JP2972177B2 - Fuel element and fuel assembly for thermal neutron reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel element and a fuel assembly for use in a thermal neutron reactor. More specifically, the present invention relates to a fuel element and a fuel assembly in which the inner portion has a higher fissile material concentration than the outer portion. The present invention relates to a fuel element and a fuel assembly for a thermal neutron reactor using double pellets comprising an inner portion made of a nuclear fuel material and an outer portion made of a fuel parent material.

[0002]

2. Description of the Related Art A fuel element used in a thermal neutron reactor generally has a structure in which a large number of fuel pellets are sealed in a cladding tube. As the fuel pellets, generally, uniform pellets made of nuclear fuel material composed of uranium dioxide or uranium-plutonium mixed oxide (MOX) are used.

In order to increase the operating rate of a nuclear reactor, it is necessary to extend the operating period of the nuclear reactor.
Therefore, it is necessary to burn the fuel assembly for a long time, and for that purpose, it is necessary to increase the concentration of fissile material in the fuel.
However, a fuel assembly using a fuel element having a high fissile material concentration in fuel simply has the following problems. The excess reactivity in the initial stage of combustion increases, and the power peaking in the core radial direction increases. Since the thermal neutron flux in the center of the fuel assembly decreases, the local power peaking coefficient in the fuel assembly increases.

As a countermeasure for the above, there is a technique using a fuel element to which a burnable poison is added in order to suppress the excess reactivity in the initial stage of combustion. Specifically, gadolinium oxide is solid-dissolved in uranium dioxide or a mixed oxide of uranium and plutonium, or a uniform pellet in which gadolinium oxide is dissolved, or a gadolinium oxide rod is inserted into the center of a pellet of uranium dioxide or a mixed oxide of uranium and plutonium. Heavy pellets are used.

In order to reduce the local output peaking coefficient of the fuel assembly, an arrangement of various types of fuel elements having different fissile material concentrations has been studied as the above countermeasure.

[0006]

However, a fuel element containing a burnable poison has a large output suppressing effect, and a fuel element having an extremely small output is unevenly distributed locally. The problem of further increase arises. In addition, in order to manufacture such a fuel element containing a burnable poison, two independent lines of a fuel element not containing a burnable poison and a line for producing the fuel element containing a burnable poison are required in a nuclear fuel manufacturing facility. This leads to an increase in manufacturing costs.

[0007] On the other hand, when using many types of fuel elements with varied fissile material concentrations, the number of campaigns in the fuel manufacturing process increases, and manufacturing and product management become complicated, which also increases manufacturing costs.

SUMMARY OF THE INVENTION An object of the present invention is to provide a fuel element which does not require a burnable poison because the excess reactivity does not increase in the initial stage of combustion, and changes the fissile material concentration in order to prevent the local power peaking coefficient in the fuel assembly from increasing. It is an object of the present invention to provide a fuel element and a fuel assembly for a thermal neutron reactor, which do not require the use of many kinds of, and whose reactivity decreases slowly with combustion and can be burned for a long period of time.

[0009]

SUMMARY OF THE INVENTION The present invention provides a thermal neutron reactor in which a large number of double pellets made of a nuclear fuel material whose fissile material concentration is higher than that of an outer portion are enclosed in a cladding tube. Fuel element. Further, the present invention is a fuel element for a thermal neutron reactor in which a large number of double pellets having an inner portion made of a nuclear fuel material and an outer portion made of a fuel parent material are sealed in a cladding tube. In any case, in the present invention, the excess reactivity is suppressed by utilizing the self-shielding effect of the outer portion of the double pellet, which is characterized in this point.

In the present invention, a fuel assembly is formed by arranging and bundling a number of such fuel elements. The above nuclear fuel material or fuel parent material is, for example, uranium,
Plutonium or oxide fuel containing thorium. Further, the nuclear fuel material or the parent material of the outer portion may be a metal fuel. Metal fuels are thermally advantageous because they have better thermal conductivity than oxide fuels.

[0011]

FIG. 1 is a cross-sectional view showing one embodiment of a fuel assembly according to the present invention. Here, an example in which the present invention is applied to an advanced conversion furnace (ATR) is shown. First, the new conversion furnace will be briefly described. The reactor body includes a calandria tank in which a number of calandria tubes 10 are arranged in a square lattice and the outside of each of the calandria tubes 10 is filled with heavy water D ₂ O, a pressure tube 12 inserted in each of the calandria tubes 10, It consists of a fuel assembly loaded in the pressure pipe 12, a shield, and the like. The fuel assembly is cooled by the light water coolant H ₂ O flowing from the lower part of the core in the pressure pipe 12, and the heated and boiled coolant becomes steam and rises toward the steam drum. In addition, between the pressure pipe 12 and the calandria pipe 10, carbon dioxide CO ₂ is supplied as a heat shield.

The fuel assembly is a cluster type in which 28 fuel elements 14 are bundled, and each fuel element 14 has a structure in which a large number of fuel pellets are filled in a cladding tube 16. The fuel elements 14 are arranged on three layers of concentric circles. The upper and lower ends are fixed by tie plates, spacers are provided for structural stability in the horizontal direction, and spacers are fixed by spacer tie rods 18.

