JPS61129593A - 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 [Technical Field of the Invention] The present invention relates to a nuclear fuel element in which fissile material and fissile-friendly material are arranged in an axially non-homogeneous structure.

[Technical Background of the Invention] Conventionally, a fast breeder reactor as shown in FIG. 3 has been used. That is, reference number 1 in FIG. 3 indicates a reactor vessel, and this reactor vessel 1 is filled with liquid sodium 2, which is a coolant. In addition, a reactor core 3 is configured in the center of the reactor vessel 1, and a core upper mechanism 4 is provided in the upper part of the reactor core 3.A primary coolant bomb 5, an intermediate heat Exchanger 6 etc. are arranged. The upper part of the reactor vessel 1 is sealed with a roof slab 7 as an upper shield, and a cover gas is sealed between the roof slab 7 and the liquid level of the liquid sodium 2. The core upper mechanism 4 is located in the center of the roof slab 7.
A position control rod drive mechanism 8 facing the is supported.

As shown in FIG. 4, the control rod drive mechanism 8 is comprised of a drive section on the roof slab 7, an upper guide tube 12 and an extension tube 13 on the lower surface side of the roof slab 7.

This control rod drive mechanism 8 moves the control rods 10 into the fuel section 9 of the reactor core 3.
The reactivity of the reactor core 3 is controlled by inserting or manipulating the reactor core 3 into the reactor core 3.

As this core 3, in an axially non-homogeneous core having a non-homogeneous structure in the axial direction, an outer core fuel assembly consisting of a large number of core fuels containing a large amount of fissile material, and a nuclear fission The core fuel contains a large amount of fissile material and the blanket fuel contains a large amount of fissile material that is converted into fissile material by neutron absorption.

By arranging the blanket fuel near the center in the axial direction, it is formed by an inner core fuel assembly sandwiched between the core fuel from above and below in the axial direction, and a control rod that contains a neutron absorbing material and controls the nuclear fission reaction. . FIG. 5 is a conceptual cross-sectional view of the outer core fuel assembly 14, and FIG. 6 is a conceptual cross-sectional view of the inner core fuel assembly 15, where 16 represents the core fuel and 17 represents the blanket fuel. .

That is, in FIGS. 5 and 6, the trumpet tube 31
Core fuel 16 and blanket fuel 17 are housed in the reactor.

In addition, in the figure, 37 is a handling head, and 38 is an entrance nozzle.

An example of the cross section of the axially non-homogeneous core 3 of a fast breeder reactor constituted by such fuel assemblies is shown in Fig. 7.
It is a diagram. That is, in FIG. 7, the outer core area 1ii! is constituted by the outer core fuel assembly 14 shown in FIG. 5! 14' (outside the thick line in the figure) surrounds the inner core region 15 constituted by the inner core fuel assembly 15 shown in FIG. 6, and includes the control rods 10A, 10B,
10C.

10X are distributed in large numbers within these areas.

The control rods 10A, 10B, and IOC are main reactor shutdown control rods among the many control rods used for the purpose of starting, stopping, and controlling the power of the reactor.

Among the many control rods, the control rod 10X is used only for the purpose of emergency shutdown of the reactor, and is a backup reactor shutdown control rod that is always fully withdrawn from the core during normal operation. In these main reactor shutdown control rods, the code 10A is located at the center of the core,
The code 10B is located at the second distance from the core center, and the code 1
0C indicates the main reactor shutdown control rods 10 located at the farthest distance from the core center. Further, as shown in FIG. 7, the control rods 10C arranged at the farthest distance from the reactor center are normally located in the inner core region 15' and the outer core region 15'.
It is often located at the border of 4'.

Next, FIG. 8 shows an overall view of a nuclear fuel element, and FIG. 9 shows an overall view of a fuel assembly in which a plurality of nuclear fuel elements are gathered together, bundled, and assembled into a trumpet tube to form a fuel assembly, and will be described in detail.

As shown in FIG. 8, the nuclear fuel element 18 is a nuclear fuel material 19.
．． 20 and 21 are filled into the cladding tube 22, and the lower end portion is sealed by welding the upper end plug 23 and the lower end plug 24 to the cladding tube 22.

Further, the nuclear fuel element 18 includes a nuclear fuel material 19°20.2
A gas reservoir 25 is provided for holding the generated gas generated by nuclear fission in the cladding tube 22. The reference numeral 26 in the figure represents one of the nuclear fuel materials.

At the nuclear fuel filling part 20.21, the nuclear fuel filling part 26 and the gas reservoir 25 are connected to each other by providing an intermediate end plug 28 having a vent hole 27. A wire spacer 29 is wound around the entire length of the outer surface of the cladding tube 22 of the nuclear fuel element 18 configured as described above, and both ends of the wire spacer 29 are fixed to upper and lower end plugs 23 and 24 by welding 30, respectively.

