JPS63117291A - Nuclear reactor fuel aggregate - Google Patents

Abstract

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

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] <Industrial Application Field> The present invention relates to a fuel assembly for a nuclear reactor containing plutonium fuel.

(Prior Art) Uranium fuel is used in light water reactors currently in operation, and plutonium is produced by burning the uranium fuel. Since the produced plutonium contains the fissile materials 239Pu and 241Pu, it is desired to use it again as nuclear reactor fuel. In terms of fuel efficiency, fast breeder reactors are preferable for using plu-1 to nium as nuclear fuel, but the development of fast breeder reactors is currently delayed and we cannot wait until their practical application is completed, so they are not currently in operation. It is being considered for use in light water reactors.

However, since light water reactors are designed to use uranium fuel, they are fueled by uranium, which has different nuclear properties than uranium.
It is expected that various problems will occur if Ni is used.

First, when we examine the isotopes of plutonium recovered from spent fuel, we find that they have compositions as shown in the table below, for example.

This is the composition after approximately one year had passed since surface fuel was taken out at a burnup of 25 GWd/st and reprocessed after a cooling period of two years. In the table, Am is generated by beta decay of 241Pu.

Among these plutonium isotopes, 239Pu and Pu are fissile materials that undergo nuclear fission by thermal neutrons. However, these have a larger neutron absorption cross section than 235U. FIG. 7 is a graph showing the neutron absorption cross sections of isotopes contained in uranium fuel and plutonium fuel, and clearly shows the above-mentioned characteristics of plutonium.

For this reason, plutonium fuel is expected to have a harder neutron spectrum than uranium fuel. Also,
In addition to not causing nuclear fission due to thermal neutrons, Put3 and 242Pu have very large resonance absorption peaks, as is clear from FIG. For this reason, plu-1-nium fuel containing these isotopes must be loaded with even more fissile material than uranium fuel. This fact results in a rough hardening of the neutron spectrum of plutonium fuel.

This hardening of the neutron spectrum in plutonium fuel causes several problems when plutonium fuel is used in light water reactors.

One of these is the value of control rods, which are located between fuel assemblies in the reactor core and have the function of controlling the reactor's excess reactivity and power distribution, as well as shutting down the reactor in emergencies. It is to decrease. Another is the use of burnable poisons (reactivity control substances) that are loaded into a specific number of fuel rods that make up a fuel assembly and have the function of suppressing the initial reactivity of the fuel. This is because the reactivity suppressing effect decreases. This is because the materials that make up the control rods and burnable poisons are particularly sensitive to low-energy neutrons. (Gadolinium Gd shown in Figure 7 is one of the commonly used burnable poisons.) As shown in Figure 7, Gd has a larger cross-sectional area than plutonium for neutrons with energy of 0.1 cV or more. Since the neutron spectrum is hardened, even if Gd is added to a fuel rod containing Pu, the amount of neutrons absorbed by Gd will be relatively smaller than when Gd is added to a uranium fuel rod.

Therefore, the reactivity suppressing effect becomes smaller and the burning rate of Gd becomes slower. Therefore, when Gd is added to Pu fuel rods, the rate of decrease in the reactivity suppression effect accompanying combustion is smaller than when Gd is added to uranium fuel rods.

In addition, since Pu isotopes are highly chemically toxic and emit α-rays and T-rays, the production and handling of Pu fuel is much more complicated than that of other fuels.

From this point of view, it is desirable to load as much PU as possible into one fuel assembly, thereby minimizing the number of fuel assemblies containing Pu, but then the characteristics of PU fuel will differ from the characteristics of U fuel. First, they will separate, making it difficult to apply them to light water reactors.

Figure 8 shows the conventionally designed U-Pu blended fuel below 1"
) i0X fuel) is required in the fuel rod layout diagram of a fuel assembly consisting of
(quoted as). In FIG. 8, 1 represents a control rod, and 2 represents a fuel assembly.

The fuel assembly 2 is connected to a channel box 3. Water rod 4. Composed of fuel rods 5, 6, 7, and 10, fuel rod 5 is a fuel rod whose fuel pellets are made of uranium dioxide, and fuel rod 6 is a fuel rod whose fuel pellets are a mixture of uranium dioxide and highly enriched plutonium dioxide. , the fuel rod 7 is a fuel rod whose fuel pellets are a mixture of uranium dioxide and plutonium dioxide of medium grade, and the fuel rod 10 is a fuel rod whose fuel pellets are a mixture of uranium dioxide and a reactivity control substance (G d20J).
), and as shown in the longitudinal section in FIG.
A homogeneous mixture 16 of 03 is filled.

