JPH0198993A - Fuel assembly - Google Patents

Abstract

PURPOSE:To shutdown the reactor even if the enrichment of fuel is heightened, and also, to improve the axial direction output distribution by eliminating a part of a fuel rod in the upper part of 4/6H or 5/6H from the lower end of a fuel effective part. CONSTITUTION:A part which has lowered remarkably the fissionable nuclide concentration is provided in a shape of a straight line or of a crossing straight line or of a solid lump on the inside of a fuel assembly so as to contain a part of a range of 4/6H or 5/6H from the lower end of a fuel effective part in height H extending from the lower end to the upper end of the fuel effective part of the fuel assembly being a heating part of a boiling water reactor. Also, between the part of 4/6H or 5/6H and the lower end of the fuel effective part, an interposed area which has lowered remarkably and substantially the fissionable nuclide concentration extending over the width of 2-8cm so as to cross roughly the fuel assembly is provided at least by one area. In such a way, the margin for shutting down the reactor is improved, and also, the quantity of fuel decreases in the upper part of the reactor core, therefore, a water-to-fuel volume ratio increases and shortage of a moderator is improved. Accordingly, an output ascends and the axial direction output distribution can be improved.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly, and in particular to a fuel assembly suitable for a boiling water nuclear reactor with a long operating cycle and a high shutdown margin. Regarding.

(Prior Art) A fuel assembly for a boiling water reactor consists of a large number of regularly arranged fuel rods filled with nuclear fuel material inside a metal cladding tube, which are housed inside a rectangular channel box. ing. In the core of a boiling water reactor, cells each consisting of one cross-shaped control rod and four fuel assemblies surrounding it are regularly arranged.

That is, the fuel assemblies and control rods are arranged so that their axes are perpendicular and parallel to each other, and the coolant, which functions as a moderator, flows from the bottom of the core to the top. has been done.

By the way, although no bubbles are generated near the effective lower end of the core, that is, the lower end of the heat generating section, a large amount of bubbles are generated from the center to the upper end of the core, and the generated bubbles flow upwards of the core. When the volume ratio occupied by bubbles, that is, the void ratio increases, the neutron moderation characteristics decrease, resulting in a decrease in thermal neutron flux and a decrease in output. In order to avoid this, measures have been taken such as increasing the concentration of fission nuclides, that is, the degree of enrichment, in areas with a high void ratio, or adding burnable poison to suppress the increase in output in areas with a low void ratio.

Therefore, in a boiling water reactor, combustion in the upper part of the core tends to be delayed, which causes the U-235 concentration to be relatively higher than in other parts, and because fissile nuclides such as PU-239 are generated due to voids. , the margin for reactor shutdown tends to be tight in the upper part of the core. Furthermore, efforts are being made to lengthen the operating cycle and improve fuel burn-up, with the main purpose of improving economic efficiency. In this case as well, the enrichment of the fuel will inevitably increase, making the margin for reactor shutdown even tighter.

Next, typical examples of fuel assemblies used in boiling water reactors and fuel assemblies expected to be used in the near future will be described with reference to the drawings.

FIGS. 16(a) and 16(b) are a perspective view of a conventional fuel assembly and a schematic vertical sectional view of a fuel rod constituting the fuel assembly, respectively.

In FIG. 16 (a), the fuel assembly has a water rod (not shown) and a fuel rod 2 fixed by an upper tie plate 4, a spacer 5, and a lower tie plate 6, and the outside thereof is surrounded by a channel box 1. As shown in FIG. 16(b), the fuel rod 2 has fuel pellets 8 disposed inside the cladding tube γ, and a spring 9 attached to the gas plenum above the cladding tube γ.
An upper end plug 10 is provided at the upper end, and a lower end plug 11 is provided at the lower end.

FIG. 17 is a cross-sectional view of the conventional fuel assembly shown in FIG. 16(a). Sixty-two fuel rods 2 and two water rods 3 are arranged in the channel box 1 to form a fuel assembly. Are the water rods 3 suppressing the shortage of water, which is a moderator, inside the core? Since the water rods 3 are uniform in the axial direction, there is a problem that there is an excess of water in the lower part of the core and a water shortage in the upper part. There is.

The fuel assembly shown in FIG. 18 was developed to improve the characteristics of the fuel assembly, and one large-diameter water rod 12 is arranged inside the assembly to introduce non-boiling water. There is. However, even in this example, there is a problem in that there is an excess of water below the core and a shortage of water above the core.

