JPH07311291A - Fuel assembly - Google Patents

Abstract

PURPOSE:To provide a fuel assembly which can suppress exceeding of neutron multiplication rate and control the multiplication rate evenly for whole operation cycle, improve cooling characteristics and does not lead to complication of assembling and disassembling work. CONSTITUTION:A multitude of fuel rods 22 without burnable poison in the fuel, a plurality of fuel rods 23 with burnable poison W the fuel and at least one thick diameter water rod 26 having larger diameter than the fuel rods except for a certain length regions at both ends are arranged in a grid. A part of the poison rods 23 is larger than the diameter of fuel rods except for a certain length region at the both ends, and the components of inner part and peripheral part are the same or different in the thick diameter multiple region poison rods. At least one rod of these thick diameter multiple region poison rods 23 is arranged adjacent to the thick diameter water rod 26 and a multitude of fuel rods 22 are arranged to surround the group of the thick diameter water rod 26 and the adjacent thick diameter multiple region poison rods 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly suitable for a light water reactor, and more particularly to a fuel assembly having improved nuclear characteristics, thermal characteristics, hydraulic characteristics, dismantling or assembling ability, and economical efficiency.

[0002]

2. Description of the Related Art For example, in a fuel assembly of a boiling water reactor (hereinafter abbreviated as BWR), generally, a large number of fuel rods are regularly arranged in a rectangular channel box, and a neutron moderator is provided between the fuel rods. Water (also referred to as cooling water or water), which also serves as a coolant, flows from below (upstream) to above (downstream). With this flow, the cooling water boils by taking away the heat generated in the fuel rods, and the generated steam is guided to the turbine for power generation.

26 to 28 exemplify a BWR fuel assembly which has recently been put into practical use.

FIG. 26A is a vertical sectional view of the fuel assembly 1. The fuel assembly 1 includes a plurality of fuel rods 2 including a fuel rod to which a burnable poison is added and a water rod 3, a plurality of spacers 4, a lower tie plate 5 as an upstream side connecting member, and a downstream side connecting member. The fuel bundle regularly arranged by the upper tie plate 6 is housed in the channel box 7.

As shown in FIG. 1B, the fuel rod 2 is constructed by accommodating a large number of fuel pellets 11 in a metal cladding tube 10 having hermetically sealed end plugs 8 and 9. There is. The cross section of the fuel rod 2 is generally circular and has a uniform diameter except for the upper and lower end plugs 8 and 9, and usually all the fuel rods 2 have the same outer diameter.

The water rod 3 was not used in the fuel assemblies of the early BWRs, but a major goal was to improve the economical efficiency, so that it was adopted as the fuel enrichment gradually increased. Is. In the first stage, one such water rod was constructed and had an outer diameter substantially equal to that of the fuel rod. Then, in the next stage, the number of rods is increased to two, and in the next stage, it is enlarged into a large-diameter water rod (occasionally called a 4-cell water rod in the present invention) occupying a space where four fuel rods are removed. Is coming.

The improved design of the fuel assembly is in progress, and when two large diameter water rods are used in the space where seven fuel rods are removed, or one space is removed when nine fuel rods are removed. In some cases, a water rod having a large diameter (sometimes referred to as an ultra-large diameter in the present invention) having a larger cross-sectional area than a circular or square cross section of a book is adopted. It is known that such a large-diameter or ultra-large-diameter water rod has a wide wetting area for cooling water, and thus causes a relatively large pressure loss with respect to the flow of cooling water. In the present specification, the large-diameter water rod and the ultra-large-diameter water rod are collectively referred to as a large-diameter water rod unless otherwise required.

The water rod 3 shown in FIG. 26 (C) is an example of the above-mentioned 4-cell water rod. The upper part and the lower part of the water rod 3 have a partly small diameter in order to alleviate the problem of seismic resistance and the problem of pressure loss of the cooling water. In addition, inside the water rod 3, the water passage 3
Reactor water is introduced and discharged via a, the reactor water slowly flows to the extent that it does not boil, and the neutrons are effectively decelerated by the reactor water.

FIG. 27 (A) shows a cross section of the fuel assembly 1 in a portion where the water rod 3 has a large diameter, and FIG. 27 (B) shows a cross section of the fuel assembly 1 in a thin portion.

FIG. 28 shows the axial power distribution of the fuel bundle,
Also, the axial distribution of the proportion of water vapor (voids) generated with the cooling water boiling is shown in comparison with the schematic fuel assembly 1.

The fuel assembly 1 is, as shown in FIG.
For example, by using seven spacers 4 (SP1 to SP7) and upper and lower tie plates 6, 5 (UTP, LTP), a plurality of fuel rods in which burnable poisons are added to the fuel (referred to as poison rods in this specification) A large number of fuel rods including water and water rods are regularly arranged and integrated.

In the output distribution shown in FIG. 2B, the upper part and the lower part are markedly reduced, which is why the fuel enrichment in these parts is significantly reduced from the viewpoint of improving neutron economic efficiency. This is because it is a preferable design in recent years.

FIG. 3C shows a typical axial distribution of void ratio during output operation. The void ratio is high mainly in the upper part, and the neutron moderating effect is reduced. Therefore, as described above, the output distribution tends to decrease upward.

Therefore, if a part of the fuel is removed on the upper side (downstream side), the ratio of the fuel to the fuel is relatively increased, and the degree of decrease in the output on the upper side can be alleviated. The present inventor paid attention to this point, and
The concept of "ength rod (PLR)" has already been disclosed in Japanese Patent Laid-Open No. 52-50498. Here, since the lower end of the shortened fuel rod is aligned with the lower end of a non-shortened normal fuel rod (referred to as a long fuel rod or a normal fuel rod in this specification), the fuel rod at the upper portion (downstream portion) is Will be missing. This space is sometimes referred to as the "Vanishing Rod" (herein referred to as VR or VR space). This V
Due to the formation of the R space, the pressure loss of the cooling water is significantly alleviated, the change of the pressure loss due to the disturbance of the flow is also reduced, and the thermo-hydraulic stability is improved, and as a result, the nuclear stability is also improved. There are merits. Further, when the cooling characteristics of the long fuel rod deteriorate, the deterioration of the cooling characteristic can be avoided by shortening the length. Furthermore, by making the fuel rods shorter, the subcriticality at the time of reactor shutdown becomes deeper, and it becomes possible to increase the shutdown margin.