As shown in an enlarged view, the present invention uses a double pellet 20 as a fuel pellet. For example, the inner portion 22 is made of a nuclear fuel material having a high fissile material concentration, and the outer portion 24 is made of a nuclear fuel material having a low fissile material concentration. A large number of such double pellets 20 are sealed in the cladding tube 16 to form the fuel element 14.

As another example, the fuel element can be formed by using a double pellet whose inner portion is made of a nuclear fuel material and whose outer portion is made of a fuel parent material.

According to the present invention, there is a configuration in which only such a fuel element consisting of double pellets is bundled to form a fuel assembly, but there is also a configuration in which such a double pellet and a uniform pellet are combined. . In the latter case, for example, a fuel element in which uniform pellets made of uranium oxide or uranium-plutonium mixed oxide are enclosed in a cladding tube is disposed in the center, and a fuel element in which double pellets are enclosed in a cladding tube is disposed outside. There is an example of disposing a fuel assembly. Alternatively, uniform pellets made of uranium oxide or uranium-plutonium mixed oxide are located in the upper and lower portions in the axial direction, and the pellets are loaded into the cladding so that the double pellets are located in the center, and bundled. Alternatively, a fuel assembly can be formed by combining both technologies.

The double pellet used in the present invention can be produced by a method in which a cylindrical or particulate inner member is inserted into a cylindrical outer member. For example, a depleted uranium-zirconium alloy fuel is formed with a through-hole along the central axis, and a small-diameter uranium-plutonium mixed oxide fuel having a high fissile material concentration is inserted into the through-hole, or particulate uranium is inserted. -A method such as filling with plutonium mixed oxide fuel can be adopted.

[0017]

EXAMPLES Hereinafter, in Examples 1 to 4, the results of a comparative study of a fuel assembly using double pellets according to the present invention (Example) and a fuel assembly using uniform pellets (Comparative Example) will be described. explain.

Example 1 Example 1 is a case of a fuel assembly using a double pellet made of a nuclear fuel material in which the concentration of fissile material in the inner part is higher than that in the outer part. Example 1 is a case of a fuel assembly using uniform pellets. The assumed reactor is a new conversion reactor,
The structure of the fuel assembly is as shown in FIG. Table 1 shows details of the fuel pellets used in the analysis.

FIG. 2 shows the change in burnup of the effective multiplication factor of a fuel assembly constituted by using the two types of fuel pellets shown in Table 1. The excess reactivity of the fuel assembly of Comparative Example 1 in the initial stage of combustion becomes large, and it is necessary to add a burnable poison such as gadolinium oxide to suppress the excess reactivity. However, Example 1
This fuel assembly does not need to be used, and the decrease in reactivity accompanying combustion is moderate. This is due to the neutron shielding effect of the fuel assembly of the first embodiment, because the outer portion of the fuel element cross section contains a large amount of the fuel parent material.

[0020]

[Table 1]

FIG. 3 shows a comparison of the burn-up change of the local output peaking coefficient of the two fuel assemblies constructed as described above. In the fuel assembly of Comparative Example 1, since the fissile material concentration was uniform in the fuel assembly, large output peaking occurred in the outer layer of the fuel assembly, and the local output peaking coefficient was excessively increased. In order to reduce this, it is necessary to use many types of fuel elements with varied fissile material concentrations. However, in the fuel assembly of the first embodiment,
Since the local output peaking coefficient is small, it is not necessary to use many types of fuel elements with varied fissile material concentrations. In addition, the output peaking caused by the combustion of the outer fuel element is moderately reduced, which is advantageous from the viewpoint of heat removal. These are because, for the same reason as described above, the fuel element used in the first embodiment has a gradual decrease in output accompanying combustion. Further, in the fuel assembly according to the first embodiment, the combustion proceeds with a small local output peaking coefficient, that is, a flat power distribution. Therefore, it is possible to prevent the combustion of only some of the fuel elements from extremely proceeding. The mechanical and thermal integrity of the fuel element can be ensured.

Example 2 Example 2 is a case of a fuel assembly using double pellets in which the inner part is made of a fissile material and the outer part is made of a fuel parent material. Comparative Example 2 is a uniform pellet. This is the case of the fuel assembly using the fuel assembly. The assumed nuclear reactor is a new type of converter, and the structure of the fuel assembly is shown in Fig. 1.
Is the same as shown in FIG. Table 2 shows details of the fuel pellets used for the analysis.

[0023]

[Table 2]

FIG. 4 shows the change in the burnup of the effective multiplication factor of the two types of fuel assemblies configured as described above. FIG. 5 shows a comparison of changes in the burnup of the local output peaking coefficient. These also have the same tendency as in the first embodiment.

(Embodiment 3) Embodiment 3 is also similar to Embodiment 2,
This is the case of a fuel assembly using double pellets in which the inner part is made of fissile material and the outer part is made of a fuel parent material. The assumed nuclear reactor is a new type of converter, and the configuration of the fuel assembly is the same as that shown in FIG. Table 3 shows details of the fuel pellets used in the analysis.

