JPS5923396B2 - Nuclear fuel rod with tag gas capsule - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel rod containing a tag gas capsule suitable for operation management of a nuclear reactor.

Nuclear fuel rods are generally made by filling a Zircaloy or stainless steel cladding with pellets of uranium dioxide or a mixed oxide of uranium and plutonium, and closing both ends of the cladding with end plugs and welding. Collects the fission product gas generated by the nuclear fission reaction of the pellet,
and a plenum part for alleviating the increase in internal pressure due to the increase in products and extending the creep rupture life of the element due to the increase in pressure, and a shaft to prevent the position of each pellet from shifting during transportation. It is composed of a spring that regulates the direction.

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 normally monitored during operation, and if the radioactivity rises, 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 from the coolant flowing from each fuel assembly and to collect the samples 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.

As the tag gas, rare gases such as argon, xenon, neon, etc., which are inert to nuclear reactor structural materials, nuclear fuel materials, coolants, continuous materials, etc., are used singly or in combination.

Conventional nuclear fuel rods containing tag gas are made of, for example, aluminum,
It is conceivable that a tag gas of known composition is sealed in a capsule made of stainless steel, etc., inserted into a cladding tube during fuel rod manufacture, and after the cladding tube is sealed, the capsule is opened and the fuel rod is filled with the tag gas. 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, during seal welding of the cladding tube, and it requires a large number of manufacturing steps. However, the cost is high and it cannot be said to be preferable.

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.

Another known method is to construct a double-tube structure using materials with different coefficients of thermal expansion, and open the package by elongation due to heating of the material with a larger coefficient of thermal expansion.

However, in these methods, the molten sealing material comes into contact with the fuel pellets and the 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 useful life of the fuel. There are drawbacks.

Furthermore, the method of utilizing the difference in elongation of the double tubes has the disadvantage that it is difficult to obtain a large difference in expansion, and therefore the capsule must be designed to be long, which reduces the effective volume of the plenum.

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 a tag gas is sealed in a space inside a cladding tube loaded with nuclear fuel material, and a thin film part is provided on one end surface of the capsule,
A bimetal made of two types of materials with different coefficients of thermal expansion is thermally deformed by the hollow piercing portion opposite to this, which generates a driving force in the axial direction, thereby perforating the thin film portion and opening the capsule.

This tag gas-containing nuclear fuel is characterized by utilizing the fact that the bimetal is pre-processed into a figure 8 shape, and the curved portion of the bimetal changes its curvature when heated, thereby elongating in the axial direction.

An embodiment of the present invention will be described in detail below.

FIG. 1 shows the structure of a tag gas capsule according to the present invention.

The capsule tube 1 has one end sealed with a thin film part 4, and a plug 10 having a tag gas introduction lower at the other end sealed by welding or the like, and after the tag gas is sealed into the capsule from the gas introduction lower, the tip end is sealed.

On the other hand, the curved bimetal 2, which is the unsealing mechanism, is constructed in a figure-eight shape, with the high expansion side facing inward as shown in the figure.

Each intersection point 8 is joined and fixed by a welding point 6.

One end of the bimetal 2 is in contact with the plug 10 of the capsule, and the penetrator 3 is arranged at the other end with a gap 9 between it and the thin film part 4.

As shown in FIG. 2, a capsule configured in this manner may be designed by flattening a portion of the capsule tube to stabilize the position of the bimetal 2 in the capsule.

In this constructed tag gas capsule, the high expansion side of the bimetal 2 is on the inside due to heating by the furnace loading or some kind of preheating equipment, and the elongation is relatively larger than the outside on the low expansion side.As a result, as shown in FIG. The amount of change in the axial direction of l occurs as shown in .

Since such elongation occurs at each curved part of the bimetal, they advance the penetrator 3 and fill the gap 9,
As a result of further displacement, an opening occurs in the membrane 4 and the tag gas is released into the fuel rod.

Therefore, the pushing load of the penetrator at the time of opening and the thickness of the thin film part are determined from the temperature conditions during use and manufacturing aspects, but in general, the pushing load due to deformation of the bimetal is determined by selecting various thicknesses for the same material. You can design the opening time.

Furthermore, the thickness of the thin film part ranges from several μm to several μm due to condition selection.
It can be varied within a range of several 100 μm.

Furthermore, it is necessary to consider the bimetal material to be a material that does not corrode or react with the fuel body or cladding material during use.

Therefore, it is desirable that the bimetallic material be a composite material of two types of materials containing Ni, Cr, and Fe as main components.

As described above, according to the present invention, it is possible to open the capsule filled with tag gas for detecting damage to fuel rods in the reactor at any temperature in the reactor. Furthermore, the effective volume of the plenum can be reduced. The rate of corrosion is also relatively small, and the tag gas capsule can be reliably opened in the furnace without worrying about corrosive effects on fuel pellets and cladding tubes.

The production of nuclear fuel rods with tag gas capsules having this structure does not require a special process for opening the capsules on the standardized production line, and therefore capsules can be recovered even from inspected defective nuclear fuel rods. It is possible to provide a nuclear fuel rod containing a tag gas capsule that can be reused and is effective for automating the assembly of nuclear fuel rods.

[Brief explanation of drawings]

FIG. 1 is a longitudinal section of the capsule of the present invention, FIG. 2 is a cross-sectional view of the capsule of the present invention, and FIG. 3 is an explanatory diagram of a modified bimetallic structure. 1... Capsule tube, 2... Bimetal, 3... Penetrator, 4...
・Thin film part, 6...Welded part of bimetal intersection.

Claims (1)

[Claims] 1. A capsule container having an opening so that the capsule is communicated with the space inside the cladding tube in a nuclear fuel rod that is sealed by enclosing a capsule filled with tag gas in the space inside the cladding tube loaded with nuclear fuel material. and a thin film part provided to close the opening of the capsule container, and a perforation in the thin film part provided opposite to the thin film part by piercing the thin film part from the inside of the capsule container. and a hollow part provided with a tag gas flow part that allows the tag gas in the capsule container to flow in the cladding tube when the hole is perforated, and two sheets disposed between the hollow part and the inner bottom of the capsule container. A fixed part and a curved part are provided in the bimetal so that the high expansion sides face each other and form a figure 8 shape in the longitudinal direction,
A nuclear fuel rod containing a tag gas capsule, characterized in that the movable body is enclosed within the capsule container so as to have a driving force in the direction of perforating the thin film portion due to thermal deformation.