[0015]

However, when the VR space is formed, the upward flow of the cooling water is biased at the lower end position of the VR space, which may deteriorate the cooling characteristics of some of the fuel rods. .

Further, it is understood that the cooling water tends to flow in a layered manner in contact with the inner surface of the channel box which does not generate heat, so that the contribution of the fuel rods to the cooling water is small. In other words, it can be seen that there is room for improving the cooling characteristics by using the water in contact with the inner surface of the channel box.

From the viewpoint of improving the fuel economy, that is, increasing the burnup of the fuel, it is indispensable to increase the fissionable nuclide (Fissile) concentration in the fuel rod (U-235 enrichment in uranium fuel). . In addition, mixed oxide fuel (M with added accumulated plutonium to uranium)
The effective adoption of OX fuel) has recently become an important issue.

As described above, when the fissionable nuclide concentration in the fuel rod is increased or the MOX fuel is used, the neutron multiplication factor of the fuel assembly becomes excessive, and the effect of suppressing the neutron multiplication effect by the poison rod decreases. Since it becomes easier, it becomes more necessary to dispose a large number and a high concentration of poison rods in the fuel assembly as compared with the conventional case. Therefore, it is desired to design a fuel assembly that can effectively suppress the neutron multiplication factor of the fuel assembly, which tends to be excessive, and as uniformly as possible over the entire operation cycle.

The present invention has been made in view of the above circumstances, and it is possible to uniformly control the multiplication factor over the entire operation cycle while suppressing the neutron multiplication factor from becoming excessive.
It is an object of the present invention to provide a fuel assembly which has improved cooling characteristics and which does not complicate disassembling and assembling work.

[0020]

In order to achieve the above-mentioned object, the invention according to claim 1 comprises a large number of fuel rods in which no burnable poison is added to the fuel and a plurality of fuel rods in which the burnable poison is added to the fuel. Using a plurality of spacers, an upstream side connecting member and a downstream side connecting member, a poison rod and at least one large-diameter water rod having a diameter larger than the diameter of the fuel rod except for a certain length range at both ends are provided. In a fuel assembly that is arranged in a lattice and integrated into a fuel bundle, a part of the poison rod is larger than the diameter of the fuel rod except for a certain length range at both ends, and the composition is the same in the inside and the outside. Alternatively, different large-diameter multi-zone poison rods may be used, and at least one of the large-diameter multi-zone poison rods is arranged adjacent to the large-diameter water rod, and the large-diameter water rod and the large-diameter multi-zone adjacent thereto. A large number of fuel rods in an arrangement that surrounds a group with area poison rods And it said that it has set.

The invention according to claim 2 is the fuel assembly according to claim 1, characterized in that the large-diameter water rod contains a burnable poison in its tube wall portion.

According to a third aspect of the present invention, in the fuel assembly according to the first aspect, at least a part of the large-diameter multi-region poison rod has a coolant flow-through portion through which the coolant flows. Characterize.

According to a fourth aspect of the present invention, in the fuel assembly according to the first aspect, the concentration of the burnable poison added to at least a part of the large-diameter multi-region poison rod is the large-diameter multi-region poison. It is characterized by a high inside of the rod and a low outside.

The invention according to claim 5 is the fuel assembly according to claim 4, wherein the combustible poison added to the inside of at least a part of the large-diameter multi-region poison rod is a boron compound, It is characterized in that the added burnable poison is a gadolinium compound.

According to a sixth aspect of the present invention, in the fuel assembly according to the first aspect, at least a part of the large-diameter water rod or the large-diameter multi-region poison poison rod has a constant length from the downstream side of the coolant flow. The fuel rods adjacent to the portion where the concentration of the fissile nuclides is higher than that of uranium 235 of natural uranium in the portion where the diameter is reduced. It is characterized by having a drift mechanism that biases the flow of the fuel in the direction of the adjacent fuel rods.

According to a seventh aspect of the present invention, a large number of fuel rods in which no burnable poison is added to the fuel, a plurality of poison rods in which the burnable poison is added to the fuel, and the fuel except for a fixed length range at both ends are provided. At least one large-diameter water rod having a diameter larger than the diameter of the rod is arranged in a lattice using a plurality of spacers, an upstream side coupling member and a downstream side coupling member, and integrated into a fuel bundle. In a fuel assembly provided with a metal channel box in a surrounding arrangement, the fuel rod or poison rod arranged at a position facing the channel box is within a certain length range from the downstream end of the flow of cooling water. The flow of the coolant is directed to the part where the enrichment of the fissionable nuclides of the fuel rod adjacent to that part is higher than that of the uranium 235 of natural uranium. adjacent And having a drift mechanism to flow polarization in the direction of the charge bars.

The invention according to claim 8 is the invention according to claim 6 or 7.
In the fuel assembly described in (1), a portion that is reduced in diameter on the downstream side of the large-diameter multi-region poison rod and engages with the spacer, and a portion that is reduced on the downstream side of the fuel rod or poison rod facing the channel box and engages with the spacer. A short length with an outer diameter that is approximately the same as the diameter of the non-thinned portion in at least one of the thinned portion and the portion that is made thinner on the downstream side of the large diameter water rod and engages with the spacer. It is characterized by being fitted with a tube.

[0028]

According to the fuel assembly of the first aspect of the invention, the large neutron multiplication factor suppressing effect can be obtained by arranging the large diameter multi-region poison rod at a position where the neutron flux is high. It is possible to reduce the number of poison rods having a structure in which a burnable poison is added to the. As a result, the output peaking in the fuel assembly can be mitigated, more plutonium can be loaded, and the flow of cooling water that tends to collect around the large diameter water rod is adjusted, which is effectively used for cooling the fuel rod. Therefore, the cooling characteristic of the fuel is improved.

According to the fuel assembly of the second aspect of the present invention, since the pipe wall portion of the large diameter water rod in which the position where the thermal neutron flux becomes high has a large diameter contains the burnable poison. The effect of suppressing the neutron multiplication factor of the fuel assembly is increased. Furthermore, since the surface area of the large diameter water rod facing the water on the outer peripheral portion and the inner peripheral portion is significantly larger than the surface area of a conventional poison rod, the neutron multiplication suppressing effect of one large diameter water rod containing a poison is It is several times (for example, five times) the size of conventional poison sticks. Therefore, the number of normal poison rods can be reduced by that amount, and a normal fuel rod can be used instead. During the period in which the neutron absorption action of the large diameter multi-zone poison rod continues in the reactor operation cycle, the neutron absorption action of the large diameter water rod is suppressed, so the excessive suppression effect of the neutron multiplication factor during that period is significant. Is alleviated.

