The sealing material is heated to a temperature higher than the melting point and melted, and the tag gas is introduced into the cladding tube through the window. Nuclear fuel elements are known that contain capsules that can be released into the atmosphere. This nuclear fuel element has a simple structure for releasing the tag gas from the capsule, is easy to manufacture, can reliably open the seal under given temperature conditions, and is advantageous in terms of mechanical strength with less reduction in the effective volume of the plenum. be. However, when a low melting point metal is used as a sealing material, the metal adheres to the cladding tube, causing corrosion of the cladding tube and causing breakage.
Particularly when austenitic stainless steel is used for the cladding tube, low melting point metals such as Cd, Ga, Sn, and Bi-Pb are highly corrosive, so direct contact with the cladding tube is undesirable.

The present invention has been made to eliminate the above-mentioned drawbacks, and provides a tag gas-filled nuclear fuel element that has a simple structure, can reliably release tag gas into the cladding tube, and does not require consideration of the corrosivity of the cladding tube. It's about doing.

Hereinafter, one embodiment of a nuclear fuel element according to the present invention will be described with reference to FIG. In FIG. 1, the reference numeral 1 in the figure is a long cladding tube made of, for example, stainless steel, and inside this cladding tube 1 are a plurality of fuel pellets 2, which are cylindrical compacted uranium dioxide powder and sintered. It is loaded. 2/3 to 3/4 of the fuel pellets 2 are loaded leaving a plenum 3 at the top, and both ends of the cladding tube 1 are sealed with end plugs 4 and 5. Plenum springs 6 and 7 and a tag capsule 8 are inserted into the plenum portion 3 to prevent the fuel pellets 2 from shifting and to fix them. Figure 2 shows the tag capsule 8 in Figure 1.
FIG. 2 is an enlarged cross-sectional view. tag capsule 8
The device includes a support portion 9, an opening portion 10, an expandable portion 11 made of a shape memory alloy, a hollow cylindrical piercing portion 12, an opening portion 13, and a tag gas introduction portion 14. Note that a cutting edge 15 is formed on the piercing portion 12,
A diaphragm 16 is formed in the open portion 13. Shape memory alloys have a crystalline structure that can transform and reverse into an atomic arrangement known as martensite under specific temperatures or stress;
These include titanium alloys, iron/nickel alloys, nickel/aluminum alloys, iron/platinum alloys, etc. Copper-zinc-aluminum alloys and nickel-titanium alloys of about 4-10% aluminum, about 10-30% zinc, and the balance copper are suitable for the telescoping member 11 of the present invention, although other alloys may be used. A specific method for manufacturing the expandable portion 11 will be shown for example using a nickel-titanium alloy. A strip of nickel-titanium alloy is heated to about 650℃ to stabilize it. Next, it is rapidly cooled to about 50° C. or lower, and bent at that temperature into L-shapes, Z-shapes, U-shapes, and Σ-shapes as shown in FIGS. 3a to 3d. Alternatively, a wire-shaped nickel-titanium alloy may be rapidly cooled and wound into a coil. Other shape memory alloys can be manufactured in exactly the same way except for selecting appropriate temperature conditions. The stretchable part 12 manufactured in this way is inserted into the tag capsule 8, and then the piercing part 12 is inserted. Next, the unsealing part 13 is inserted at a position where the fully extended length of the expandable part is sufficiently shorter than the sum of the lengths of the piercing parts. After filling the tag gas, the tag gas introduction part 14 is welded to complete the tag capsule.

Next, the process of opening the tag capsule is shown. As a specific example, a tag capsule having an extension part made of a nickel-titanium alloy will be shown. As shown in FIG. 1, a nuclear fuel element enclosing a tag capsule is heated in a nuclear reactor by heat of self-reaction, or is heated in advance by external heating. When the temperature of the tag capsule, especially the stretchable part, reaches about 71°C, the shape memory alloy begins to undergo reverse transformation, and the stretchable part tries to become straight. However, because of the support part 9, the elastic part 11 presses the piercing part 12 against the opening part 13. The force of the shape memory alloy is more than 200 times the force of a bimetal element of the same size, and the diaphragm 16 of the opening part 13 easily
is broken by the cutting edge 15 as shown in FIG. 4, and the tag gas passes through the opening 10 and fills the fuel element.
Opening of the tag capsule is almost certainly achieved at temperatures above 77°C. If a copper-zinc-aluminum alloy is used, it is possible to open the package at even lower temperatures.

As explained above, according to the present invention, the tag capsule filled with tag gas can be reliably opened at a relatively low temperature by the force of the shape memory alloy, and the tag gas can be released. Furthermore, the structure is simpler than the conventional example, and there is no problem of cladding corrosion.

[Brief explanation of the drawing]

FIG. 1 is a vertical sectional view showing one embodiment of the nuclear fuel element according to the present invention, FIG. 2 is an enlarged vertical sectional view of the tag capsule in FIG. 1, and FIG. FIG. 4 is a vertical cross-sectional view showing the tag capsule in FIG. 1 in an unsealed state. 1...Claying tube, 2...Nuclear fuel pellets, 4,5
... end plug, 8 ... tag capsule, 9 ... support section, 10 ... hole section, 11 ... expansion and contraction section made of shape memory alloy, 12 ... piercing section, 13 ... opening section, 1
4...Tag gas introduction part.

Claims (1)

[Scope of Claims] 1. A nuclear fuel element in which a capsule filled with tag gas is enclosed in a plenum part of a long cladding tube loaded with nuclear fuel material, in which an expandable part is formed of a shape memory alloy on at least one end surface of the capsule. and means for unsealing the capsule in which the piercing portion is arranged. 2. The nuclear fuel element according to claim 1, wherein the expanding/contracting portion of the capsule is arranged by bending or winding a shape memory alloy that remembers a fully expanded state, for example, into a coil shape.