JPH07128473A - Core of reactor - Google Patents

Abstract

PURPOSE:To increase core average enrichment, in addition, enlarge taking-out burnup of an initial charge fuel assembly, particularly, a minimum enrichment fuel assembly and improve fuel economy. CONSTITUTION:A minimum enrichment fuel assembly is disposed at a core outermost circumference or for radial output distribution leveling and a middle enrichment fuel assembly 2C is arranged in a control cell. The middle enrichment fuel assembly 2C is arranged more than the usual middle enrichment fuel assembly in number by two Gd rods or more and the Gd rods are arranged inclinedly and concentratedly on the side of W/W. Thereby, core average enrichment can be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a boiling water reactor (hereinafter,
BWR) core, and more particularly to a core of a nuclear reactor capable of increasing the take-out burnup and improving fuel economy.

[0002]

2. Description of the Related Art As an initially loaded core of a BWR, a plurality of types of fuel assemblies having different average enrichments are loaded to improve the take-out burnup of the initially loaded core. In such a core, the fuel assembly whose reactivity has been lowered is replaced with a new fuel assembly each time the operation cycle is updated, and the operation is continued, whereby the transition to the equilibrium cycle can be performed quickly.

Here, the operation by the initially loaded core is called the first cycle, but while partially exchanging the fuel assembly as described above, the operation cycle is repeated from the second cycle to the third cycle. The equilibrium cycle is a cycle in which the fuel components of the entire core become almost constant between adjacent cycles after several cycles of operation.

When this equilibrium cycle is reached, the thermal characteristics of the adjacent cycle (maximum line power density, minimum limit power ratio (MCPR), power peaking, etc.), the number of fuel assemblies to be replaced after the end of the cycle, and the fuel in the core The aggregate loading arrangement and control rod pattern during cycle operation are almost equal and stable.

In a nuclear reactor having a core as described above,
The reactor is shut down after each cycle of operation, the fuel assembly with the lowest reactivity is replaced with a new one, and the next operation cycle starts. The reactor operation is continued while repeating this, but if the thermal characteristics of each cycle are poor or the target burnup is not achieved, the integrity of the fuel assembly, the reactor core And the economical efficiency of the fuel assembly.

From the viewpoint of the integrity of the fuel assembly, the economical efficiency of the reactor core and the fuel assembly, the intermediate cycle of the process of transition from the first cycle to the equilibrium cycle, in other words, the thermal characteristics and cycle in the transition cycle. The acquired burnup is
It should be comparable to those of the equilibrium cycle, or converge rapidly towards them.

In such a nuclear reactor, as a prior art of BWR having excellent fuel economy, there is little variation in the thermal characteristics and the acquired burnup during the transition cycle in the process of transitioning from the first cycle to the equilibrium cycle and the fuel economy is excellent. Japanese Patent Fair 3-45358
It is disclosed in the publication.

According to this publication, when the replacement fuel assembly stays in the equilibrium core for N cycles, N types of initially loaded fuel assemblies having different equilibrium enrichments are loaded in the initially loaded core, and each initially loaded fuel assembly is loaded. The average enrichment of each initially loaded fuel assembly is adjusted so that the infinite multiplication factor of the fuel assembly when it does not contain burnable poisons is almost equal to the infinite multiplication factor of the replacement fuel assembly with different in-core stay cycles in the equilibrium core. It is set. The average enrichment of each initially loaded fuel assembly allowed a variation range of ± 0.2 wt% above and below the value obtained by the above setting.

[0009] By the way, the take-out burnup of an initially loaded core using a plurality of types of enrichment is a method of increasing the core average enrichment. Even if the core average enrichment is constant, Studies have shown that the method of increasing the dispersion parameter can also increase.

[0010]

[Equation 1] n _f : number of i-type fuel assemblies e _f : enrichment of i-type fuel assemblies

Further, in order to increase the take-out burnup and improve the fuel economy, the enrichment of the replacement fuel assembly is increased, and the number of batches of the fuel assembly in the equilibrium cycle is about 3 batches in the past. Is over 4 batches. An example of such a high burnup fuel assembly is shown in FIG.