Then, when the neutron absorbing action of the large diameter multi-region poison rod is weakened, the neutron absorbing action of the thick water rod containing the poison is exerted. Poisoning is generally minimized shortly before the end of the operating cycle, in which case it simply acts as a large diameter water rod to improve the neutron multiplication effect that tends to be insufficient at the end of the cycle of the fuel assembly. You can Depending on the design conditions, it is possible to leave the toxic effect even at the end of the cycle.

According to the fuel assembly of the third aspect of the present invention, by introducing a coolant, that is, reactor water into the large-diameter multi-zone poison rod, the fast neutron is decelerated and the thermal neutron is easily absorbed by the poison. Therefore, the effect of suppressing the neutron multiplication factor of the fuel assembly is further increased.

According to the fuel assembly of the fourth aspect of the present invention, the concentration of the burnable poison inside the large-diameter multi-zone poison rod is made higher than that at the outer peripheral portion, so that the fuel assembly at the beginning of the operation cycle is first obtained. After completing the multiplication suppression effect during the period when the body's neutron multiplication factor tends to be particularly high, the neutron absorbing effect of the burnable poison in the outer peripheral part disappears, and the internal burnable poison slowly exerts the multiplication suppression effect. Will be done. That is, with this configuration, the effect of keeping the effective value of the neutron multiplication factor of the fuel assembly substantially constant is obtained.

According to the fuel assembly of the fifth aspect of the present invention, the action according to the fourth aspect of the invention can be specifically realized by specifying the large-diameter multi-region poison rod. However, in the boron compound added inside, only B-10 is a combustible poison, but the neutron absorption cross section of B-10 is Gd.
It is much smaller than those of -155 and Gd-157, and the neutron absorbing action often continues gently even at the end of the operation cycle, so it is necessary to design in consideration of its characteristics.

According to the fuel assembly of the sixth aspect of the present invention, by the action of the cooling water uneven flow mechanism attached to the narrowed portion, the fuel assembly collects in the vicinity of the large diameter water rod and the large diameter multi-region poison rod. Since the cooling water, which tends to occur, is unevenly distributed toward the fuel rods in the vicinity, the fuel cooling effect is improved.

According to the fuel assembly of the seventh aspect of the invention, among some of the fuel rods or poison rods facing the channel box, the fuel rod has a small thermal margin downstream of the flow of the cooling water. By narrowing the diameter of the portion and providing a cooling water non-uniform flow mechanism therein, the cooling water on the inner surface of the channel box is non-uniformly flowed toward the adjacent fuel rods by the cooling water non-uniform flow action, and the adjacent fuel rods are the same as the invention of claim 6. Can contribute to cooling. In the present specification, the “channel box” is not limited to the channel box of the BWR fuel assembly, but broadly refers to the structural material that directly surrounds the fuel bundle in various nuclear reactors.

According to the fuel assembly of the eighth aspect of the present invention, the portion of the large diameter multi-region poison rod, the fuel rod, the poison rod or the thick water rod downstream of the thin portion which engages with the spacer is reduced. Since a short tube with the same diameter as the part that has not been thinned on the upstream side is installed, it is not necessary to change the spacer itself, and even when assembling the fuel assembly or disassembling work when necessary, The work can be performed almost in the same manner as in the past.

The cooling characteristics can be improved by tapering the upstream side and the downstream side of the short tube. For example, by providing a taper part so that the outer diameter is smoothly connected in a conical shape between the narrowed portion and the short tube, the flow passage area change in the spacer part becomes smooth, and the disturbance to the flow is prevented. Can be suppressed. Also,
Since the cooling water is directed to the adjacent long fuel rods by the tapered portion, the cooling efficiency of the long fuel rods is improved. If necessary, the swirling direction of the cooling water can be reversed in the short pipe portion or the like to cancel the rotational stress generated as a reaction of the unnecessary swirling of the cooling water.

[0038]

Embodiments of the fuel assembly according to the present invention will be described below with reference to the drawings.

1 to 9 show a first embodiment.

The fuel assembly 20 of this embodiment is a BWR fuel assembly, and as shown in FIGS. 1 and 2A to 2E, a normal channel rod 21 is provided inside a rectangular channel box 21. (Long fuel rod) 22 and combustible poison rod 23 having the same outer diameter as that of the rod (marked with G. Hereinafter referred to as G poison rod. Ordinary gadolinia (Gd ₂ O ₃ ) is replaced with urania (UO ₂ ) And sintered pellets 24 are filled in a metal tube 25 like a normal fuel rod (Fig. 2).
(A))), a large diameter water rod 26 (Fig. 2 (B)), a solid large diameter 2 area poison rod 27 (Fig. 2 (C)), and a medium water large diameter 2 area poison rod 28 (Fig. 2 (D)) and the channel box 21
A part of the short-length fuel rod 29 (FIG. 2 (E)) with a non-uniform flow mechanism facing the inner surface of the fuel cell (referred to as a short-flow non-uniform fuel rod in this specification) is regularly arranged at the fuel bundle corner and the center. There is.

As shown in FIG. 2B, the large diameter water rod 26 is
The outer tube 26a and the inner tube 26b have a double tube structure, and a gadolinium (Gd) tube 26c is watertightly inserted therebetween. Inside the inner pipe 26b, reactor water flows gently through a water passage hole 26d as a coolant flow-through portion, as shown by an arrow a, so that it does not boil from below to above (this burnable poison is a burnable poison). The large-diameter water rod 26 to which Gd is added is hereinafter referred to as a large-diameter water rod containing poison.) The solid large-diameter two-region poison rod 27 is, as shown in FIG.
The outer pipe 27a and the inner pipe 27b have a double structure,
The annular poison pellet 27c having a low gadolinia addition rate (for example, 1 to 3% by weight) is provided between the a and the inner pipe 27b.
Further, inside the inner tube 27b, core poison pellets 27d with a high gadolinia addition ratio (for example, 5% by weight or more) are hermetically inserted. The inner pipe 27b may be omitted. The core poison pellet 27d is an annular pellet 27c.
The cross-sectional area is significantly smaller than that of, and the rate of decrease in the amount of fuel loaded into the core is small even without filling with fuel substances such as uranium. Therefore, if necessary, without using urania, using gadolinia It is filled inside the inner tube either alone or as a cermet using zirconium or as a mixed oxide sintered body with alumina (Al ₂ O ₃ ) or zirconia (ZrO ₂ ) (here, pelletized And also include granular).

