JPH01189591A - Fuel assembly - Google Patents

Abstract

PURPOSE:To increase a subcriticality of a nuclear reactor during a reactor shut down, by inserting inclusions of a certain constant length having a significantly degraded concentration of fissile nuclides into a part of fuel rods and by arranging the fuel rods having the inclusions in an outside region of a fuel assembly. CONSTITUTION:A fuel rod having an inclusion 31 is formed so as to an appropriate length of the inclusion to be less than 1/3H and a height of the inclusion at its center to be around 3/4H from the lowest end of an effective length of the fuel. The reason is that a subcriticality is especially shallow in a range of around 1/4H at a height of around 3/4H. Moreover, for prevention of an output spike, an output spike suppressor 36a is inserted next to the inclusion 36, and in case of a boiling water nuclear reactor, the fuel rods 31 are arranged in this surface for the purpose of utilization of an subcriticality increasing phenomenon which the fuel rods 31 have, mainly for the side where control rods are not inserted, and water gaps between adjacent fuel assemblies are expanded, and consequently, a subcriticality of a reactor during its shut down can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly, and particularly to a fuel assembly suitable for a nuclear reactor with a long operating cycle and a high shutdown margin.

(Prior Art) As an example, a fuel assembly for a boiling water reactor will be described.

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 and housed inside a rectangular channel box. In the core of a boiling water reactor,
Usually, cells consisting of one cross-shaped control rod and four fuel assemblies surrounding it are regularly arranged. In other words, the fuel assemblies and control rods in the core of a boiling water reactor 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. It is structured so that it flows in the opposite direction.

Although no bubbles are generated near the effective lower end of the core, that is, the lower end of the heat generating part, deadly bubbles are generated from the center to the upper end of the core, and these generated bubbles flow upwards of the core. Therefore, the proportion of bubbles is particularly high above the center 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 fission nuclide concentration, that is, the concentration, 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 U-23s1 degrees to be relatively higher than other parts, and voids cause p u-239
It is well known that the margin for shutdown of a nuclear reactor tends to be tight from the center to the upper part of the reactor core due to the generation of fissile nuclides such as Generally speaking, 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 currently 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. 24(a) and 24(b) are a perspective view of a conventional fuel assembly and a schematic longitudinal sectional view of a fuel rod constituting the fuel assembly, respectively.

In Figure (a), the fuel assembly includes a water rod (not shown) and a fuel rod 2 connected to an upper tie plate 4. It is fixed by a spacer 5 and a lower tie plate 6, and is surrounded by a channel box 1 on the outside. Fuel rod 2 is shown in the same figure (
As shown in b), fuel pellets 8 are disposed within the cladding tube 7, a spring 9 is provided in the gas plenum above the cladding tube, an upper end plug 10 is provided at the upper end, and a lower end plug 11 is provided at the lower end.

FIG. 25 is a cross-sectional view of the conventional fuel assembly shown in FIG. 24. There are 62 fuel rods 2 in the channel box 1.
and two water rods 3 are arranged to form a fuel assembly. The water rods 3 suppress the shortage of water, which is a moderator, inside the reactor assembly, but since these water rods 3 are uniform in the axial direction, there is a problem that there is an excess of water below the core and a lack of water above the core. There is a point.

The fuel assembly shown in FIG. 26 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. 27 is also an improvement on the fuel assembly shown in FIG. A non-boiling moderator water region is provided in the cross-shaped gap 14 to flatten the horizontal power distribution, but this type of fuel assembly also suffers from excess water below the core and insufficient water above it. There is a problem.

The fuel assembly shown in FIG. 28 was developed as an improved version of the fuel rod shown in FIG. 27. 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 generated everywhere else. Moreover, the generated bubbles flow upward (downstream). Therefore, 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
Its purpose is to make it possible to shut down the reactor even with high fuel enrichment, improve the axial power distribution, and reduce the pressure loss of the coolant. It's about offering your body.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a fuel assembly constituted by regularly arranging a large number of fuel rods filled with nuclear fuel material inside a metal cladding tube. In some fuel rods, an intervening member of a certain length that significantly reduces the concentration of fissile nuclides is inserted into the metal cladding tube, and the length of the intervening member is designed to prevent thermal neutrons during reactor power operation. An intervention in which the diffusion distance is equal to or longer than the diffusion distance, and the insertion position of the intervening member in the axial direction within the fuel rod is set to a position that includes a region where the degree of subcriticality becomes shallow at a point during the reactor operating period when the margin for reactor shutdown becomes severe. The present invention is characterized in that the fuel rod is a member-inserted fuel rod, and the intervening member-inserted fuel rod is disposed at least on the outer periphery of the fuel assembly.