The fuel assembly shown in FIG. 19 is an improvement of the fuel assembly shown in FIG. 17, and is provided with four small channel boxes 13. Sixteen fuel rods 2 are arranged in this small channel box 13 to form a boiling moderator water region, and a cross-shaped gap 14 between the small channel boxes 13 is a non-boiling moderator water region. , which aims to flatten the horizontal power distribution, but this type of fuel assembly also has the problem of excess water below the core and insufficient water above it.

The fuel assembly shown in FIG. 20 was developed as an improved version of the fuel rod shown in FIG. 19. This fuel assembly is composed of nine subassemblies 15, and each subassembly 15 is composed of nine fuel rods 2.

A rather wide gap 16 is provided between the subassemblies 15. In the case of this fuel assembly as well, the problem of excess and shortage of water in the upper and lower parts of the core has not been resolved.

(Problem to be solved by the invention) As mentioned above, although no bubbles are generated at the bottom end of the fuel assembly, which is the heat generating part of a boiling water reactor (BWR), bubbles are not generated anywhere else in the boiling water reactor (BWR). The generated bubbles flow upward (downstream).Therefore, the BW
The bubble ratio (void ratio) of R becomes higher above the core.

As a result, the rate of nuclear fission decreases because the moderation characteristics of neutrons decrease. That is, combustion proceeds below the core and is delayed above the core. Therefore, it has been proposed to increase the concentration of fission nuclides above the reactor core in order to suppress the decrease in power above the reactor core.

However, increasing the void ratio and fission nuclide concentration above the core will reduce the degree of subcriticality at the top of the core when the reactor is shut down.

On the other hand, in order to lengthen the operating cycle and improve economic efficiency, it is necessary to further increase the enrichment of the fuel, but this will make the subcriticality in the upper part of the core even shallower, and eventually There may be cases where the reactor cannot be shut down. In other words, this point has become a bottleneck, and conventional nuclear reactor cores have had the problem of not being able to extend the operating cycle.

The present invention was made to solve the above problems, and
The object of the present invention is to provide a fuel assembly constituting the core of a boiling water reactor that allows the reactor to be shut down even when the fuel enrichment level is high and that improves the axial power distribution.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention is constructed by regularly arranging a large number of fuel rods filled with nuclear fuel material inside a metal cladding tube. In the fuel assembly, a straight line is drawn inside the fuel assembly so as to include a part of the range of 476H to 5/6H from the lower effective fuel end of the height H from the lower effective fuel end (BAF) to the upper end (TAF). The portions with significantly reduced fissile nuclide concentration are arranged in a straight or intersecting linear shape or in a block shape, and further the above 476H to 576
Between the portion H and the effective lower end of the fuel, there is at least one intervening region substantially across the entire fuel assembly with a width of 2 cm to 8 cm that substantially reduces the concentration of fissile nuclides, thereby reducing the atomic concentration. This improves the shutdown margin of the reactor, improves the axial distribution of output, and eliminates the fuel rods in the portion including 476H to 576H from the effective lower end of the fuel, further reducing the pressure loss of the coolant. It is characterized by contributing to stability.

(Function) The fuel assembly of the present invention has at least the following two remarkable functions.

■Improved reactor shutdown margin (most noticeable) k during cold state. f
f can be significantly reduced, and the present invention can reduce ke during high temperature operation.
It is also possible to design the ff value to be increased somewhat.

(2) Improved power distribution In BWR, moderator is insufficient during operation due to high void ratio in the upper part of the core. According to the present invention, since the amount of fuel is reduced in the upper part of the core, the water-to-fuel volume ratio increases. That is, the shortage of moderator is improved. The power is therefore increased and the axial power distribution is improved. Also, since there is more water in the upper part, the void coefficient (excessively negative) is also alleviated.

Further, in the present invention, 4/6 from the effective lower end of the fuel
When a part of the fuel rods in the upper part (downstream side of the coolant) is removed from H to 5/6H to form a vanishing region, the following third effect occurs. In other words, (1) Reduction of coolant pressure loss. Since the coolant flows from the bottom to the top of the aggregate, voids do not occur in the lower part of the aggregate, but they do occur in other parts, and the void ratio is particularly high in the upper half. . This means that the coolant flow rate of the gas-liquid mixture increases significantly.

Since the pressure loss is roughly proportional to the square of the flow velocity, the pressure loss at the upper part of the assembly is large. Pressure loss depends on the wetted area of fuel rods and channel boxes, and the structure and number of spacers (number of stages).
It changes depending on.