As shown in FIG. 2D, the medium water large diameter 2 area poison rod 28 has the same outer tube 2 as the solid large diameter 2 area poison rod 27.
8a, an inner pipe 28b, and a ring-shaped poisonous substance pellet 28c, but there is no core poisonous substance, and the reactor water is flown into the inner pipe 28b through the water passage hole 28d as shown by an arrow b to such an extent that it does not boil. They differ in points. When the inner diameter of the inner pipe 28b is increased, the reactivity effect as a poison rod increases, but the combustion is exhausted at an early stage. Therefore, a design considering this point is necessary.

As shown in FIG. 2 (E), the drift short fuel rod 29 has a structure in which the metal pipe 25 is filled with the pellets 24 as in the case of the normal fuel rod 22, but it extends over a fixed length range on the upper end side. The portion is a small-diameter tube 30. It should be noted that a small-diameter fuel substance can be adopted for the small-diameter pipe 30, and a solid rod structure may be adopted instead of the pipe. As a non-uniform flow mechanism 31, a non-uniform flow member 31a made of a spiral wire or the like similar to a wire spacer used in a fast reactor is wound around the outer peripheral portion of the small-diameter pipe 30. The cooling water is supposed to create a swirling flow. This swirling flow allows water having a high cooling capacity on the surface of the channel box 21 to be directed toward the adjacent fuel rods 22. In addition, if the number of the non-uniform flow short fuel rods 29 is too large,
The number of fuels should be a little small, because various problems such as reduced fuel inventory, loss of reactivity, and excessive output peaking are likely to occur. In this embodiment, eight channels are provided along the channel box 21. Of these, it may be considered that it is better to provide only the short fuel rods 29a located at the bundle corners and replace the central one 29b with a normal fuel rod.

FIG. 3 is a side view schematically showing the fuel bundle 20a with the channel box 21 removed from the fuel assembly 20 of FIG.
6, the thick two-region poison rods 27 and 28 and the drift short fuel rod 29 are extracted and shown. Code LTP is lower tie plate 32, UTP is upper tie plate 33, SP1-S
P7 indicates the lattice-shaped spacers 34, respectively.

In the BWR fuel assembly, it is known that the margin of the cooling capacity of the cooling water tends to be small near the spacers (SP6, SP7) on the upper end side. The large diameter two-region poisoning rods 27 and 28 are both formed into a small diameter pipe 35 on the upper side (downstream side) from this vicinity. A swirl vane 37 with a short pipe 36 shown in FIG. 4 (A) and a conical flow control material 38 shown in FIG. 4 (B) are attached to the small-diameter pipe 35 and directed toward the adjacent fuel rod 22 side. Cooling water is supplied as indicated by arrow c. Here, the spacer 34 is brought into contact with or held by the short tube 36, and the diameter thereof is the same as the diameter of the non-thinned portion, so that the bundle is assembled or disassembled in the same manner as in the case where the diameter is not reduced. Therefore, the conventional fuel assembly apparatus or dismantling apparatus can be used as it is. The upstream and downstream ends 36a, 36 of the short pipe 36
In order to avoid unnecessary pressure loss of the cooling water, it is desirable that b has a tapered cross section or a streamline structure as shown in FIG.

FIG. 4C shows a modification of FIG. 4A. That is, both the upstream and downstream ends of the short pipe 36 have a conical outer shape and smoothly change. Since the outer shape changes smoothly in this way, the cooling water is unevenly flown to the outer peripheral side as shown by the arrow d, and the cooling efficiency for the neighboring fuel rods is improved.

FIG. 4D shows the above-mentioned non-uniform flow short fuel rod 29.
The small-diameter tube 30 is shown in an enlarged manner. As shown in the figure, a drift member 31a as a cooling water swirl mechanism is attached. Other variations of this mechanism are possible.
For example, a relatively deep screw or groove may be formed at a moderate pitch on a rod-shaped object.

A short pipe similar to that shown in FIG. 4A (however, the spacer holding member is not attached) is attached to the spacer contact portion of the non-uniform flow short fuel rod 29. Further, the cooling water swirling mechanism supplies the cooling water flowing on the inner surface side of the channel box 21 to the adjacent fuel rods 22.

FIGS. 5 to 9 are schematic and more detailed views showing the longitudinal structure of various rod-shaped objects which constitute the fuel assembly 20 of the present invention. That is, FIG. 5 is an overall view of the large diameter water rod 26 shown in FIG.
6c is sealed inside a double pipe composed of an outer pipe 26a and an inner pipe 26b. In addition, FIG. 6 shows an ordinary long fuel rod 22.
7 shows a solid large-diameter two-region poison rod 27, FIG. 8 shows a medium water large-diameter two-region poison rod 28, and FIG. 9 shows a drift short fuel rod 29. Since the configuration of each member shown in FIGS. 5 to 9 has already been described,
2 to 4 (and FIG. 26 and the like) are designated by the same reference numerals, and the description thereof will be omitted.

According to the fuel assembly 20 of the first embodiment described above, a large neutron multiplication factor suppressing effect can be obtained by disposing the thick diameter 2 region poison rods 27 and 28 at the position where the neutron flux is high. As a result, it is possible to reduce the number of poison rods 23 having a structure in which a combustible poison is added to the fuel. As a result, the output peaking in the fuel assembly 20 can be mitigated, more plutonium can be loaded, and the flow of cooling water that tends to collect around the large diameter water rod 26 is adjusted to cool the fuel rod 22. Since it is effectively used, the cooling characteristics of the fuel are improved.

Further, since the thick water rod 26 having a large position where the thermal neutron flux becomes high contains the combustible poisonous substance such as the Gd cylinder 16c in the tube portion of the large diameter water rod 26, the neutron of the fuel assembly 20 is The effect of suppressing the multiplication factor becomes large. Furthermore, a large diameter water rod 26
Since the surface area of the outer peripheral portion and the inner peripheral portion facing the water is significantly larger than the surface area of the conventional poison rod, the neutron multiplication suppressing effect of one large diameter water rod 26 containing a poison is equal to that of the conventional poison rod. Several times (for example, five times). Therefore, the number of normal poison rods 23 can be reduced by that amount, and the normal fuel rods 22 can be used instead. During the period in which the neutron absorption action of the large diameter multi-zone poison rod continues in the reactor operation cycle, the neutron absorption action of the large diameter water rod is suppressed, so the excessive suppression effect of the neutron multiplication factor during that period is significant. Is alleviated.