Incidentally, the insertion length of the intervening member is 1/3 or less of the length of the axial heat generating portion of the fuel assembly, and the appropriate position thereof in the axial direction is 1/2 or more from the lower end of the heat generating part. Roughly speaking, most of the intervening member insertion fuel rods are arranged at the outer periphery of the fuel assembly facing the water gap where the cross-shaped control rods are not inserted or removed.

Further, the intervening member is a uranium pellet whose both ends contain a burnable poison in the entire pellet, the axial center of the pellet, or the outer periphery of the pellet, which has a lower concentration of fissile nuclides than the fuel pellet 1 adjacent in the axial direction, The parts of about 0.5 to 3 cm at both ends are Hafnia (H
f02 ) and the rare earth oxide Yttria (Yb20
:+) is a mixed sintered pellet. Roughly speaking, the intervening members, except for both ends of the fuel rod in the axial direction, are constructed so that the neutron absorption characteristics are as small as possible at the end of the reactor operation cycle. The region into which the intervening member is inserted, except for both ends in the axial direction, is made of hollow or solid graphite, a non-sealable Zircaloy tubular body, hollow or solid beryllium, or alumina.

Composed of zirconia, depleted uranium pellets, or a combination thereof, which may or may not contain burnable poisons; The concentration is such that it is almost burnt out at the end of the operating cycle. Further, the region into which the intervening member is inserted may be a gas plenum portion.

Furthermore, for intervening member inserted fuel rods that have a gas plenum in the intervening member insertion region, the gas plenum at the top is reduced or deleted, and the outer diameter is correspondingly reduced or removed (vanishing), and the coolant passage area is is configured to expand.

Further, in the fuel rod adjacent to the intervening member insertion fuel rod, the fuel enrichment at the side surface portion of the intervening member is not higher than the fuel enrichment at the portion of the adjacent fuel rod excluding the intervening member side surface portion.

Furthermore, the intervening member inserted fuel rods that are adjacent to each other in the reactor loaded state are configured such that the heights in the intervening member insertion axial direction are, in principle, equal to each other. In a fuel assembly in which the control rod gathers rod-shaped absorbing elements into a cluster and inserts them into the center of the fuel assembly in a distributed manner, the inclusion-inserted fuel is inserted almost symmetrically around the outer circumference of the assembly. Most of the sticks are placed here.

(Function) As described above, according to the fuel assembly of the present invention, neutron interactions occur on both sides of the intervening region, particularly in the direction perpendicular to the fuel assembly axis, across the intervening region where the concentration of fissile material is particularly low. A phenomenon occurs in which the (coupling effect) weakens when cold and becomes stronger during high-temperature operation, especially when voids occur. 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 (about 1.0)
Since this 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'j/cr.
It becomes n3. The distance traveled by thermal neutrons in water is 110 when it is cold.
．． 74 (= 1.35 > times larger. Furthermore, the density of air-water temperature coalescence at the time of void generation decreases to about 0.3, and as a result, the thermal neutron diffusion distance in the air-water mixture becomes 1.
Increases by 10.3 (;3) times.

As a result, neutron interaction on both sides of the intervening region increases, and neutron multiplication properties increase.

By using the above-mentioned effect, by introducing the intervening region, the multiplication factor is reduced in the cold state, that is, the reactor shutdown margin (subcriticality) is reduced.
During high-temperature operation, the multiplication factor can be prevented from decreasing even if the amount of fuel is reduced by introducing an intervening region.
With suitable design, it is even possible to increase the multiplication factor compared to the case without the intervening region.

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 zone structures II each 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)
Therefore, it is difficult to obtain a wide water gap area. That is, making the water gear knob wider within a given range of external shape means that the fuel area becomes narrower, and the heat generation area becomes narrower.

In the present invention, since the intervening region is inserted in a direction perpendicular to the axis of the fuel assembly, it is necessary to clarify the characteristics of the narrow intervening region. Figure 3 (C) is based on this idea, and Figure 3 (B)
This is an enlarged view of part 0 of . Theoretically calculated values for the case where a water gap of about 3 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 the multiplication factor increases in the positive direction with the water gap width, resulting in an effective multiplication factor k. ff increases), and when the water gap width exceeds about 1 cm in the cold state, k becomes noticeable. ff
It can be seen from this figure that the water gap 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 if we consider the infinite multiplication factor of the fuel assembly and the long-known four convicts, It can also be explained separately. In this method, the curve of FIG. 3(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 as in the example shown in Figure 28, the gap between other fuel rods must be reduced, and this will cause the fuel rods to absorb resonance. Increases the shielding effect of resonance neutrons between each other, resulting in the effect of increasing the probability of escaping resonance, while decreasing the thermal neutron flux ratio of the fuel region to water gap, resulting in a decrease in the thermal neutron utilization rate. This has the effect of reducing the FIG. 3(C) is almost determined by the offsetting effects of the above two effects, water density dependence and water gap width dependence.

