JP2002357686A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an economical fuel assembly capable of ensuring proper operation tolerance and safe tolerance, facilitating a spectrum shift operation, and thus attaining the saving of uranium. SOLUTION: In this fuel assemble, when the fuel assembly is divided into an upper region U located on the upper side occupying about 30% of a heating effective length except the upper and lower ends and a lower region L located on the lower side, and the lower region L is further divided into a lower-side upper region L1 located on the upper side therein and a lower-side lower region L2 located on the lower side, a combustible poison-containing fuel rod 2 contains a combustible poison in the effective length region except the upper and lower ends, and it formed of a first combustible poison-containing fuel rod 2a containing the combustible poison of the highest concentration in the assembly in the lower-side lower region L2, and a second combustible poison-containing fuel rod 2b containing the combustible poison of the lowest concentration in the assembly not in the lower-side upper region L1 but in the lower-side lower region L2. The combustible poison concentration averaged excepting the combustible poison contained in the second combustible poison-containing fuel rod 2b is larger in the order of the upper region U, the lower-side upper region L1, and the lower-side lower region L2, and the average concentration of fissile material in the lower region L is larger than in the upper region U.

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 use in a boiling water reactor, and more particularly, to a replacement fuel assembly for high burnup.

[0002]

2. Description of the Related Art In a boiling water reactor, in order to improve economic efficiency, 1) a prolonged operation cycle aiming at an economic effect by improving a facility utilization rate (operating rate) of a plant, and 2) a fuel assembly It is known that an effective means of increasing the removal energy per one body is to increase the average removal burnup in order to improve the economic efficiency. For these two purposes, increasing the average enrichment of fissile material represented by uranium 235 and increasing the concentration of burnable poisons represented by gadolinia are effective and indispensable means. I have. At present, 9 × 9 grid fuel is known as one of the candidates.

FIG. 6 is an explanatory view showing a conventional example in which a water rod is arranged in a region corresponding to nine fuel rods in a fuel assembly having a 9 × 9 lattice. This conventional example uses a technique for increasing the enrichment under the limitation of the maximum enrichment of the pellet of 5 wt% or less.
Also, as a target of high burn-up, the average discharge burn-up of about 50G
The fuel assembly was aimed at Wd / t and had an average enrichment of 4 wt% or more.

[0004] The means of this technique is to use a fuel rod (type 2 in the figure) that contains the most enriched pellets in most of the lower region and that contains less enriched pellets in most of the upper region. The fuel rod is disposed at a fuel rod position adjacent to both the corner rod and the water gap, and further includes a fuel rod containing gadolinia-containing pellets (type G1 in the figure) in at least a part or most of the lower region adjacent to fuel rod type 2. It is located at a position that is not laterally adjacent to the water gap.

[0005] One of the aims of this prior art is to prevent the output peaking of the fuel rod type 2 from becoming too large in the early stage of combustion and from becoming too small in the middle stage of combustion in the lower region where the linear power density tends to be large. By locating the fuel rods, the maximum linear power density during operation is kept within the allowable range, and the linear output of gadolinia-containing fuel rods is relatively reduced as compared to the uranium fuel rods.
Even if a high concentration of gadolinia is added to a highly enriched nuclear fuel substance, it is an object of the present invention to ensure the thermomechanical integrity of a fuel rod containing gadolinia.

[0006] Further, in this prior art, another aim is to improve the limit output to the boiling transition. this is,
For type 2 fuel rods located at positions where the relative output of the fuel rods is likely to increase, the enrichment in the upper region is set to less than the maximum enrichment, thereby lowering the relative output of the fuel rods and suppressing a decrease in marginal output. It is to be.

[0007]

However, in the case of this prior art, the maximum linear power density during operation can be kept within an allowable range, but when focusing on the initial stage of combustion,
The maximum linear power density tends to have a smaller operating margin than in the middle stage of the cycle. This utilizes the fact that high-concentration gadolinia can be added by the conventional technique, and arranges pellets with high-concentration gadolinia added below the effective heat generating portion, thereby suppressing the reactivity of the lower fuel portion in the middle stage of combustion. On the other hand, sufficient measures have not been taken against the initial stage of combustion in which the maximum linear output density increases in the early stage of the cycle.

