JPS5880593A - 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 to a nuclear fuel element loaded with hollow fuel pellets as a fuel body.

Nuclear fuel elements such as those shown in the m1 diagram are generally used as fuel for nuclear reactors such as boiling water reactors that use fluid as a coolant. That is, a small-diameter, cylindrical metal sheathed tube 1 is loaded with fuel pellets 2 in which uranium dioxide or plutonium dioxide powder is formed into pellets, and a spring is used to press down the fuel pellets 2 with an appropriate force. After inserting 3, fill in gas with good thermal conductivity and plug both ends with end plug 4.
It has a sealed structure.

In such a nuclear fuel element, during normal operation of the nuclear reactor, the fuel pellet 2 reaches a high temperature of 1,800° C. or more at the center, and a temperature gradient of nearly 12,000° C. occurs between the center and the surface. For this reason, the fuel pellet 2 has a difference in thermal expansion between the center and the surface, and as shown in FIG. 2, the fuel pellet 2 is thermally deformed into a chain shape and at the same time, a large number of cracks occur due to thermal stress.

The ridge portion 5 of the fuel pellet 2 has the largest thermal deformation.
causes strong contact with the cladding tube l. This mechanical interaction between the fuel pellets 2 and the cladding tube 1 is called PCI.
This PCI causes local stress effects and strain concentration in the cladding tube 1, and there is a risk that the nuclear fuel element will be damaged.

FIG. 3 shows a nuclear fuel element loaded with hollow fuel pellets 6 instead of the fuel pellets 2 described above.

The hollow fuel pellet 6 can reduce the fuel temperature without reducing the heat output per unit length of the nuclear fuel element. Therefore, using the hollow fuel pellet 6 has many advantages, such as not only being able to reduce the amount of thermal deformation described above, but also being able to reduce the fission product gas released from the fuel pellet.

Although the hollow fuel pellet 6 has many advantages as described above, it has the following drawbacks.

The first point is that there is a risk that the internal pressure of the nuclear fuel element will increase because the gas sealed in the center hole 7 during production and the gas produced by nuclear fission become high temperatures. This increases the tensile stress of the cladding tube 1 caused by PCI and the like, which is unfavorable in terms of the integrity of the nuclear fuel element.

The second point is that the broken fuel pellet fragments 8 fall into the center hole 7 due to thermal stress caused by the temperature gradient.

This may have a large impact on the operating characteristics of the nuclear reactor, as the nuclear fuel material becomes non-uniform on one side of the nuclear fuel element.

An object of the present invention is to provide a hollow fuel pellet-type nuclear fuel element that reduces the internal pressure of the nuclear fuel element to reduce the tensile stress generated in the cladding tube, and at the same time prevents fuel pellet fragments from falling into the central hole. It's about doing.

The present invention suppresses the increase in the internal pressure of the nuclear fuel element and eliminates the initially sealed gas and fission product gas from the center hole, where the temperature increases even when the rupture occurs, in a nuclear fuel JIi cable loaded with hollow fuel pellets. A cylindrical body is provided in the center hole of the hollow fuel pellet as a means for reducing the applied stress generated in the tube and at the same time preventing fuel pellet fragments from falling into the center hole.

In the embodiments of the present invention, nuclear fuel elements used in Nateng water reactors will be described.

FIG. 4 shows the structure of a nuclear fuel element in which the present invention is embodied. That is, after loading several bales of hollow fuel pellets 6 into a cladding tube 1 and inserting a spring 3 to press down the hollow nitrogen fuel pellets 6 with an appropriate force, the hollow fuel element is sealed at both ends with plugs 4. □This is a counterfeit product in which the cylindrical body 9 of the present invention is provided in the center hole 71/J of all the fuel pellets 6. ) The conditions required for the cylindrical body 9 are as follows.

(11# & point is sufficiently high compared to the maximum temperature of the nuclear fuel element.

(2) The coefficient of thermal expansion is equal to or lower than that of hollow fuel pellets.

(3) From the perspective of neutron economy, the neutron absorption depth should be small.

First, the melting point of (]) above will be discussed. For small hollow fuel pellets, the temperature at the center can be lowered by increasing the hollow diameter, but since this involves a reduction in the weight of the nuclear fuel material, it is practical to set the hollow diameter to 3 times the outer diameter of the fuel pellet.
It is desirable to set it to about 0%. In this case, the temperature at the center is expected to be approximately 1400C. Therefore, it is safe that the melting point of the cylindrical body is 1500C or higher. (2
), if the condition is not satisfied that the coefficient of thermal expansion is equal to or lower than that of the hollow fuel pellets, the hollow fuel pellets will be pushed outward by the cylindrical chamber, creating a risk of increased PCI.

If conditions (1) and (2) are satisfied, and considering the neutron economy in (3), zirconium dioxide (ZrO□) or the like is suitable as the material for the cylinder.

According to an embodiment of the present invention, in the hollow fuel pellet type nuclear fuel element shown in FIG.
Since the initial filler gas and fission product gas can be removed from the
Since the internal pressure of the nuclear fuel element can be lowered as shown in FIG. 5, it has the effect of reducing the stress applied to the cladding tube 1, and furthermore, it is possible to prevent fuel pellet fragments 8 from falling into the center hole 7. It also has effects.

As a modification ν of the above invention, upper and lower th are used instead of the cylindrical body.
It is possible to use a sealed cylindrical tube.

In this case, there is an advantage in terms of improving the economics of neutrons.

According to the present invention, in a hollow fuel pellet type nuclear fuel element, it is possible to completely reduce the internal pressure of the nuclear fuel element and at the same time prevent fuel pellet fragments from falling into the center hole.

[Brief explanation of drawings]

Figure 1 is a vertical cross-sectional view of a nuclear fuel element manufactured using conventionally used solid fuel pellets, and Figure 2 is a modified view of a nuclear fuel element using conventionally used solid fuel pellets. 3 is a longitudinal sectional view of a nuclear fuel element using hollow fuel pellets instead of solid fuel pellets; FIG. 4 is a longitudinal sectional view of a nuclear fuel element according to an embodiment of the present invention; FIG. 5 is a comparison diagram of the internal pressure of a nuclear fuel element according to the present invention and the prior art. DESCRIPTION OF SYMBOLS 1...Claying tube, 2...Fuel pellet, 3...Spring, 4...End plug, °5...Ridge part, 6...
・Hollow fuel pellet, 'jF l Figure v-2 Figure 477- No. 3 謬i! + Figure Y5 Figure WjjLll tetwet/l)

Claims (1)

[Claims] 1. In a nuclear fuel element consisting of a cladding tube sealed with both end plugs and a plurality of hollow fuel pellets loaded into the cladding tube, a cylindrical body is provided in the center hole of the hollow fuel pellet. Nuclear fuel elements.