In order to fix the fuel rod spacing and widen the water gap, fuel material must be removed from within the fuel rods. 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 moderator shortage, so introducing a water gap will alleviate this, and as a result, the probability of escaping resonance will increase. increases. The change in thermal neutron utilization rate is almost the same as in the above example.

In the present invention, the latter is mainly used, and the former is also used in combination as necessary. It is an object of the present invention to provide a fuel assembly that effectively produces the effects of the present invention without reducing the amount of fuel loaded in the assembly as much as possible (so as not to reduce the absolute amount of output).

(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.
) is a longitudinal sectional view taken along line A-A in the same figure (b), and the same figure (b)
is a plan view.

In the fuel assembly of this embodiment, a rectangular water rod 32 is arranged in the center, and except for this central part, fuel rods 30 and 31 are regularly arranged in 9 rows and 9 columns, and the outside is surrounded by a channel box. 33, and its upper and lower ends are roughly fixed by an upper tie plate 34 and a lower tie plate 35, respectively.

The fuel rod 31 is an intervening member insertion fuel rod p. Interposed member 3
The preferred length l of 6 is 1/3H or less, and in this example 1
It is about 5 to 60 cm. The height of the center of the intervening member is 3/4 H from the lower end of the effective fuel length. This is because the degree of subcriticality becomes particularly shallow around 374■ and within about 174■. The details of this intervening member 36 will be shown later, but representative examples include insertion of a hollow tube, insertion of graphite, insertion of ZrH2 pellets, and introduction of water.

Further, as will be described later, an output spike suppressing material 38a is inserted adjacent to the intervening member 36 to prevent output spikes.

As shown in FIG. 1(b), in a boiling water reactor, a cross-shaped control rod is inserted and removed in the water gap between fuel assemblies (on the left and top side of the figure). Insertion of control rods suppresses the neutron multiplication factor of the fuel assembly, but the degree of suppression is not uniform throughout the fuel assembly; it is larger on the side facing the control rods, and is larger on the side facing the water gap where control rods cannot be inserted or removed. It's small on the side where it is. In the present invention, paying attention to this point, in order to utilize the subcriticality increasing effect of P fuel during reactor shutdown mainly on the side where no control rods are inserted, P fuel rods 31 are arranged on this side and adjacent The water gap between the fuel assembly and the fuel assembly (not shown) is expanded, and the value to the right of the cold state curve in FIG. 3(C) is utilized. In addition to the existing water gap, the P fuel arrangement effectively expands the water gap width, so during cold shutdown the neutron multiplication factor decreases and the degree of subcriticality increases), and conversely during high temperature operation, the neutron increase increases. It can be seen from FIG. 3(C) that the magnification increases.

There are also three P fuels 31 in the corner facing the control rod center axis.
is located. The reason for this is that this portion also makes a relatively large contribution to the reduction in multiplication factor during stoppage and the increase in multiplication factor during high-temperature operation.

In this embodiment, as shown in FIG. 1(b), 22 P fuel rods 31 are arranged. The effective length (heat generating section) of a normal fuel rod 30 is about 3.6 to 3.7 m.

In the operation management of boiling water reactors, combustion management is performed by dividing the effective length into 24 equal parts, so 1/2 of 3.6 to 3.7 m
It is convenient to use 4 lengths (approximately 15 cm, which is called 1)-do (7 beads) as the unit length for insertion of the inclusion. However, this is merely a matter of convenience and is not necessary.

Now, let us consider the case where a 37-dore (approximately 45 cm) barge inclusion is inserted into the fuel rod 30.

This length is about the same as the length 2' of the gas plenum provided at the top of the normal fuel rod 30, so if the insertion position is fully utilized as a gas plenum, the gas plenum will be placed at the top of the P fuel rod. It is not necessary to provide Figure 1 (
In a), the diameter is reduced to 36C in consideration of this point. Reducing the diameter expands the coolant flow path, which contributes to reducing coolant pressure loss and improving channel stability.

This contributes to reducing the volume of the recirculation pump and improving operability accordingly.