Further, it has been found that, when the enrichment difference is hardly provided in the axial direction, the spectrum shift operation hardly works effectively, and there is room for improvement in economic efficiency.

Here, the economical improvement by the spectrum shift operation is as follows. In a boiling water reactor, due to the generation of voids, the spectrum in the upper part of the reactor core is particularly hard and plutonium is liable to accumulate during power operation due to the generation of voids. Spectral shift operation is to increase the void fraction by lowering the core flow rate from the beginning of the cycle to the middle period, and by operating a few control rods deeply into the core so that the core power distribution becomes the lower peak. Promotes the accumulation of plutonium in the upper core,
On the other hand, at the end of the cycle, the operation method that lowers the void fraction by pulling out the control rods and increasing the core flow rate, and shifting the power distribution of the core to the upper side, to effectively use the accumulated plutonium and improve economic efficiency is there.

[0010] In order to make the spectrum shift operation more effective, it is desirable that the power distribution of the core be a lower peak from the early stage to the middle period of the cycle, so that the reactivity of the fuel assembly becomes higher on the lower side of the core. However, in the prior art described above, in particular, measures for reducing the maximum linear power density at the beginning of the cycle are not sufficient, and the lower peaking is increased by actively performing the spectrum shift operation. Accordingly, the maximum linear output density increases, which causes a problem that a sufficient operating margin cannot be secured.

According to the present invention, when the average enrichment of the fuel assembly is set to 4 wt% or more for the purpose of increasing the burnup and prolonging the operation cycle, even if the enrichment to be used has an upper limit, It is an object of the present invention to obtain a fuel assembly which can secure a sufficient operating margin and a safety margin, and can easily perform a spectrum shift operation, thereby achieving uranium saving and high economic efficiency. Further, it is another object of the present invention to obtain a fuel assembly having a further improved stop margin especially at the beginning of a cycle.

[0012]

According to the first aspect of the present invention, there is provided a fuel assembly for a boiling water reactor, wherein nuclear fuel material pellets having a maximum enrichment in a range of 4.9 to 5.0 wt% are used. The fuel rod group filled in the cladding tube is regularly arranged in a square lattice arrangement of 9 rows and 9 columns or more, and further includes a large-diameter water rod occupying a region corresponding to the plurality of fuel rods, and the fuel rod group is flammable. In a fuel assembly for a boiling water reactor having an average enrichment of fissile material of 4 wt% or more comprising a poison-free fuel rod 1 and a burnable poison-containing fuel rod 2, the upper and lower ends of the fuel assembly are removed. It is divided into an upper side area U located on the upper side occupying about 30% of the effective heat generation length and a lower side area L located on the lower side, and further, a lower upper area located on the upper side in the lower side area L. L1 and the lower lower region L2 located on the lower side. When divided, the burnable poison-bearing fuel rod 2 contains the burnable poison in the effective length region except the upper and lower ends and the burnable poison of the highest concentration in the assembly only in the lower lower region L2. A burnable poison-containing fuel rod 2a;
A second burnable poison-bearing fuel rod 2b that does not include the lowest concentration of burnable poison in the lower region-side upper region L1 and also includes the lowest concentration of burnable poison in the lower region L2. The average burnable poison concentration excluding the burnable poison contained in the burnable poison-containing fuel rod 2b is larger in the order of the upper region U, the lower upper region L1, and the lower lower region L2, and further, the lower side. The average enrichment of fissile material in region L is in upper region U
It is configured to be larger than that.

In the fuel assembly for a boiling water reactor according to the second aspect of the present invention, the second burnable poison-containing fuel rod 2b according to the first aspect has the lowest concentration in the assembly. The burnable poison is contained only in the lower side lower region L2.

According to a third aspect of the present invention, there is provided a fuel assembly for a boiling water reactor, wherein the second burnable poison-containing fuel rod 2b according to the first aspect has the lowest concentration in the assembly. The burnable poison is contained in at least a part of the upper region U and the lower lower region L2 of the upper region U.

According to a fourth aspect of the present invention, there is provided a fuel assembly for a boiling water reactor, the lower portion of the first burnable poison-containing fuel rod 2a according to any one of the first to third aspects. The maximum concentration of the burnable poison of L2 is 1.5 wt% or more higher than the concentration of the other burnable poison in the fuel assembly, and the lower lower region of the second burnable poison-containing fuel rod 2b. L2
Is lower than the concentration of other burnable poisons in the aggregate by 1.5% by weight or more, and the number ratio of the first burnable poison-containing fuel rods 2a is This is 2 to 6% of the total number of fuel rods inside.