In the fuel assembly 1, the long fuel rods 2, the short fuel rods 3, and the large diameter water rods 6 are arranged in 9 rows 9 by spacers 8.
A bundle of fuel rods is formed by bundling them in a square lattice in a row and fixing them to the upper tie plate 4 and the lower tie plate 5, and the fuel rod bundle is surrounded by a channel box 7. 21 (A) shows a fuel assembly, FIG. 2 (B) is a BB sectional view of (A), and FIG. 2 (C) is a CC sectional view of (A).

22A shows the structure of the long fuel rod 2, and FIG. 22B shows the structure of the short fuel rod 3. That is, in these fuel rods 2 and 3, a plurality of fuel pellets 10 are loaded in the cladding tube 11, and both ends of the cladding tube 11 are sealed by the upper end plugs 12 and the lower end plugs 13. Spring 15 in upper plenum 14
Is provided to press the fuel pellet 10. Incidentally. The short fuel rod 3 is also provided with a plenum 14 at the lower part.

The short fuel rod 3 expands the coolant passage above the fuel assembly to reduce the loss pressure and improve the reactor shutdown margin. Further, the position of the short fuel rod 3 is selected by selecting a position where boiling transition easily occurs, which contributes to the improvement of the limit output.

As an example of the equilibrium core 9 loaded with a high combustion fuel assembly, a 1/4 plan view of a BWR core having an electric output of 1.35 million KW is shown in FIG. In the figure, one □ represents one fuel assembly, and the core is composed of 872 fuel assemblies 1. The number in □ is the number of in-core stay cycles of each fuel assembly.

In this core, a first cycle fuel (new fuel) assembly indicated by 1 in the box, a second cycle fuel assembly also indicated by 2, a third cycle fuel assembly indicated by 3 and a 4 indicated by 4 in the same manner. Cycle fuel assemblies are 200 each,
Similarly, 72 fifth-cycle fuel assemblies indicated by 5 are loaded. The average enrichment of the replacement fuel assemblies used in this core 9 is 3.7 wt%.

The core 9 is equipped with 205 control rods, and one control rod and the fuel assembly 4 surrounding the control rod are provided.
The body together is called a cell. However, there is a fuel assembly that does not form cells in a part of the outermost periphery of the core.

Further, in order to control the excess reactivity of the core during the cycle operation by the control rod, the enrichment is low so that the output distribution distortion of the fuel assembly adjacent to the control rod due to the movement of the control rod is alleviated. A control cell is a cell in which four fuel assemblies with low reactivity or advanced combustion have been arranged.
, And are arranged discretely in the furnace. Control cell
The number and position of 16 vary depending on the excess reactivity of the core and the control rod pattern, but in this core, it is composed of 21 indicated by the thick frame.

To construct the first cycle suitable for such an equilibrium cycle based on the above-mentioned conventional technique, five types of initially loaded fuel assemblies having different average enrichments are used. However, since the fuel assembly having the lowest average enrichment corresponding to the fifth cycle fuel assembly has a small number of loaded bodies, it is usually substituted with the first loaded fuel assembly having the second lowest average enrichment and four types of fuel assemblies are used. An initially loaded fuel assembly is used.

An example of such a first cycle is shown in FIG. Table 1 shows the breakdown of the fuel assembly. This core is the same BWR core as in FIG. 23. In FIG. 24, the symbol S in □ is the lowest enrichment fuel assembly corresponding to the 4th cycle fuel assembly, and L is the average enrichment corresponding to the 3rd cycle fuel assembly. Fuel assembly with the second lowest degree, M is the fuel assembly with the lowest average enrichment corresponding to the second cycle fuel assembly, and H is the highest enrichment fuel assembly with the same enrichment as the replacement fuel assembly. It is the body.