By the way, as already explained, in this example, there are 22 P fuel rods, and assuming that the length of each intervening member region is 3 nodes (approximately 45 cm), the uranium inventory is 22.
．． 75 pieces (22X 3/24> decrease.

In this example, the uranium inventory is reduced by 5 rods of rectangular thick water rods in the center of the assembly, so the inventory is reduced by 7.75 rods in total. On the other hand, in a 9x9 aggregate as in this example, the use of water rods that usually excludes about 9 rods is being put into practical use, and compared to this, the uranium inventory in this example is 1.25 rods higher. It turns out. This means that the calorific value per unit volume of the nuclear reactor can be increased, or the power density can be reduced accordingly, contributing to improved fuel integrity.

Figure 2 (a) is a schematic cross-sectional view of the fuel assembly of the present invention applied to a boiling water reactor, and Figure 2 (1) is a diagram showing the void ratio and subcriticality distribution in the core axis direction. It is. The hatched portion in FIG. 2(a) is the arrangement position of the intervening member.

It is usually effective to make the heights of the intervening members within the fuel rods the same. The axial length of the intervening member varies depending on the number and arrangement of the fuel rods into which the intervening member is placed within the assembly.
It is usually in the range of about 15 to 90 cm. If it is 15 cm or less, the effect is small, and if it is longer than 90 cm, the effect becomes (relatively) small in cases v1 where the reduction in the amount of calorific material (fuel) loaded is large.

Since the intervening member is disposed at the portion where the degree of subcriticality is the shallowest, the present invention works effectively.

FIG. 4 is a plan view of a second embodiment of the invention.

It is assumed that the center of the cross-shaped control rod is returned to the upper left side of the assembly, and the intervening member inserted fuel rod p.

pl and p2 are appropriately adopted from the intervening member insertion fuel rods shown in FIGS. 23(a) to (d), which will be described later, but the numbers are added because the arrangement within the assembly is different (the same applies hereinafter). . In addition, the same parts as in the first embodiment already described are given the same reference numerals and will be explained. The same applies to each of the following examples.

In the figure, the four-water intervening member insertion fuel rod p2 is replaced with four fuel rods 30 around the thick water rod 32, since the area around the thick water rod 32 is also effective in improving the stop margin.

These four intervening member inserted fuel rods p2 are ke during high temperature operation.
Since the function of increasing ff is large (because there is enough water in the thick water rod and it is easy to output), the arrangement is designed to effectively utilize this function. Furthermore, the size of the large-diameter water rod is effectively increased by the fuel rod p2, and the degree of subcriticality during cold shutdown can be increased. In order to prevent a reduction in uranium inventory, a design may be considered in which inclusions in the p1 fuel rods are removed and normal fuel is used.

In this fuel rod, if a burnable poison (such as Gd) is inserted into the center only at the end of the cycle to the extent that the poisonous effect disappears, the above objective can be achieved without causing an output spike.

FIG. 5 is a plan view of a third embodiment of the present invention, in which the intervening member insertion fuel rod p is centered around the central water rod 32.
2 are arranged in a cross shape along the diagonal line of the central water rod 32 in addition to the p fuel of the above embodiment. Therefore, in this embodiment, there are a total of 28 intervening member insertion fuel rods p and 48 fuel rods 30.

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

This embodiment is applied to a conventional fuel assembly (a system using two thin water rods 37 instead of a large diameter water rod). Therefore, in this assembly, the number of intervening member inserted fuel rods p is 15,
The fuel rods 30 are composed of 47 pieces.

Note that if the length of the intervening member is 30 cm, the uranium inventory will decrease by 1.25 rods, and if the length of the intervening member is 45 cm, the uranium inventory will decrease by 1.88 rods.

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

This embodiment has two small diameter water rods 31 as well as the embodiment shown in FIG.
This is the case when applied to the method used herein. In this embodiment, the intervening member insertion fuel rods p are also arranged near the center axis of the control rod, and the intervening member insertion fuel rods p are 14 pieces and the fuel rods 30 are composed of 48 pieces. Note that the length of the intervening member is 30
Cm, the uranium inventory is 1.17 pieces, 45
cm, the uranium inventory decreases by 1.75 rods, but has almost the same effect as the embodiment shown in FIG.

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 37, similar to the embodiments shown in FIGS. 6 and 7. In this embodiment, the intervening member inserted fuel rods p are arranged in a block shape in the central part of the assembly, and the intervening member inserted fuel rods p are composed of 21 pieces, and the fuel rods 30 are composed of 41 pieces. In addition,
If the length of the intervening member is 30 cm, the uranium inventory decreases by 1.75 rods, but the binding/separation action also occurs inside the handle.