When the neutron absorbing action of the thick diameter two-region poison rod 27 is weakened, the neutron absorbing action of the poison-containing large diameter water rod 26 is exerted. Shortly before the end of the operation cycle, the poisoning action is generally minimized, but in that case, it acts simply as the large diameter water rod 26, and improves the neutron multiplication effect that tends to be insufficient at the end of the cycle of the fuel assembly 20. Can be made. Depending on the design conditions, it is possible to leave the toxic effect even at the end of the cycle.

Further, when a coolant, that is, reactor water is introduced into the large diameter two-region poison rod 28, the fast neutrons are decelerated to become thermal neutrons that are easily absorbed by the poisons, so that the neutron increase of the fuel assembly 20 is increased. The magnification suppression effect is further increased.

Moreover, if the concentration of the burnable poison 27d inside the large diameter two-region poison rod 27 is made higher than that at the outer peripheral portion,
First, after the effect of suppressing the multiplication factor is completed in the period in which the neutron multiplication factor of the fuel assembly 20 tends to be particularly high in the early part of the operation cycle, the neutron absorbing action of the burnable poison 27c in the outer peripheral portion disappears, and the internal flammability is increased. The poison 27d gently exerts the multiplication factor suppressing effect. That is, this configuration has an effect of keeping the effective value of the neutron multiplication factor of the fuel assembly 20 substantially constant.

Further, according to the present embodiment, by the action of the cooling water distribution mechanism 31 attached to the narrowed portion,
The cooling water, which tends to gather near the large-diameter water rod 26 and the large-diameter multi-region poisoning rod, flows unevenly toward the peripheral fuel rods 22, so that the fuel cooling effect is improved.

Further, of the part of the fuel rods 22 or poison rods 23 facing the channel box 21, the portion of the fuel rods 22 having a small thermal margin is made thin at the downstream side of the flow of the cooling water, and the diameter is reduced there. By providing the cooling water non-uniform flow mechanism 31, the cooling water on the inner surface of the structural member is non-uniformly flowed toward the adjacent fuel rods 22 due to the cooling water non-uniform flow action, which can contribute to the cooling of the adjacent fuel rods 22.

Further, the diameter of the large diameter two-region poison rods 27, 28, the fuel rod 22, the poison rod 23, or the thick water rod 26, which is reduced in diameter on the downstream side and engages with the spacer, is reduced on the upstream side. Since the short tube 36 having substantially the same diameter as that of the unattached portion is attached, it is not necessary to change the spacer 34 itself,
Further, even in the case of assembling the fuel assembly 20 and the disassembling work when necessary, the work can be performed almost in the same manner as the conventional one.

By forming the tapered portions 36a and 36b on the upstream side and the downstream side of the short pipe 36, the cooling characteristic can be improved. That is, by providing the tapered portions 36a and 36b so that the outer diameter is smoothly connected in a conical shape between the small diameter pipe 35 and the short length pipe 36, the flow passage area change in the spacer portion becomes smooth, and the disturbance to the flow is disturbed. Can be suppressed. In addition, the tapered portions 36a and 36b
As a result, the cooling water is directed to the adjacent long fuel rods, so that the cooling efficiency of the long fuel rods 22 is improved.

The first embodiment described above can be modified in various ways as described below.

(1) Use a large-diameter water rod as a single tube and remove combustible poisons. (2) All the large diameter two-region poison sticks are solid or solid water poison sticks. (3) Two or more large diameter poisonous rods are partially or entirely made into three or more regions. (4) A combustible poison such as zirconium boride (ZrB ₂ ) is applied to the surface of pellets or the like filled in the thick two-region poison rod. (5) A boron compound, an oxide of a rare earth element, or the like is filled in the inner tube of the solid large-diameter two-region poison bar, or without using the inner tube. (6) On the downstream side, the range of reducing the diameter of water rods, poison rods, short-distance drift fuel rods, etc. is variously changed. (7) The position, the number, or the range of narrowing of the non-uniformly distributed short fuel rods are variously changed, and further these are appropriately combined. (8) Change the position, concentration, number, etc. of solid burnable poisonous rods.

FIG. 10 shows a second embodiment of the present invention. In this embodiment, the arrangement of the fuel rods 22 is changed so that all the fuel rods 22 face a relatively wide gap in which the cooling water flows.

That is, in this fuel assembly 20, four medium-water large-diameter two-region poison rods 28 and four G poison rods 23 indicated by the symbol G are arranged around the large-diameter water rod 26. Has been done.
The concentration of Gd ₂ O ₃ added to UO ₂ in the G poison rod 23 is made relatively high (for example, 8 to 10% by weight), and the core of the large diameter water rod 26 containing poison and the large diameter medium water 2 region poison rod 28 is added. The excessive reactivity increase problem of the fuel assembly at the end of the cycle, which is caused by the physical characteristics, is avoided.

According to the structure of the present embodiment as described above, the cooling characteristic is good, the probability of escaping the resonance absorption of neutrons is increased, and there is the reactivity gain during the output operation. A short-flow drift fuel rod 29 is adopted as the fuel rod 22 of the channel corner to prevent the cooling characteristic from deteriorating and the stop margin to be improved. Further, there may be a case where the type of fuel enrichment that constitutes the fuel assembly 20 can be reduced.

FIG. 11 shows a third embodiment of the present invention. This embodiment is similar to the second embodiment, but is suitable for the case where the concentration of fuel is relatively low, and the G poison rod is arranged around the large diameter water rod 26 containing poison. Instead, the normal fuel rods 22 are arranged symmetrically with respect to the 45-degree diagonal. With such a configuration, the Gd ₂ O ₃ concentration of the medium-water large-diameter two-region poison rod 28 is relatively low (for example, 2 to 4% by weight) and high (for example, 5 to 5%).
8% by weight) can alleviate the excessive reactivity increase problem of the fuel assembly at the end of the cycle.

