JPS5918678B2 - Boiling water reactor core - Google Patents

Description

[Detailed description of the invention] The present invention relates to the core of a boiling water nuclear reactor.

In the core of a boiling water reactor, four fuel assemblies are arranged in a lattice around one control rod to form a unit lattice, and this unit lattice is further arranged in a lattice to form the reactor core. ing.

Generally, the operation of a nuclear reactor is stopped and the fuel is replaced at regular intervals, for example every 12 months.

In general, about 1/3 of all fuel assemblies are replaced with new fuel assemblies in one fuel replacement cycle.

By the way, in the past, one or two fuel assemblies at predetermined positions in each unit grid were replaced with new fuel assemblies for each fuel replacement cycle.

In this case, there must be a margin for reactor shutdown so that the core remains subcritical even if any one control rod is withdrawn, and the power distribution of each unit cell must be made as uniform as possible. For this reason, in the conventional system, it was necessary to change the arrangement of almost all the old fuel assemblies every time the fuel was replaced, and the refueling work was troublesome and took a long time.

The present invention has been made based on the above circumstances, and its purpose is to reduce the number of fuel assemblies that need to be rearranged during fuel exchange, and to make the power distribution of the core uniform and constant. The aim is to obtain a boiling water reactor core in which only a child needs to change the arrangement of fuel assemblies in this pattern, and the refueling operation is simple and efficient.

The present invention will be explained below according to an embodiment shown in the drawings.

First, two types of basic patterns within the unit cell are
This will be explained with reference to the figures and FIG.

FIG. 1 shows a unit cell 1 of the first pattern, and 3 in the figure is a control rod.

Around this control rod 3 is a fuel assembly 4 with a burnup of 1.
2 a, 4a, 2 fuel assemblies 4c, 4c with burnup level 3
The fuel assembly 4a has a total of four fuel assemblies 4a, 4a, 4c, and 4c arranged in a grid pattern, and has a burnup of 1.

4a are located diagonally to each other, and fuel assemblies 4c, 4c having a burnup of 3 are also located diagonally to each other.

Further, FIG. 2 shows the unit cell 2 of the second pattern, and 3 in the figure is a control rod.

Around this control rod 3, there is a fuel assembly 4 with a burnup of 2.
There are two fuel assemblies 4a and 4b, and two fuel assemblies 4at4c with burnup 1 and burnup 2 respectively, making a total of 4 fuel assemblies 4a and 4b.
b.

4 b t 4 c are arranged in a grid, fuel assemblies 4b, 4b with burnup of 2 are arranged diagonally to each other, and fuel assemblies 4a with burnup of 1 and fuel assemblies 4c with burnup of 3 are arranged diagonally to each other. are placed diagonally to each other.

Note that the fuel assemblies 4a... with a burnup of 1 refer to new fuel assemblies that are newly loaded, and the fuel assemblies 4b... with a burnup of 2 refer to fuel assemblies with a burnup of 1. For example, the fuel assembly 4c with a burnup of 3 is one that has been burned for a fuel exchange cycle of 12 months.
of fuel assemblies burned for two refueling cycles, eg, 24 months.

Therefore, when burned for one fuel exchange cycle, the burnup is 1.
The fuel assembly 4a has a burnup of 2 and the fuel assembly 4b has a burnup of 2, and the fuel assembly 4b has a burnup of 2 and a fuel assembly 4c has a burnup of 3.
Furthermore, the fuel assembly 4c with a burnup of 3 becomes a fuel assembly 4d with a burnup of 4.

The unit lattices 1 and 2 of the first pattern and the unit lattice 2 of the second pattern are arranged in a checkerboard shape in the core central part 5, as shown in FIG. . . are arranged in a grid pattern so that the unit grids 2 of the second pattern are located at diagonal positions, and the unit grids 2 of the second pattern are located at diagonal positions.

Further, in the unit lattice T... arranged in the core peripheral area 6, fuel assemblies are arranged in a pattern different from the unit lattice 1... of the first pattern and the unit lattice 2 of the second pattern, and these Fuel assemblies 4b with a burnup of 2 are arranged in unit grids 7 in the core peripheral area 6.

Note that the numbers written inside the fuel assembly in the figure indicate the burnup of that fuel assembly.

One embodiment of the present invention configured as described above has unit lattices 1 of the first pattern and unit lattices 2 of the second pattern regularly arranged in a checkerboard shape in the central part 5 of the reactor core. As a result, reactor shutdown margin is reliably obtained, and the power distribution is also uniform, resulting in good core characteristics.