This embodiment has a greater effect than the embodiments shown in FIGS. 6 and 7.

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

This embodiment is applied to a system using two small-diameter water rods, similar to the embodiments shown in FIGS. 6 and 7. In this embodiment, the intervening member insertion fuel rods p are arranged symmetrically around the entire outer periphery of the assembly. In this embodiment, there are 28 intervening member insertion fuel rods p and 34 fuel rods 30.
Since the number of intervening member inserted fuel rods p is large, if the length of the intervening member is 30 cm, the uranium inventory decreases by 2.33 fuel rods. Therefore, it is necessary to make the length of the intervening member a little short so that the reduction in υ02inben1~ does not become disadvantageous. Note that this embodiment produces a large coupling/separation effect. That is, during reactor shutdown, the action of the p-fuel facing the control rods is smaller than that of other p-fuels, but the symmetrical arrangement provides good characteristics during high-output operation.

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

This embodiment is a case where large-diameter water rods 38 for four central fuel cells are used, and the number of intervening member inserted fuel rods p is 16.

The fuel rods 30 are composed of 44 pieces. If the width of the intervening member is 30 cm, the uranium inventory will decrease by 1.33 pieces. Effects similar to those of the embodiments shown in FIGS. 6 and 7 are produced.

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

This embodiment is almost the same as the embodiment shown in FIG. 6, in which square water rods 39 for four central fuel cells are used, and the number of intervening member insertion fuel rods p is 16, and the number of fuel rods 30 is 4/1. Consists of books. If the length of the intervening member is 30 cm, the uranium inventory will decrease by 1.33 pieces. Almost the same effect as the embodiment shown in FIG. 10 is produced.

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

In this embodiment, the fuel cells are arranged in 11 rows and 11 columns, and a large diameter water rod 40 for nine fuel cells is used in the center.

There are 21 intervening member insertion fuel rods p on the side where the cross-shaped control rod is not inserted and removed, and one on the side facing the central axis of the control rod for a total of 22 pieces, and 90 fuel rods 30. The length of the intervening member is 3
If it is 0Cm, the uranium inventory will decrease by 1.83 pieces.

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

In this embodiment, the fuel cells are arranged in 11 rows and 11 columns.
This is a modification of the embodiment shown in FIG. 2. A rectangular water rod 41 corresponding to nine fuel cells is used in the center, and the number of fuel rods 30 is 90, with 22 intervening member insertion fuel rods ph. Almost the same effect as the embodiment shown in FIG. 12 is produced.

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

In this embodiment, rectangular water rods 42 for five fuel cells are arranged in the center at 45 degrees with respect to the fuel bundle, roughly 15 intervening member insertion fuel rods p, 10 intervening member insertion fuel rods p1, Using one intervening member insertion fuel rod p2, the fuel rods p1 are arranged in a cross shape, and the inside of the fuel bundle is divided into four sub-regions (
Furthermore, P fuel is placed on the two sides that do not face the cross-shaped control rods. Fuel rod 30 is 48
Consists of books. In addition to the effect of p-fuel alone, the distance between the sub-regions can be increased at the height of the intervening member insertion of the central cross-shaped intervening member insertion fuel rod p1, which has the effect of increasing the effective multiplication factor hot keff at high temperatures and the effective It has the effect of significantly lowering the multiplication factor cold 'KeH (large margin for reactor shutdown). Also, an intervening member insertion fuel rod p.

Even if the lengths of the intervening members pl and p2 are made relatively short, there is a great effect. If the length of the intervening member is 30 cm, the uranium inventory will decrease by 2.33 pieces.

FIG. 15 is a plan view of a thirteenth embodiment of the present invention.

In this embodiment, a rectangular water rod 32 for five fuel cells is arranged in the center at 45 degrees with respect to the fuel bundle, and the whole is composed of nine sub-regions 43, with a space between each sub-region 143. A slightly wider gap 44 is provided. Fifteen intervening member inserted fuel rods p are arranged on two sides not facing the control rod, and one intervening member insertion fuel rod p faces the control rod central axis. The fuel rods 30 are composed of 60 pieces. If the length of the intervening member is 30 cm, the uranium inventory will decrease by 1.33 pieces. This embodiment also has the effect of increasing the effective multiplication factor hotkeff at high temperature and the effect of lowering the effective multiplication factor coldkeff at low temperature (large margin for reactor shutdown), but it is slightly smaller than the previous embodiment.