FIG. 12 shows a fourth embodiment of the present invention. In this embodiment, unlike the above-described embodiments, two large water rods 26 containing poisonous substances are arranged adjacent to each other in the central portion of the fuel assembly 20, and two large diameters of medium water are adjacent to each other so as to be orthogonal to each other. A two-zone poison stick 28 is arranged.

If a fuel rod having a normal diameter or a conventional poison rod is arranged at this position, the cooling water flow passage around the fuel rod is wide, so that the cooling water is likely to collect and the cooling characteristics are deteriorated. According to this embodiment, the adverse effect is eliminated.
In this embodiment, the water rod has a circular cross section, but it is not limited to this, as is the case with other embodiments.

FIG. 13 shows a fifth embodiment of the present invention. In this embodiment, two adjacent large-diameter water rods 26 containing poisons
Two solid large-diameter two-region poison sticks (concentration is high inside and low at the outer periphery) 27 are arranged orthogonally to. Since it is a low enrichment fuel assembly, eight G poison rods are used in the case of FIG. 12, whereas four G poison rods are used in the example of FIG.

The configuration of this embodiment is suitable when the fuel enrichment is relatively low.

FIG. 14 shows a sixth embodiment of the present invention. This embodiment is similar to the fourth embodiment, except that the conventional G poison rod 23 is disposed close to the drift short fuel rod 29, and the solid large diameter 2 is used instead of the medium water large diameter. Area poison stick 2
7 (however, gadolinia poison is not contained in the outer peripheral portion) is used.

According to this embodiment, although the reactivity suppressing effect is slightly smaller than that shown in FIG. 10, the change in the neutron multiplication characteristics over the combustion cycle can be easily reduced.

FIG. 15 shows a seventh embodiment of the present invention. This embodiment is structurally similar to the sixth embodiment, but one of the two large diameter water rods 26 contains gadolinium and the other 26 'contains a boron compound.

According to the present embodiment, since the neutron absorption cross section of boron is smaller than that of Gd, the burning rate is slow, and therefore the excessive increase of the neutron multiplication factor at the end of the cycle can be suppressed. The boron compound may be used by inserting an Al ₂ O ₃ —B ₄ C mixed sintered body between the outer tube 26a and the inner tube 26b, or by inserting it on the inner surface of the outer tube 26a and / or the outer surface of the inner tube 26b. Means such as applying ZrB ₂ are possible.

FIG. 16 shows an eighth embodiment of the present invention. This embodiment is similar to the fifth embodiment, but the fuel assembly 20 has two poisonous large diameter water rods 26 and 2 inside.
3 x fuel rods surrounding a large-diameter two-region poison rod 27 (there are pellets containing Gd inside and the fuel pellets on the outer periphery are coated with ZrB ₂ )
It is composed of eight sub-handles separated by a water gap in groups of three.

According to this embodiment, the probability of escape from resonance is higher than that in the fifth embodiment, so that the neutron multiplication factor can be slightly increased. Further, the third fuel rod 22 from the corner is the channel box 21 in the fifth embodiment.
On the other hand, in the present embodiment, the cooling characteristic is also improved because it faces a wide water gap.

FIG. 17 shows a ninth embodiment of the present invention. This embodiment is also similar to the fifth embodiment, except that
The arrangement of the fuel rods 22 is not uniform, and the fuel rods 22 are spaced apart from each other and have many water gaps as in the cases of FIGS. 10 and 11. According to this example, a fuel assembly having good cooling characteristics and a slightly higher neutron multiplication factor than in the case of FIG. 13 is obtained, which is advantageous for improving the burnup.

FIG. 18 shows a tenth embodiment of the present invention. In this embodiment, the fuel rods 22 in the fuel assembly 20 are not uniformly arranged, but are arranged in a 4-1-4 type coarse and dense structure. 2 large diameter water rods with 2 poisons in the center
6 are arranged adjacent to each other, and six large-diameter two-region poison rods 27 and 28 are arranged so as to sandwich them. Of these 6 poison rods, the central one 28 of the three sets is a medium water 2 area thick poison rod, and the ones on both sides 27 are solid 2 area thick poison rods. The solid one has a low concentration of gadolinia added to the outer periphery. Of the fuel rods 22 facing the channel box 21, every other eight fuel rods 22 are shortened, and cooling water with a large cooling margin is supplied from the inner surface of the channel box 21 to the adjacent fuel rods.

According to this embodiment, the effect of suppressing the neutron multiplication factor of the fuel assembly 20 by the burnable poison is large and lasts long. Therefore, it is suitable for a fuel assembly having a relatively high enrichment. Further, since the fuel rods 22 are densely arranged, it is advantageous to increase the reactivity during operation as described above.

FIG. 19 shows an eleventh embodiment of the present invention. This embodiment is similar to the embodiment shown in FIG. 10, but the fuel rods 22 are arranged in 10 × 10 instead of 9 × 9.

According to this embodiment, the operation and effect are substantially the same as those in the case of FIG. However, every two fuel rods 22 facing the channel box are shortened so that the cooling characteristics are further improved.

FIG. 20 shows a twelfth embodiment of the present invention. In this embodiment, the fuel rods 22 are arranged in 11 × 11, one large diameter water rod 26 containing poison is arranged in the center, and eight solid large diameter two-region poisons surrounding the same are arranged. A bar 27 is arranged. The number of fuel rods in the fuel assembly was 6 × 6 in the initial BWR, but 7 × 7, 8 ×
It has been increasing to 8 × 9 × 9, and is expected to further increase to 10 × 10 and 11 × 11. This embodiment corresponds to that.

FIG. 21 shows a thirteenth embodiment of the present invention. The fuel assembly 20 of the type of this embodiment is basically developed in Europe, and the fuel bundle 20a of the fuel assembly 20 is divided into four sub-bundles 20'a, and in between. Slit-shaped non-boiling water area 4
0 is provided.

In this example, a large diameter water rod 26 containing a poison is arranged in the center. The structural material of the slit part is made of Zircaloy, like the channel box. It is also possible to insert a neutron absorbing plate added with a burnable poison in the slit-shaped non-boiling water region 40, if necessary.
In addition, 4 large diameter 2
Area poison sticks 27, 28 are arranged. Two of them 28 are medium-water large-diameter two-region poison rods, and the other two 27 are solid large-diameter two-region poison rods. Since the cooling characteristics of the fuel rods 22 at the respective corners of the fuel assembly 20 are generally poor, the drift short fuel rods 29 are arranged.

