JPS6042434B2 - nuclear fuel elements - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel element, and more particularly to a nuclear fuel element equipped with a capsule that is inserted into a space within a cladding tube and has a structure that allows the capsule to be easily opened.

In a typical nuclear reactor, a core consists of a number of nuclear fuel elements each containing a fissile material housed in a heat-resistant cladding tube, which are spaced apart from each other, and a coolant is circulated inside the core. The thermal energy released from the nuclear fuel material during the fission reaction is extracted outside the reactor and utilized.

Each fuel element is, for example, U-23, U-23 & or P
It is generally composed of a substance at U-23. The cladding has two primary purposes: first, it prevents the chemical reaction between the fuel element and the coolant or moderator, and second, it prevents the fission products, which are highly radioactive, from being cooled. The aim is to avoid release into the fuel or moderator. For these purposes, the cladding tube is made of zirconium alloy, stainless steel, or the like. Local failure of the cladding can lead to contamination of the coolant or moderator with long-lived fission products, posing a problem to plant operation and management. Furthermore, there is a risk that the heat exchanger through which the coolant passes and external equipment such as the turbine may be 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,
Normally, the total radioactivity of the reactor coolant and generated gases is monitored during operation, and any rise in radioactivity would indicate a leak in the cladding somewhere within the system, so excessive It is common practice to shut down reactor operations before such contamination of the coolant occurs. Of course, in the method of monitoring total radioactivity,
It is not possible to determine the location of a damaged nuclear fuel element from the large number of nuclear fuel elements loaded. Conventional methods involve taking samples continuously or intermittently from the coolant flowing out of the nuclear fuel assembly inside the reactor, and measuring the radioactivity of each sample in a sample chamber located away from the reactor. Attempts are being made to monitor the However, if samples are taken intermittently,
Significant failures can occur between samplings for a given nuclear fuel assembly, resulting in significant contamination of the coolant.

In addition, in order to continuously collect and test samples from each of the hundreds of nuclear fuel assemblies, an enormous amount of equipment, a complex array of sample collection tubes, and complex analysis equipment are required. In order to reduce these difficulties, a gas mixture with a different composition (generally called tag gas) is sealed in advance for each nuclear fuel element, and the composition of the gas sample taken from inside the reactor is analyzed. A method is being considered to determine which nuclear fuel element has been damaged.

The sealed mixed gas must be inert to reactor structural materials, nuclear fuel materials, coolants, and moderators, and neon, argon, etc. are used, for example. As a method of enclosing these mixed gases into a nuclear fuel element, for example, predetermined mixed gases are sealed in a capsule made of aluminum or stainless steel, inserted into a cladding tube at the time of manufacturing the nuclear fuel element, and the cladding tube is sealed. After that, the capsule is opened and filled with nuclear fuel elements.

This is because the fuel elements are manufactured on a standardized mass production line, which makes it difficult to fill the prescribed amount of gas with a different composition for each fuel element when sealing the cladding tube, which requires a lot of man-hours. This is because the cost is extremely high. 1st
The figure shows an example of a nuclear fuel element used in a fast breeder reactor, etc. Ceramic fuel pellets 2, which are nuclear fuel elements, are stacked and stored in a long and thin cladding tube 1, and the gas generated during nuclear fission is A plenum section 3 for storing fission products (FP gas) is provided, and end plugs 4 are installed at both ends of the cladding tube 1.
, 5 are welded together to form a sealed structure.

In the plenum part 3, nuclear fuel pellets 2 are emitted as a result of a nuclear fission reaction. This space has an appropriate volume and is designed to accommodate the nuclear fission product gas generated and to prevent the pressure within the cladding tube 1 from rising too much during its lifetime. Normally, the plenum part 3 is designed to prevent the fuel pellets 2 from moving within the cladding tube 1 after the nuclear fuel elements are processed and before being loaded into the reactor.
It is held by a plenum spring 6 and a compression body 7. A capsule 8 containing a tag gas is normally placed between a plenum spring 6 and a compression body 7 for retention within the cladding tube. Therefore, the method for opening the capsule 8 after sealing the cladding tube is as follows (Japanese Unexamined Patent Publication No. 49-50391).
)It has been known.

That is, as shown in FIG. 2, a part of the capsule 8 is sealed with a low melting point alloy 15, and then melted at high temperatures during reactor operation. This method has the advantage that the structure for releasing the tag gas 12 from the capsule is simple, easy to manufacture, and that the capsule can be reliably opened under given temperature conditions. In addition, since the structure is simple, the effective volume of the gas plenum portion is less reduced, which is advantageous in terms of the mechanical strength of the nuclear fuel element. However, using such a low melting point alloy as a sealing material requires thorough consideration of the coexistence of the alloy with the cladding tube and the sealing performance. That is, in the fuel element installed in the reactor, the capsule is opened by melting the low melting point alloy used to open the capsule, but if the low melting point alloy adheres to the inner surface of the cladding tube, As the nuclear fuel element stays in the reactor, corrosion of the cladding tube progresses and there is a risk that the cladding tube will be damaged, which poses a problem. In order to reliably seal the tag gas inside the capsule during the manufacture of nuclear fuel elements or until the nuclear fuel element is installed in a reactor, it is necessary that the low melting point alloy has good sealing properties. However, when dissimilar metals are alloyed, their mutual bonding strength increases, which improves the sealing performance, but on the other hand, it also increases corrosion if it adheres to the cladding tube. In other words, it is extremely difficult to develop a low melting point alloy that satisfies the contradictory requirements of sealing performance and corrosion resistance at the same time, especially for fast breeder reactors where the normal operating temperature is around 600°C. In the case of fuel, consideration must be given to corrosion, and in general, low melting point metals are particularly susceptible to corrosion. Therefore, in the conventional example shown in FIG. 2, when the low melting point metal 15 of the capsule melts, the tag gas 12 in the capsule is released into the nuclear fuel element, and the low melting point metal 15 is prevented from scattering into the nuclear fuel element. To prevent this, a porous plug 14 is installed at the end of the capsule 11, but due to the adhesion of molten low melting point metal,
The porous plug may become clogged and the tag gas may not be released sufficiently. It is also possible that the porous plug may not completely prevent diffusion of low melting point metals into the nuclear fuel element. The present invention has been made in consideration of the above-mentioned circumstances, and aims to obtain a nuclear fuel element having a capsule mechanism that has a simple structure and operates reliably without using a low-melting point metal.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