According to a fifth aspect of the present invention, there is provided a fuel assembly for a boiling water reactor, wherein the second burnable poison-containing fuel rod 2b according to any one of the first to fourth aspects is arranged such that All of the burnable poison-bearing fuel rods 2 in the body are not adjacent to each other in the vertical or horizontal direction.

In the fuel assembly for a boiling water reactor according to the invention as set forth in claim 6, the lower lower region L2 according to any of claims 1 to 5, wherein the lower side lower region L2 is 30% or less of the effective heat generation length. There is something.

According to a seventh aspect of the present invention, there is provided a fuel assembly for a boiling water reactor according to the first aspect of the present invention. The cross-sectional average enrichment gradually increases from the upper part to the lower part.

In the fuel assembly for a boiling water reactor according to the invention described in claim 8, the large-diameter water rod according to any one of claims 1 to 7 occupies nine fuel rod regions. It is a square tubular water rod.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a burnable poison-bearing fuel rod 2 contains burnable poison in an effective length region excluding upper and lower ends, and burns the highest concentration of burnable poison in the lower part of the assembly. First burnable poison-bearing fuel rod 2 included only in region L2
a, and a second burnable poison-bearing fuel rod 2b that does not include the lowest concentration of burnable poison in the lower region side upper region L1 and that does not include the lowest concentration in the lower region L2 in the assembly. The average burnable poison concentration excluding the burnable poison contained in the second burnable poison-containing fuel rod 2b increases in the order of the upper region U, the lower upper region L1, and the lower lower region L2, and furthermore The lower region L is configured such that the average enrichment of fissile material is higher than that of the upper region U. For this reason, an appropriate operation margin and a safety margin can be ensured, and a spectrum shift operation is easily performed, so that a uranium saving can be achieved and a highly economical fuel assembly can be obtained.

In a preferred embodiment, the second burnable poison-bearing fuel rod 2b contains only the lowest concentration of burnable poison in the lower lower region L2 in the assembly, and the second burnable poison-containing fuel rod 2b has the lowest concentration of burnable poison in the assembly. It is proposed that the poison is contained in at least a part of the upper region U and the lower lower region L2 of the upper region U.

FIG. 1 is an explanatory view schematically showing a configuration which is a basic part of the present invention. FIG. 3A shows a first burnable poison-containing fuel rod 2a and a second burnable poison-containing fuel rod 2b ₁ and 2b.
₂ is shown, and FIG. B shows the configuration of the fuel assembly. As the burnable poison, gadolinia is typical, and therefore, the description will be made below using gadolinia. That is, the first
The gadolinia-containing fuel rod 2a includes gadolinia except for the upper and lower ends, and also includes gadolinia having the highest concentration in the assembly only in the lower lower region L2 corresponding to the lowermost portion. The second gadolinia containing fuel rods 2b includes gadolinia only the lower-side lower region L2, its concentration and the fuel rods 2b ₁ is the lowest in the aggregate, a portion of the top of the upper region U wherein on the region and the lower-side lower region L2, its concentration is the fuel rod 2b ₂ which is the lowest in the aggregate.

As shown in FIG. 2B, the distribution of the average gadolinia concentration in the fuel assembly of the present invention increases from the upper part to the lower part. As a result, the output distribution of the replacement new fuel assembly in the middle stage of the cycle can be prevented from being excessively lower peaking. In order to maintain the suppression effect during the cycle period, the lower lower region L2 needs to include the highest concentration of gadolinia and maximize the average concentration. At this time, in calculating the average concentration of gadolinia in the lower side lower region L2, the lowest concentration gadolinia included in the second gadolinia-containing fuel rod 2b is not considered due to its nature. This is because the second gadolinia-containing fuel rod 2b having the lowest concentration of gadolinia is effective for adjusting the power distribution mainly at the initial stage of the cycle, but does not contribute to the suppression of the reactivity after the middle stage of the cycle, as described later. is there.