In the above embodiments, the case of the BWR fuel assembly has been described by giving many examples, but the present invention is not limited to the case of the BWR fuel assembly.
Pressurized water reactor (PWR) and boiling light water cooling / heavy water moderator thermal neutron reactor (SGHWR, developed in the United Kingdom and Japan
ATR) etc. can also be used.

Hereinafter, the fuel assembly for PWR and the ATR,
A case of a fuel assembly for a pressure tube type reactor such as CANDU and SGHWR will be briefly described.

FIG. 22 shows a fourteenth embodiment of the present invention.
1 illustrates a PWR fuel assembly. 15 × fuel rod 22
15 large water rods 26 containing poisonous substances are arranged symmetrically. The central water rod has a smaller diameter than the other four due to structural conditions, and four solid large-diameter two-region poison rods 27 are arranged around the water rod. The gadolinia concentration of this poison stick is high inside and low at the outer periphery. PWR
Since the channel box is not arranged around the fuel assembly, the non-uniform flow short fuel rods are not arranged.

Also in this embodiment, the same effect as that of the BWR fuel assembly is obtained.

23 to 25 show three kinds of fuel assemblies 20 for a pressure tube type reactor as a fifteenth embodiment of the present invention. In this embodiment, the fuel rods 22 are concentrically arranged so as to have a circular outer shape at substantially equal intervals to form a fuel bundle (often called a fuel cluster). The fuel bundle is arranged in the circular pressure pipe 41 to form the fuel assembly 20. In reality, there is a gas heat insulating layer (not shown) around the pressure pipe 41, and a calandria pipe surrounds the heat insulating layer. Unevenly distributed short fuel rods 29 are arranged for every five fuel rods 22 arranged so as to face the inner surface of the pressure pipe 41, and water having a cooling margin near the inner face of the pressure pipe 41 is sprinkled on the adjacent fuel rods 22. It is configured to. At the center of the fuel assembly 20, a large diameter water rod 26 containing poison and a large diameter 2 area poison rod 2 surrounding the water rod 26 are arranged.
7, 28 are arranged.

In the example of FIG. 23, a solid large diameter two area poison stick 27 is used.
24, and in the example of FIGS. 24 and 25, it is necessary when the fuel enrichment is high, and 3 out of 6 are solid large diameter 2 area poison rod 27 and the other 3 are medium water large diameter 2 It is designated as area poison stick 28. In the example of FIG. 25, the poisonous rod 23 shown by G of the conventional type is arranged, which is suitable when the fuel enrichment is higher than that of FIG.

The concept of the present invention is based on the above-mentioned BWR and PW.
The present invention can be applied not only to the fuel assembly of the pressure tube type heavy water reactor developed in R, UK, Canada, Japan, etc., but also to the fuel assembly of the high conversion type light water reactor which is being researched and developed.

[0090]

As described above in detail, according to the fuel assembly of the present invention, the large-diameter multi-region poison rod is arranged at a position where the neutron flux is high, and the large-diameter water rod is also required as necessary. Since a combustible neutron poison is added to the fuel assembly, the effect of suppressing the neutron multiplication factor of the fuel assembly can be significantly increased compared to the conventional case. As a result, the number of conventionally used poison rods, that is, the number of fuel rods obtained by adding a burnable poison such as gadolinia to a nuclear fuel substance can be reduced. Conventional poison rods of the same kind are difficult to add at high concentrations because it is necessary to suppress the decrease in thermal conductivity, and it is considered to be a difficulty in designing fuel assemblies for higher burnup in the future. When the number of rods increases, it becomes difficult to design a flat power distribution, and in the fuel assembly using plutonium, manufacturing complexity and cost increase,
It was extremely disadvantageous to add a burnable poison to the plutonium fuel rod for reasons such as a decrease in the neutron multiplication factor suppression effect, but such a drawback is greatly alleviated by the above-described configuration of the present invention. The number of conventional poison sticks can be reduced to the minimum necessary.

Further, since the large-diameter multi-zone poison rod is arranged at the side of the large-diameter water rod where cooling water is likely to collect, such wasteful concentration of cooling water is eliminated and the cooling characteristics of the fuel rod are improved. it can.

Furthermore, the cooling water having a high cooling margin facing the structural material surrounding the fuel bundle of the channel box,
Since it can be used for cooling the neighboring fuel rods by the action of the short-flowing fuel rods having the non-uniform flow mechanism, the cooling characteristics of the fuel rods can be improved. In addition, the effect of relatively easy assembly and dismantling work can be achieved.

[Brief description of drawings]

FIG. 1 shows a first embodiment of a fuel assembly according to the present invention and is a cross-sectional view of a BWR fuel assembly.

2 is an enlarged view of the parts in FIG. 1, (A) is a normal fuel rod and poison rod, (B) is a large water rod containing poison, (C)
Is a solid large-diameter two-zone poison rod, (D) is a medium-water large-diameter two-zone poison rod, and (E) is a longitudinal cross-sectional view of an essential portion showing a drift short fuel rod equipped with a cooling water drift mechanism.

FIG. 3 is a vertical side view showing a state where the channel box is removed from the fuel assembly in the above-mentioned embodiment, conceptually showing an axial height relationship between a large diameter water rod, a large diameter multi-region poison rod and a drift short fuel rod. Fig.

FIG. 4A illustrates the concept of a short pipe part having a cooling water swirl mechanism at a position where the diameter of the water rod is reduced in the downstream part and engaged with the spacer, and a spacer holding piece is further attached. FIG. 6B is a view showing a flow control member that is attached to a reduced diameter portion of a water rod, a poison rod, and a short fuel rod, and shows a flow control material that causes the cooling water to flow to the side surface instead of the swivel mechanism. 8A is a diagram showing a modified example of FIG. 9A, and FIG. 9D is a diagram showing an example of a flow control material having a turning mechanism.

FIG. 5 is a diagram showing a large diameter water rod according to the embodiment.

FIG. 6 is a diagram showing a fuel rod and a poison rod according to the embodiment.

FIG. 7 is a view showing a solid large-diameter two-region poison stick according to the embodiment.

FIG. 8 is a view showing a medium-water large-diameter two-region poison stick according to the embodiment.

FIG. 9 is a view showing a drift short fuel rod according to the embodiment.

FIG. 10 is a cross-sectional view showing a second embodiment of the fuel assembly according to the present invention.

FIG. 11 is a cross sectional view showing a third embodiment of the fuel assembly according to the present invention.