In other words, in general, the infinite effective multiplication factor koo of a fuel assembly increases as the burnup progresses from 1 to 2 as shown in Figure 4, reaches its maximum at a burnup of 2, and decreases as the burnup progresses from 3 to 4. , exhibits characteristics such that burnup 1 and burnup 3 are approximately equal.

Therefore, in the unit grid 1 of the first pattern, two fuel assemblies 4a, 4a with a burnup of 1 and two fuel assemblies 4cy40 with a burnup of 3 are arranged, so that they are operated during one fuel exchange cycle. Then, the fuel assembly 4 a has a burnup of 1.

4a reaches a burnup of 2 and koO increases, and the fuel assemblies 4c and 4c of burnup of 3 reach a burnup of 4 and koO decreases, so these two cancel each other out and the overall unit cell 1 increases as shown in FIG. The average infinite effective multiplication factor koO remains constant during the refueling cycle.

Furthermore, in the unit grid 2 of the second pattern, there are two fuel assemblies 4b and 4b with a burnup of 2, and a fuel assembly 48 with a burnup of 1.
Since one fuel assembly 4c with a burnup of 3 and one fuel assembly 4c with a burnup of 3 are arranged, when operated during one fuel exchange cycle, the fuel assemblies 4b and 4b with a burnup of 2 progress to a burnup of 3; The fuel assembly 4c progresses to burnup 4, and the infinite effective multiplication factor koO
decreases.

Further, the fuel assembly 4a with burnup of 1 progresses to burnup of 2, and the infinite effective multiplication factor koO increases.

Therefore, as shown in FIG. 6, the average infinite effective multiplication factor Voo of the entire unit cell 2 decreases slightly during the fuel exchange cycle, but becomes substantially uniform, and is also approximately equal to that of the unit cell 1 of the first pattern.

Therefore, if these unit grids 1..., 2... are set in advance to provide the necessary reactor shutdown margin, the necessary reactor shutdown margin will be ensured when these unit grids are arranged in a checkerboard pattern. In addition, the power distribution becomes uniform, and excellent core characteristics are obtained.

In addition, since the reactivity tends to be low in the core peripheral area 6, the first pattern of unit lattices 1...
Since the unit lattices 7... are arranged in a pattern different from the second pattern of unit lattices 2... and in which the fuel assemblies 4b... of burnup 2, which has the highest degree of reactivity, are arranged around the reactor core. A uniform reactivity is obtained even in the core peripheral area 6, resulting in even better core characteristics.

Next, a method for exchanging fuel in the core will be described with reference to FIGS. 7 to 19.

FIG. 7 shows the state after an arbitrary n-th refueling, and a core with the same arrangement as described above is formed.

When the fuel assembly is operated during one fuel exchange cycle, the burnup of each fuel assembly increases and becomes arranged as shown in FIG.

To perform the next (n+1)th fuel exchange, first remove the fuel assemblies 4b, 4b that have reached burnup 3 around the core, as shown in Figure 9, and replace the unit lattice of the second pattern during the previous fuel exchange. A fuel assembly 4b in which a burnup of 1 has progressed to a burnup of 2 in a unit cell that was 2...
4b to this core peripheral area 6, and each unit cell 1.
..., 2..., six fuel assemblies 4d... that have reached burnup level 4 are removed.

In this case, two fuel assemblies 4b and 4b, which have progressed from burnup 1 to burnup 2, are placed diagonally in the unit cell which was previously unit cell 1 of the first pattern. These fuel assemblies 4b, 4b are not moved.

In addition, two fuel assemblies 4e and 4c, which have progressed from burnup 2 to burnup 3, are placed diagonally in the unit lattice that was unit lattice 2 of the second pattern last time. These fuel assemblies 4c, 4c are not moved.

Next, as shown in FIG. 10, the fuel assembly 4c y4c with a burnup of 3 removed from the core peripheral area 6 is replaced with the unit lattice that was the unit lattice 1 of the first pattern at the time of the previous fuel exchange, that is, the unit lattice with a burnup of 2. Fuel assemblies 4b... Two new fuel assemblies 4a are loaded, one in each unit grid, arranged diagonally, and six new fuel assemblies 4a with a burnup of 1 are loaded in the remaining locations.
Load...

