JP2000019282A - Fuel assembly for light-water reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify the manufacture process, and at the same time, to improve the controllability of a reactor such as the optimization of surplus reactivity by reducing the amount of absorption of neutron per combustible poisonous granule-mixed fuel rod, and at the same time, by increasing the usage ratio. SOLUTION: In the fuel assembly for a light-water reactor, a fuel rod where a combustible poisonous granule 23 with a large grain diameter is thinly dispersed in a fuel granules 16 for mixing and is sealed to a covering pipe 15 by oscillation filling or the like is used, for example, 60% of the total fuel rods. The combustible poisonous granule 23 should be a solid solution, for example, with depleted uranium dioxide containing approximately 45% gadolinia, and has a nearly spherical shape and an average grain diameter of 0.5 mm or more, for example, approximately 1 mm. The density of the number of the combustible poisonous granules 23 in the fuel rod is set so that the ratio of the total area of a granule to the surface area of the fuel part of the fuel rod is set to 1 or less, for example, approximately 1/2. When the surface area is reduced, the amount of the absorption of a neutron can be reduced. Also, when the grain diameter is increased, the suppression of excess reactivity is ensured from the beginning to the end of a cycle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a nuclear reactor, and more particularly to a fuel assembly for a light water reactor using a fuel rod mixed and filled with a granular nuclear fuel material and a burnable poison.

[0002]

2. Description of the Related Art A fuel assembly for a boiling water reactor (hereinafter, referred to as BWR) is formed by sintering uranium dioxide or a mixed oxide of uranium and plutonium (hereinafter, referred to as MOX) into a columnar shape having a diameter of about 1 cm. Pellets, these are sealed in a Zircaloy cladding tube to form fuel rods, bundled in a square lattice to form an aggregate, and further covered with a Zircaloy channel box, and the boiling water flow path between the fuel rods Is provided.

In a conventional fuel rod, as shown in FIG. 9, a large number of sintered pellets 1 are stacked and loaded in a fuel cladding tube 2.
The upper end plug 3 and the lower end plug 4 are hermetically sealed. In this case, a heat insulating pellet 5 such as alumina is inserted between the lowermost pellet and the lower end plug, and a coil spring 7 spot-welded to a metal holding plate 6 is inserted between the uppermost pellet and the upper end plug. Is provided.

FIG. 10 is a longitudinal sectional view of a BWR fuel assembly, particularly an example of a fuel assembly designed for high burn-up, and FIG. 11 is a sectional view taken along line AA of FIG. . The fuel assembly 13 is composed of a bundle of 74 fuel rods 8 and two water rods 9 bundled in a square grid of 9 rows and 9 columns by spacers 10 and fixed by an upper tie plate 11 and a lower tie plate 12. The fuel rod bundle is surrounded by a channel box 13.

[0005] In the BWR core, a water gap is provided between the fuel assemblies. However, since the moderator distribution is not uniform in the horizontal cross-sectional direction, the local output is determined by using several types of uranium enrichment or plutonium enrichment of the fuel rods. The distribution is flattened.

Further, in the early stage of combustion, gadolinia (Gd) is used as a burnable poison in some of the combustion rods in order to suppress excess fission reaction and appropriately control the excess reactivity.
Pellets containing a burnable poison to which a neutron absorber such as ₂ O ₃ ) is added are enclosed. In a fuel rod having such a burnable poison, most of the neutrons are absorbed by the burnable poison surface, and the diameter of the effective burnable poison initially having the same outer shape as the fuel rod becomes smaller in proportion to the burnup. Therefore, the neutron absorption effect decreases linearly with the burnup. The amount of burnable poison added depends on fuel economy.
At the end of the driving cycle, the residual amount is designed to be almost zero.

FIG. 12 is a graph schematically showing the combustion change of the fuel assembly at infinite multiplication factor and the combustion change of the gadolinia reactivity when gadolinia is contained as a burnable poison and when gadolinia is not contained. FIG. 13 is a graph schematically showing a combustion change of the excess reactivity of the equilibrium cycle core loaded with these fuel assemblies. Here, N is the number of cycles in which the fuel assembly stays in the core. N is the number of replacement batches and is generally a non-integer. In FIG. 12, symbols d1, d2, and dN indicate periods of the first, second, and Nth cycles, respectively. The solid line of reference numeral 51 and the reference numeral 52
Are the combustion changes of the infinite multiplication factor when gadolinia is included and when gadolinia is not included, and the solid line of reference numeral 53 is a combustion change of gadolinia reactivity corresponding to the case of the solid line of reference numeral 51. This shows that the neutron absorption effect by gadolinia in the fuel assembly appears remarkably in the first cycle.