If, as in the present invention, a part of the fuel rods in the upper part (downstream side of the coolant) is removed from 476H to 5/6H from the fuel effective lower end to provide a vanishing region, the pressure loss will be large in the upper part of the assembly. Since the number of fuel rods is reduced,
The wetted area is reduced, and as a result, pressure loss can be reduced. This allows for a reduction in recirculation pump power and also improves channel stability, resulting in
The flow within the channel is stabilized and fluctuations in the void ratio are reduced, improving the stability of the reactor core.

Next, the above effect (2) will be explained in more detail.

According to the fuel assembly of the present invention, the neutron interaction (coupling effect) on both sides of the region where the content of fissile nuclides is significantly reduced, that is, the intervening region, is weakened when it is cold, and during high-temperature operation, the neutron interaction (coupling effect) is weakened. A phenomenon occurs that becomes stronger when it occurs. This phenomenon can be mainly explained by the action of thermal neutrons, which have a short diffusion distance. In other words, when it is cold, the density of water (
(approximately 1.0) is large, the diffusion distance of thermal neutrons becomes short, and the interaction between neutrons on both sides of the intervening region decreases, resulting in a decrease in neutron multiplication characteristics. During high-temperature operation, even when no voids occur, the water temperature (standard value) in a boiling water reactor is approximately 286°C, and the density of water is approximately 0.74.
'I/cm3. The distance traveled by thermal neutrons in water increases by 110.74 (=1.35) times when it is cold.

Furthermore, the density of the air-water mixture when voids occur decreases to about 0.3, and as a result, the thermal neutron diffusion distance in the air-water mixture increases by a factor of 110.3 (:3). As a result, neutron interactions on both sides of the intervening region increase,
Neutron multiplication properties increase.

By utilizing the above-mentioned effect, by introducing the intervening region, the multiplication factor can be reduced in the cold state, that is, the reactor can be shut down (subcriticality).
and prevent the multiplication factor from decreasing even if the amount of fuel is reduced by introducing an intervening region during high-temperature operation.
With suitable design, it is even possible to increase the multiplication factor compared to the case where such a region does not exist.

Next, the operation of the present invention will be explained with reference to FIG. As shown in FIG. 5A, it is assumed that there are two fuel regions (1) and (2) having a rectangular cross section, and a water gap of width W exists between them. Also in the fuel area ■.

The width Wf in the same direction as the water gap width W in (2) is sufficiently wider than the water gap width W. The relationship between the water gap width W and the change in the neutron multiplication factor at this time is as shown in the same figure (b).
Shown below. Here, the "change in neutron multiplication factor" at high temperature (
Both the broken line) and the cold state (solid line) indicate changes from the neutron multiplication factor when the water gap width is O. Directions perpendicular to the axial direction within the fuel assembly (usually horizontal in light water reactors)
When configuring a water gap in a large area, it is difficult to have a wide water gap area. That is, widening the water gap within a given range of the external shape means that the fuel area becomes narrower, and the heat generation area also becomes narrower.

In the present invention, a narrow intervening region is inserted in the direction perpendicular to the axis of the fuel assembly, and a slightly wider intervening region is inserted in the horizontal direction. Therefore, it is necessary to clarify the characteristics of narrow intervening regions.

Based on this idea, FIG. 2(C) is an enlarged view of part 0 of FIG. 2(b). Theoretically calculated values for the case where a water gap of about 2 cm at most is provided also give a curve almost similar to that shown in Figure (C). That is, during high-temperature operation (when voids occur), the change in multiplication factor increases in the positive direction with the water gap width, and the effective multiplication factor keff increases), and during cold operation, it becomes noticeable when the water gap width exceeds about 1 cm. It can be seen from this figure that keff decreases as the water gap width increases, which helps increase the degree of subcriticality during reactor shutdown.

In addition, in the explanation of the above effect, we looked at it as a change in neutron interaction between two fuel regions sandwiching a water gap, but the infinite multiplication factor of the fuel assembly is divided into four factors that have been known for a long time. It can also be explained in terms of method. In this way, the curve of FIG. 2(C) is also mainly explained by changes in the characteristics of the thermal neutron utilization rate and the probability of escaping resonance. If the water gap is widened within the fuel assembly without reducing the number of fuel rods, the gap between the fuel rods must be reduced, which increases the shielding effect of resonance neutrons between the fuel rods during resonance absorption. As a result, the probability of escaping resonance increases, while the thermal neutron flux ratio of the fuel region to the water gap decreases, resulting in a decrease in the thermal neutron utilization rate. Figure 2 (C) is the above 2
It is almost determined by the offsetting effects of water density dependence and water gap width dependence.