With the fuel shown in FIG. 8, in order to obtain the necessary and sufficient reactivity suppressing effect of the Gd-containing fuel rods and not to reduce the value of the control rods, it is necessary to
Only 22 can be loaded. In order to further increase the number of PU-containing fuel rods, it may be possible to use only the fuel rods facing the channel box as U fuel rods.In that case, 34 PU fuel rods can be loaded, which is different from the conventional example (■). 22
It's about 50% more expensive than the book. However, in this case, it is desirable that the Gd-containing fuel rod be adjacent to the outermost corner round in order to maintain the local peaking coefficient at an appropriate value.

The fuel rod arrangement described above is shown in FIG. 10 (hereinafter referred to as "prior art example (■)"). Codes 1 to 1 in Figure 10
7 and 10 are the same as those in FIG. 8, and 8 is a fuel rod whose fuel pellets are a mixture of uranium dioxide and low enrichment plutonium dioxide.

However, such an arrangement lowers the value of the control rod, increases the neutron absorption effect per gadolinia rod, and increases the rate of change in void reactivity, reducing flexibility in fuel design.

(Problems to be Solved by the Invention) The present invention has been made in view of the above circumstances, and is capable of loading as many plutonium fuel rods as possible in a fuel assembly for a boiling water reactor, and also has fuel characteristics. The purpose is to provide a fuel assembly that does not deteriorate.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides a system in which a large number of fuel rods are regularly arranged in a channel box, and the large number of fuel rods carry a mixed fuel of plutonium fuel and uranium fuel. In a nuclear reactor fuel assembly consisting of encapsulated mixed fuel rods and uranium fuel rods encapsulated only with uranium fuel, a flammable reactivity control substance is inserted into the shaft core of some of the uranium fuel rods to reduce the reactivity. As a control fuel rod,
The present invention relates to a fuel assembly for a nuclear reactor, characterized in that the reactivity control fuel rods are arranged at the outermost position of a fuel rod array.

(Function) The thermal neutron distribution in the horizontal section of the fuel assembly is
As shown in the figure, it is higher in the fuel rod facing the water gap and lower in the center. In FIG. 11, 3 represents a channel box, 12 represents each fuel rod, and a water gap portion 11 is located outside the channel box 3.

Therefore, when a fuel rod containing a flammable reactivity controlling substance is placed on the outermost periphery as in conventional example (■), the neutron absorption effect of the flammable reactivity controlling substance becomes greater than in conventional example (■).
It also burns faster. For example, when the amount of the control substance added is the same, the neutron absorption effect and burning rate of Conventional Example (2) will increase by 10 to 20 percent compared to that of Conventional Example (2).

However, in a reactivity control fuel rod like this one, in which a flammable reactivity control substance is inserted only in the core of the uranium fuel, thermal neutrons are generated more inside the pellet than on the surface, even if it is placed at the outermost periphery of the pellet. Since the flux is reduced, the neutron absorption effect does not increase as in the conventional example (2). Furthermore, since it is less susceptible to changes in the thermal neutron flux distribution when the in-channel void ratio changes, the void coefficient becomes smaller. Roughly speaking, since the amount of neutron absorption per reactivity control fuel rod becomes smaller, fine adjustment of the reactivity suppression level becomes possible.

(Example) Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a fuel rod layout diagram of an embodiment of the fuel assembly of the present invention. In the figure, 1 represents a control rod, and 2 represents a fuel assembly. The fuel assembly 2 is a V channel box 3. Consists of water rod 4 and fuel rods 5°6, 7.8 and 9. The fuel rod 5 is a fuel rod whose fuel pellets are made of uranium dioxide. The fuel rod 6 is a fuel rod whose fuel pellets are a mixture of uranium dioxide and highly enriched plutonium dioxide.The fuel rod 7 is a fuel rod whose fuel pellets are a mixture of uranium dioxide and medium-enriched plutonium dioxide. The fuel rod 8 is a fuel rod whose fuel pellets are made of a mixture of uranium dioxide and low-enrichment plutonium dioxide.The fuel rod 9 is a fuel rod whose fuel pellets are made of a mixture of uranium dioxide and low-enrichment plutonium dioxide. These are fuel rods containing flammable reactivity control substances.

A longitudinal cross-sectional view of the reactivity-controlled fuel rod 9 is shown in FIG. As shown in FIG. 2, this fuel rod is surrounded by a cladding 13 and loaded with fuel pellets made of uranium dioxide t, 10214, and its core is filled with a flammable reactivity control substance 15. has been done.

The combustible reactivity control substance is a mixture of Gd2O3 and UC2, a mixture of Gd2O3 and A1□03, a mixture of Gd2O3 and zirconium oxide ZrO2, etc.

The components of the fuel pellets filled in each of the above fuel rods are as shown below.