A solid line 56 and a broken line 57 in FIG. 13 schematically show changes in the burnup of the excess reactivity when gadolinia is included and when gadolinia is not included, respectively. Although the excess reactivity when gadolinia is not included decreases linearly, the presence of gadolinia makes it possible to control the excess reactivity to be low and almost constant from the beginning to the end of the cycle.

When a fuel rod containing gadolinia is used, the magnitude of the suppression reactivity at the beginning of the cycle is proportional to the number of fuel rods containing gadolinia, and the length of the period in which the reactivity is to be suppressed is proportional to the gadolinia concentration. Are known. That is, when the number of fuel rods including gadolinia is increased, the initial gadolinia reactivity is increased as shown by a broken line 54, and the gadolinia concentration is increased by increasing the number of fuel rods including gadolinia, as indicated by the solid line 53 in FIG. Increases, the time until the gat linearity becomes zero becomes longer as indicated by the broken line 55. Utilizing this relationship, the gadolinia is set so that the excess reactivity of the core becomes flat for a long time to simplify the operation of the reactivity control using the control rods, and that the gadolinia remains unburned at the end of combustion. Appropriately set the number and concentration of fuel rods to be included.

In recent years, high burnup (high enrichment) of uranium fuel assemblies has been promoted in order to improve fuel economy. In particular, in fuel assemblies having high burnup, a thermal margin is secured. In order to achieve this, the number of fuel rods is increased from the prior art, and the number of fuel arrays is set to 9 rows and 9 columns. In parallel with this, the so-called pluthermal project, in which plutonium obtained by reprocessing spent uranium fuel is reused in light water reactors, is being pursued in order to make effective use of plutonium. In order to increase the plutonium loading per unit, a fuel assembly in which most fuel rods are MOX fuel rods has been designed.

In order to increase the burnup of these fuel assemblies, it is necessary to increase the uranium enrichment and plutonium enrichment of each fuel rod, and accordingly, the number of gadolinia-containing fuel rods is increased and the surplus is increased. It is necessary to suppress the reactivity, but doing so increases the difference in local power.To flatten this, prepare many types of fuel rods with different enrichment or plutonium enrichment. do it,
They must be properly positioned and adjusted, which complicates the manufacture of fuel heating elements and increases costs.

In the MOX fuel assembly, since the neutron flux spectrum is hard, the neutron absorption effect of gadolinia is lower than that in the case of the uranium fuel assembly. For this reason, in order to suppress the excess reactivity, it is necessary to arrange the number of fuel rods including gadolinia larger than that of the uranium fuel assembly.

FIG. 14 is a diagram showing an example of a fuel rod arrangement of a conventional MOX fuel assembly using uranium fuel rods containing gadolinia for suppressing excess reactivity. In FIG. 14, symbol 8 is a fuel rod, symbol 9 is a water rod, and symbol 13 is a channel box. And the symbol G _U is uranium fuel rods containing gadolinia, the symbol M ₁ ~M ₄ is MOX
Fuel rods are shown, and plutonium enrichment is larger in the order of subscript numbers. The base material in the MOX fuel rods uses depleted uranium, and the weight fraction of fissile plutonium in the total fissile material in the fuel assembly is less than 70%. That is, for the total amount of uranium and plutonium, fissile material is
4.6% by weight, of which fissile plutonium is 3.1%
% By weight. Also, among the 74 fuel rods of the fuel assembly,
The region except the outer peripheral portion is arranged fuel rods G _U containing 22 present gadolinia concentration of gadolinia is 1.5 wt%.

In conventional fuel designs, fuel rods containing gadolinia do not use MOX pellets containing gadolinia,
Uranium fuel rods containing gadolinia molded by mixing gadolinia with uranium are adopted. Therefore, further increasing the number of fuel rods G _U containing gadolinia in MOX fuel assembly shown in FIG. 14, the number of MOX fuel rods is reduced first, plutonium loading amount per fuel assembly is reduced . Moreover, when further increasing the number of fuel rods G _U containing gadolinia 14 will be positioned adjacent the fuel rods G _U together containing gadolinia. Then the burnable poison is rather excessive at that location,
Problems such as a reduction in the neutron absorption effect per fuel rod and an increase in the residual gadolinia after combustion occur. Thus in the design of high burnup fuel assembly increases plutonium enrichment is difficult to arrange the fuel rods G _U containing more gadolinia than the case shown in FIG. 14.

FIG. 15 is a graph showing the change of the infinite multiplication factor of the fuel assembly shown in FIG. 14 depending on the burnup. The solid line 58 in FIG. 15 indicates the infinite multiplication factor of the fuel assembly, and the broken line 52 indicates the infinite multiplication factor when gadolinia is not included.
The symbols d ₁ , d ₂ , d ₃ and d ₄ indicate the first, second, third and fourth burnup ranges, respectively. Further, the graph of FIG. 16 shows an example of a combustion change of the excess reactivity of the equilibrium cycle core loaded with the fuel assembly. Further FIG.
The graph of 7 shows the combustion change of the local output peaking coefficient.

