JPS63149588A - Fuel aggregate for nuclear reactor - 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 [Object of the Invention] (Industrial Application Field) The present invention relates to a nuclear reactor fuel assembly, and particularly to a nuclear reactor fuel assembly suitable for increasing burnup.

(Prior art) Fuel assemblies used in current commercial nuclear reactors are made up of fuel rods in which fuel elements made by sintering enriched uranium dioxide into pellets are filled in zirconium alloy cladding tubes in a square lattice structure. The fuel rods are arranged in rows and bundled together using spacers to maintain the spacing between the fuel rods. Roughly speaking, some of the fuel rods that make up this fuel assembly are filled with pellets of a mixed oxide (gadolinia) of enriched uranium and gadolinium, a neutron absorbing substance, in order to control the excess reactivity of the fuel. (hereinafter referred to as Gadolinia dedication).

FIG. 7 shows a layout diagram of a conventional uranium fuel assembly for a boiling water reactor. In this fuel, fuel rods (numbered in descending order of enrichment) are arranged in an 8 x 8 square grid, and two water rods W through which cold oil and water flow are arranged in the center. . Of the 62 fuel bodies, 8 are gadolinia rods G, and the remaining 54 are ordinary fuel bodies 1 to 1 filled with enriched uranium oxide pellets.
It is 4. Fig. 8 shows the combustion change of the infinite multiplication factor of the fuel H in Fig. 7 described above.
It increases with combustion until then, and decreases thereafter. When this fuel is loaded into a 3-badge replacement core with an operating period of 10 GWd/l (one-third of the entire core is replaced with new fuel at the time of fuel replacement), the part where the infinite multiplication factor increases (first cycle fuel) and Part that decreases (fuel after 2nd cycle)
are exactly balanced, and a nearly flat effective magnification is achieved throughout the operating period.

Such fuel can be obtained by appropriately designing the number of gadolinia rods and the gadolinia concentration. The former determines how much to suppress the infinite multiplication factor, and the latter determines how long to control it. In particular, the gadolinia concentration must be determined so that gadolinia is completely burned out at the end of the operating cycle (hereinafter abbreviated as EOC). This is because if gadolinia remains in the EOC, neutrons will be wasted and the fuel economy will deteriorate.

(Problem to be solved by the invention) In recent years, from the perspective of improving fuel economy, designs have been advanced in which the number of replacement bodies per fuel assembly is reduced by increasing the content of fissile material per fuel assembly. There is. For this purpose, it is necessary to suppress the excess reactivity of the fuel assembly at the initial stage of combustion, so it is necessary to increase the number of gadolinia than before.

On the other hand, since the neutron absorption capacity of gadolinia rods is large, the conventional design concept is not to arrange them adjacent to each other within the 11 East coalescence in order to avoid strong interference of neutron absorption between gadolinia rods. The degree of freedom in arranging the gadolinia rods within the fuel assembly is restricted.

However, as described above, when the number of gadolinia tubes increases due to the increase in burnup, there will be no space for placement within the assembly, and the necessary number of gadolinia tubes may not be able to be placed. As a countermeasure to this problem, if the gadolinia concentration is increased and the gadolinia is left unburned by Li'EOC, the infinite multiplication factor at the beginning of combustion of the second cycle fuel becomes small, and the initial operation cycle (hereinafter referred to as B
The reactivity of the core at the OC (abbreviated as OC) is reduced by that amount, and the initial surplus reactivity can be suppressed. However, on the other hand, E.O.
Since gadolinia is wasted and left unburned in C, there is a problem in that the reactivity of the reactor core is reduced, the operating cycle length is shortened, and fuel economy is reduced.

In addition, Figure 6 is a layout diagram in which the location of gadolinia rods is particularly restricted.
(Mixed Oxide) fuel rods are arranged in the center of the fuel assembly, which is called an island-type MOX fuel conduit assembly. What is shown in the figure is an example of a design that is especially compatible with high burnup. As a result, the number of fuel bodies is two fewer than the conventional fuel shown in FIG. 7. Here, the uranium fuel rods are designated by numbers 3, 2°1 in order of decreasing enrichment, and the MOX fuel rods are designated by P.

In order to simplify the fuel production process, Gadolinia G uses uranium as a base material and does not contain plutonium.

When such a design is adopted, it is necessary to avoid placing the gadolinia rod in the central part. Furthermore, in conventional design methods, the arrangement of the outermost circumference of the gadolinia rod along the water gap is avoided for reasons such as the fact that the combustion characteristics of gadolinia change more greatly than when it is arranged inside.

Because of the above, the location where the gadolinia rod can be placed is extremely limited. The MOX fuel assembly shown in the figure is
Although the reactivity life is equivalent to that of a uranium fuel assembly with an average enrichment of about 3.5w10, more than 10 gadolinia rods were required to suppress excess reactivity at the initial stage of combustion. However, as shown in the figure, the limit was 10 rods, the gadolinia concentration was 3.0W10, and there was a problem that the design had to leave the gadolinia rods G1 unburned in the EOC [Structure of the invention] (Problem Means for Solving the Problem) In order to solve the above problem, in the fuel assembly for a nuclear reactor of the present invention, at least a part of the gadolinia shale arranged in the fuel assembly is arranged in a position adjacent to each other. The gadolinium (155Gd, 157Gd) is characterized by ensuring the required number of gadolinia rods and reducing the gadolinia concentration in at least one of the adjacent gadolinia rods (function). Neutrons generated within the fuel body and flowing from the adjacent fuel body l
By absorbing neutrons generated by ν, excess reactivity of the fuel assembly is suppressed. 155Gd and 157G
, 1 absorbs neutrons and has a small neutron absorption cross section.
Since the Gd value changes to 158 Gd, the neutron absorption capacity of the gadolinia rod gradually decreases with the burnup, and the EO
It is designed to burn out in C.