The number of gadolinia-containing fuel rods is increased in the lower lower region L2. This is performed by the second gadolinia-containing fuel rod 2b including the lowest concentration of the burnable poison in the assembly at least in the lower side lower region L2. With this configuration, it is possible to suppress an increase in the lower output peaking of the replacement new fuel assembly at the beginning of the cycle. Even if the concentration of gadolinia is increased in the lower region, the effect of suppressing the reactivity is small at the beginning of combustion. On the other hand, when the number of gadolinia-containing fuel rods is increased in the lower region, the reactivity suppression effect can be sufficiently expected at the beginning of combustion. Moreover, in this configuration, the lower number of gadolinia is the lowest concentration gadolinia,
The toxic effect disappears quickly and acts only at the beginning of the cycle, but does not contribute to the suppression of reactivity in the middle of the cycle.

According to the present invention, the second cycle is mainly used at the beginning of the cycle.
The presence of the gadolinia-containing fuel rods 2b and the first gadolinia-containing fuel rods 2a after the middle of the cycle suppress the increase in the lower output peaking, particularly for new fuel assemblies whose reactivity is controlled by gadolinia. This has the effect of reducing the maximum linear output density throughout the cycle period. That is, the first effect of the present invention is a reduction in the maximum linear output density. Note that the fuel assemblies in the second and subsequent loading cycles in which the gadolinia poisoning action has disappeared usually have a sufficiently reduced relative output of fuel rods, so that the linear output density does not become severe.

Further, in addition to the above-mentioned gadolinia distribution, the enrichment distribution of the fuel assembly of the present invention is reduced with respect to the upper region U located at the upper side, while being reduced at the lower region. In the lower region L, the degree of concentration is increased. This has the following second and third effects.

Next, the second effect is that the concentration is reduced in a region where the output has an upper peak at the time of cold temperature.
That is, the reactivity in this region is reduced, and the stop margin can be increased. This has the effect of improving the stop margin over the cycle period. Furthermore, improvement in shutdown margin becomes remarkable by using the second gadolinia containing fuel rods 2b ₂ containing burnable poison the lowest concentration in the aggregate in the upper side region U.

By the way, in general, new fuel has a strong gadolinia poisoning action, so that it has a role of increasing the stop margin at a cold temperature when compared with the second cycle fuel. If the burnup of fuel is increased, the number of fuel replacement bodies tends to decrease, so that it becomes difficult to secure a stop margin at the beginning of the cycle. In view of such a tendency, in the present invention, the second gadolinia-containing fuel rod 2b ₂ containing the lowest concentration of burnable poison in the aggregate in at least a part of the upper region U and the lower lower region L2 is included. Is used to further improve the stop margin, especially at the beginning of the cycle.

That is, in the fuel assembly of the present invention, the number of gadolinia-containing fuel rods is increased in the upper region by the _second gadolinia-containing fuel rods 2b2. For this reason, the upper peak at the time of cold temperature is reduced, the reactivity of the core is effectively suppressed, and the target stop margin in the first half of the cycle is improved. Since the second gadolinia containing fuel rods 2b ₂ is a very low concentration, no burning remaining upper region gadolinia in the cycle end, there is no fear that impairs the economics.

Finally, the third effect is that, as a result of increasing the enrichment on the lower side, the power distribution of the fuel assembly in the core tends to be lower peaking during the power operation, and the spectrum shift operation is more effectively performed. Work and economics are to be improved. Generally, in the replacement core, fuel assemblies having different burnups are mixed in the furnace. However, since the fuel assembly of the present invention has a high enrichment in the lower region, in particular, the replacement core in which the poisoning effect of gadolinia has disappeared. It is easy to maintain the state of peaking of the lower output for fuel assemblies other than the fuel assembly. Of course, for new fuel assemblies with a higher maximum linear power density,
, The lower power peaking is particularly effectively suppressed, but usually in the core aiming at high burnup,
Since the ratio of the replacement new fuel assembly is about 1/4 of the entire core, even when the lower power peaking of the new fuel assembly is suppressed as in the present invention, many other fuel assemblies are used. The spectrum shift operation works effectively on the body, and can bring about a sufficient improvement in economic efficiency.

In the upper region, the poisoning effect of gadolinia is hard to be eliminated because the spectrum is hard during the output operation. However, the fuel assembly of this configuration has a gadolinia in the upper region corresponding to the improvement of the stop margin by the second effect. The concentration can also be reduced, resulting in less gadolinia unburned at the end of the cycle, which also contributes to improved economics.