In order to solve the above-mentioned difficulty in arranging gadolinia-containing fuel rods, as shown in FIG.
Leave slightly ensure residual amount of gadolinia in the fuel rods G _U containing gadolinia at the end of the cycle, by decreasing the infinite multiplication factor in the second cycle the initial, is a method of suppressing the excess reactivity of the entire core is there. However, this method is not desirable from the viewpoint of fuel economy because reactivity loss occurs due to slight residual gadolinia at the end of the first cycle. Further, in this configuration due to the increased concentration of gadolinia-containing fuel rods G _U, increases the output difference between the fuel rods, power peaking factor is slightly larger. Although this is within the design goals, it leaves room for reducing the output peaking factor and improving the power distribution.

The above-described pellet fuel is widely used, has a proven track record, and is highly reliable. However, the manufacturing process is considerably complicated as illustrated in FIG.
In particular, the high production cost of MOX fuel is a drawback.

A so-called vibration-filled fuel rod, in which a granular nuclear fuel material is vibration-filled and sealed in a metal clad tube, instead of the above-described fuel rod laminated and filled with nuclear fuel pellets, is known. As illustrated in FIG. 19, the manufacturing process is greatly simplified as compared with the case of stacking the pellet fuel, and further, the automation and remote control of the process are easy, and the manufacturing cost can be significantly reduced. It is also easy to make a fuel rod by mixing a burnable poison such as gadolinia with MOX granules.

FIG. 20 shows an example of a conventional vibration-filled fuel rod. In the fuel cladding tube 15 shown in FIG. 9, granular nuclear fuel material oxide 16 is used instead of pellet.
Is enclosed. In this case, the granules of nuclear fuel material oxide are filled to form an effective heat generating portion of the fuel rod,
At its upper and lower ends, a buffer section 17 made of metal wool is provided. The upper end plug 18, the lower end plug 19, the holding plate 20, and the coil spring 21 have basically the same structure as the fuel rod shown in FIG.

[0021] As shown in FIG. 21, the fuel granules 1 of this vibrating filled fuel rod are usually set to have a high filling density.
By combining and filling the particles by optimizing the combination of plural kinds of particle diameters of No. 6 and mixing ratio, it is possible to make the average fuel density (smear density) in the cladding tube approximately equal to that of pellet fuel. The nuclear properties are equivalent to those of conventional pellet fuel. Similarly, as shown in FIG. 22, by adding gadolinia fine particles 22 and mixing them uniformly, characteristics equivalent to those of a conventional pellet fuel containing gadolinia can be obtained. Therefore, for example, in the conventional pellet fuel assembly shown in FIG. 14, even when the pellet fuel rods are simply replaced with vibration-filled fuel rods, the fuel assembly nuclear characteristics are almost the same as those of the pellet fuel assemblies, and so-called back-fitting is achieved. It is possible to use the fuel assembly without changing the dimensions of the fuel assembly. Moreover, the manufacturing cost can be significantly reduced.

However, even with a vibratingly filled fuel rod,
For fuel assemblies with high burnup such as high enrichment and high enrichment, similar to the pellet fuel design already described,
It is difficult to increase the number of fuel rods mixed with burnable poisons to suppress excess reactivity, and manufacturing cost by increasing enrichment or plutonium enrichment type to flatten local power distribution The problem of rising exists.

[0022]

SUMMARY OF THE INVENTION In view of the above-mentioned problems in the prior art, the present invention provides a fuel assembly having a high burnup such as a high enrichment or a high enrichment, which contains a burnable poison. To provide a fuel assembly for a light water reactor capable of suppressing the excess reactivity by increasing the number of fuel rods and also flattening the local power distribution without increasing the enrichment or plutonium enrichment type. With the goal.

[0023]

A fuel assembly for a light water reactor according to the present invention is a fuel assembly in which a plurality of fuel rods are arranged in a grid, wherein a part or all of the fuel rods are formed in granular form in a cladding tube. A fuel rod containing a mixture of a nuclear fuel material and a burnable poison granule, wherein the fuel rod mixed and filled with the burnable poison granule selects the particle size and the mixing amount of the burnable poison granule to form the burnable fuel rod. By controlling the neutron absorption amount and neutron absorption period per one toxic poison granule-mixed fuel rod, and making the majority of the fuel rods in the fuel assembly the burnable poison granule-mixed fuel rod, the fuel assembly In the equilibrium cycle core loaded with, the excess reactivity is flattened and the local power distribution is made uniform.

Further, in the present invention, the burnable poison granules of the fuel rod mixed and filled with the burnable poison granules are characterized by being substantially spherical and having an average particle diameter of 0.5 mm or more.