The number of loaded fuel assemblies H, M, L is 2 each.
The number of loaded fuel assemblies of the fuel assembly S is 00 and 272. There are 21 control cells 16 and the fuel assembly S with the lowest enrichment is the outermost periphery of the core in order to suppress the leakage of neutrons from the core and improve the economic efficiency, like the control cell and the equilibrium cycle shown in FIG. Is loaded into.

[0022]

[Table 1]

[0023]

Recently, from the viewpoint of improving the fuel economy by increasing the take-out burnup and improving the fuel economy, under the condition that the replacement fuel assembly is enriched for 13 months.
The number of batches of fuel assemblies in the equilibrium cycle has conventionally increased from about 3 batches to more than 4 batches.

As a result, the core thermal characteristics from the first cycle to the equilibrium cycle, especially the radial power peaking, can be easily operated in compliance with the thermal limit maximum line power density (MLHGR) and minimum limit power ratio (MCPR). In order to achieve this, on the basis of the above-mentioned prior art, four types of enrichment are prepared as the types of the initially loaded fuel assemblies, or three enrichment type fuel assemblies are prepared and the minimum enrichment fuel assembly is prepared. It becomes necessary to disperse the body widely in the central region of the core to suppress radial power keeping of the high enrichment fuel assembly from the middle to the end of the first cycle.

In this case, the control cell 16 is usually provided with the third or fourth fuel assembly counting from the high-enrichment side in a 3 or 4 type enriched fuel assembly core. This is because the fuel assemblies arranged in the control cells are delayed in burnup due to the control rods inserted into the core for a long period during the cycle period to control the excess reactivity of the core during operation, and The burnup in the cross-sectional view of the fuel assembly is also due to the effect of delaying the progress on the control rod side.

When the average enrichment of the initially loaded core is 2.2 wt% or more, and when the control rods of the control cell are pulled out at the end of the cycle, the maximum limiting power ratio (MLHGR) to the fuel assembly adjacent to the control rods due to these effects. If the average enrichment of the fuel assembly is set so as not to occur, it is required that the enrichment is 1.2 to 1.4 wt% or less.

As a result, the low enrichment fuel assemblies are required for the number of control cells 16 and the number for suppressing the radial power distribution, and the average enrichment of the core is lowered.

By the way, it is considered that it is better to increase the core average enrichment and to increase the above-mentioned enrichment dispersion parameter for the same core average enrichment from the viewpoint of improving the burnup. Has a higher enrichment of the fuel assembly with the highest enrichment and a greater number of
In addition, the enrichment of the lowest enrichment fuel assembly is lower, the number of bodies is small so as not to lower the average core enrichment, and if possible, the number of bodies is almost the same as that of the fuel assemblies taken out after the first cycle. The point is to reduce the types of body enrichment.

When the operation of the first cycle shown in FIG. 24 is completed, the reactor is lowered, the fuel assembly having the lowest reactivity is replaced with a new fuel assembly, and then the operation of the second cycle is started. In this case, in the above-mentioned conventional example, the initially loaded fuel assemblies S having the lowest enrichment are sequentially taken out.

However, since the control rod 16 is inserted in the control cell 16, the output of the initially loaded fuel assembly S having the lowest enrichment, which is disposed in the control cell 16 in the first cycle, is suppressed. Combustion is not progressing sufficiently.

Therefore, in the above-mentioned conventional example, the initially loaded fuel assembly S having the lowest enrichment is replaced with a fresh fuel assembly while leaving a large amount of uranium 235 that has not yet burned. It is still insufficient in terms of improving the economical efficiency of the fuel assembly. This means that the initially loaded fuel assembly S of the lowest enrichment that was placed on the outermost periphery of the core in the first cycle
Is also the same.