In addition, by reducing the unburned gadolinia, the excess reactivity from the middle to the end of the cycle is increased, albeit slightly. As a result, even if the flow rate is reduced or the control rod is inserted deeper. Since the criticality can be maintained, the spectrum shift operation is facilitated, and the economy is further improved.

In order to further enhance the above first to third effects, the present invention further proposes the following configuration.

That is, in the present invention, the maximum concentration of the burnable poison in the lower lower region L2 of the first burnable poison-containing fuel rod 2a is higher than the concentration of the other burnable poison in the fuel assembly. The minimum concentration of the burnable poison in the lower lower region L2 of the second burnable poison-containing fuel rod 2b is 1.5 wt% or more than the concentration of the other burnable poison in the assembly.
%, And the number ratio of the first burnable poison-bearing fuel rods 2a is 2 to 6% of the total number of fuel rods in the assembly.

That is, the gadolinia contained in the first burnable poison-bearing fuel rod 2a has the highest concentration in order to adjust the output distribution in the middle stage of the cycle, but is at least 1.5 wt% higher than the concentration of other burnable poisons. Preferably, by increasing the amount by 2% by weight or more, even when a long-term operation such as continuous operation for 18 months is performed, the output distribution can be adjusted extremely appropriately from the middle to the end of the cycle, and particularly, the maximum linear power density in the middle of the cycle can be appropriately adjusted. Value can be maintained.

Further, the lowest concentration of gadolinia contained in the second burnable poison-bearing fuel rod 2b contributes to a reduction in the linear power density at the beginning of the cycle, and particularly, for a new replacement fuel having a high linear power density. It is configured to work. In addition, in order to perform the spectrum shift operation over a longer period, it is preferable that the poisoning effect of the second burnable poison-containing fuel rod 2b is effective only in the first half of the operation cycle. Therefore, the second burnable poison-containing fuel rod 2
It is desirable that the gadolinia concentration contained in b be a low concentration, preferably 1.5 wt% or less. Also, the number of the second burnable poison-bearing fuel rods 2b may be any number as long as the linear output density of the new fuel assembly in the early stage of the cycle is sufficiently reduced. Desirably, it is about 2 to 6% of the rod.

In another embodiment of the present invention, the second burnable poison-bearing fuel rods 2b are arranged so as not to be adjacent to all the burnable poison-bearing fuel rods 2 in the fuel assembly either vertically or horizontally. .

That is, if the gadolinia-containing fuel rods are adjacent to each other, the neutron spectrum in the vicinity becomes hard and the elimination of the poisoning action is delayed. It is better not to be adjacent to the rod.

Further, in another embodiment of the present invention, the lower side lower region L2
About 30% or less of the effective heat generation length. That is, the lower side lower region L2 is a region for optimizing the linear output density during operation by the first burnable poison-containing fuel rod 2a and the second burnable poison-containing fuel rod 2b. Normally, the lower output increases when the spectrum shift operation is performed. Therefore, it is sufficient to deal with the maximum linear output density only on the lower side where this becomes severer. May not be expected. Therefore, it is preferable to set the length of the lower side lower region L2 to 30% or less of the effective fuel heat generation length where the linear output density is likely to be increased by the spectrum shift operation.

In another aspect of the present invention, the average cross-sectional enrichment of the nuclear fuel material in the heat generating effective portion excluding the upper and lower ends of the fuel assembly is increased stepwise from the upper part to the lower part. That is, in the present invention, the maximum linear power density can be optimized by the first and second burnable poison-containing fuel rods 2a, 2b, so that the average enrichment of the fissile material in the fuel assembly except for the upper and lower ends is reduced. It is also possible to increase the height stepwise from the upper part to the lower part in the axial direction. In this case, the lower output can be further increased, and the economics of the spectrum shift operation are further improved.

Further, in another aspect of the present invention, the large-diameter water rod is
One rectangular tubular water rod occupying nine fuel rod areas. That is, the large-diameter water rod used in the fuel assembly of the present invention has a size corresponding to nine large-diameter water rods so that the non-boiling water region during operation can be sufficiently secured without excessively increasing the pressure loss of the fuel assembly. It is desirable to have a single rectangular tubular water rod occupying the fuel rod area. Since the water rod of this configuration has a large non-boiling water area, it has the effect of flattening the relative power distribution of the fuel rods in the cross section of the fuel assembly, reducing the local peaking coefficient, and consequently, the linear power density during operation. It is suitable for spectrum shift operation because it is easy to realize the reduction of noise.

