JPS64636Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a nuclear fuel rod, and more particularly to a nuclear fuel rod that prevents mechanical interaction between pellets and cladding and increases the plenum volume.

[Technical background of the invention and its problems]

The configuration of a conventional nuclear fuel rod will be explained using FIG. 2. The cladding tube 4 is filled with a plurality of pellets 3 made of fired nuclear fuel material and a plenum spring 2 that presses and fixes the pellets 3. The pellets 3 are loaded into a cladding tube 4 whose lower end is welded to a lower end plug 5, and the upper end of the cladding tube 4 is sealed by welding the upper end plug 1 to the upper end.

Generally, a plurality of nuclear fuel rods are bundled together and transported as a nuclear fuel assembly from a manufacturing plant to a power plant facility.

The plenum spring 2 prevents the pellets 3 in the nuclear fuel rods from moving in the axial direction during transportation and colliding with each other.
Alternatively, it is inserted for the purpose of preventing the pellets 3 from being damaged due to vibration in the radial direction and colliding with the cladding tube 4, and to restrain the movement of the pellets 3.

During operation of the nuclear reactor, the pellets 3 become hot due to nuclear fission and undergo large thermal expansion. Furthermore, the fission products generated by the nuclear reaction are accumulated in the space of the plenum part 6, and some of the fission products accumulate in the pellet 3 and cause irradiation growth, so that the pellet 3 Expand. As mentioned above, the pellet 3 thermally expands in the axial or radial direction during irradiation.
It expands by irradiation.

At the beginning of combustion, a gap exists between the pellets 3 and the cladding tube 4. However, as the combustion progresses, the cladding tube 4 also thermally expands, but the pellet 3 exhibits an expansion amount exceeding that due to thermal expansion or irradiation expansion, so that the cladding tube 4 gradually closes during the initial stage. Furthermore, when the pellet 3 expands, it comes into contact with the inner surface of the cladding tube 4 and mechanical interaction begins, and if excessive expansion occurs, a large stress is generated in the cladding tube 4, which may lead to fuel failure. .

As mentioned above, the pellet 3 also expands in the axial direction. However, the presence of the plenum spring 2 prevents this free expansion, restricting the axial movement of the pellet 3 and causing it to expand in the radial direction.
Therefore, stress due to mechanical interaction is further generated in the cladding tube 4.

In addition, as mentioned above, during reactor operation, pellets 3
Fission products are generated by a nuclear reaction and accumulated in the space of the plenum section 6, but the volume of the plenum section 6 is lost due to the presence of the plenum spring 2. When the volume of the plenum part 6 decreases,
As some of the fission products accumulate, the internal pressure increases, generating large stress in the cladding tube 4, which may lead to fuel failure.

In this way, in the prior art, the plenum spring 2, which is inserted for the purpose of maintaining the physical integrity of the pellet 3 during fuel transportation, promotes mechanical interaction between the pellet 3 and the cladding tube 4 during reactor operation. Furthermore, there is a drawback that this results in a loss of volume in which fission products are accumulated in the plenum section 6, increasing the risk of fuel failure.

Although the plenum spring 2 is a necessary component for maintaining the physical integrity of the pellet 3 during fuel transportation, it is rather undesirable since it increases the risk of fuel damage during reactor operation.

Typically, the forces acting on the plenum spring 2 during transportation are small.

[Purpose of invention]

The first purpose of this invention is to prevent the movement of the pellet 3 due to a small force during transportation, but this function is lost when the temperature rises to a certain extent due to a nuclear reaction, so that the free expansion of the pellet 3 is not restricted. We will provide nuclear fuel rods that have been developed.

The second purpose of the present invention is to improve the spring of the plenum part above the nuclear fuel rod and increase the plenum volume.

[Summary of the idea]

In the present invention, a cladding tube 4 in a nuclear fuel rod is filled with pellets 3 in a layered manner, and both ends of the cladding tube 4 are provided with end plugs 5.
The present invention provides a nuclear fuel rod which is sealed using a fuel rod and is constructed with a fixed cap 7 made of an irreversible shape memory alloy between the upper end plug 1 and the final end of the pellet 3 stack.