The present invention has been made to solve the above-mentioned problems, and a fuel assembly having an enrichment of 3.0 wt% or more, which becomes a replacement fuel assembly of about 3 batches or more in an equilibrium core, is initially loaded. Highest enrichment of a fuel assembly When used as a fuel assembly, a reactor capable of satisfying the thermal characteristics of the first and second cycles, improving the fuel economy, and reducing the manufacturing cost of the initially loaded fuel assembly. To provide the core of.

Further, the present invention provides an initial loading core suitable for a core of a nuclear reactor using a highly enriched exchangeable fuel assembly having a batch number of about 4 batches or more in an equilibrium core, and providing an initial loading core. An object of the present invention is to provide a core of a nuclear reactor which can improve the fuel economy by increasing the burnup of the fuel assembly.

[0034]

According to the present invention, a fuel assembly having an enrichment of 3.0 wt% or more, which is a replacement fuel assembly of about 3 batches or more in an equilibrium core, has a maximum enrichment of an initially loaded fuel assembly. When used as a fuel assembly, in an initially loaded core with three enrichments, a medium enrichment fuel assembly is further divided into two types according to the amount of burnable poison, and the difference in the number of burnable poison-containing fuel rods is two. Because it is more than a book. Further, the medium enriched fuel assembly containing the highly combustible poison is arranged at the control cell position.

Here, in order to control the excess reactivity of the core during the cycle operation by the control rod, the reactivity is adjusted so that the output distribution distortion of the fuel assembly adjacent to the control rod is relaxed by the movement of the control rod. Is considerably low, for example, a fuel assembly with a low degree of enrichment or advanced combustion, which is arranged around four control rods, is a fuel assembly-control rod unit and is discretely arranged in the reactor. There are 9 if there are few and 37 if there are many.

The medium-enriched fuel assembly containing high burnable poison has more burnable poison-containing fuel rods than the medium-enriched fuel assembly containing low burnable poison by two or more, and the fuel assemblies are arranged. The fuel assembly is offset from the center of the cross section to the control rod side and the average enrichment of the fuel assembly is 1.5 to 2.8 wt%.

Further, according to the present invention, in the core of a nuclear reactor constituted by arranging a large number of cells each consisting of one control rod and four fuel assemblies surrounding the control rod, different average enrichments are obtained. Using multiple types of initially loaded fuel assemblies, the first loaded fuel assemblies with the lowest average enrichment in the first cycle can only be found in positions other than the control cells provided for reactivity control during reactor operation. Deploy. More preferably, the initially loaded fuel assemblies having the lowest average enrichment are arranged so as not to be adjacent to the cells inside the core other than the outermost periphery of the core and not adjacent to the control cells.

[0038]

[Function] For example, in the initial load enrichment type 3 type core,
If the highest enrichment fuel assembly is the same as the replacement fuel assembly with 3.5 wt% enrichment, for example, the enrichment type is 3.
5 (Type 1), 2.3 (Type 2), 1.3 (Type 3) wt
It has a concentration distribution like%.

On the other hand, according to the present invention, each type of enriched fuel assembly is the same, but the medium enrichment is 2.3 wt%.
For the (type 2) fuel assembly, two types of fuel rods with a small number of gadolinia-containing fuel rods added as burnable poisons (type 2) and many (type 2C) are prepared.

By setting the difference in the number of gadolinia-containing fuel rods to two or more, the gadolinia-containing fuel rods in the cross section of the fuel assembly are further arranged such that the center of gravity is offset to the control rod side.

As a result, the operating standard of the maximum linear power density can be satisfied even for the high-gadolinia fuel assembly (type 2C) of the medium enrichment fuel assembly even when the blade history of the control cell of the first cycle is taken into consideration.

Moreover, by designing the required enrichment levels to be common among different types of gadolinia designs of the initially loaded fuel assemblies of medium enrichment, the enriched uranium powder required for the initially loaded fuel assemblies is enriched. It can be handled without increasing the number of types.

As a result, in the enrichment 3 type core of the present invention, the number of low enrichment fuel assemblies (type 3) in the initially loaded core can be reduced as compared with the conventional one, and as a result, the average enrichment of the core can be increased. This can increase the take-out burnup of the initially loaded core.