In order to fix the fuel rod spacing and widen the water gap, the fuel rods (fuel material) must be removed. In that case, the change in the probability of escaping resonance absorption described above is not due to the shielding effect of resonant neutrons, but is due to an increase in the moderating effect, which increases the probability of escaping resonance. In other words, if a nuclear reactor is operated at high temperatures and voids are generated, there is a lack of moderator, so the introduction of a water gap alleviates this, and as a result, the probability of escaping resonance increases. do. The change in thermal neutron utilization rate is almost the same as in the above example.

In the present invention, the portion that constitutes the intervening region perpendicular to the axial direction of the fuel assembly is mainly the latter (exclusion of fuel rods),
The former (widening the water gap without reducing the number of fuel rods inside the fuel assembly) is also used as needed. The concentration of fissile nuclides is significantly reduced in the portion parallel to the axial direction, that is, the portion constituting the intervening region in the vertical direction.

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

FIG. 1 is a schematic diagram of an embodiment of the present invention, and FIG.
is a vertical cross-sectional view along line A-A in the same figure (b), and the same figure (bL
(C) and (d) are the lines BB in (a) of the figure, respectively.

It is a top view along the CD line and the DD line.

In the fuel assembly of this embodiment, a rectangular water rod 32 is arranged in the center, and except for this central part, long fuel rods 30° and short fuel rods 31 (hereinafter referred to as P in the plan view) are arranged regularly at 9 They are arranged in rows and nine columns, and are surrounded by a channel box 33 on the outside, and their upper and lower ends are fixed by an upper tie plate 34 and a lower tie plate 35, respectively.

For the short fuel rods 31, an upper plenum 40 is arranged at the top of a stack (fuel pellets) 36 via an output spike suppressor 37, and a lower plenum 41 is arranged at the bottom of the stack 36.
It is configured with . The space above the short fuel rods 31 is occupied by coolant and air bubbles, forming a space in which the fuel rods are erased, ie, a vanishing region. Also,
The main part of the gas plenum is provided on the lower side of the stack, and a short plenum is provided on the upper side as an auxiliary part. Note that the upper part of the short fuel is attached to a vanishing rod.
) It is called 42.

In the CC section of FIG. 1(a), as shown in FIG. 1(C), intervening members 38 of 2 to 8 cm are inserted into all the fuel rods. This length is said to be about or greater than the thermal neutron diffusion distance in a cold state, and about or smaller than the thermal neutron diffusion distance when voids occur during high-temperature operation.

Output spike suppressing material 39 is inserted so as to sandwich intervening member 38 from above and below. The characteristics required of this restraining material 39 are basically the same as those of the restraining material 37 at the top of the stack of short fuel rods.

In this embodiment, five pellets from the top of the short fuel rod 31 are used.
Since the output tends to locally increase within 1 cm, particularly within 1 cm, an output spike suppressing material 37 is inserted at the top of the pellet stack in order to suppress this. Similarly, the output spike suppressing material 3 is applied to the fuel pellets on both sides of the part where the intervening member is inserted in the axial direction of the fuel assembly.
Insert 9.

Examples of configurations of the output spike suppressing material include depleted uranium pellets with a length of 0.5 to 5 cm, usually 1 to 2 cm, natural uranium pellets, and materials containing burnable poisons in the center of circular pellets, that is, (Gd203 -UO2, Gd203-ZrO2゜Gd2
03-Al2O2, Hf02-Yb2O3.

HfO2-DV203, etc.) or pellets of non-fuel material = 1
9-(ZrO2, ZrO2-GdO3, A2203.

Affi203-Gd203, j-1f02-Tb2
03.

HfO2-DV203, etc.) is used.

A gradual increase in output also occurs in the long fuel rods 30 adjacent to the vanishing rod portions 42 above the short fuel rods. Usually, no special measures are required in this case, but
For future assemblies with progressively higher fuel enrichments, a number of known techniques can be used, including slightly lowering the enrichment and using annular fuel pellets with GCJ203 inserted into the center. .

Assuming that the output spike suppressing material is not interposed, if a relatively long intervening region (in the axial direction) is inserted in a direction perpendicular to the axis at a relatively upper part of the aggregate (D in Fig. 1 (a) There is a tendency that the output spikes above and below the relatively long intervening region in the axial direction (vertical direction) of the C-C cross section tend to be larger than the maximum output spike at the -0 section). For this reason, the latter is placed in areas where sufficient cooling water cooling capacity remains (excluding the upper part of the core) to ensure fuel integrity in the event of an emergency.