As shown in FIG. 9, a plurality of nuclear fuel elements 18 are collected and bundled and assembled into a trumpet tube 31, and a handling head 37 is installed at the upper end of the trumpet tube 31, and a rainbow transformer is installed at the lower end of the trumpet tube 31. Nozzles 38 are connected to form a nuclear fuel assembly. In this nuclear fuel assembly, coolant enters the reactor through an entrance nozzle, flows between the nuclear fuel elements 18, and exits from the handling head 37.

[Problems in the Background Art] In the axially non-homogeneous nuclear fuel element having the above configuration, due to its characteristic power distribution shape, cesium (hereinafter referred to as C3), a fission product produced by nuclear fission, is irradiated into the wave cladding tube. and the gap between the fuel pellets in the axial direction. In particular, the internal bracket fuel area sandwiched between the core fuel
Acts as a sink. The C3 that has moved to the internal blanket fuel region reacts with uranium dioxide, especially at the boundary with the core fuel region, to generate reactive compounds of the Cs-tJ-0 system (for example, Cs 2 UO4, Cs 2
U207 etc.). This is usually uranium dioxide (UO2+x
), the oxygen to uranium ratio (2+x) is the stoichiometric composition ratio (
Since uranium dioxide pellets that are larger than 2,00> by X are used, excess oxygen is involved.

There is a concern that this reactive compound fills the gap between the cladding tube and the fuel pellet, applies a load from the inside of the cladding tube, and affects the life of the fuel. FIG. 10 shows the output distribution of the nuclear fuel element and the distribution of C3 in relative values. As is clear from FIG. 10, the amount of Cs produced increases in the blanket fuel sandwiched between the core fuels.

[Object of the Invention] The present invention has been made in order to solve the problems of the prior art described above, and is aimed at suppressing the reaction compounds of the Cs-U-0 system or reducing the load on the cladding tube due to the generation of the reaction compounds. The object of the present invention is to provide a nuclear fuel element that extends the life of the fuel by eliminating factors that affect the life of the fuel.

[Summary of the Invention] The present invention is constructed by filling a cladding tube with a core fuel consisting of a large number of plutonium-uranium mixed oxide pellets and a blanket fuel consisting of a large number of uranium dioxide pellets, and the blanket fuel is In an axially heterogeneous nuclear fuel element having a structure in which it is sandwiched by the core fuel from above and below in the axial direction by placing it near the center of the reactor core, oxygen and uranium in the uranium dioxide of the blanket fuel at the boundary with the core fuel are removed. This is a nuclear fuel element characterized in that one or more uranium dioxide pellets having a composition ratio close to the stoichiometric composition are arranged at each of the upper and lower boundaries.

Furthermore, the present invention reduces the outer diameter of each plutonium-uranium mixed oxide pellet and uranium dioxide pellet at the boundary between the core fuel and the blanket fuel, and makes the gap between the cladding tube and the fuel pellet larger than in other areas. It is a nuclear fuel element characterized by:

The nuclear fuel element according to the present invention suppresses the generation of C3-U-O-based reactive compounds characteristic of nuclear fuel elements having an axially non-homogeneous structure, or alleviates the load on the cladding due to the reactive compounds. , can extend the life of nuclear fuel elements.

[Embodiments of the Invention] Hereinafter, embodiments of the nuclear fuel element according to the present invention will be described with reference to FIGS. 1 and 2. In both FIGS. 1 and 2, parts that are the same as those in FIG. 8 are indicated by the same reference numerals, explanations of overlapping parts are omitted, and only essential parts are shown.

The nuclear fuel element 32 of the first embodiment in FIG. 1 is sandwiched between core fuel 20 made of plutonium-uranium mixed oxide pellets filled in a cladding tube 22, and blanket fuel 19.33 made of uranium dioxide pellets. is filled in the cladding tube 22.

Here, among the blanket fuel 19.33, the uranium dioxide pellets constituting the blanket fuel 33 at the boundary with the core fuel 20 have an oxygen to uranium ratio of stoichiometric composition (2,00). Similar items are arranged and configured.

The ratio of oxygen to uranium in the uranium dioxide pellets normally used for blanket fuel is 2.00 to 2.02, and the ratio of oxygen to uranium in the uranium dioxide pellets used in the present invention as blanket fuel 33 is 2.00 to 2.02. .00~2.
By selectively using those of 01, Cs −U
The generation of -0 type reaction compounds can be considerably reduced compared to the conventional method.

Of course, the ratio of oxygen and uranium is the stoichiometric composition ratio (2,0
It is possible to produce uranium dioxide pellets of 0). By controlling the production of uranium dioxide, it is possible to use uranium dioxide pellets in which the ratio of oxygen to uranium is extremely close to the stoichiometric composition.