Conventionally, in the initially loaded core of the nuclear reactor, the fuel assemblies having the lowest enrichment were arranged in the control cell and the outermost periphery of the core, so that the output thereof was suppressed during the operation of the reactor, and the fuel burned sufficiently. Was being replaced with a fresh fuel assembly after the end of the first cycle.

On the other hand, according to the present invention, the initially loaded fuel assembly having the lowest average enrichment extracted from the core after the end of the first cycle is arranged at a position excluding the control cell and the outermost periphery of the core. Therefore, the combustion has progressed sufficiently, and it is possible to increase the take-out burnup of the initially loaded fuel assembly.

Further, the fuel assemblies having the lowest average enrichment are arranged in the cells not newly installed in the control cell so as not to be adjacent to each other, so that the fuel assemblies having the highest average enrichment are arranged next to each other. Since excess neutrons can be supplied, the combustion can be further advanced. Further, in the initially loaded fuel assembly having a high average enrichment, which was arranged in the control cell or the outermost periphery of the core, the fuel assembly is suppressed in the first cycle.

Since these fuel assemblies remain in the core after the second cycle, more unburned uranium 235 is carried over to the second cycle, and the new fuel required for the second cycle operation is correspondingly carried over. Since the number of assemblies can be reduced, fuel economy is further improved. As a result, even with the same core average enrichment as that of the core constructed by the conventional technique, a higher take-out burnup of the initially loaded core is achieved.

[0048]

Embodiments of the core of a nuclear reactor according to the present invention will be described with reference to the drawings.

(First Embodiment) FIG. 1A shows an example of fuel arrangement in a 1/4 90 ° rotationally symmetric initially loaded core according to the first embodiment of the present invention. In the first embodiment, three kinds of fuel assemblies having different average enrichment of fuel assemblies (type 1 fuel assembly of high enrichment fuel, type 2 and 2C of medium enrichment fuel) are used.
Fuel assembly, type 3 fuel assembly of low enrichment fuel) is used. The average enrichment and the number of fuel assemblies are shown in the table below.

[0050]

[Table 2]

[0051]

[Table 3]

In the core of the present embodiment, for example, when the average enrichment of the replacement fuel assembly is 3.5 wt%, the types of enrichment of the initially loaded core are 3.5 (type 1) and 2.3 (type 2,
Type 2C), 1.3 (Type 3) wt%, and 2.3 wt% (Type 2, Type 2C) fuel assemblies, the number of fuel rods with gadolinia added as combustible poisons Prepare two types, one with a small amount (type 2) and one with a large amount (type 2C), and make the difference in the number of gadolinia-containing fuel rods two or more. Moreover, it is more convenient for fuel production if the design of the required enrichment level is made common among fuel assemblies having different gadolinia designs of the initially loaded fuel assemblies of medium enrichment level.

In the first embodiment shown in FIG. 1 (A), the type 3 fuel assembly 3p is provided at the outermost periphery of the core (here, even if the same design as the type 3 fuel assembly arranged in the center of the core is used, from the first cycle). The symbol 3p is attached to the type 3 fuel assembly at the outermost periphery of the core so that the fuel exchange and movement to the second cycle can be easily understood.

Further, in the central region of the core, a control cell C composed of four type of 4C fuel assemblies around the control rods (dedicated control rod cells for controlling reactivity and power distribution during power operation) The fuel assemblies around the control rods are low-reactivity fuel assemblies). From the outermost circumference to the second layer and the third layer, only the type 1 fuel assemblies having the highest enrichment are arranged, or most of them are type 1 fuel assemblies.

In the other remaining positions, the type 1 fuel assemblies are in principle distributed so as to face the type 2 or type 3 fuel assemblies. For example, in the control cell facing the control cell C, in principle, the type 2 or type 3 fuel assembly and the type 1 fuel assembly are alternately arranged in a checkerboard shape. In principle, the control rod cells not facing the control cell C are provided with three type 2 and type 3 fuel assemblies and one type 1 fuel assembly.

