JPS5920117B2 - nuclear fuel elements - Google Patents

Description

[Detailed description of the invention] The present invention relates to nuclear fuel elements encapsulating taglass.

In a reactor core where a large number of nuclear fuel assemblies are arranged in a nicomb shape, early detection of where fission products are being released from the nuclear fuel assemblies arranged in the reactor is critical to the safety of the reactor. This is important for monitoring safety, operating conditions, and preventing radioactive contamination of the surrounding area.

The release of fission products is mainly caused by damage to the cladding of the nuclear fuel elements in the 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 method.

As the taglas, rare gases such as argon, xenon, and neon, which are inert to nuclear reactor structural materials, nuclear fuel materials, coolants, moderators, etc., are used alone or in combination.

Conventionally, a window was provided in a part of the capsule that sealed the tag gas,
Nuclear fuel elements are known in which the window is sealed with a sealing material of a low melting point metal, and the sealing material is heated to a temperature above its melting point to melt and incorporate a capsule that allows tag gas to be released from the window into the cladding tube. .

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 autenite stainless steel is used for the cladding tube, low melting point metals such as Cd, Ga, Sn, and B1-Pb are highly corrosive, so direct contact with the cladding tube is not preferred.

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, nutless steel, and inside this cladding tube 1 are a plurality of fuel pellets 2, each of which is made of uranium dioxide powder compacted into a cylindrical shape and sintered. It is loaded.

The fuel pellet 2 is 2/3~ leaving the prenub part 3 at the top.
The cladding tube 1 is 3/4 loaded, and both ends of the cladding tube 1 are sealed with end plugs 4 and 5.

Plenum nup rings 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.

FIG. 2 is a sectional view showing the tag capsule 8 in FIG. 1.

The tag capsule 8 has a hollow cylindrical piercing part 11, an expandable part 12 made of a shape memory alloy, and a tag gas tube surrounded by a sleeve 9 and a diaphragm part 10 attached to the end of the sleeve 9. It is configured to accommodate an introduction sealing part 13.

Shape memory alloys have a crystalline structure that can transform and reverse into an atomic arrangement known as martensite under specific temperatures or stresses, and include copper-zinc-aluminum alloys, nickel-titanium alloys, and iron alloys. - Types include nickel alloy, nickel-aluminum alloy, and iron-platinum alloy.

Copper-zinc-aluminum alloys and nickel-titanium alloys of about 4 to 10 parts aluminum, about 10 to 30 parts zinc, and the balance copper are suitable for the telescoping section 12 of the present invention, although other alloys may be used.

As an example, a specific method of manufacturing the expandable portion 12 made of nickel-titanium alloy will be explained.

A wire-shaped nickel-titanium alloy is heated to approximately 650°C and stabilized.

Next, the wire is rapidly cooled to about 50° C. or less, and the wire is formed into a coil at that humidity.

This stretchable portion may be formed by forming a band-shaped material into an L-shape, Σ-shape, U-shape, or Σ-shape as shown in FIG. 3 using a method similar to that described above.

Other shape memory alloys can be manufactured in exactly the same way, except for selecting appropriate humidity conditions.

The elastic part 12 manufactured in this way is inserted into the sleeve 9, and then the piercing part 11 is inserted.

Next, the diaphragm part 10 is inserted at a position shorter than the sum of the fully extended length of the expandable part 12 and the length of the piercing part.

Next, a predetermined amount of tag gas is filled from the tag gas introduction part 13, and then the tag gas introduction part 13 is welded to complete the tag capsule 8. Next, a process of opening the tag capsule will be shown.

As a specific example, a tag capsule having an elastic part made of nickel-titanium alloy will be shown.

As shown in FIG. 1, a nuclear fuel element enclosing a tag capsule is exposed to heat of self-reaction in a nuclear reactor or is exposed to external heat in advance.

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 return to its original shape, that is, straight.

The lower part of the elastic part 12 is the sleeve 9. Since it is supported at the bottom, the telescopic portion 12 presses the piercing portion 11 against the diaphragm portion 10.

The restoring force of the shape memory alloy is more than 200 times the force of a bimetal element of the same size, and the diaphragm 10 is easily broken as shown in Figure 4, allowing the tag gas to pass through this break and release the fuel element. Fill inside.

Opening of the tag capsule is reliably achieved at temperatures above 77°C.

When a copper-zinc-aluminum alloy is used, it is possible to open the package at even lower temperatures.

Furthermore, FIG. 5 shows another embodiment of the present invention.

The extensible portion 12 is made of a band-shaped plate of shape memory alloy bent into a U-shape.

As the temperature rises, the shape memory alloy in the expandable part 12 begins to undergo reverse transformation and tries to become straight, but the support base 1
4, it is possible to push the piercing portion 11 and break through the diaphragm portion 10.

As explained above, according to the present invention, the tag capsule filled with tag gas can be reliably opened from the inside of the capsule at a relatively low temperature by the restoring 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 fear of cladding corrosion.

Furthermore, since the opening mechanism is housed inside the capsule, handling becomes easy and the number of manufacturing steps for the nuclear fuel element can be reduced.

Therefore, there is an advantage that nuclear fuel elements can be manufactured at low cost.

[Brief explanation of the drawing]

FIG. 1 is a vertical sectional view showing an embodiment of the nuclear fuel element according to the present invention, FIG. 2 is an enlarged vertical sectional view of the tag capsule shown in FIG. 1, and FIG. FIG. 4 is a vertical cross-sectional view showing an example of the expandable part shown in FIG. 2, and FIG. 5 is a vertical cross-sectional view showing the tag capsule shown in FIG. FIG. 1... Cladding tube, 2... Nuclear fuel pellet, 4, 5... End plug, 8... Tag capsule, 10... Diaphragm. Part, 11...
- Piercing part, 12...Extension part.

Claims (1)

[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, wherein the capsule is formed of a shape memory alloy in a bottomed sleeve. 1. A nuclear fuel element, comprising a telescopic portion and a piercing portion, and a diaphragm portion is sealed to an opening of the bottomed sleeve facing the piercing portion. 2. The nuclear fuel element according to claim 1, wherein the extensible portion is formed by forming a wire shape memory alloy into a coil shape.