[0042]

FIG. 2 is an explanatory view showing the structure of one embodiment of the fuel assembly of the present invention. Note that FIG. 2 shows an example in which the present invention is applied to the conventional example having the 9 × 9 lattice arrangement shown in FIG. 6 described above. There is no difference in the structure of the body.

In this embodiment, the upper region U is at the node 18/24 to 23/24 excluding the natural uranium at the upper end, and the lower region L is 2/2 excluding the natural uranium at the lower end.
4th to 17 / 24th node. The length of the upper region U occupies 6/24 of the effective heat generation length, that is, approximately 30% of the region. The lower region L is divided into two by the boundary between the seventh node and the eighth node, and the length of the lower lower region L2 is set to 6/24 of the effective heat generation length, that is, 30% or less.

The burnable poison-free fuel rod 1 in this embodiment four, burnable poison-containing fuel rods 2 types 1, 2, 3 and 4 were configured with type G1, G2 and G3 ₁ three. The type G2 corresponding to the first gadolinia-containing fuel rod 2a contains 9.0 wt% gadolinia in the lower lower region L2, the concentration of which is the highest in the assembly, and the next highest gadolinia concentration of 7.0 wt%. 2.0w than
t% higher, that is, a difference of 1.5 wt% or more was provided. Further, the second gadolinia containing fuel rods 2b ₁ falls type G3 ₁ containing gadolinia only the lower-side lower region L2 of the second gadolinia containing fuel rods 2b comprises a gadolinia only the lower-side lower region L2, the The concentration is 1.5wt%
Is the lowest in the aggregate and the next lowest 3.5 w
The difference was provided by 2.0 wt% lower than the gadolinia concentration of t%, that is, 1.5 wt% or more. The second number of the gadolinia containing fuel rods 2b ₁ has two, i.e. percentage of the total fuel rods is about 2.8%, was in the range of 2 to 6%.

In the axial direction of the fuel assembly, first, the average enrichment is larger in the lower region L than in the upper region U. Here, the average enrichment of the upper area U occupying the area for 6/24 nodes is 4.67 wt%, while
The average enrichment of the lower side region L occupying the area for the / 24 node is 4.76 wt%, which is a difference of 0.09 wt% which is larger than the enrichment difference 0.06 wt% of the conventional example shown in FIG. . In other words, the enrichment of the lower region is increased so that the lower output is easily generated. Further, in this embodiment, the upper side area U
The enrichment difference is also provided in the axial direction also in the lower region L and the average enrichment is increased stepwise from the upper part to the lower part of the fuel assembly. The above was carried out by providing an axial enrichment difference for the burnable poison-free fuel rod 1 containing no gadolinia.

Next, the second gadolinia-containing fuel rod 2b ₁
In the axial distribution of gadolinia density except for, the average density increases in the order of the upper region U, the lower upper region L1, and the lower lower region L2. This configuration is the same as that of the conventional example shown in FIG. 6, but in this embodiment, by including 3.5 wt% of gadolinia in the upper region U located at the top, the average gadolinia concentration in this region is reduced. It was lower than the conventional example. Such a configuration can be achieved by the second effect described above.

FIG. 3 shows the effect of this embodiment in comparison with the conventional example of FIG. FIG. 3 is an explanatory diagram for explaining an example of analysis of the equilibrium core design of the embodiment shown in FIG. 2 for 18 months of operation and an average extraction burnup of 50 GWd / t, wherein FIG. 3A is the maximum linear power density, and FIG. The stop margin and the graph c show the evaluation results of the core average void ratio. In the figure, the solid line shows the analysis result of this embodiment, and the broken line shows the analysis result of the conventional example.

The core flow rate in the core design in this embodiment was lower than that of the conventional example from the beginning to the middle of the cycle. When the spectrum shift operation is performed more positively in this manner, generally, the lower output increases and the maximum linear output density also increases in accordance with the increase, but the fuel assembly of the present embodiment is loaded. In the core,
Due to the first effect described above, the increase in the maximum linear output density is suppressed as compared with the conventional example, and the maximum value is reduced as compared with the conventional example, and the operation limit value can be sufficiently satisfied.

