JPS58186078A - Nuclear fuel element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel element, and more particularly, fuel pellets are filled in a cladding tube in a layered manner, and the upper and lower ends of the cladding tube are sealed with end plugs, respectively, and the fuel pellets are inserted between these end frames. It relates to nuclear fuel elements held fixed by springs between them.

In general, a fuel element used in a boiling water reactor is made by filling a zirconium alloy cladding tube 1 with uranium dioxide fuel pellets 2 in a layered manner, as shown in Figure 1.
The upper and lower ends of the tube are sealed with an upper end plug 3 and a lower end plug 4, respectively. A space (Blenheim part) 5 is provided above the fuel element to hold the fission product gas released from the fuel pellet 2 by the fission reaction.
A stainless steel Blenheim spring 6 is built in to suppress and fix the fuel pellet 2 so that it does not vibrate, and a stainless steel cylindrical container 7 containing a getter is provided inside the Blenheim spring 6. There is.

Further, known shapes of the fuel pellets 2 include a dish pellet as shown in FIG. 2, a Hiko chamfer pellet as shown in FIG. 3, and a hollow pellet as shown in FIG. 4.

When a fuel element filled with such a pellet is loaded into a nuclear reactor and the reactor is operated, a temperature distribution occurs within the pellet. In this temperature distribution, the temperature T
is high at the center C of the pellet, and the surface L1. It becomes low at Lt.

Furthermore, if a hollow pellet as shown in Figure 4 is used, it is expected that the temperature distribution will be somewhat flattened as shown by the broken line in Figure 6, but for any of the pellets, the heat distribution in the radial direction is The fuel design is designed to be symmetrical.

However, it is unlikely that the radial heat distribution of all fuel rods in the assembly is axially symmetrical. 41! j
In the case of LC boiling water reactors, the flow path within the channel and the bypass flow path are completely separated by a channel box, and while there is almost no boiling in the bypass flow path, there is a high void ratio within the channel. It is expected that M conversion of neutrons will be better on the bypass flow path side and worse on the inside of the channel. The heat distribution within the fuel pellet is affected by the thermal neutron flux I# surrounding the fuel element, so if we compare the heat distribution within the pellet between the fuel elements on the outer periphery of the assembly and the fuel elements inside the assembly, it is possible to determine the bypass flow. It is thought that the distortion from the axial symmetry of the heat generation distribution increases in the fuel elements near the outer periphery of the assembly near the road, and particularly in the fuel elements near the control position when no control rods are inserted. If the deviation of the heat distribution within the pellet from the axis symmetry becomes large, a local peak output will occur, and it is expected that the temperature of the fuel cladding tube in the vicinity will increase, resulting in performance deterioration.

As described above, in conventional fuel elements, the heat generation distribution deviates from the axially symmetrical distribution, resulting in the above-mentioned problems. As examples of pellets with different degrees of concentration in the radial direction, there are also known pellets having a structure consisting of cylindrical pellet layers 8, 9, and 10 with mutually different degrees of concentration, as shown in FIG. These are aimed at flattening the temperature distribution (FIG. 6), and are not effective in improving the asymmetry of the heat generation distribution.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art and provide a nuclear fuel element that can improve the asymmetry of heat distribution within the pellet and improve the integrity and safety of the fuel cladding. .

That is, in the present invention, fuel pellets filled in a layered manner in a cladding tube are held via springs by end plugs that seal the upper and lower ends of the cladding tube, respectively, and are arranged in a channel box to form a fuel assembly. In a nuclear fuel element configured to form a body, each nuclear fuel element is formed by two fuel pellet parts that are symmetrical with respect to the tube axis of the cladding tube and have mutually different enrichments, and when placed in the channel box, these two fuel pellet parts are formed. The present invention is characterized in that the two fuel pellet portions can be arbitrarily positioned in the direction of the inside and outside of the box.

Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 7 shows a fuel cladding tube 1 containing a fuel element 12 of the present invention.
1 is a diagram schematically showing a state in which a large number of channels 1 are arranged in a channel box 13. FIG. The basic structure of the fuel element 12 is the same as that shown in FIG. Two fuel pellet parts A
and B. In the figure, A indicates a fuel pellet portion with a low enrichment concentration, and when the cladding tube 11 is disposed in the channel box 13 at a position near the bypass flow path (not shown) on the outer periphery of the assembly, the area outside the box, That is, it is positioned and arranged on the side facing the bypass flow path. On the other hand, B indicates a fuel pellet portion having a higher concentration than A, and is arranged so as to face the inside of the channel box 13 according to the positioning of the pellet portion. Note that the positioning of the fuel pellet parts A and B in the inner and outer directions of the channel box 13 when forming the fuel assembly is carried out by placing predetermined marks inside the cladding pipe when loading these pellet parts A and H into the cladding pipe 11. This can be easily done by attaching the following.

As described above, in this embodiment, the fuel pellets 12 filled inside each cladding tube 11 are formed by two fuel pellet portions B and B that are symmetrical about the tube axis of the cladding tube and have different concentrations, Among these fuel pellets 12, which are located on the outer circumferential side of the assembly near the bypass flow path and are susceptible to axial distortion in the heat distribution, the fuel pellet portion with a lower enrichment concentration faces toward the bypass flow path. Therefore, since the mounting is arranged so that the higher enrichment fuel pellet portion B faces the inside of the box, the axial distortion of the heat distribution in these fuel elements 12 is reduced. It is possible to improve the uniformity and prevent damage to the cladding tube 11 due to local peak power.

In the embodiment shown in FIG. 7, the fuel elements 12 are arranged symmetrically in two semi-cylindrical pellets A.

Although the nuclear fuel element according to the present invention is formed by B, the nuclear fuel element according to the present invention is not limited to such a certificate, but can be made in various forms. For example, as shown in FIG. 8, the fuel element 12 may be formed by dividing a hollow cylindrical pellet along the diameter direction into brim-shaped pellets A and H.

Incidentally, the fuel element 12 consisting of the fuel pellet portion B
When loading the cladding into the cladding tube 11, for example, as shown in FIG. It is sufficient to fill it with a semi-cylindrical beret (B). Further, in the case of a fuel element having the shape shown in FIG. 8, a smooth round bar 15 may be used as the holding member. These thin plates 14 and round bars 15 as holding members are made of pellets A and H.
For example, the thin plate 1 may be pulled out after loading is completed.
4. If a material with good neutron economy, high melting point, and good thermal conductivity is used, there is no need to remove it after loading.

As described above, according to the nuclear fuel element according to the present invention, the integrity and safety of the fuel element and cladding can be improved by improving the asymmetry of the heat distribution inside the pellet and preventing local peak output. I can do it.

[Brief explanation of the drawing]

Fig. 1 is a partially longitudinal front view of the cladding tube, Figs. 2 to 5 are explanatory diagrams showing the shape of a conventional fuel pellet, respectively, Fig. 6 is an explanatory diagram showing the state of temperature distribution inside the fuel pellet, 7
The figure is a schematic cross-sectional view of nine fuel assemblies to which an embodiment of the present invention is applied, FIG. 8 is a perspective view showing an outline of another embodiment of the present invention, and FIGS. It is an explanatory view showing an assembly mode of an example. 11...Claying tube, 12...Fuel pellet, 13...
・Cha Figure 7! T:J 8 Kyoudai 9 Figure 70 Figure 1

Claims (1)

[Claims] 1. The fuel pellets filled in the cladding tube in a layered manner are held via springs by end plugs that seal the upper and lower ends of the cladding tube, respectively, and the fuel pellets are arranged in a channel box. In the nuclear fuel element, each nuclear fuel element is formed by two fuel pellet parts that are symmetrical with respect to the tube axis of the cladding tube and have mutually different enrichments, and each nuclear fuel element is arranged in the channel hock. The above-mentioned nuclear fuel element, characterized in that sometimes these two fuel pellet parts can be arbitrarily positioned with respect to the inside and outside directions of the box.・