In addition, one or more uranium dioxide pellets at the boundary are required at the upper and lower boundaries, but the most effective number is 2 to 2.
There are about 3 pieces.

Note that the blanket fuel 19 sandwiched between the core fuel 20
．． The ratio of oxygen to uranium is 2.00 to 2.00 for all 33
It goes without saying that the use of 2.01 uranium dioxide pellets is within the scope of the present invention.

FIG. 2 shows a second embodiment of the invention.

The nuclear fuel element 34 of the second embodiment in FIG. 2 forms core fuel 36 with uranium dioxide pellets forming blanket fuel 35 located at the upper and lower boundaries of blanket fuel 19.35 and core fuel 20.36, respectively. The outer diameter of the plutonium-uranium mixed oxide pellets is 20% smaller than that of other fuels.
．． These pellets are smaller than No. 19 pellets. In this embodiment, by making the gap 39 with the cladding tube 22 larger than other areas, the blanket fuel 35 and the core fuel 3
This gap 39 absorbs the Cs-U-0-based reaction compound generated at the boundary between the holes 6 and 6, thereby relieving the load on the cladding tube.

[Effects of the Invention] As explained above, according to the present invention, since the oxygen and uranium ratio used in the blanket is close to the stoichiometric composition, there is little surplus oxygen, and C3 generated and transferred during irradiation is The amount of reaction compounds produced can be suppressed to a small level.

Alternatively, the generated reaction compounds can be absorbed in the gap between the fuel pellets and the cladding tube, thereby relieving the load on the cladding tube. This makes it possible to eliminate factors that affect the lifetime and extend the lifetime of the nuclear fuel element.

[Brief explanation of the drawing]

1 and 2 are longitudinal cross-sectional views showing examples of the nuclear fuel element according to the present invention, FIG. 3 is a vertical cross-sectional view showing an outline of a fast breeder reactor, and FIG. 4 is a vicinity of the reactor core in FIG. 3. Figures 5 and 6 are cross-sectional views schematically showing conventional nuclear fuel elements, and Figure 7 is an enlarged view of the reactor core in Figure 3. 8 is a longitudinal sectional view showing a conventional nuclear fuel element, FIG. 9 is a longitudinal sectional view partially showing a nuclear fuel assembly incorporating the nuclear fuel element of FIG. 8, and FIG. The figure is a curve diagram showing the power distribution and cesium distribution of a conventional nuclear fuel element. 1...Reactor vessel 2...Liquid sodium 3.
...Reactor core 4...Core upper mechanism 5...-
Secondary coolant pump 6... Intermediate heat exchanger 7... Roof slab 8...
・Control rod drive mechanism 9...Fuel part 10...Control rod
11...Lower guide tube 12...Upper guide tube
13... Extension tube 10A... Control rod ios, ioc, iox... Control rod 14'... Outer core region 15'... Inner core region 14... Outer core fuel assembly 15... - Inner core fuel assembly 16... Core fuel 17... Blanket fuel 18... One nuclear fuel element... Blanket fuel 20... Core fuel 21... Blanket fuel 22... Cladding tube 23 ...Top end plug 24...
・Lower end plug 25...Gas reservoir 26...Nuclear fuel filling part 27...Vent hole 28...Middle end plug 29
...-wire space 30... welding part 31.
...Trumpet tube 32...Nuclear fuel element 33 of the first embodiment...Blanket fuel 34 of the first embodiment...Nuclear fuel element 35 of the second embodiment...Blanket fuel 36 of the second embodiment ...Core fuel 37 of the second embodiment...Handling head 38...Entrance nozzle 39...Gap

Claims (2)

[Claims] (1) Nuclear fuel material is filled in a cladding tube, both upper and lower ends of the cladding tube are sealed with end plugs, and the nuclear fuel material is a core fuel consisting of a large number of plutonium-uranium mixed oxide pellets and a large number of uranium dioxide pellets. In an axially heterogeneous nuclear fuel element having a structure in which the blanket fuel is placed near the center of the reactor core and is sandwiched vertically by the core fuel in the axial direction, the blanket fuel and the core fuel are Blanket fuel uranium dioxide (UO 2+x
), the ratio of oxygen to uranium (2+x) is made close to the stoichiometric composition ratio (2.00, X 0.00), and X 0
．． A nuclear fuel element characterized in that at least one 00 uranium dioxide pellet is arranged above and below the boundary. (2) The outer diameters of the uranium dioxide pellets forming the blanket fuel and the plutonium-uranium mixed oxide fuel pellets forming the core fuel located at the upper and lower boundaries between the blanket fuel and the core fuel are reduced to reduce the outer diameter of the cladding tube. 2. The nuclear fuel element according to claim 1, wherein the gap between the two areas is larger than that in other areas.