Further, in the present invention, the number of burnable poisons contained in the burnable poison-mixed fuel rod is such that the total surface area of the burnable poison granules does not exceed the fuel part surface area of the fuel rod. Features.

In the fuel assembly of the present invention, the neutron absorption per fuel rod is reduced by increasing the particle size of the burnable poison granules mixed and filled in the fuel rod and reducing the mixing amount. Instead, a majority of the fuel rods in the fuel assembly are fuel rods containing burnable poisons. In this way, the neutron absorption by the burnable poison granules can be widely distributed in the fuel assembly without being concentrated locally.

Further, by doing so, in the fuel assembly having a high enrichment or a high enrichment and a high burnup, an appropriate excess reactivity can be suppressed, and the local power distribution can be flattened. .

If the burnable poison granules are, for example, substantially spherical and have an average particle size of 0.5 mm or more, neutrons can be absorbed from the beginning to the end of the cycle to suppress the excess reactivity.

The neutron absorption amount per fuel rod can be reduced by making the total surface area of the burnable poison granules contained in the fuel rod not to exceed the fuel part surface area of the fuel rod. it can.

The fuel-filled fuel rod of the present invention may contain, in addition to the above-mentioned components, other components such as granules of a non-fuel substance having a small neutron absorption cross-sectional area. .

The fuel assembly for a light water reactor according to the present invention is characterized in that the granular nuclear fuel material is at least one of granules of uranium dioxide and granules of a mixed oxide of uranium and plutonium.

In the present invention, the effects of the present invention can be obtained by using at least one of granules of uranium dioxide and granules of a mixed oxide of uranium and plutonium as a nuclear fuel. It is possible to cope with high burnup such as high burnup.

In the fuel assembly for a light water reactor according to the present invention, at least one burnable poison is selected from gadolinium granules and gadolinium atom-containing oxide ceramic granules.
It is characterized by being a kind. Further, the present invention is characterized in that the oxide ceramic is uranium dioxide. Further, the present invention is characterized in that the oxide ceramic is at least one selected from zirconia and alumina. In the present invention, by using gadolinia granules or gadolinium atom-containing oxide ceramic granules as a burnable poison,
Surplus reactions can be controlled by neutron absorption of gadolinium atoms.

When oxide solids containing gadolinium atoms, for example, solid solution granules of gadolinia and uranium dioxide as a solid solution of gadolinia and ceramics are used as burnable poisons, the specific gravity of the granules is almost equal to that of MOX fuel granules. It is possible to easily distribute the fuel uniformly in the fuel rods.

By using zirconia or alumina as the ceramic, stable physical properties without self-heating can be used, and the excess reactivity can be appropriately controlled.

The fuel assembly for a light water reactor according to the present invention is characterized in that the burnable poison is at least one selected from gadolinium granules and gadolinium-containing metal alloy granules having a small neutron absorption cross-sectional area. . Further, the present invention is characterized in that the metal containing gadolinium and having a small neutron absorption cross-sectional area is zirconium.

According to the present invention, gadolinium metal and its alloy can be used as the burnable poison. By using an alloy with zirconium metal having a small neutron absorption cross section, there is no self-heating, stable physical properties can be used, and granules are easy to be produced, deformation is less likely to occur, and the excess reactivity is appropriately adjusted. Can control.

The fuel assembly for a light water reactor according to the present invention is characterized in that the number of burnable poison mixed fuel rods is 60% or more of the total number of fuel rods in the fuel assembly for a light water reactor.

According to the structure of the present invention, for example, in a fuel assembly having a large number of MOX fuel rods, a large number of fuel rods containing a mixture of burnable poisons are arranged to appropriately suppress the excess reactivity at the beginning of an operation cycle. Can be.

The fuel assembly for a light water reactor according to the present invention is characterized in that the fuel rods disposed at the corners of the fuel assembly are not mixed with burnable poisons. Further, the present invention is characterized in that the fuel rod disposed at the corner of the fuel assembly is a fuel rod in which uranium dioxide pellets are sealed.

According to the structure of the present invention, the fuel rods arranged at the corners of the fuel assembly, that is, at the places where the moderator density is high and the thermal neutron flux is particularly high, do not contain burnable poisons. In addition, the neutron absorption of each fuel rod containing a combustible poison can be substantially the same. If fuel rods containing uranium dioxide pellets are used as the fuel rods arranged at the corners, the types of MOX fuel rods with different enrichment used for the fuel assemblies can be reduced, and the production can be simplified. Thus, the manufacturing cost can be reduced.

Further, the fuel assembly for a light water reactor according to the present invention is characterized in that the fuel rod disposed on the water rod portion of the fuel assembly does not have a burnable poison.

According to this configuration of the present invention, the output at the center of the fuel assembly can be increased, and the local power distribution can be flattened.