FIG. 3(a) is a schematic cross-sectional view of the fuel assembly of the present invention applied to a boiling water reactor, and FIG. 3(b) is a diagram showing the void ratio and subcriticality distribution in the core axis direction.

The hatched part A in FIG. 3(a) is the burnishing area of the short fuel rod. The hatching part is located in the intervening region.

This vanishing area, that is, the vertical intervening area and the horizontal intervening area (intervening member insertion area) are usually at the same height, but
Since they are long in the axial direction, there is no need to always align them. For example, the lower end may be changed to gently improve the axial power distribution. A height difference may be provided within a bundle, or a step may be provided between bundles. However, since the horizontally interposed region is short in the vertical direction, it is desirable to make the heights the same.

FIG. 3(b) compares the present invention with the conventional one, and shows that the subcriticality of the present invention is flatter and larger than that of the conventional one, and the void ratio is small at the upper end of the core.

FIG. 4 is a schematic diagram of a second embodiment of the present invention, and FIG.
a) is a vertical cross-sectional view taken along line A-A in figure (b);
), (c), (d) and (e) are shown in the same figure (
a) 8-13. FIG. 3 is a plan view taken along lines CC, DD, and EE.

Incidentally, the same parts as in the first embodiment already described are given the same reference numerals, and detailed explanation thereof will be omitted. The same applies to each of the following examples.

As shown in FIG. 4, a set of three long intervening member-enclosed fuel rods 43 (hereinafter referred to as P in the plan view) are arranged in a cross shape around a large-diameter water rod 46 in the center, and a bundle The position (2, 2) near the corner is a short fuel rod 44 (hereinafter referred to as PV in the plan view).

In this embodiment, although it is not necessarily necessary to use short fuel rods 44 (PV), the height of the long intervening member, that is, the width of the water gap across the vertical intervening area is widened, and the coolant collects there. As a result, it is conceivable that the cooling capacity of the coolant near the corners may be insufficient. In such a case, if the fuel rod near the corner rod, at the (2,2) position in this embodiment, is made into a short fuel rod 44 (PV), such a problem can be improved.

= 22- In addition, the effects (1) to (2) described above occur. In this assembly, 16 fuel rods P with long inclusions and 16 short fuel rods Pv (hereinafter simply referred to as fuel rods P) are used to form a horizontally interposed region. ) consists of 60 pieces. In this embodiment, the gap width of the vertical intervening region is widened compared to the previous embodiment, so that the effect is increased with the same amount of fuel.

FIG. 5 is a plan view of a third embodiment of the invention.

This embodiment uses a conventional type aggregate (4 thin water rods instead of a large diameter water rod).
7), and the fuel rod P
are arranged in a cross shape. Therefore, in this assembly, there are 14 short fuel rods or long intervening member pairs, that is, 14 fuel rods P, and 719 fuel rods 45. Which fuel rod P to select is determined by design, and each has its advantages and disadvantages. For example, the use of short fuel rods is particularly effective in reducing pressure drop, but coolant flow is affected. Although the pressure drop reduction effect is small when a long intervening member-enclosed fuel rod is used, it is easy to put into practical use because it does not disturb the flow of coolant. The same applies to the following examples.

FIG. 6 is a plan view of a fourth embodiment of the present invention.

This embodiment has two small diameter water rods 47 as well as the embodiment shown in FIG.
This is the case when applied to the method used herein. In this example, the fuel rods P are arranged diagonally, and the number of fuel rods P is 14.
The fuel rods 45 are composed of 48 pieces.

FIG. 7 is a plan view of a fifth embodiment of the invention.

This embodiment has two small diameter water rods 47 as well as the embodiment shown in FIG.
This is the case when applied to the method used herein. In this embodiment, the fuel rods P are arranged in two rows parallel to the diagonal line inside the assembly. In this embodiment, there are eight fuel rods P and 54 fuel rods 45.

FIG. 8 is a plan view of a sixth embodiment of the present invention.

This embodiment is applied to a system using two small diameter water rods 47. In this embodiment, the fuel rods P are arranged in a double cross shape, and the number of fuel rods P is 26 and the number of fuel rods 45 is 36. In the vertically intervening region, the assembly is substantially composed of four 3×3 subbundles, providing a large reactor shutdown margin.

FIG. 9 is a plan view of a seventh embodiment of the present invention.

This embodiment uses large-diameter water rods 46 for four central fuel cells in the embodiment shown in FIG. 8, and also replaces the fuel rods P on the channel box side with fuel rods 45.