Fuel rods 5 (4 pieces) Uranium dioxide (2JRV average enrichment 2.2% by weight) Fuel rods 6 (14 pieces) Plutonium dioxide (fissile Pu enrichment 3.83% by weight) Uranium dioxide (235U average enrichment 0 .71%) Fuel rod 7 (16 pieces) Plutonium dioxide (fissile Pu enrichment 2.80% by weight) Uranium dioxide (235U average enrichment 0.71%) Fuel rod 8 (4 pieces) Dioxide Plutonium (fissile Pu enrichment 2.00% by weight) Uranium dioxide (235U average enrichment 0.71% by weight) Fuel rods 9 (24 pieces) Pellet core diameter 0.3cm Gadolinium oxide 0.9Mcc Aluminum oxide pellets Outer periphery uranium dioxide (235u average enrichment 3650% by weight) FIG. 3 is a fuel rod layout diagram showing an embodiment of the present invention.

In this embodiment, the number of IOX fuel rods (from the first embodiment) is increased, and the core type 9 and conventional type 1 are used as reactivity control fuel rods.
0 is also used.

In FIG. 4, the same number of HOX fuel rods as in the first embodiment is used, and the diameter of the core part (reactivity υ control material part) of the reactivity control fuel rod 9 is increased to 4.5 mm. This is an example in which the number of reactivity control fuel rods is reduced accordingly.

Figure 5 shows the diameter of the core part of the core type reactivity control fuel rod, which is 5 m.
This is an example in which two types are used: m (!J1) and 3.5 mm (g2).

Next, the effects of comparing the fuel assembly of the first embodiment with the conventional example will be described.

Figure 6 shows the relationship between void change and reactivity change in the first embodiment of the present invention.
2 is a graph showing fuel assemblies of Example 1, Conventional Example (2), and Conventional Example (2). As shown in this figure, although the fuel assembly of the present invention (solid line a) has more IOX fuel than the conventional example (2) (-dotted chain line b), the reactivity change due to void change is small. Further, in the conventional example (dotted line C), the number of mox fuel rods is the same as that of the present invention, but the reactivity change is large. A similar relationship exists for control rod values.

It should be noted that although the above description has been made with respect to an 8×8 fuel array assembly, the present invention can also be practiced with respect to other fuel rod arrays.

[Effects of the Invention] As explained above, the fuel assembly of the present invention has the effect that the fuel characteristics do not deteriorate even if a large amount of PU gold-containing mOX fuel is loaded.

[Brief explanation of the drawing]

1 and 3 to 5 are fuel rod layout diagrams of a fuel assembly for explaining the present invention in detail, FIG. 2 is a longitudinal sectional view of a reactivity control fuel rod used in the present invention, and FIG. The figure is a graph showing reactivity changes due to void changes in the fuel assembly of the present invention and the conventional fuel assembly. Fig. 7 is a graph showing the neutron absorption cross sections of various isotopes. Figs. FIG. 9 is a longitudinal cross-sectional view of a conventional reactivity-controlled fuel rod, and FIG. 11 is a diagram showing the thermal neutron distribution in a horizontal cross-section of the fuel assembly. 1...Control rod, 2...Fuel assembly 3...Channel box 4...Water rod 5...Uranium fuel rod. 6.7.8...t (OX fuel rod 9...
Axial reactivity control fuel rod 10...Conventional reactivity control fuel rod 11...Water gap, 12...
...Fuel rod 13...Clad. 14...Uranium dioxide fuel 15...Flammable reactivity control substance (8733) Agent Patent attorney Yoshiaki Inomata (1 idiot) Figure 1 Figure 2 Figure 3 4 Figure 7 σlψware v'turf'-id, 6%,
Ito Mejichimi I-Shihoido Hukimemi Degree J
[Ishio, oo+ o, o+ o, +
to +o,. Medium and low 7: Energy (eV) Figure 7 Figure 8 Figure 9

Claims (3)

[Claims] (1) A mixed fuel rod in which a large number of fuel rods are regularly arranged in a channel box, and the large number of fuel rods contains a mixed fuel of plutonium fuel and uranium fuel, and a uranium fuel rod in which only uranium fuel is sealed. In a nuclear reactor fuel assembly consisting of uranium fuel rods, a flammable reactivity control substance is put into the core of some of the uranium fuel rods to form reactivity control fuel rods, and the reactivity control fuel rods are used as reactivity control fuel rods. A fuel assembly for a nuclear reactor, characterized in that it is arranged at an outermost position. (2) The fuel assembly for a nuclear reactor according to claim 1, wherein the flammable reactivity controlling substance is gadolinium. (3) The fuel assembly for a nuclear reactor according to claim 1, wherein the reactivity control fuel rods are arranged at all outermost positions except the corner positions.