Further, in the fuel assembly for a light water reactor according to the present invention, the fuel rod disposed at the corner of the fuel assembly has a burnable poison, and the content of the fissile material is such that the fuel assembly contains the fissile material. It is characterized by being smaller than the average value of the rates.

According to the configuration of the present invention, for example, even when the amount of plutonium loaded in the fuel assembly is increased to increase the fuel, the excess reactivity can be suppressed and the local power distribution can be flattened.

The fuel assembly for a light water reactor according to the present invention is characterized in that 70% or more of the total fissile material of the fuel assembly is fissile plutonium.

According to this configuration of the present invention, the loading of fissile plutonium can be increased, and the power generation of the pluthermal reactor can be promoted.

Further, the fuel assembly for a light water reactor according to the present invention is characterized in that the number of fuel rods arranged is 9 rows and 9 columns.

According to this configuration of the present invention, it is possible to construct a fuel assembly having a particularly high burnup and a high plutonium enrichment so that the operation of the present invention can be effectively functioned.

Next, the main operation of the present invention will be described.

In the fuel assembly according to the present invention, a fuel rod containing a mixture of burnable poisons filled with vibration is used, and the diameter of the burnable poison granules mixed and filled in the fuel rods is made large and sparse. The neutron absorption per fuel rod is reduced by reducing the total surface area.

Instead, the majority of the fuel rods in the fuel assembly are made up of fuel rods containing burnable poisons, so that the neutron absorption by the burnable poison granules does not concentrate locally on the fuel assembly. It allows for wide distribution in the aggregate.

By doing so, it becomes possible to appropriately suppress the excess reactivity and to flatten the local output distribution. Therefore, it is possible to cope with a fuel assembly having a high burnup such as a high enrichment or a high enrichment.

[0054]

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

FIG. 1 is a schematic cross sectional view showing a fuel rod mixed and filled with a burnable poison constituting a fuel assembly according to an embodiment of the present invention.

In the cladding tube 15 of the fuel rod shown in FIG. 1, burnable poison granules 23 having a large particle size as burnable poison are sparsely dispersed and mixed in the fuel granules 16.

In a fuel rod mixed and filled with a conventional burnable poison, gadolinia fine particles, which are a burnable poison,
The neutrons are uniformly dispersed in the fuel rod, so that neutrons are absorbed gadolinia on the surface of the fuel rod. On the other hand, in the fuel rod of the present invention in which the burnable poison granules shown in FIG. 1 are mixed and filled, neutrons penetrate into the fuel rods, reach the burnable poison granules, and are absorbed by the surface. Become.

In this embodiment, the outer diameter of the fuel section is 9.8 mm, and the burnable poison granules have a gadolinia of about 4 mm.
It is a solid solution of gadolinia and uranium dioxide depleted uranium containing 5% by weight, the granules are almost spherical, the average particle size is about 1 mm, and the number density in fuel rods is about 30 / cm ³ . With this configuration, the neutron absorption amount can be reduced by reducing the total surface area of the burnable poison granules per fuel rod to about 1/2 of the fuel part surface area of the fuel rod. By increasing the particle size of the burnable poison granules, it is possible to absorb neutrons and suppress excess reactivity from the beginning to the end of the cycle.

Here, gadolinia is added to the burnable poison granules for about 4 g.
By making the solid solution of gadolinia and uranium dioxide depleted uranium containing 5% by weight, the specific gravity of the burnable poisonous granules can be made almost the same as the specific gravity of the nuclear fuel particles. Therefore, the burnable poisonous particles are uniformly mixed and filled into the nuclear fuel particles. can do.

As the burnable poison granules, in addition to the above,
By using a solid solution with a non-nuclear fuel material such as zirconia (ZrO ₂ ) or alumina (Al ₂ O ₃ ), which does not generate heat due to nuclear fission, it can be used taking advantage of stable physical properties. In addition, by using the granules of gadolinia itself as the burnable poison granules, the excess reactivity can be suppressed with a smaller mixing amount. Further, as the burnable poison granules, granules of gadolinium metal or an alloy of gadolinium and zirconium, which can be easily produced and have little deformation, can be used.

Embodiment 1 FIG. 2 is an arrangement view of a fuel rod shown in a horizontal cross section at the center of a fuel assembly according to an embodiment of the present invention. The fuel rods _{U 1, U 2, M 1} G M which is indicated by the symbol 8 in FIG. 2 are arranged in the channel box 13 having a water rod 9. U ₁ and U
₂ is a fuel rod both vibration filling, fuel rods U ₁ and U ₂ is a different uranium fuel rods having enrichment, U ₁
Is more concentrated than U ₂ . The M ₁ is MOX fuel rods containing no gadolinia, is G _M is MOX fuel rods obtained by mixing gadolinia granules described in the embodiment of FIG. 1 as a burnable poison.