FIG. 12 is a cross sectional view showing a fourth embodiment of the fuel assembly according to the present invention.

FIG. 13 is a cross sectional view showing a fifth embodiment of the fuel assembly according to the present invention.

FIG. 14 is a cross sectional view showing a sixth embodiment of the fuel assembly according to the present invention.

FIG. 15 is a cross sectional view showing a seventh embodiment of the fuel assembly according to the present invention.

FIG. 16 is a cross sectional view showing an eighth embodiment of the fuel assembly according to the present invention.

FIG. 17 is a cross sectional view showing a ninth embodiment of the fuel assembly according to the present invention.

FIG. 18 is a transverse sectional view showing a tenth embodiment of the fuel assembly according to the present invention.

FIG. 19 is a cross-sectional view showing an eleventh embodiment of the fuel assembly according to the present invention.

FIG. 20 is a cross-sectional view showing a twelfth embodiment of the fuel assembly according to the present invention.

FIG. 21 is a transverse sectional view showing a thirteenth embodiment of the fuel assembly according to the present invention.

FIG. 22 is a cross-sectional view showing a fourteenth embodiment of the fuel assembly according to the present invention.

FIG. 23 is a cross sectional view showing a fifteenth embodiment of the fuel assembly according to the present invention.

FIG. 24 is a diagram showing a modification of the embodiment shown in FIG. 23.

FIG. 25 is a diagram showing another modification of the embodiment shown in FIG. 23.

26A is a vertical sectional view showing a conventional BWR fuel assembly, FIG. 26B is a view showing a fuel rod, and FIG. 26C is a view showing a large-diameter water rod.

27 (A) is a cross-sectional view showing a conventional fuel assembly of the same as above, and FIG. 27 (B) is a cross-sectional view showing another example.

28A is a vertical side view of the fuel assembly with the channel box removed, FIG. 28B is an output distribution map corresponding thereto, and FIG. 28C is a void distribution map.

[Explanation of symbols]

 20 Fuel Assembly 20a Fuel Bundle 21 Channel Box 22 Long Fuel Rod 23 Burnable Poison Rod 24 Pellet 25 Metal Tube 26 Large Diameter Water Rod 27 Solid Large Diameter 2 Region Poison Rod 27a, 28a Outer Tube 27b, 28b Inner Tube 27c , 28c Annular poison pellet 27d Core poison pellet 28 Medium water large diameter 2 area poison rod 28d Water passage hole 29 Distorted short fuel rod 30 Small-diameter pipe 31 Distortion mechanism 31a Dispersion member 32 Lower tie plate 33 Upper tie plate 34 Spacer 35 Thin Diameter tube 36 Short tube 36a, 36b Tapered portion 37 Swirling blade 38 Flow control material 40 Slit-shaped non-boiling water region 41 Pressure tube

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G21C 3/326 GDB 3/62 GDB K G21C 3/30 GDB P GDB K GDB Y (72) Inventor Toru Mitsutake 8 Shinsita-cho, Isogo-ku, Yokohama-shi, Kanagawa Stock company Toshiba Yokohama office

Claims (8)

[Claims] 1. A large number of fuel rods in which no burnable poison is added to the fuel, and a plurality of poison rods in which the burnable poison is added to the fuel,
At least one large-diameter water rod having a diameter larger than the diameter of the fuel rod except for a certain length range at both ends is arranged in a lattice using a plurality of spacers, an upstream-side coupling member and a downstream-side coupling member. In the fuel assembly integrated into a single fuel bundle, a part of the poison rod is larger than the diameter of the fuel rod except for a certain length range at both ends, and the composition is the same or different between the inside and the outside. A large-diameter multi-zone poison rod, at least one of the large-diameter multi-zone poison rods is disposed adjacent to the large-diameter water rod, and the large-diameter water rod and the large-diameter multi-zone poison stick adjacent thereto. The fuel assembly is characterized in that a large number of fuel rods are arranged so as to surround the group. 2. The fuel assembly according to claim 1, wherein the large-diameter water rod contains a burnable poison in its tube wall portion. 3. The fuel assembly according to claim 1, wherein at least a part of the large-diameter multi-zone poison rod has a coolant flow-through portion through which the coolant flows. 4. The concentration of the combustible poison added to at least a part of the large-diameter multi-zone poison rod is high inside the large-diameter multi-zone poison rod and low in the outer peripheral portion. The fuel assembly according to item 1. 5. The burnable poison added to the inside of at least a part of the large-diameter multi-zone poison rod is a boron compound,
The fuel assembly according to claim 4, wherein the burnable poison added to the outer peripheral portion is a gadolinium compound. 6. A large-diameter water rod or at least a part of a large-diameter multi-zone poison rod is thinned over a certain length range from the downstream side of the flow of the coolant, and the diameter is reduced. In the portion where the enrichment of fissionable nuclides in the fuel rod adjacent to that portion is higher than that of natural uranium 235,
The fuel assembly according to claim 1, further comprising a drift mechanism that biases the flow of the coolant toward the adjacent fuel rods. 7. A large number of fuel rods in which no burnable poison is added to the fuel, and a plurality of poison rods in which the burnable poison is added to the fuel,
At least one large-diameter water rod having a diameter larger than the diameter of the fuel rod except for a certain length range at both ends is arranged in a lattice using a plurality of spacers, an upstream-side coupling member and a downstream-side coupling member. In a fuel assembly in which a metal channel box is provided so as to surround the fuel bundle, a fuel rod or a poison rod disposed at a position facing the channel box is a cooling water. The uranium 2 of natural uranium has a reduced diameter over a certain length range from the downstream end of the flow, and the enriched concentration of the fissile nuclides of the fuel rods adjacent to the reduced diameter is 2
35. A fuel assembly characterized by having a drift mechanism for biasing the flow of the coolant in the direction of the adjacent fuel rods, in a portion of the fuel cell 35 having a higher concentration than that of 35. 8. A portion of a large-diameter multi-zone poison rod that is reduced in diameter downstream side and engages with a spacer, and a portion of a fuel rod facing a channel box or a portion of a poison rod that is reduced in diameter downstream side and engages with a spacer. , And a portion of the portion that is reduced in diameter on the downstream side of the large diameter water rod and engages with the spacer, at least one of the portions has a short pipe with an outer diameter that is approximately the same as the diameter of the non-thinned portion. The fuel assembly according to claim 6, which is mounted.