[Example of idea]

Embodiments of the present invention will be described below with reference to the drawings.

In the nuclear fuel rod of the present invention, as shown in Fig. 1, a cladding tube 4 is filled with pellets 3 in a layered manner, and a fixed cap 7 made of a shape memory alloy is attached to the pellet ₃₀ at the final end.
It is characterized by having been installed.

The fixed cap 7 has a truncated conical shape, and the tip is smaller than the inner diameter of the cladding tube 4, and the upper end is slightly larger. The side surfaces form slits and come into contact with the inner surface of the cladding tube 4 due to elastic force, restraining the movement of the pellets 3 and maintaining their physical integrity.

On the other hand, during reactor operation, the outer diameter of the upper end of the fixed cap 7 contracts due to the heat generated by the nuclear reaction of the pellets 3, and the inner diameter of the cladding tube 4 becomes smaller. Therefore,
Free expansion is allowed and the pellets 3 are not subjected to compressive forces and therefore do not increase mechanical interaction with the cladding tube 4. In addition, the volume of the plenum is 30 to 40% larger than the conventional 25 to 31 cm ² , which alleviates the rise in internal pressure of the nuclear fuel rods and reduces fuel rod breakage.

As the shape memory alloy constituting the fixed cap 7 according to the present invention, a shape memory alloy made of nickel or titanium, which has excellent spring elasticity and high corrosion resistance, is preferably used.

The fixed cap 7 of the present invention is inserted into the fuel rod as follows. That is, a cylindrical shape memory alloy is previously processed into a cylindrical shape with some grooves as shown in FIG. Then add this to 80-100
The shape shown in FIG. 4 is memorized by immersing it in boiling water at ℃. Thereafter, it is taken out from the hot water and expanded into a conical shape at room temperature to form the shape shown in FIG. This is inserted into the final end of the stacked pellets, and the upper end is welded together with the end plug to complete the assembly.

[Effect of idea]

The fuel rod of the present invention assembled in this way is designed to withstand a small force in the axial direction, so the movement of the pellets 3 due to vibrations during fuel transportation is restricted and the physical integrity is reduced. It is maintained.

On the other hand, even if the pellet 3 undergoes thermal expansion and irradiation expansion during reactor operation, the heat from the nuclear reaction remains at 80 to 100℃.
When the temperature reaches , the fixed cap 7 of the shape shown in FIG.
is restored to the shape shown in FIG.

In this way, even if the axial expansion of the pellet 3 increases, the pellet 3 can expand freely, and the cladding tube 4
The advantage is that there is no increase in mechanical interaction with the

[Brief explanation of the drawing]

Figure 1 shows an embodiment of the nuclear fuel rod of the present invention, and the fixed cap is a vertical cross-sectional view showing the shape during transportation. Figure 2 is a schematic diagram of a conventional nuclear fuel rod. Figure 3 is the nuclear fuel rod of the present invention. Fig. 4 is a perspective view of the fixed cap of the present invention, and Fig. 5 shows another embodiment of the fixed cap shown in Fig. 4 when the fixed cap is expanded. FIG. 1...Top end plug, 2...Plenum spring,
3... Pellet, 4... Cladding tube, 5... Lower end plug, 6... Plenum part, 7... Fixed cap.

Claims (1)

[Scope of utility model registration request] A cladding tube, pellets filled in the cladding tube in a layered manner, upper and lower end plugs that seal the upper and lower parts of the cladding tube, and a pellet inserted between the pellets and the end plug in the cladding tube, and at room temperature. A nuclear fuel rod comprising a fixed cap made of a shape memory alloy that expands in diameter to prevent the movement of pellets, and contracts in diameter when the temperature rises so as not to restrict the movement of the pellets in the longitudinal direction of the cladding axis.