This fuel assembly is composed of four types of fuel rods. First, the fuel rods U ₂ is disposed at the position indicated by the reference numeral 25 in the fuel assembly corners, it is arranged fuel rods U ₁ in the fuel assembly side portion continuous thereto. The periphery of the water rod 9 also is in the position shown by reference numeral 26 sandwiched between two water rods arranged G _M, are arranged M ₁ in the remaining positions facing the water rods . G _M is arranged at all other positions.

In this manner, this fuel rod assembly has 46 G _M , 8 M ₁ , and U ₁ out of a total of 74 fuel rods.
But 16, and U ₂ is constituted by four. Therefore, 73% of all fuel rods are MOX fuel rods,
% Is the fuel rod mixed with gadolinia. Further, of the total amount of uranium and plutonium in this fuel assembly, the proportion of fissile material is 4.5% by weight, and the proportion of fissile plutonium is 71%.

The following effects are observed in this embodiment.

First, since the fuel rods are manufactured by vibration filling, the manufacturing is simplified. In particular, the production of MOX fuel rods including gadolinia has been greatly simplified.

Further, by using a fuel rod in which gadolinia granules having a large particle size and a small total surface area are mixed and dispersed, the neutron absorption amount per fuel rod is reduced, so that the flammability per fuel assembly is reduced. The number of poison-mixed fuel rods can be greatly increased, and as a result, appropriate excess reactivity can be suppressed, and the local power distribution is flattened.
In addition, uranium fuel rods that do not contain burnable poisons are located in the corners of the fuel assembly, where the moderator density is high and the thermal neutron flux is particularly high, and gadolinia mixed MOX fuel rods that are located in the rest Neutron absorption can be made almost the same, and as a result, the types of MOX fuel rods used can be reduced. In FIG. 14, which shows an example of a conventional MOX fuel assembly, four types of MOX fuels are used. In this embodiment, the number of MOX fuel rods can be reduced to half including two gadolinia mixed MOX fuel rods. Significant simplification has been obtained.

Further, at the position facing the water rod, M
By arranging OX fuel rods and arranging gadolinia mixed fuel rods at all other positions except for the corners of the fuel assemblies and the vicinity thereof, the local output at the center in the horizontal direction is relatively increased, and the local power distribution is increased. Is flattened. According to this nuclear fuel assembly arrangement, the plutonium utilization rate in the pluthermal can be increased by increasing the content of fissile plutonium in the MOX fuel. In this case, instead of enriched uranium as the base material for MOX fuel granules,
Depleted uranium or natural uranium can be preferably used.

The solid line shown in FIG. 3 represents the infinite multiplication factor combustion change of the fuel assembly of this embodiment. Further, the solid line shown in FIG. 4 is a combustion change of the excess reactivity of the equilibrium cycle core loaded with the fuel assembly. The broken lines in these figures show the values of the MOX fuel assembly of FIG.

In this embodiment, instead of increasing the number of gadolinia mixed fuel rods from the conventional 22 to 46, the fuel rods 1
Reduced the amount of gadolinia per book. As a result, the gadolinia content of the fuel assembly of this example is reduced by 5% as compared with the case of FIG. With this configuration, as shown in FIGS. 3 and 4, the excess reactivity in the early stage of combustion is reduced and optimized as compared with the conventional case, and the unburned residue of gadolinia at the end of the first cycle combustion is reduced. FIG.
As shown in the above, according to the present embodiment, the excess reactivity of the reactor core is flattened during the operation cycle, and the operation of the reactor is simplified. Further, FIG. 5 shows a combustion change of the local output peaking coefficient of the fuel rod assembly of the present embodiment. While the conventional local output peaking coefficient is around 1.3,
In this embodiment, it is about 1.2, and especially in the latter stage of combustion, it is 1.
It is about 1, which is much improved as compared with the related art, and the output distribution is flattened.

The diameter, concentration and number density of the burnable poison granules in the burnable poison-containing fuel rod shown in FIG. 2 are not limited to those in the above-described embodiment, and a combination thereof can be appropriately selected. When the granule diameter is reduced to 0.5 mm, the gadolinia concentration and the number density of the granules at which the neutron absorption effects become almost equal are 90% by weight and 12%, respectively.
Although the number is 5 / cm ³ , the same nuclear characteristics as in the above embodiment can be obtained. Therefore, the object of the present invention is achieved by appropriately selecting the burnable poison concentration and the number density by setting the diameter of the burnable poison granules to 0.5 mm or more.

Embodiment 2 FIG. 6 is a horizontal sectional view of a fuel assembly according to another embodiment of the present invention. In this embodiment, U ₁ and U ₂ disposed in the corner of the fuel assembly and the vicinity thereof in the first embodiment are replaced with M ₃ and M ₄ , respectively, in which gadolinia is not mixed.
With this configuration, it is possible to obtain substantially the same reactivity characteristics and operation effects as those of the first embodiment, and to obtain MOX in the fuel assembly.
The loading ratio can be increased.