FIG. 16 is a plan view of a fourteenth embodiment of the present invention.

This embodiment is a modification of the embodiment shown in FIG. 15, in which a rectangular water rod 45 larger than that of the previous embodiment is arranged in the center, and one fuel rod 30 is further placed in each position facing the rectangular water rod 45. This is an additional arrangement.

Therefore, there are 17 intervening member insertion fuel rods p, one intervening member insertion fuel rod p2, and 58 fuel rods 30. Since the central rectangular water rod 45 is made larger, the reactor shutdown margin is greater than in the previous embodiment.

FIG. 17 is a plan view of a fifteenth embodiment of the present invention.

This embodiment is a modification of the embodiment shown in FIG. 14, in which the large-diameter water rod 46 and the cross-shaped wide gear 47 are offset with respect to the fuel bundle, and the water gap width on the outer periphery of the assembly is different. This embodiment can be effectively applied to a reactor core (called a BWR-D lattice).

In such a core, it is preferable that the cruciform control rod has its central tie rod located on the upper left side of the figure. In other words,
Place more fuel rods on the side with the widest water gap. There are 17 intervening member insertion fuel rods p, and 1 intervening member insertion fuel rod p2.
The fuel rods 30 are composed of 59 pieces.

FIG. 18 shows a plan view of a sixteenth embodiment of the present invention.

This embodiment has four sub-bundles 48, with a cross-shaped gap 49 between each sub-bundle serving as a non-boiling moderator water region, and 15 lines of p-fuel and control rods in the water gap not connected to the control rods. One p2 fuel is placed around the center axis of the rod. The fuel rod 11 30 is composed of 48 pieces.This embodiment has the effect of increasing the effective multiplication factor at high temperatures and the effect of lowering the effective multiplication factor at low temperatures (large margin for reactor shutdown).

FIG. 19 is a plan view of a seventeenth embodiment of the present invention.

This embodiment is constructed entirely of nine sub-bundles 50, each sub-bundle 50 having nine fuel rods 3.
0, and there is a slightly wide gap 5 between each sub-bundle 50.
1 is provided. Seventeen P fuels are inserted facing the water gap that does not face the control rods, and one fuel rod P2 is inserted facing the control rod central axis, but in addition to these, there are Since all the sub-bundles in the center have intervening member insertion fuel rods p1, the intervening member insertion fuel rods p are 17.
Book, 9 PL, 1 PL, total 27, 30 fuel rods
consists of 54 pieces. This embodiment has the effect of increasing the effective multiplication factor at high temperatures and the effect of lowering the effective multiplication factor at low temperatures (large margin for reactor shutdown).

FIG. 20 is a plan view of the eighteenth embodiment of the present invention.

In this embodiment, the whole is composed of nine sub-bundles 52, but in the center there is a large water bundle 53 corresponding to five fuel rods.
At the same time, the corner fuel is removed and the channel is chamfered, and the fuel rod 30 is arranged in the gap 54 between the large water rod 53 and the sub-bundle 52. This arrangement can be made thinner because stresses are reduced.

This makes it possible to increase the effective multiplication factor, leading to economic improvement. Also, since the number of fuel rods p inserted into the intervening member is 14, if the length of the intervening member is 30 cm, the uranium inventory is equivalent to 1.17 rods, and the length of the intervening member is 45 cm.
Cm, the uranium inventory will decrease by 1.75 pieces. Since there are 5 thick water rods, 1.75 rods are added to this, resulting in an inventory loss of 6.75 rods, but normally in a 9x9 type, there are approximately 9 water rods, so 2. .25 pieces of inventory have been obtained.

FIG. 21 is a plan view of a nineteenth embodiment of the present invention.

The arrangement of the fuel rods 30 is basically almost the same as in FIG. 14, but two thick water rods 55A and 55B are arranged as shown in the figure, and five intervening member insertion fuel rods p1 are arranged between them. There is. There are 17 intervening member insertion fuel rods p and one intervening member insertion fuel rod p2, and a gap 56 is provided between the subbundles 57 as in the embodiment shown in FIGS. 15 and 16. . At the height where the intervening region exists in the intervening member insertion fuel rod p1, the thick water rod 55A.

55B and the intervening member inserted fuel rod p1, a wide space without fuel is formed, and a large margin for reactor shutdown is created.

FIG. 22 is a plan view of a twentieth embodiment of the present invention.

This embodiment is applied to a PWR fuel assembly.

This is an example of a 17X17 type fuel rod 30, 31 has 264 water rods, 25 water rods 37, and 64 fuel rods 31 arranged on the outer periphery are used as intervening member insertion fuel rods p.