There are 16 fuel rods P and 44 fuel rods 45.

FIG. 10 is a plan view of an eighth embodiment of the present invention.

A rectangular water rod 48 corresponding to four fuel cells is arranged in the center, and the fuel rods P are arranged so as to cross in the diagonal direction of the fuel assembly. There are 12 fuel rods P and 48 fuel rods 45.

FIG. 11 is a plan view of a ninth embodiment of the present invention.

A large approximately rectangular water rod 49 is placed in the center rotated 45 degrees to the bundle surface, and one fuel rod is placed on each side of this rectangular water rod 49, and fuel is also placed at each corner of the fuel handle. A stick P is placed. Therefore, there are 16 fuel rods P and 60 fuel rods 45.

FIG. 12 is a plan view of a tenth embodiment of the present invention.

This embodiment is a modification of the embodiment shown in FIG. 5, in which the water gap between the large-diameter water rod 46 and the subbundle is offset with respect to the fuel handle, and the width of the water gap around the outer periphery of the assembly differs depending on the direction. This embodiment can be effectively applied to a reactor core (called a BWR-D lattice). The left side and upper side of this figure are arranged in the core so that there is a wide gap.

A favorable effect can be obtained by flattening the output distribution. Fuel rod P is 1
The fuel rods 45 are made up of 63 pieces.

FIG. 13 is a plan view of an eleventh embodiment of the present invention.

This embodiment is an example of application of the present invention to a conventional type. That is, the collection in this example consists of four sub-bundles 50.
, the cross-shaped gap 51 between each sub-bundle 50 is a non-boiling moderator water region, and each sub-bundle 50 has a configuration in which fuel rods P are arranged in a cluster at a corner portion located at the center of the assembly. It is.

Therefore, there are 12 fuel rods P and 52 fuel rods 45.

FIG. 14 is a plan view of a twelfth embodiment of the present invention.

This embodiment is also an example of application of the present invention to the conventional type. That is, the assembly of this embodiment is entirely composed of nine subbundles 52, each subbundle 52 consists of nine fuel rods 45, and a slightly wide gap 53 is provided between each subbundle 52. It is being Since all the sub-bundles in the center of this fuel assembly are made up of fuel rods P, there are nine fuel rods P and 72 fuel rods 45.

FIGS. 15(a) to 15(e) are longitudinal sectional views of different fuel rods according to the present invention.

That is, the fuel rod shown in FIG.
Graphite 21 is inserted. The intervening member for the vertical intervening area is generally long, from 15 to 906 m.
The intervening member for the horizontal intervening region is generally short and has a length of 2
~8 crn. The same applies to the following examples. Graphite 21 is an optimal example because it has excellent high-temperature properties, absorbs few thermal neutrons, and also functions as a moderator. Low density (porous) A2203, Zr
Although O2 and the like do not have excellent moderation properties, they have good heat resistance properties, and such substances with low neutron absorption can also be used. Instead of solid graphite, hollow graphite, hollow Aj2203, ZrO2, hollow natural uranium,
The hollow part may be used as a gas plenum by using hollow depleted uranium or the like.

The most important characteristic required for this region is that the thermal neutron absorption rate at the end of the cycle is smaller than the fuel region that sandwiches this region. In the fuel material adjacent to this graphite 21, an output peak (spike) occurs in a range of about 2 cm (at most 5 cm), which is disadvantageous in terms of the health of the fuel, so the burnable poison 22a is contained only in the vicinity of the axis. Two pellets 22 (approximately 2 cm> each) are arranged.

Since the outer periphery of these pellets 22 does not contain poisonous substances, the output power remains relatively constant over the entire operating cycle. The design is such that as the end of the cycle approaches, the absorption characteristics of toxic substances disappear and the output of this part gradually increases.

The neutron interaction (coupling effect) in the horizontal fuel region sandwiching the region with low concentration of fissile nuclides (hereinafter referred to as the intervening region) is reduced, and as a result, the subcriticality of the reactor during shutdown can be increased. .

The difference between the fuel rod shown in FIG. 15(b) and the fuel rod shown in FIG. 15(a) is that a tube 24 made of Zircaloy, which has a small thermal neutron absorption cross section, is inserted instead of graphite 21. Many variations on this example are possible. (1) When used as a gas plenum, use unsealed pipes.