Therefore, as shown in FIG. 10, the unit cell which was the unit cell 1 of the first pattern last time becomes the unit cell 2 of the second pattern, and also the unit cell 2 of the second pattern last time.
The unit cell that was ... is the unit cell 1 of the first pattern.
...becomes.

Therefore, the unit lattices 1 of the first pattern and the unit lattices 2 of the second pattern are arranged in a chessboard shape, and the fuel assemblies 4b of burnup 2 are arranged in the core peripheral area 6.
・The reactor core structure is arranged as follows.

If one fuel cycle operation is performed in this state, the 11th
As shown in the figure, each fuel assembly 4a..., 4b..., 4
The burnup of c... progresses, and at the next fuel change, that is, n+
At the time of the second fuel exchange, the fuel exchange is performed in the same manner as described above, as shown in FIGS. 12 and 13.

In this case, the unit lattice that was the unit lattice 1 of the first pattern at the previous time, that is, the n+1th fuel change, becomes the unit lattice 2 of the second pattern, and the unit lattice 1 of the second pattern becomes the unit lattice 1 of the second pattern. The unit lattice that was there becomes the unit lattice 2 of the first pattern, so the arrangement of the unit lattice 1 of the first pattern and the unit lattice 2 of the second pattern is the same as that at the nth fuel exchange. It will be the same, but each unit cell 1...
・.

2... fuel assemblies 4a..., 4b...
The arrangement of y4c... is optically symmetrical to that at the nth fuel exchange.

When one more fuel exchange cycle is operated, the burnup of each fuel assembly 4a..., 4b..., 4c... progresses and the arrangement becomes as shown in FIG. 14.

Then, the (n+3)th fuel exchange is performed in the same manner as described above, as shown in FIGS. 15 and 16, and in this case, the unit lattice 1 of the first pattern and the unit lattice 2 of the second pattern...
The arrangement of . and is the same as in the case of the nth time, that is, the n+1th time.

After one more fuel cycle, each fuel assembly 4a... as shown in FIG.

As the burnup of 4b...y4c... progresses and the n+4th fuel exchange is performed as shown in Figs. 18 and 19, the unit lattice 1 of the first pattern and the unit lattice 2 of the second pattern... The arrangement of ... is reversed again and becomes the same as at the nth fuel change, and each unit cell 1 ..., 2 ...
- The arrangement of the fuel assembly within is optically symmetrical to that at the (n+2)th fuel exchange.

Therefore, by the n + 4th fuel exchange, the arrangement of unit cells 1..., 2... and the unit cells 1...,
2... fuel assemblies 4a..., 4b...,
4c... will be exactly the same as in the case of the nth fuel exchange, and such a fuel exchange pattern can be repeated infinitely.

In such a core structure, during any fuel exchange, the fuel assemblies 4b, . )4
c... does not need to be moved, so the number of fuel assemblies to be replaced with new fuel assemblies or moved is half of the total number, and the fuel assemblies 4 that are diagonally located do not need to be moved.
Since the control rods 3... are supported by b..., 4b..., there is no need to install blade guides for the control rods 3... at the time of fuel exchange.

Furthermore, since the same fuel loading pattern can be repeated, the refueling operation is extremely simple and efficient, and since the fuel loading pattern is the same for each refueling cycle, constant core characteristics can be maintained.

゛In addition, at each fuel exchange, four unit cells 1..., 2
..., two fuel assemblies 4b with burnup of 2 are left over, and two fuel assemblies 4c with burnup of 3 are required, but the remaining burnup of 2 fuel assembly 4
B... is placed in the core peripheral area 6 to advance the burnup to 3 in one fuel exchange cycle, and this is used at the next fuel exchange, so the unit grid 1... is placed at each fuel exchange.

2...4 fuel assemblies with burnup of 4 4d...6
It is only necessary to remove the body and load six new fuel assemblies 4a with burnup of 1, and the fuel cycle becomes rational.

Note that the present invention is not limited to the above embodiment.

For example, fuel assemblies with a burnup of 2 do not necessarily need to be placed around the core; the point is that at least the unit cells of the first pattern and the unit cells of the second pattern should be placed in a chessboard shape in the center of the core. It's good.