The number of the type 3 fuel assemblies is larger than that of the first replacement fuel assemblies. Especially in this example,
As can be seen from the core fuel arrangement diagram of the second cycle in FIG. 1B, the outermost type 3 fuel assembly is also arranged in the outermost periphery in the second cycle, and is arranged inward of the outermost periphery in the first cycle. Only Type 3 fuel assemblies are replaced with the first replacement fuel assemblies.

FIG. 2 shows the axial enrichment distributions and flammability of Type 1, Type 2, 2C, and Type 3 fuel assemblies when the effective lengths of the fuel rods included in the fuel assemblies are the same at least in the enrichment region. An example of the poison axial distribution is shown. In such a fuel assembly, as shown in FIGS. 3A and 3B, the fuel effective portion as shown in FIG. 22A is constituted by only the standard length long fuel rods 2. There are examples of the fuel assemblies 1a and 1b.

As shown in FIG. 2, this initially loaded fuel assembly has blanket regions (fuel effective region using natural uranium, depleted uranium or reprocessed recovered uranium) at the upper and lower ends of the effective fuel length L. However, the length is L / 24 to L /
Twelve. In the type 1, type 2, and 2C fuel assemblies, the enrichment region ¨L _e has an axial distribution of enrichment, and the enrichment region ¨L _e of the type 3 fuel assembly is uniform in the axial direction of enrichment. is there.

Type 1 Fuel Assembly and Type 2, 2C
The fuel assembly is approximately L / 3 to L / 2 from the lower end of the effective fuel length L.
There is a partition boundary a of enrichment at the position of, and the enrichment of the upper part above and below the boundary a is about 0.2 wt% higher than the lower part.

Here, the type 1 fuel assembly and the type 2 fuel assembly
(Both 2, 2C) The boundary a of the fuel assembly may be shifted in the axial direction. Type 2 (both 2, 2C) when shifting
The boundary a of the fuel assembly is better than that of the type 1 fuel assembly.
Set above L / 12 or above.

Further, the type 1, type 2, and 2C fuel assemblies have combustible poison fuel rods, and the number thereof increases in the order of type 2, type 2C, and type 1 fuel assemblies. Here, as the burnable poison, a mode in which gadolinia is added to the fuel pellet is considered. The design of the axial distribution of the burnable poison is such that the burnable poison is added to the concentration region L _e in the effective fuel length L, and the design is uniform or distributed in the region L _e. Conceivable.

As an example having a distribution, as shown in FIG. 2, the gadolinia concentration of the burnable poison-added fuel rod is uniform within that region, or the amount of burnable poison is at the same position as the boundary a of the enrichment classification. There is a difference, and in the gadolinia axial direction design of the entire fuel assembly, the gadolinia amount is small on the upper side of the boundary a and is large on the lower side as shown in FIG.

Further type 1 fuel assemblies and type 2
Both either or both (2,2C of both) the fuel assemblies, approximately from the upper end of the upper enrichment region ¨L _e ¨ the boundary a
It has a low flammable poison area, L _LG, which has a length of L / 12 to L / 6 and is low in burnable poison.

The means for reducing the amount of burnable poison is type 1
The fuel assembly is smaller than the area immediately below the low burnable poison regions ¨L _LG ¨ the concentration of gadolinia. For example, the gadolinia concentration is set to a low concentration of 1.5 to 4 wt%. Alternatively, reduce one gadolinia-added fuel rod. Alternatively, both means may be used.

[0065] Also, the type 1 fuel assemblies or the lowest enrichment to enrichment of the portion corresponding to the low burnable poison regions ¨L _LG ¨ in enriched region, or below the enrichment of boundary a The degree of enrichment may be about the same.