Since no water gap is provided between the PWR fuel assemblies, the p fuel rows of the S′X2 rows are aligned between the assemblies and form a fuel-free portion at the inclusion insertion portion. During shutdown, the water in this portion acts to isolate the aggregates, and during high temperature operation, the water density decreases (65 to 70% of that during cold shutdown), increasing the binding effect. Since voids do not occur in PWR, wide spacing between non-fuel portions is disadvantageous due to lack of binding effect even during high temperature operation. Therefore, this embodiment is good. The p-fuel row can also be provided inside the assembly. Although the axial height of the intervening region is different from that of BWR, the center or upper part is often effective (effects such as lower output from the upper part due to control rods).

The length of the intervening member of the 64 intervening member inserted fuel rods p is 30c.
m, it will be 0020th for 5.33 trains, and the fuel
264 bottles will be reduced to 258.9 bottles (about 2% decrease), but this can be compensated for by increasing the concentration by 2%. For example 3
．． 5wt% to 3.57wt% J, a slight increase in sea urchin.

FIGS. 23(a) to 23(d) are longitudinal sectional views of different fuel rods according to the present invention.

That is, the fuel rod shown in FIG.
m, and graphite 21 is inserted. Graphite 21 is one of the best examples as it has excellent high temperature properties, absorbs few thermal neutrons, and also functions as a moderator. Low density (porous) Aj203, Zr
Although O2 and the like have poor deceleration properties, they have good heat resistance properties, and such substances with low neutron absorption can also be used. Hollow graphite instead of solid graphite.

Hollow A-poor 203, ZrO2, hollow natural uranium, hollow depleted uranium, etc. may be used, and the hollow portion may be used as a gas plenum. Although it is also possible to use solid natural uranium, depleted uranium, etc., the features of the present invention tend to be somewhat diluted in that case.

The most important characteristic required for this region is that the thermal neutron absorption rate at the end of the cycle is lower 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 pellets containing burnable poison only near the axis 22 (approximately 2 cm) each. These pellets 22 do not contain toxic substances on their outer periphery, so
The output does not particularly increase over the entire driving cycle, and there is relatively little fluctuation. 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 further exacerbated. I can do it.

The difference between the fuel rod shown in FIG. 23(b) and the fuel rod shown in FIG. 23(a) is that a Zircaloy tube 24 having a small thermal neutron absorption cross section is inserted instead of the graphite 21. Many variations on this example are possible. In other words, (1) When used as a gas plenum, use non-sealed 1" pipes. In this case, the plenum above the fuel is unnecessary or can be shortened, so the diameter can be reduced, the coolant flow path area can be expanded, and pressure loss can be reduced.

(2) When packing ZrH2 (zirconium high try!~, zirconium hydride, etc.) at high density, Zr
To be precise, H2 should be interpreted as ZrHx (0<x×2>), which is desirable for the purpose of water-sprinkling inventions where X is large, but as X becomes large, it tends to become brittle, so it is generally best to seal it in a tube. Desirably, a relatively small air gap is provided within the tube to serve as a gas plenum for the slight release of 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.

A small heat insulating material pellet 25 is placed between the Zircaloy tube 24 and the fuel pellet 23. Al2O3, Zr0z.

Yb203-Hf 02 and a depleted uranium cap are interposed to improve fuel integrity. Insulation pellets! -2
No. 5 preferably has a low thermal neutron absorption characteristic at the end of the operating cycle. Therefore, in A with burnable poison added, 2
03-Gd2O3, depleted uranium UO2-Gdz 03 pellets are preferred. Zircaloy pipe 24
For fuel pellets adjacent in the axial direction of
It is preferable to arrange the pellets 22 containing the burnable poison up to a length of about 5 cm (about 5 cm at most).

Although FIG. 23(b) shows the fuel pellet 22 in which a small-diameter Gd pellet is inserted, Gd may be mixed into the entire pellet, and the fuel rods shown in FIG. 23(a) and FIG. 23(C) Similarly, Gd may be mixed into the entire pellet.

The difference between the fuel rod shown in FIG. 23(C) and the fuel rod shown in FIG. 23(b) is that they are configured to introduce water.

That is, water holes 26 are provided above and below in the cladding tube 20 of the fuel rod in the portion where the Zircaloy tubes are inserted, as shown in FIG. However, pellets 22 containing burnable poison are provided roughly above and below, and then fuel pellets 23 are placed above and below, respectively.

The difference between the fuel rod shown in FIG. 23(d) and the fuel rod shown in FIG. 23(a) is graph 1 (Afz 03 , ZrO2).