[0026]

[Table 3]

FIG. 6 shows a change in the burnup of the effective multiplication factor of the two fuel assemblies configured as described above, and FIG. 7 shows a comparison of a change in the burnup of the local output peaking coefficient. These also have the same tendency as in the first and second embodiments.

(Embodiment 4) Embodiment 4 is a case of a fuel assembly using a double pellet made of a metal fuel of depleted uranium having an inner portion made of fissile material and an outer portion having good thermal conductivity. . Comparative Example 4 is a case of a fuel assembly using uniform pellets, and is set so that Pu weights are equal. The channel output is the same under all conditions. The assumed nuclear reactor is a new type of reactor, and the fuel assembly has a spacer support tube inserted in the center, 12 fuel elements in the inner layer, 18 fuel elements in the intermediate layer, and 24 fuel elements in the outer layer arranged concentrically. This is the configuration. Table 4 shows details of the fuel pellets used in the analysis.

[0029]

[Table 4]

FIG. 8 shows a change in fuel reactivity caused by irradiation when two types of fuel assemblies having the above-described structures are used.
FIG. 9 shows the power distribution in the fuel assembly. In Example 4, a metal fuel was used, and the weight of the nuclear material was different from that in Comparative Example 4. Therefore, the burn-up was converted into the number of irradiation days and comparative evaluation was performed. These also have the same tendency as in the first to third embodiments.

From the analysis of each of the above embodiments, the fuel assembly of the present invention can suppress the excess reactivity in the early stage of combustion and increase the reactivity even after the middle stage of combustion. You can see that it can be done. It can also be seen that the power distribution in the fuel assembly is flattened throughout the combustion period. These effects become larger as the proportion of the parent material in the outer part in the cross section of the fuel element increases, and the effect of U-2
It can also be seen that Th-232 is larger than 38.

[0032]

According to the present invention, in the cross section of a fuel element used in a thermal neutron reactor, the concentration of fissile material in the inner part of the fuel is made higher than that in the outer part, or the outer part is made of a fuel parent material. The fuel assembly is burned for a long time by increasing the fissile material concentration in the fuel without using burnable poisons such as gadolinium oxide, and without using various types of fuel elements with varied fissile material concentrations. It becomes possible. Therefore, the operating rate of the nuclear reactor can be increased, the fuel production cost can be reduced, and a fuel element and a fuel assembly that are excellent in overall economic efficiency can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of one embodiment of a fuel assembly according to the present invention.

FIG. 2 is a graph showing a change in burnup of an effective multiplication factor in Example 1.

FIG. 3 is a graph showing a change in burnup of a local output peaking coefficient in Example 1.

FIG. 4 is a graph showing a change in burnup of the effective multiplication factor in Example 2.

FIG. 5 is a graph showing a change in burnup of a local output peaking coefficient in Example 2.

FIG. 6 is a graph showing a change in burnup of the effective multiplication factor in Example 3.

FIG. 7 is a graph showing a change in burnup of a local output peaking coefficient in Example 3.

FIG. 8 is a graph showing a change in effective multiplication factor accompanying irradiation in Example 4.

FIG. 9 is a graph showing a change in local output peaking coefficient due to irradiation in Example 4.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Calandria pipe 12 Pressure pipe 14 Fuel element 16 Cladding pipe 20 Double pellet 22 Inner part 24 Outer part

Continued on the front page (72) Inventor Kazuya Komori 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki Pref. Power reactor and Nuclear Fuel Development Corporation Tokai Works, affiliated to Nuclear Technology Co., Ltd. (56) References JP-A-4-4 256892 (JP, A) JP-A-2-6786 (JP, A) JP-A-64-110292 (JP, A) JP-A-9-101396 (JP, A) (58) Fields investigated (Int. Cl. ^6, DB name) G21C 3/28 G21C 3/60 G21C 3/62 G21C 3/30

Claims (3)

(57) [Claims] In a fuel element for a thermal neutron reactor , wherein a plurality of double pellets made of a nuclear fuel material having a higher fissile material concentration in an inner portion than a fissile material concentration in an outer portion are enclosed in a cladding tube, The pellet is the inner part of nuclear fuel
Material contains uranium, plutonium, or thorium
The outer portion of the nuclear fuel material
Metals containing orchid, plutonium, or thorium
A fuel element for a thermal neutron reactor, comprising a fuel. 2. The thermal neutron reactor fuel element according to claim 1 , wherein a plurality of double pellets having an inner portion made of a nuclear fuel material and an outer portion made of a fuel parent material are sealed in a cladding tube.
The pellets consist of uranium and plutonium
Oxide fuels containing thorium or thorium
Uranium, plutonium, and
Or metal fuel containing thorium
Fuel element for thermal neutron reactor. 3. A fuel assembly for a thermal neutron reactor, wherein the fuel elements for a thermal neutron reactor according to claim 1 or 2 are arranged and bundled.