As shown in FIG. 3, in the nuclear fuel element of the present invention, a plurality of fuel pellets 2 are filled in a cladding tube 1 in a layered manner, the openings at both ends of the cladding tube 1 are sealed with end plugs 4 and 5, and fuel The pellet 2 is held by a plenum spring 6 and a compression body 7, and a tag gas-filled capsule 8 is arranged between the plenum spring 6 and the compression body 7. FIG. 4 shows an embodiment of the structure for manufacturing a tag gas capsule according to the present invention.

In this structure, a tag gas release mechanism 26 is inserted into the lower part of the tag gas capsule body 21, and the capsule is sealed by closing the tag gas inlet 23. The tag gas release mechanism 26 is inside the capsule 2 in which the tag gas is sealed.
2 and a permanent magnetic plug 16 separating the capsule external plug 25.
and a permanent magnet plug 1 shown in FIG.
6 is comprised of an auxiliary stopper 19 that holds it in the fallen state. Further, the upper end 24 and the ring 17 are welded together, and a metal seal 20 is inserted between the ring 17 and the permanent magnet plug 16. Figure 6 shows the capsule after manufacturing.
This shows another example of the manufacturing structure of the tag gas capsule, which aims to improve the sealing performance until it is inserted into the nuclear fuel element. A screw 28 is attached to the.

The capsule is filled with tag gas through the inlet 28 and stored until it is inserted into a nuclear fuel element, and each nuclear fuel element is assembled into a fuel assembly having the form of a large number of bundles. When the fuel assembly is loaded into the reactor core, the capsule inserted into the nuclear fuel element is exposed to the high temperature of the reactor core, and the permanent magnet plug 16 exceeds the Curie temperature and is demagnetized, causing permanent magnetization as shown in FIG. The magnetic stopper 16 falls due to gravity,
Enclosed in nuclear fuel elements. The tag gas, which is heavier than the helium present, exits through the small holes 18 and diffuses into the nuclear fuel element.

The modification shown in FIG. 6 takes into account the case where the magnetic force of the permanent magnet stopper 16 alone is insufficient for sealing, and the screw 28 is tightened to prevent the tag gas from entering the nuclear fuel element of the capsule. Improves adhesion until insertion. In this case, when sealing helium into the nuclear fuel element, remove the screw 28, insert the capsule into the nuclear fuel element,
Prevent harmful outside air from entering. FIG. 7 shows experimental data showing how the magnetization of a ferromagnetic material (nickel) disappears as it approaches the Curie temperature, and Table 2 shows the Curie temperature of the ferromagnetic material.

Me(T) indicates magnetization at temperature T, and Tc indicates Curie temperature. The materials for the permanent magnet 16 and the ring 17 are materials that have an appropriate Curie temperature considering the usage conditions in the core, generate sufficient magnetic force, and are compatible with the cladding tube at high temperatures, such as the materials shown in Table 2. Choose from among them. Also,
As the material for the metal seal 20, a metal O-ring made of stainless steel or the like is used, for example, in consideration of corrosion resistance at high temperatures. According to the present invention, the unsealing mechanism is constructed of ferromagnetic materials such as nickel and ferrite materials that are stable at high temperatures, instead of using low melting point metals that have problems with coexistence with the cladding tube at high temperatures as in the conventional example. Since it takes advantage of the demagnetization property of ferromagnetic material at the Curie temperature and the fall due to gravity, the operating time can be shortened and a reliable opening operation can be expected. In addition, if it is considered that magnetic force alone is insufficient for sealing, the auxiliary mechanism shown in Figure 6 can be used to prevent this from occurring during storage after filling the tag gas and manufacturing the capsule until it is inserted into the fuel element. It can be supplemented.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view showing a conventional nuclear fuel element, FIG. 2 is a vertical cross-sectional view showing a conventional tag gas-filled capsule, FIG. 3 is a vertical cross-sectional view showing an embodiment of the nuclear fuel element of the present invention, and FIG. , FIG. 5 is a longitudinal sectional view showing the main part of the present invention, FIG. 6 is a longitudinal sectional view showing the main part of another embodiment of the invention, and FIG. 7 is an explanatory diagram showing temperature change in magnetization. . 1... Cladding tube, 2... Fuel pellets, 4, 5...
End plug, 6... Plenum spring, 7... Compression body,
8... Capsule, 16... Stopper, 17... Ring,
19... Auxiliary plug, 27... Screw hole, 28... Screw.

Claims (1)

[Claims] 1. A nuclear fuel element comprising a capsule filled with tag gas and a plurality of fuel pellets filled in a cladding tube, wherein the capsule includes a ring made of a ferromagnetic material fixed to cover an end of the capsule; A permanent magnet plug is attached to the ring by magnetic force so as to seal the opening of the ring, and the Curie temperature of the permanent magnet is higher than the temperature of the permanent magnet when the nuclear fuel element is assembled. high,
A nuclear fuel element characterized in that the temperature is lower than the normal operating temperature of the permanent magnet.