The difference is that the intervening layer 28 is formed by adding a burnable poison to Af203-ZrO2, etc.). This embodiment provides manufacturing advantages because no burnable poison is added to the fuel.

[Effects of the Invention] As explained above, the present invention provides the following effects.

(1) When the reactor is shut down, the water temperature is low and the water density is high, so the diffusion distance of thermal neutrons is small; however, according to the fuel assembly of the present invention, when the reactor is shut down, the concentration of fissile material is low. The neutron interaction (coupling effect) in the horizontal fuel region across the (intervening region) is reduced, and as a result, the degree of subcriticality of the reactor during shutdown can be increased.

(2) During high-temperature operation, the average density of water is significantly reduced, so the thermal neutron diffusion distance is significantly extended (2 to 3 times). As a result, the coupling effect across the intervening regions is improved, and the effective multiplication factor can even be increased even though there are regions where the fissile material concentration is significantly reduced. That is, the introduction of the intervening region can be advantageous.

(3) *In the invention, the burnable poison is effectively placed in a limited portion of the fuel axially adjacent to or intervening, so local power peaks (power spikes) do not occur; Therefore, the integrity of the fuel is maintained.

[Brief explanation of the drawing]

FIGS. 1(a) and (b) are schematic vertical cross-sectional views and plan views of one embodiment of the present invention, and FIGS. 2(a) and (b) show the fuel assembly of the present invention used in a boiling water reactor. Figure 3 is a diagram for explaining the present invention in detail, and Figures 4 to 22 are diagrams showing the applied schematic cross-sectional view and the void ratio and subcriticality distribution in the axial direction of the core. 23(a) to 23(d) are longitudinal cross-sectional views of different fuel rods according to the present invention, and FIG. 24(a) and (b) are respectively plan views of the conventional fuel assembly. A perspective view and a schematic vertical cross-sectional view of the fuel rods constituting the fuel assembly F4, FIG. 25 is a cross-sectional view of the fuel assembly shown in FIG. 24, and FIGS. FIG. 3 is a cross-sectional view of the +1 aggregate. 30...1 fuel rod 31...intervening member insertion fuel rod p, pI, p2.
32.37-42.45.46.53.55A, 5
5B...Water rod 33...Channel box 34...Upper tie plate 35...Lower tie plate 36...Intervening member 43, 48.50.52.57...Sub bundle/1
7.49...Cross-shaped gap 44, 51.54.56...Gap (8733) Agent Patent attorney Yoshiaki Inomata (and others)
1 person) Figure 1 %<Aj÷2. , % (ur) (C) Fig. 8 Fig. 9 Fig. 10 Fig. 11 Fig. N ■2 Fig. I3 Fig. ○ 0) -t Nl'V"* 22nd Melt (a) (b) (c)
(d) Figure 23 Figure 26 Figure 27 Figure 28

Claims (5)

[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, some of the fuel rods have a certain length that significantly reduces the concentration of fissile nuclides. An intervening member is inserted into the metal cladding tube, and the length of the intervening member is equal to or longer than the thermal neutron diffusion distance during reactor power operation, and the axial insertion position of the intervening member within the fuel rod is an atomic The interposed member inserted fuel rod is located at a position including a portion where subcriticality becomes shallow at a time when the reactor shutdown margin becomes tight during the reactor operation period, and the interposed member inserted fuel rod is placed at least in the outer peripheral portion of the fuel assembly. A fuel assembly characterized in that: (2) The insertion length of the intervening member is 1/3 or less of the length of the axial heat generating portion of the fuel assembly, and the position thereof is 1/2 or more of the axial position from the lower end of the heat generating part. A fuel assembly according to claim 1. (3) A fuel assembly according to claim 1, characterized in that most of the intervening member inserted fuel rods are arranged on the outer periphery of the fuel assembly facing a water gap in which no cross-shaped control rods are inserted or removed. body. (4) In a fuel assembly in which the control rod collects rod-shaped absorption elements into a cluster and inserts and removes them inside the fuel assembly in a decentralized manner, the inclusion-inserted fuel is inserted almost symmetrically around the outer circumference of the assembly. The fuel assembly according to claim 1, characterized in that most of the rods are arranged. (5) For intervening member insertion fuel rods with a gas plenum provided in the intervening member insertion region, the gas plenum at the top is reduced or deleted, the outer diameter is correspondingly reduced or removed (vanishing), and the coolant passage is The fuel assembly according to claim 1, characterized in that the fuel assembly is configured to increase the area.