[0066] For the type 2,2C fuel assemblies gadolinia added fuel rods of low-burnable poison region ¨L _LG ¨ 1~3
The gadolinia concentration is reduced to a low concentration of 1.5 to 4 wt% at the same time. Further, enrichment of the portion corresponding to the low burnable poison regions ¨L _LG ¨ is the same as the enrichment of the lower.

FIG. 4 shows an example of a structure having a 9 × 9 fuel rod and a water rod 6 for a 3 × 3 fuel rod cell in the center, and a cross-sectional enrichment of a type 2 fuel assembly and a type 2C fuel assembly. Degree and gadolinia distribution design examples are shown.

In the type 2 fuel assembly of FIG. 4, four gadolinia-containing fuel rods (G1) are arranged discretely and having a center of gravity at the center of the cross section of the fuel assembly. On the other hand, in the type 2C fuel assembly shown in FIG. 5, the number of gadolinia-containing fuel rods is increased by four and the gadolinia addition concentration is 7.5 wt% or more, and seven high-concentration fuel rods are adjacent to each other.

The center of gravity of the arrangement is also offset to the control rod (WW side) side from the center of the cross sectional view of the fuel assembly. Also, W-
As for the outermost fuel rod on the W side, the type 2C fuel assembly has a larger number of low enrichment fuel rods than the type 2 fuel assembly.

It is easier in terms of fuel production to make the fuel rod production types in the enrichment design of the initially loaded fuel assembly the same as the type 2 fuel assembly and the type 2C fuel assembly in terms of enrichment and number, respectively. As shown in FIG. 6, even if the average enrichment is slightly different, the design may be such that only the number is different with the same enrichment.

Type 2C fuel assembly fuel rod position (8.
The gadolinia-added fuel rod of 8) may be arranged with a thin gadolinia concentration, for example, 1.0 wt%. In addition, the concentration of other gadolinia-added fuel rods may be increased to about 10 wt%.

7 and 8 show examples of cross-sectional design of the type 1 fuel assembly and the type 3 fuel assembly of the present invention. As shown in FIG. 8, the type 3 fuel assembly has a more simplified enrichment distribution, and is designed so that high local power peaking easily occurs around the outer circumference fuel rods and large water rods of the fuel assembly.

Next, the operation of the first embodiment of the present invention will be described with reference to FIG.
(A), (B) A comparison with a conventional enrichment type 3 type core will be described with reference to FIGS. 10 and 13 (A), (B).

FIG. 10 shows how the infinite multiplication factor of each type of fuel assembly changes in the first cycle of the enrichment 3 type core.

Since the first cycle length is longer than that of the replacement core by a start test etc. (2000 to 3000 MWd / st) (first cycle length is about 11000 MWd / st), the type 1
The gadolinia concentration of the type 2 and type 2C fuel assemblies is higher than that of the replacement fuel assemblies. Infinite multiplication factor type 1 and type 2 fuel assembly curves in Figure 10 are 7.
The calculation is for a gadolinia addition concentration of 5 wt%. As a result, the maximum infinite multiplication factor of the Type 1 fuel assembly was smaller than that of the replacement fuel assembly, and the peak position was 3000 to 5000 MWd /
It occurs after about st.

In the conventional example, the first of each type placed in the core
When the burnup at the end of cycle of the fuel assembly of is plotted in Fig. 10, it becomes a vertical bar. In this way, the type 3 fuel assemblies arranged in the control cells are compared with other type 3 fuel assemblies arranged in the center of the core.
The burnup is about 0.8 times (8000MWd / st) only.

Further, the type 3 fuel assembly arranged on the outermost periphery also advances only about 0.5 times the burnup (5000 MWd / st). When this core shifts to the second cycle,
As shown in (A) and (B), the Type 3 fuel assemblies with advanced burnup in the central region of the core were taken out and the remaining Type 3 fuel assemblies were taken out.
The fuel assemblies with the most advanced combustion are preferentially placed on the outermost periphery of the core.