Further, the stop margin sufficiently satisfies the design target similarly to the conventional example due to the second effect. In this embodiment, the gadolinia concentration in the upper portion is lower than that in the conventional example, and in particular, the stop margin from the middle stage to the end of the cycle tends to be smaller than that in the conventional example.
On the other hand, in the present invention. The effect of improving the stop margin throughout the cycle period by lowering the concentration of the upper portion compensated for this, and as a result, the minimum value of the stop margin throughout the cycle period could be made larger than in the conventional example.

Further, as a result of the active spectrum shift operation, the core average void ratio can be increased from the initial stage to the end of the cycle as compared with the conventional example, so that the accumulation of plutonium is promoted. At the end, more plutonium accumulated in the upper part can be burned, and a greater reactivity gain can be obtained. As a result, the average enrichment of the fuel assembly according to the present embodiment is about 0.4% higher than that of the conventional example shown in FIG.
The amount can be reduced by 4.40 wt%, which is smaller by 04 wt%, and an improvement in fuel economy and a reduction in uranium can be achieved. The above is due to the third effect.

FIG. 4 is an explanatory view showing the structure of another embodiment of the fuel assembly of the present invention. Note that for Examples the figure 4 with a 9 × 9 matrix arrangement shown in FIG. 2 described above, at least one of the gadolinia fuel rod upper region U that corresponds to the second gadolinia containing fuel rods 2b ₁ Second gadolinia-containing fuel rod 2 included in a part of upper region and lower side lower region L2
It is modified to b _2. The other configuration is completely the same as that of FIG. 2, and a description related to this portion will be omitted.

FIG. 5 shows the effect of the embodiment shown in FIG. 4 in comparison with the conventional example shown in FIG. In FIG. 5, the analysis method and notation are the same as those shown in FIG. In the core loaded with the fuel assembly of the present embodiment, although the improvement effect is slightly smaller than that of the previous embodiment, the maximum value of the maximum selection power density is reduced by the first effect as compared with the conventional example. The limit value was sufficiently satisfied. In addition, the first effect allows the minimum value of the stop margin throughout the cycle period to be larger than that of the conventional example. In particular, for the first half of the cycle, shutdown margin by reactivity suppressing effect of the upper by the fuel rods G3 ₂ it could be further improved over the previous prior art. Further, the same effects as in the previous embodiment can be obtained for the improvement of economic efficiency and uranium saving by the third effect.

The above two embodiments are for the purpose of comparison with the conventional example, and 9 × 9 as a fuel assembly suitable for high burnup.
A fuel assembly having one square tubular large diameter water rod occupying nine fuel rod regions in a lattice arrangement is shown as an example. However, the fuel assembly of the present invention may be a fuel assembly having a 10 × 10 lattice arrangement, and the large-diameter water rod may be not only a square tube but also a round tube. Further, even in the case of a fuel assembly having a so-called partial-length fuel rod whose effective heat generation length is shorter than other fuel rods, the function and effect of the present invention are not lost.

[0054]

As described above, the present invention increases the average enrichment of a fuel assembly for the purpose of increasing the burnup and prolonging the operation cycle, even if the enrichment used has an upper limit. In addition, it is possible to provide a fuel assembly for a boiling water reactor that can secure appropriate operation margin and safety margin, and can easily perform spectrum shift operation, thereby achieving uranium savings and high economic efficiency. Has the effect of contributing significantly to the development of

[Brief description of the drawings]

FIG. 1 is an explanatory diagram schematically illustrating a configuration that is a basic part of the present invention. Fig. a shows the configuration of the first burnable poison-containing fuel rod 2a and the second burnable poison-containing fuel rod 2b, and Fig. b shows the configuration of the fuel assembly.

FIG. 2 is an explanatory view showing an embodiment in which water rods are arranged in a region corresponding to nine fuel rods in a fuel assembly having a 9 × 9 lattice.

FIG. 3 is an explanatory diagram for explaining an example of analysis of an equilibrium core design for the embodiment shown in FIG. 2 for 18 months of operation and an average extraction burnup of 50 GWd / t, wherein FIG. 3A is the maximum linear power density, and FIG. Shows the margin for stopping, and FIG. C shows the evaluation result of the average core void ratio.