Embodiment 3 FIG. 7 is a horizontal sectional view of a fuel assembly according to still another embodiment of the present invention. This embodiment is a case where the average enrichment of the fuel assembly is increased in the second embodiment. All the fuel rods are gadolinia mixed fuel rods, and the fuel rods are arranged in the corners and the vicinity thereof in the second embodiment. M ₃
And M ₄ are replaced by G _M2 and G _M3 , respectively, with a fissile material content lower than the average value of the fuel assemblies.

With this configuration, it is possible to obtain substantially the same reactivity characteristics and operation effects as those of the second embodiment, and it is possible to increase the MOX loading ratio in the fuel assembly.

Fourth Embodiment FIG. 8 is a horizontal sectional view of a fuel assembly according to a fourth embodiment of the present invention. This embodiment is a modification of the second embodiment, in which a square water rod is disposed at the center, and the number of fuel rods is seven.
Although the number is two, this configuration provides substantially the same reactivity characteristics and operation and effect as those of the second embodiment.

The fuel rod mixed with the burnable poison described in the above-mentioned embodiment is the same as the fuel rod mixed with the burnable poison granule in terms of the particle size, content and number density, and the burnable poison mixed in the fuel assembly. Is not limited to the above embodiment, and can be appropriately selected from the viewpoint of appropriately suppressing the excess reactivity, flattening the local power distribution, and minimizing the unburned residue of the burnable poison. .

In the above embodiments, the case where the fuel assembly for BWR and the MOX fuel are used has been described. However, the present invention can be applied to the fuel assembly for pressurized water type and the uranium fuel assembly. The present invention is not limited to the arrangement of the 9 × 9 type fuel assemblies described in the above-described embodiment.
It goes without saying that the present invention can be applied to a 0 × 10 type arrangement, for example, a 17 × 17 type pressurized water reactor fuel assembly.

[0077]

As described above, according to the present invention,
Combustible poison granules are mixed by increasing the particle size of the burnable poison granules and reducing the total surface area of the burnable poison granules in the fuel rods mixed and filled with the burnable poison granules by using the fuel rods that are vibration-filled in the fuel assembly. By reducing the amount of neutron absorption per fuel rod and increasing the number of burnable poison granule-mixed fuel rods used in the fuel assembly, the manufacturing process of the fuel assembly is simplified, and the initial operation cycle is further reduced. The excess reactivity can be appropriately controlled to eliminate unburned burnable poisons at the end of the operation cycle, and to flatten the power distribution inside the fuel assembly over the entire operation cycle to improve the thermal margin.

Thus, it is possible to cope with a fuel assembly having a high burnup such as a high enrichment or a high enrichment.
Reactor operation with higher fuel economy can be stably performed.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a fuel rod mixed and filled with a burnable poison constituting a fuel assembly according to an embodiment of the present invention.

FIG. 2 is an arrangement view of fuel rods shown in a horizontal cross-sectional view of a central portion of a fuel assembly according to one embodiment of the present invention.

FIG. 3 is a diagram showing an average burn-up change of an infinite image magnification of a fuel assembly according to an embodiment of the present invention.

FIG. 4 is a diagram showing a cycle incremental burnup change of an excess reactivity of an equilibrium cycle core loaded with a fuel assembly according to an embodiment of the present invention.

FIG. 5 is a diagram showing an average burn-up change of a local output peaking coefficient of a fuel assembly according to an embodiment of the present invention.

FIG. 6 is a plan view of a fuel rod according to another embodiment of the present invention, which is shown in a horizontal cross-sectional view at the center of a fuel assembly.

FIG. 7 is a plan view of a fuel rod according to another embodiment of the present invention, which is shown in a horizontal cross-sectional view of a central portion of a fuel assembly.

FIG. 8 is an arrangement view of fuel rods shown in a horizontal cross-sectional view of a central portion of a fuel assembly according to still another embodiment of the present invention.

FIG. 9 is a longitudinal sectional view showing a fuel rod filled with a conventional fuel pellet.

FIG. 10 is a longitudinal sectional view showing a BWR fuel assembly.

FIG. 11 is a sectional view taken along the line AA of FIG. 11;

FIG. 12 is a diagram schematically showing a combustion change at an infinite multiplication factor and a combustion change of a gadolinia reactivity of a fuel assembly when gadolinia is contained as a burnable poison and when gadolinia is not contained.

FIG. 13 is a diagram schematically showing a combustion change in excess reactivity of an equilibrium cycle core loaded with a fuel assembly.

FIG. 14 is a diagram showing an example of a fuel rod arrangement of a conventional MOX fuel assembly.