As described above, in the present invention, two fuel assemblies having a burnup of 1 are arranged diagonally to each other, and two fuel assemblies having a burnup of 3 are arranged diagonally to each other to form a unit cell of the first pattern. two fuel assemblies with a burnup of 2 are arranged diagonally to each other, and one fuel assembly with a burnup of 1 and one fuel assembly with a burnup of 3 are formed.
The unit cells of the first pattern and the unit cells of the second pattern are arranged in a chessboard shape.

Therefore, by the time of the next fuel change, fuel assemblies with a burnup of 1 that are located diagonally to each other in the unit cell of the first pattern will have reached a burnup of 2, and will also be diagonally opposite to each other in the unit cell of the second pattern. The fuel assemblies with burnup of 2 at the corner positions advance to burnup of 3, so at the next fuel exchange, without moving these fuel assemblies, you only need to replace and move the other half of the fuel assemblies. What was a unit cell of one pattern can be made a unit cell of a second pattern, and what was a unit cell of a second pattern can be made a unit cell of a first pattern.

Therefore, the number of fuel assemblies to be replaced or moved during fuel exchange can be reduced, and fuel can be loaded in a fixed pattern, making the fuel exchange operation simple and efficient.

Further, since the configuration is such that the unit lattices of the first pattern and the unit lattices of the second pattern are simply arranged in a chessboard shape, it is possible to simply obtain constant core characteristics.

Furthermore, in the unit cells of the first pattern and the second pattern, there is little change in the infinite effective multiplication factor during the fuel exchange cycle, and the infinite effective multiplication factors are approximately equal to each other, so that a constant reactor shutdown margin can always be ensured. Moreover, the effects are great, such as obtaining favorable core characteristics such as uniform core reactivity and uniform power distribution.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, and FIG. 1 is a layout diagram of the unit lattice of the first pattern, FIG. 2 is a layout diagram of the unit lattice of the second pattern, and FIG. 3 is a layout diagram of the main parts of the reactor core. , FIG. 4 is a diagram showing the relationship between the burnup of the fuel assembly and the infinite effective multiplication factor, and FIG. 5 is a diagram showing the change in the average infinite effective multiplication factor of the unit cell of the first pattern during the fuel exchange cycle. FIG. 6 is a diagram showing changes in the average infinite effective multiplication factor of the unit cell of the second pattern, and FIGS. 7 to 19 are layout diagrams of the main parts of the core, each showing the fuel exchange procedure in the order of steps. 1...Unit cell of the first pattern, 2...
・Second pattern unit cell, 3...Control rod, 4
a...Fuel assembly with burnup of 1, 4b...
・Fuel assembly with burnup degree 2, 4c...Fuel assembly with burnup degree 3, 4d...Fuel assembly with burnup degree 4, 5
・・・－・・・Core center part, 6...Core periphery part.

Claims (1)

[Claims] 1 A newly loaded fuel assembly has a burnup of 1. A fuel assembly that has undergone one fuel exchange cycle has a burnup of 2. A fuel assembly that has undergone two fuel exchange cycles has a burnup of 3. In this case, two fuel assemblies with a burnup of 1 are arranged diagonally to each other in a unit cell in which four fuel assemblies are arranged in a lattice around one control rod, and the burnup is 3. Two fuel assemblies with a burnup of 2 are arranged diagonally to each other to form a unit cell of the first pattern, and two fuel assemblies with a burnup of 1 are arranged diagonally to each other. One fuel assembly and one fuel assembly with a burnup of 3 are arranged diagonally to each other to form a unit cell of a second pattern, and the unit cell of the first pattern and the unit cell of the second pattern are arranged diagonally to each other. and are arranged such that the unit cells of the first pattern are located diagonally to each other, and the unit cells of the second pattern are located diagonally to each other. Reactor core. 2 The unit cells of the first pattern and the second pattern are arranged only in the center of the core, and the unit of the pattern that is different from the first pattern and the second pattern and includes fuel assemblies with a burnup of 2 is arranged in the periphery of the core. 1. A boiling water reactor core as described in item 1 of the patent, characterized in that a lattice is arranged. 3 The fuel assemblies with burnup 2 located diagonally to each other in the unit grid of the second pattern are replaced by the fuel assemblies with burnup 1 loaded during the previous fuel exchange that have progressed to burnup 2. In addition, the fuel assemblies with burnup of 3 that are diagonally located in the unit grid of the first pattern are the same as the fuel assemblies with burnup of 1 that were loaded at the time of the previous fuel change. 2. The boiling water reactor core according to claim 1, wherein the core has reached burnup level 3 and is used without replacement.