JPS5833517B2 - nuclear fuel rod - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel rod with a tag gas capsule capable of detecting cladding failure.

Generally reactors are U-235, U-238 or PU-2
Nuclear fuel materials such as No. 39 are irradiated with neutrons to cause a nuclear fission reaction, and the resulting heat energy is used for power generation and other purposes.

Currently, in many nuclear reactors, a large number of nuclear fuel materials in the form of cylindrical pellets are sealed inside a metal cladding tube, and by circulating a coolant outside the cladding tube, the heat generated from the nuclear fuel material is transferred outside the reactor. It has a structure that allows it to be taken out.

Nuclear fuel elements generally consist of a Zircaloy or stainless steel cladding tube filled with pellets of uranium dioxide or a mixed oxide of uranium and plutonium, and both ends of the cladding tube are closed with end plugs and welded. The plenum part is designed to collect the fission product gas produced by the nuclear fission reaction of the pellet, to alleviate the increase in internal pressure due to the increase in the products, and to extend the creep rupture life of the element due to the increase in pressure. and a spring that restricts the axial direction of each pellet so that its position does not move during transportation.

The cladding has two main purposes: the first is to prevent chemical reactions between the nuclear fuel pellets and the coolant, and the second is to prevent highly radioactive fission products from being released into the coolant. It is to do so.

Localized failure of the cladding can result in contamination of the coolant with fission products that have long half-lives, resulting in significant disruption to plant operation and management.

Furthermore, external devices such as heat exchangers and turbines through which the coolant passes are also contaminated, so it is necessary to detect damage to the cladding tube at an early stage and take measures to deal with the damaged fuel.

For this reason, the overall radioactivity of the reactor coolant and generated gas is usually monitored during operation, and if the radioactivity increases, it is likely that a leak has occurred in one of the cladding tubes in the equipment, so excessive Reactor operations were shut down before coolant contamination occurred.

In order to determine which fuel element in a fuel assembly has failed, it has been necessary to continuously or intermittently take samples of the coolant flowing from each fuel assembly and to collect them at a location remote from the reactor. Attempts are being made to monitor the radioactivity of each sample in established laboratories.

However, intermittent sampling can result in significant failures between samples for a given fuel assembly, resulting in problematic contamination of the coolant.

In addition, in order to continuously collect and inspect samples from each of the hundreds of fuel assemblies loaded in the reactor, a huge amount of equipment and complicated analysis equipment for the sample collection tubes are required. There is a problem.

Therefore, in order to solve these problems, nuclear fuel rods are divided into individual fuel rods or assemblies, or into several groups, and mixed gases called tag gases, which have different mixture compositions at the isotope level, are sealed in advance in the nuclear fuel rods. Attempts have been made to sample the tag gas flowing out from the damaged fuel from the cover gas or coolant, analyze its composition, and identify the damaged fuel.

The tag gas is inert to nuclear reactor structural materials, nuclear fuel materials, coolants, moderators, etc., and includes one or a mixture of rare gases such as argon, xenon, and neon.

Conventional nuclear fuel rods containing tag gas are made of, for example, aluminum,
One idea is to seal a tag gas of known composition into a capsule made of stainless steel or other material, insert it into a cladding tube during fuel rod manufacture, seal the cladding tube, and then open the capsule to fill the fuel rod. However, on a production line for mass-produced nuclear fuel rods, it is difficult to fill each nuclear fuel rod with a predetermined amount of tag gas, which has a different composition, when the cladding tube is hermetically welded, and it also increases the number of manufacturing steps. , which is not desirable due to high cost.

Therefore, the conventional method of opening the above-mentioned capsule after sealing the cladding tube is to seal the opening of the capsule with a low-melting metal sealing material, melt the sealing material at the working temperature of the nuclear fuel rod or with local heating, and open the capsule with the tag gas. Nuclear fuel rods are also known in which nuclear fuel is released into a cladding tube.

Another known method is to apply impact force from the outside to a capsule loaded in a cladding tube to break the thin film.

However, in these methods, the molten sealant comes into contact with the fuel pellets and cladding, producing reactants and sometimes corroding the cladding, causing damage.

In addition, in order to apply an impact force, it is necessary to encapsulate a movable object or apply an impact to the nuclear fuel rod itself, which reduces the effective volume of the plenum of the nuclear fuel rod and shortens the service life of the fuel. There are certain drawbacks.

An object of the present invention has been made to eliminate the above-mentioned drawbacks.The purpose of the present invention is to reduce the rate of decrease in the volume of the gas plenum, to reliably open the seal and release tag gas into the cladding tube, and to release the tag gas into the cladding tube and fuel pellets. An object of the present invention is to provide a nuclear fuel rod containing tag gas that does not cause problems such as corrosion.

That is, the present invention provides a nuclear fuel rod in which a capsule filled with Doug gas is enclosed in the plenum part of a long cladding tube loaded with nuclear fuel material, and the tube end is hermetically sealed, in which a part of one end surface of the capsule is made of a thin film part. A protrusion is provided opposite to the thin film part, and a disc-shaped bimetal made of at least one material having a different coefficient of thermal expansion is disposed at a position where the protrusion is driven, and the bimetal is thermally deformed. The nuclear fuel rod is characterized in that the thin film portion is opened by the piercing portion.

Further, in the nuclear fuel rod, the disk-shaped bimetal is formed into a dish shape that can be reversed due to the generation of a difference in thermal expansion, and the diaphragm is opened by the displacement and impact force accompanying this reversal.

An embodiment of the present invention will be described in detail below with reference to FIGS. 1 to 3.

FIG. 1 shows a tag gas capsule 1 according to the present invention and a nuclear fuel rod 2.
2b is a cladding tube, 2C is a pellet, and 2d is a spring.