FIG. 15 is a diagram showing a combustion change of the fuel assembly of FIG. 14 at an infinite multiplication factor.

FIG. 16 is a diagram showing a combustion change in surplus reactivity of an equilibrium cycle core loaded with the fuel assembly of FIG.

FIG. 17 is a diagram showing a combustion change of a local output peaking coefficient of the fuel assembly of FIG. 14;

FIG. 18 is a diagram showing a manufacturing process of the pellet fuel rod shown in FIG.

FIG. 19 is a diagram showing a manufacturing process of a fuel rod of vibration filling.

FIG. 20 is a longitudinal sectional view showing a fuel rod of vibration filling.

FIG. 21 is a cross-sectional view showing a conventional fuel rod of vibration filling.

FIG. 22 is a cross-sectional view showing a fuel rod in which gadolinia fine particles are mixed and filled into fuel particles as a conventional burnable poison.

[Explanation of symbols]

8 ... fuel rod, 9 ... water rod, 13
…… Channel box, 15 …… Clad tube, 1
6: Nuclear fuel material granules, 23: Burnable poison granules,
25 ... fuel rod position at the corner of the fuel assembly, 26 ...
… Position of fuel rod between water rods, 41
.... Infinite multiplication factor of the fuel assembly according to the present invention, 42 ...
... surplus reactivity of the equilibrium cycle core loaded with the fuel assembly according to the present invention, 58 ... infinite multiplication factor of the conventional fuel assembly, 59 ... surplus reaction of the equilibrium cycle core loaded with the conventional fuel assembly Every time

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Jun Saeki 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture Inside the Toshiba Yokohama Works Co., Ltd. Street address Toshiba R & D Center

Claims (16)

[Claims] 1. A fuel assembly in which a plurality of fuel rods are arranged in a grid pattern, wherein a part or all of the fuel rods are mixed and filled with a granular nuclear fuel material and burnable poison granules in a cladding tube. A fuel rod mixed and filled with the burnable poison granules, the neutron absorption amount per one burnable poison granule mixed fuel rod by selecting the particle size and the mixing amount of the burnable poison granules. By controlling the neutron absorption period and the majority of the fuel rods in the fuel assembly as the burnable poison granule mixed fuel rods, the excess reactivity is flattened in the equilibrium cycle core loaded with the fuel assembly. A fuel assembly for a light water reactor, wherein the local power distribution is made uniform. 2. The fuel assembly for a light water reactor according to claim 1, wherein said burnable poison granules are substantially spherical and have an average particle size of 0.5 mm or more. 3. The number of burnable poison granules contained in the burnable poison-mixed fuel rod is such that the total surface area of the burnable poison granules does not exceed the fuel part surface area of the fuel rod. The fuel assembly for a light water reactor according to claim 1. 4. The fuel assembly for a light water reactor according to claim 1, wherein the granular nuclear fuel material is at least one of granules of uranium dioxide and granules of a mixed oxide of uranium and plutonium. 5. The fuel assembly for a light water reactor according to claim 1, wherein the burnable poison is at least one selected from gadolinia granules and gadolinium atom-containing oxide ceramic granules. 6. The fuel assembly for a light water reactor according to claim 5, wherein said oxide ceramic is uranium dioxide. 7. The fuel assembly for a light water reactor according to claim 5, wherein said oxide ceramic is at least one selected from zirconia and alumina. 8. The fuel for a light water reactor according to claim 1, wherein the burnable poison is at least one selected from gadolinium granules and gadolinium-containing granules of a metal alloy having a small neutron absorption cross-sectional area. Aggregation. 9. The light water reactor fuel assembly according to claim 8, wherein the gadolinium-containing metal having a small neutron absorption cross-sectional area is zirconium. 10. The fuel assembly for a light water reactor according to claim 1, wherein the number of said burnable poison mixed fuel rods is 60% or more of the total number of fuel rods in the fuel assembly for a light water reactor. 11. The fuel assembly for a light water reactor according to claim 1, wherein the fuel rods disposed at the corners of the fuel assembly are not mixed with burnable poisons. 12. The fuel assembly for a light water reactor according to claim 1, wherein the fuel rods disposed at the corners of the fuel assembly are fuel rods containing uranium dioxide pellets. 13. The fuel assembly for a light water reactor according to claim 1, wherein the fuel rods disposed on the water rod portion of the fuel assembly have no burnable poison. 14. A fuel rod disposed at a corner of the fuel assembly has a burnable poison, and the content of fissile material is smaller than the average value of the content of fissile material in the fuel assembly. The fuel assembly for a light water reactor according to claim 1, wherein: 15. The fuel assembly according to claim 1, wherein at least 70% of the total fissile material of said fuel assembly is fissile plutonium. 16. The fuel assembly for a light water reactor according to claim 1, wherein the number of fuel rods arranged is 9 rows and 9 columns.