(2) When ZrH2 (referred to as zirconium hydride, zirconium hydride, etc.) is densely packed, ZrH
2 should be written as ZrHx (0<X×2),
The larger X is, the more desirable it is for the purpose of the present invention, but as X becomes larger, it tends to become brittle, so it is generally desirable to seal the tube. A relatively small air gap is provided within the tube to serve as a gas plenum for the small amount of H2 released from the ZrH2.

(3) Since Be and BeO are toxic, it is preferable to put them in a tube. Since Be also generates He gas by reaction with neutrons, a small plenum (gap) for He gas is provided.

Between the Zircaloy tube 24 and the fuel pellets 23, there are small heat insulating pellets 2'5. Aj2203, Zro2° depleted uranium, etc. are used to improve fuel integrity. It is desirable that the heat insulating material pellets 25 have small thermal neutron absorption characteristics at the end of the operating cycle. Therefore, Aff203-Gd203 and depleted uranium UO2-Gd203 pellets to which burnable poison has been added are suitable. For fuel pellets adjacent to the Zircaloy tube 24 in the axial direction, the distance is approximately 2 cm from the end surface (approximately 5 cm at most).
It is preferable to arrange pellets 22 containing burnable poison until then. Although FIG. 15(b) shows the fuel pellet 22 with small-diameter Gd pellets inserted, Gd may be mixed into the entire pellet, and FIG. 15(a) and FIG. 15(C)
) Gd may be mixed into the entire pellet in the same manner.

The difference between the fuel rod shown in FIG. 15(C) and the fuel rod shown in FIG. 15(b) is that they are configured to introduce water.

In other words, water holes 26 are provided above and below the cladding tube 20 of the fuel rod in the portion where the Zircaloy tube 24 of the fuel rod is located in FIG. 25, pellets 22 containing a burnable poison were placed above and below, and fuel pellets 23 were placed above and below, respectively.

The difference between the fuel body shown in FIG. 15(d) and the fuel rod shown in FIG. 15(a) is that the fuel rod shown in FIG.
203-ZrO2 or the like) is provided with an intervening layer 28 containing a burnable poison. According to this example:
There are manufacturing advantages as no burnable poisons are added to the fuel.

The difference between the fuel rod shown in FIG. 15(e) and the fuel rod shown in FIG. 15(a) is that the intervening member is graphite (A2z 03 ).

ZrO2,l! 203-ZrO2, etc.) by specifying fuel pellets that are adjacent to each other from above and below, so that almost no output spike occurs throughout the operating cycle. That is, in this example, a composite pellet 29 is prepared in which a thin pellet 29'a made by sintering a mixed oxide of depleted uranium oxide and gadolinia is inserted into an annular pellet using natural uranium oxide, and the intervening member 21 is sandwiched between the thin pellets 29'a. Although not shown, they are wrapped in a thin metal sleeve made of pure zirconium or the like to form a cassette.

This allows the fuel assembly to be assembled accurately and quickly. Even after the neutron toxicity of Gd203 in the core is eliminated by combustion, depleted uranium has a small heat generating area, so high temperatures are not generated. Since the annular part is made of natural uranium, high heat generation does not occur.

[Effects of the Invention] As explained above, according to the present invention, the following effects are achieved. That is, ■, k in cold state. Since ff can be significantly reduced and the keff value can be increased somewhat during high temperature operation, it can contribute to improving the reactor shutdown margin.

(2) The amount of fuel decreases in the upper part of the core, so the water-to-fuel volume ratio increases. In other words, the moderator shortage is improved = 32-. Therefore, the output horn is raised and the axial power distribution is improved.

In addition, as an embodiment of the present invention, from the lower end of the effective fuel
When a part of the fuel body is removed above (downstream side) from /6H to 5/6H to form a vanishing region, the following third effect occurs. That is, (1) the number of fuel rods in the upper part of the assembly, where the pressure loss is large, is reduced, so the wetted area is reduced, and as a result, the pressure loss can be reduced. This allows for a reduction in recirculation pump power and also improves channel stability.