In the tag capsule 1, an end plate 4a having a gas introduction pipe 4 is provided at one end of a tube 1a, and a thin film portion 6 forming an opening when the seal is opened is integrally formed with the end plate 5a at the other end.

The tag gas inlet 5 of the gas inlet pipe 4 is sealed by pressure welding, welding, etc. after a predetermined amount of the tag gas 3 is introduced into the capsule 1.

In this way, a predetermined amount of the tag gas 3 is sealed in the capsule 1.

On the other hand, the barbed end 7 of the piercing member 11 is disposed facing the thin film part 6 at a constant distance (G) from the thin film part 6 at room temperature.

Furthermore, a plurality of disc-shaped bimetals 10 made of materials with different thermal expansion coefficients are stacked on the back of the piercing member 11, and each disc-shaped bimetal 10 has a material side with a high expansion coefficient (hereinafter referred to as a high expansion side). These members are housed in a housing 9 attached to the end of the capsule tube 1a on the thin film portion 6 side.

Further, the housing 9 and the bimetal 10 are provided with tag gas vents 8.8a.

By the way, as shown in FIG. 2a, the disk-shaped piemetal 10 has a high expansion side 10a and a low expansion side 10b as described above, so when it is heated, it bends into a disk shape as shown in FIG. The bimetal 10 deforms as if it were a disc spring.

As a result, a material having a thickness hl) at room temperature increases to an apparent thickness h2) in a heated state in proportion to the temperature.

In this way, due to the displacement of each bimetal 10 stacked back to back, the distance q from the thin film part 6 shown in FIG. 1 disappears, and finally the barbed end 7 penetrates the thin film part 6 as shown in FIG. The tag gas 3 in the capsule 1 can be diffused into the nuclear fuel rod 2 by doing so.

Here, the length of the thin film portion 6 and the one-curve coefficient, thickness, and number of bimetallic parts 10 can be arbitrarily designed in accordance with the opening temperature conditions of the tag gas capsule 1.

In addition to these conditions, the setting of the distance G between the barbed end I and the thin film portion 6 can also be selected taking into consideration the manufacturing process and handling of the capsule and fuel rod.

Next, another embodiment of the present invention will be described with reference to FIGS. 4 and 5.

As shown in FIG. 4, a disc-shaped bimetal 20 having a ventilation hole 8a in the center is formed into a slightly dish-shaped shape using a die 15 and a punch 16.degree. 17 having a hemispherical convex and concave tip.

At this time, the high expansion side 20a is made into a concave shape facing toward the outer surface of the arc.

Next, as shown in FIG. 5a, this dish-shaped disc-shaped bimetal 20 is inverted and displaced so that the concave shape is opposite to that when molded, that is, the high expansion side is inside the arc (as shown in the figure), and a The member 11a having the barbed end 7 is attached.

This is placed near the thin film portion 6 so that the peripheral edge of the disc-shaped bimetal 200 is supported by the protrusion 21 machined into the housing 9a.

The dish-shaped disc-shaped bimetal 20 is thus reversely displaced.
The barbed end 7 gradually deforms as it is heated, and finally, at the moment when it is displaced into a dish shape during molding, the barbed end 7 shown in FIG.
penetrates the thin film portion 6 and can diffuse the tag gas 3 into the plenum portion 2a within the nuclear fuel rod 2.

The bimetal used here is Fe. A composite material of two types of alloys containing Ni and Cr as all components may be used.

As described above, according to the present invention, a capsule filled with tag gas for detecting damage to a fuel rod in a nuclear reactor can be opened at any temperature in the reactor.

Furthermore, the effective volume of the plenum part is reduced at a reduced rate, and the tag gas capsule can be reliably opened in the furnace without fear of corrosion to the fuel pellets and cladding.

The production of nuclear fuel rods with tag gas capsules having this structure does not require any special process for opening the capsules on the standardized production line, and therefore it is possible to recover and reuse capsules from defective nuclear fuel rods during inspection. It is also possible to provide each fuel rod containing a tag gas capsule, which is also effective for making the assembly of fuel rods more efficient.

[Brief explanation of the drawing]

FIG. 1 is a partial vertical sectional view showing an embodiment of the present invention, FIGS. 2a and b are diagrams explaining thermal displacement of the opening bimetal used in FIG. 1, and FIG. FIG. 4 is an enlarged cross-sectional view to explain the unsealed state of the unsealing structure; FIG. 4 is a diagram illustrating the forming process of a dish-shaped disc-shaped bimetal used in another embodiment of the present invention; FIG. 5a , b are sectional views showing enlarged main parts of other embodiments of the present invention. 1... Capsule, 2... Nuclear fuel rod, 2
a...Plenum part, 2c...Pellet, 3...Tag gas, 4...Gas introduction pipe, 6...Thin film part, 7 ...barbed end, 8.
8a...Vent hole, 9...Housing,
10゜20・・・Disc-shaped bimetal.

Claims (1)

[Scope of Claims] 1. In a nuclear fuel rod in which a capsule filled with tag gas is sealed in the plenum part of a long cladding tube loaded with nuclear fuel material and the tube end is tightly plugged, a part of one end surface of the capsule is sealed. A thin film part is provided, a protrusion is provided opposite to the thin film part, and a disc-shaped bimetal made of at least one material having a different coefficient of thermal expansion is disposed at a position where the protrusion is driven. A nuclear fuel rod containing a tag capsule, characterized in that the thin film part is opened by the piercing part by thermal deformation. 2. The nuclear fuel rod according to claim 1, wherein the disc-shaped bimetal has a dish shape that is reversely displaced due to a difference in thermal expansion.