FIG. 4 is an explanatory view showing another embodiment in which water rods are arranged in a region corresponding to nine fuel rods in a fuel assembly having a 9 × 9 lattice.

FIG. 5 is an explanatory view illustrating an analysis example of another equilibrium core design aiming at an average removal burnup of 50 GWd / t for 18 months of operation of the embodiment shown in FIG. 4;
Fig. b shows the margin for stopping, and Fig. c shows the evaluation result of the average core void ratio.

FIG. 6 is an explanatory view showing a conventional example in which water rods are arranged in a region corresponding to nine fuel rods in a fuel assembly having a 9 × 9 lattice.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference)

Claims (8)

[Claims] 1. A fuel rod group filled with nuclear fuel material pellets having a maximum enrichment in the range of 4.9 to 5.0 wt% in a cladding tube is regularly arranged in a square lattice arrangement of 9 rows and 9 columns or more. A large-diameter water rod occupying an area equivalent to the plurality of fuel rods, wherein the fuel rod group comprises a fuel rod 1 containing no burnable poison and a fuel rod 2 containing burnable poison, and has an average fissile substance enrichment of 4 wt. % Or more of the fuel assembly for a boiling water reactor, wherein the fuel assembly is divided into an upper region U located on the upper side occupying approximately 30% of an effective heating length excluding upper and lower ends and a lower region located on the lower side. When the fuel rod is divided into a lower region L1 located on the upper side in the lower region L and a lower region L2 located on the lower side, the burnable poison-containing fuel rod 2 is burnable poison in the effective length area except upper and lower ends A first burnable poison-bearing fuel rod 2a that contains only the burnable poison of the highest concentration in the lower region L2 and the lowest concentration of burnable poison in the lower region of the assembly. A second burnable poison-containing fuel rod 2b that is not included in the upper side area L1 and is also included in the lower side lower area L2, and further includes a burnable poison that is included in the second burnable poison-containing fuel rod 2b. The burnable poison concentration averaged by exclusion is larger in the order of the upper region U, the lower upper region L1, and the lower lower region L2, and the average enrichment of the fissile material in the lower region L is the upper region. A fuel assembly for a boiling water reactor, wherein the fuel assembly is configured to be larger than U. 2. The second burnable poison-containing fuel rod 2b.
2. The fuel assembly for a boiling water reactor according to claim 1, wherein the fuel assembly contains the lowest concentration of burnable poison in only the lower lower region L <b> 2 in the assembly. 3. 3. The second burnable poison-containing fuel rod 2b.
Reduces the burnable poison having the lowest concentration in the assembly to at least a part of the upper region U and the lower lower region L of the upper region U.
2. The fuel assembly for a boiling water reactor according to claim 1, wherein the fuel assembly includes: 4. The maximum concentration of the burnable poison in the lower lower region L2 of the first burnable poison-containing fuel rod 2a is 1.5 wt% higher than the concentration of the other burnable poison in the fuel assembly. The minimum concentration of the burnable poison in the lower lower region L2 of the second burnable poison-containing fuel rod 2b is 1.5 wt% or more higher than the concentration of the other burnable poison in the assembly. 4. The fuel rod of claim 1, wherein the number of the first burnable poison-bearing fuel rods 2a is 2 to 6% of the total number of fuel rods in the assembly. Fuel assembly for boiling water reactors. 5. The second burnable poison-bearing fuel rod 2b.
The fuel for a boiling water reactor according to any one of claims 1 to 4, wherein all of the burnable poison-containing fuel rods 2 in the fuel assembly are not adjacent to each other in a vertical or horizontal direction. Aggregation. 6. The fuel assembly for a boiling water reactor according to claim 1, wherein the lower lower region L2 is 30% or less of an effective heat generation length. 7. The method according to claim 1, wherein the average cross-sectional enrichment of the nuclear fuel material in the heat generating effective portion excluding the upper and lower ends of the fuel assembly increases stepwise from the upper part to the lower part. A fuel assembly for a boiling water reactor according to any one of the above items. 8. The boiling water type according to claim 1, wherein said large-diameter water rod is a single rectangular tubular water rod occupying nine fuel rod regions. Reactor fuel assemblies.