If the number of gadolinia rods is increased and some of them are placed adjacent to each other, excess reactivity at the beginning of the cycle can be suppressed, but neutron absorption per gadolinia rod decreases due to mutual interference between the gadolinia rods. will be delayed.

As a solution to this problem, if the gadolinia degree is made sufficiently small so that at least one of the gadolinia rods placed adjacent to each other burns out in the first half of the operating cycle,
Other gadolinia rods are burned out at EOC, which can improve fuel economy.

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

Figure 1 shows the layout of a fuel assembly according to an embodiment of the present invention. This is an example in which two gadolinia rods G2 <aml, 5W10) are arranged, and by increasing the number of gadolinia rods, the initial excess reactivity was suppressed, and the gadolinia concentration of gadolinia rod G1 was able to be reduced to 2.5W10. .

Figure 2 is a layout diagram of an embodiment of the present invention. As shown in the figure, the number of gadolinia rods G1 is reduced to eight, and the number of Gadolinia rods G1 is increased to four instead. The symmetry of the degree/enrichment distribution was improved, and the local power distribution was made flatter than in the above embodiment. Gadolinia concentration is 2 in G1
．． 5W10, G2 is 1°5W, which is 60% less than the concentration of G1.
10, it was possible to roughly reduce the total amount of gadolinia.

FIG. 3 is a diagram showing the combustion change of the fuel assembly of the present invention shown in FIG. 2 and the conventional fuel 1"I assembly shown in FIG. 6 at an infinite multiplication factor. FIG. This figure shows the excess reactivity during power operation and cold reactor shutdown margin of the inventive core and the conventional core in Figure 6.This figure shows the cold reactor shutdown margin designed with an effective multiplication factor of 0.99 or less. It is shown that while meeting the standards, the surplus reactivity during power operation in the early stage of combustion is lowered, the combustion change is flattened, and the operation cycle length is extended.

From this, it can be seen that this example suppresses the excess reactivity of the core and fuel at the early stage of combustion, reduces the amount of unburned gadolinia at EOC, and improves fuel economy.

FIG. 5 is a layout diagram of another embodiment of the present invention, and shows an example of a uranium fuel assembly with a roughly high burnup. In this embodiment, one tarrod W is arranged in the center of the fuel assembly, which has a fuel assembly arranged in 9 rows and 9 columns, and has large diameters corresponding to 9 fuel bodies. The water-to-fuel volume ratio is increased. Here, numbers 3, 2
．． Fuels with low enrichment are arranged in order of 1 to make the fuel assembly average enrichment about 6W10, and 20 gadolinia rods G1 with a concentration of 6.0W10 are arranged, and roughly 8 gadolinia rods G2 with a concentration of 1.5W10 are arranged. It is placed.

In this embodiment, the low-concentration sumo linear rod G2 is placed at the second corner of the outermost periphery of the fuel assembly, where the neutron flux is high, and at a location where one side faces one tarod to enhance neutron absorption of gadolinia.

In the embodiments described above, the low-concentration gadolinia fuel was limited to the corner part of the second line from the outermost periphery of the aggregate or the part facing the 4-talorado, but by appropriately adjusting the gadolinia concentration and number, other It can be placed anywhere, increasing the flexibility of location.

[Effects of the Invention] As explained above, according to the nuclear reactor fuel assembly of the present invention, it is possible to arrange the necessary number of gadolinia in the fuel assembly, and it is possible to eliminate unburned gadolinia at the end of the operation cycle. This has the excellent effect of improving fuel economy.

[Brief explanation of the drawing]

FIG. 1 is a layout diagram of one embodiment of the present invention, FIG. 2 is a layout diagram of another embodiment of the present invention, and FIG. 3 is a fuel assembly of the present invention shown in FIG. 2 and a conventional type shown in FIG. 6. Figure 4 is a diagram comparing the infinite multiplication rate combustion changes of MOX fuel assemblies, and Figure 4 shows the surplus reactivity and reactor during power operation of the fuel assembly of the present invention in Figure 2 and the conventional MOX fuel assembly in Figure 6. Figure 5 is a layout diagram of still another embodiment of the present invention, Figure 6 is a layout diagram of a conventional MOX fuel assembly, and Figure 7 is a diagram comparing combustion changes in stop margin. FIG. 8 is a layout diagram of a uranium fuel assembly for a reactor, and a diagram showing the combustion change of the burner 1 of FIG. 7 at an infinite multiplication factor. 1-4...Fuel rod G1. G2...Gadolinia rod W...Water rod (8733) Agent Patent attorney Yoshiaki Inomata (Idiot)
1 person) Driving out and facing Muo So double prison #, P Ai Shao Zeng command

Claims (3)

[Claims] (1) In a fuel assembly in which a large number of fuel rods and water rods filled with pellet-shaped fuel elements are arranged in a grid, at least some of the fuel rods to which burnable poison has been added are located adjacent to each other. 1. A fuel assembly for a nuclear reactor, wherein at least one fuel rod therein has a low burnable poison content. (2) The position of the fuel rod to which the burnable poison at a low concentration is added is located at the corner of the second line from the outermost periphery of the fuel assembly or at the water rod. fuel assembly for nuclear reactors. (3) The fuel assembly for a nuclear reactor according to claim 1, wherein the low concentration of gadolinia is 60% or less of the gadolinia rod having the highest concentration.