In addition, in the unlikely event that the output spike suppressor does not function as designed and an unexpected output spike occurs, the horizontally intervening region where the output spike could potentially be larger should be placed in the coolant flow relative to the Since it is disposed on the upstream side, the heat generating part caused by an unexpected output spike can be effectively cooled with cooling water having sufficient cooling capacity, and the integrity of the fuel is ensured.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an embodiment of the present invention, and FIG.
is a vertical cross-sectional view along line A-A in the same figure (b), and the same figure (bL
(C) and (d) are lines 8-8 in (a) of the same figure, respectively. Plan view along C-C line and D-D line, Fig. 2(a)
~(C) is a diagram for explaining the present invention in detail, FIG. 3 (
a) and (b) are schematic cross-sectional views when the fuel assembly of the present invention is applied to a boiling water reactor, and a diagram showing the void ratio and subcriticality distribution in the core axis direction. FIG. 2 is a schematic diagram of Example 2, and FIG.
A vertical cross-sectional view along the line, FIG. (C), (d) and (e) are the same figure (a) respectively.
5 to 14 are plan views taken along lines B-B, C-C, D-D, and E-E, and FIG. 15 (a) is a plan view of various embodiments of the present invention. , ) to (e) are longitudinal sectional views of different fuel bodies according to the present invention, and FIG.
) and (b) are respectively a perspective view of a conventional fuel assembly and a schematic vertical cross-sectional view of a fuel body constituting the fuel assembly, FIG. 17 is a cross-sectional view of the fuel assembly of FIG. 16, and FIG. Both figures are cross-sectional views of conventional fuel assemblies. 30...Long fuel rod 31.44...Short fuel rod 32, 46-49...Water rod 33...Channel box 34...Upper tie plate 35...Lower tie plate 36... - Stacks 37, 39... Output spike suppressing material 38... Intervening member 40... Upper plenum 41... Lower plenum 42... Vanishing rod 43... Long intervening member enclosed fuel rod 45...・Short intervening member-enclosed fuel rods 50, 52... Subbundle 51.53... Gap (8733) Agent Patent attorney Yoshiaki Inomata (Idiot)
1 person) -ni- Reef Ginko Figure 17 Figure 19

Claims (9)

[Claims] (1) In a fuel assembly constructed by regularly arranging a large number of fuel rods filled with nuclear fuel material inside a metal cladding tube, the height H from the lower end to the upper end of the effective fuel range is 4/6H or more from the lower end. A vertically intervening region in which the fissile nuclide concentration is significantly reduced is arranged in a straight line or in an intersecting straight line or in a block shape inside the fuel assembly so as to include a part of the range 4/6H, and or 5/
It is characterized by providing at least one horizontally intervening region between the part 6H and the effective lower end, which substantially reduces the concentration of fissile nuclides over a width of 2 cm to 8 cm almost across the fuel assembly. fuel assembly. (2) The vertically intervening region that significantly reduces the concentration of fissile nuclides, which is arranged to include part of the range from 4/6H to 5/6H from the effective lower end, reduces the concentration of fissile nuclides within the fuel rod. 2. A fuel assembly according to claim 1, characterized in that the fuel assembly is filled with a material having a significantly reduced concentration. (3) The vertically intervening region, which is arranged to include part of the range from 4/6H to 5/6H from the effective lower end and which significantly reduces the concentration of fissile nuclides, shortens the fuel rod and The fuel assembly according to claim 1, characterized in that the fuel assembly is constructed without fuel rods. (4) The vertically intervening region, which is arranged to include a part of the range from 4/6H to 5/6H from the effective lower end and which significantly reduces the concentration of fissile nuclides, contains zirconium hydride (hydrogenated The fuel assembly according to claim 1, characterized in that it is a water rod or a gas plenum into which a solid moderator such as zirconium or graphite or water is introduced. (5) A patent claim characterized in that the intervening region in at least one of the vertical direction and the horizontal direction is filled with a solid moderator such as graphite, beryllium, or zirconium bitite (zirconium hydride) inside the fuel rod. The fuel assembly according to item 1. (6) At least one of the vertical and horizontal intervening regions constitutes a gas plenum that functions as a spacer to maintain fuel pellet spacing within the fuel rod. Fuel assembly as described. (7) At least one of the vertical and horizontal intervening regions is a heat-resistant material containing a burnable poison at a concentration such that its neutron absorption properties burn out and almost disappear as the end of the cycle approaches, and the heat-resistant material A fuel assembly according to claim 1, characterized in that the neutron absorption properties are not greater than those of the fuel pellets. (8) The fuel assembly according to claim 7, wherein the heat-resistant material is graphite beryllium oxide, alumina, zirconia, depleted uranium pellets, or the like. (9) 1 cm to 1 cm adjacent in the axial direction of the fuel rod to the vertically intervening region that significantly reduces the concentration of fissile nuclides and is arranged so as to include a part of the range from 4/6H to 5/6H from the effective lower end. 2. The fuel assembly according to claim 1, wherein the fuel pellets within 5 cm contain a burnable poison at a concentration such that the neutron absorbing property burns out and almost disappears as the end of the cycle approaches.