This is the second cycle (cycle length of about 9500
MWd / st) core reactivity and the fuel extraction efficiency of type 3 fuel assemblies. However, about half of the Type 3 fuel assemblies placed in the control cells in the first cycle were placed in the outermost periphery in the second cycle,
About half are taken out.

The type 3 fuel assemblies arranged at the outermost periphery in the second cycle can obtain a burnup of about 4000 MWd / st, and other than the control cells at the outermost periphery of the first cycle and the center of the second cycle core. It is about the same as the 5000 + 8000 MWd / st that a Type 3 fuel assembly placed at a position can obtain. As a result, the burnup acquisition loss of the type 3 fuel assembly with the control cell arrangement taken out after the first cycle can hardly be recovered in the second cycle.

According to the first embodiment of the present invention, in the first cycle, the type 3 fuel assemblies having the lowest enrichment and containing no combustible poisons are except the control cells in the outermost core and the central core region. And the number of type 3 fuel assemblies is
By setting the number of refueling fuel assemblies to the total number or more, the first refueling is performed only for the type 3 fuel assembly in the center of the core. The type 3 fuel assembly in the center of the core is taken out without suffering loss of burnup acquisition by the control cell.

In the fuel assemblies loaded in the control cells, the control rod insertion operation continues for a long time, the combustion of the fuel rods on the WW side is delayed, and the fuel rods on the diagonally opposite side (NN side) are burned. Combustion progresses more. When the excess reactivity of the core decreases at the end of the first cycle, the control rod of the control cell is pulled out.

At this point, the local output peaking of the fuel rod on the WW side increases stepwise, and the linear power density of the fuel rod on the WW side is reduced due to the loss of reactivity control by the control rod. Grows very large. Even if this result is taken into consideration, it is necessary to keep the maximum linear power density within the operating limit, so conventionally, the average enrichment of the type 3 fuel assembly was set to 1.3 wt% or less without combustible poisons, The outermost fuel rod is also designed to have a relatively low enrichment.

In this embodiment, since the type 2C fuel assembly arranged in the control cell in the first cycle has a large number of gadolinia fuel rods, and a plurality of gadolinia-added fuel rods are arranged adjacent to each other in the fuel assembly. Due to the effect of slowing the burning of gadolinium due to the thermal neutron flux shielding, the infinite multiplication factor in the first cycle is suppressed by about 10% Δk or more as compared with the type 2 fuel assembly.

This suppression effect is such that the higher the concentration of gadolinia added, the longer the reactivity can be suppressed. Therefore, 7.5 wt% (type 2C fuel assembly indicated by a broken line in FIG. 10) or more 10 wt% (type 2C indicated by a chain line in FIG. 10) It is convenient to have a fuel assembly).

Further, in the type 2C fuel assembly, more fuel assemblies having low enrichment were arranged at the outermost peripheral position on the WW side, and the high concentration gadolinia-added fuel rods were also arranged on the control rod side. Designed with patterns.

Therefore, even if the control rod is pulled out at the end of the cycle, the importance of the fuel rod on the WW side increases, but the gadolinium of the high-concentration gadolinia-added fuel rod arranged in the vicinity thereof still has the residual reactivity. Because it has
It works in the direction of flattening the power distribution in the cross section of the fuel assembly by suppressing the increase of the local power on the WW side.

In the second cycle, the residual gadolinium in the type 2C fuel assembly is also burned out, and the infinite multiplication factor decreases as the combustion proceeds.

On the other hand, the type 2 fuel assembly shows little change in the infinite multiplication factor from the first cycle to the end of the first cycle, maintains the reactivity close to the infinite multiplication factor after the two cycles of combustion in the equilibrium core, and is combustible. It shows a transition in which the reactivity increase due to the combustion of poisonous substances and the reactivity decrease due to the combustion of U ²³⁵ are balanced.

Type 1 fuel assemblies have the same enrichment as replacement fuel assemblies, but gadolinia-added fuel rods have 5 to 6 enrichment.
There are few books. Therefore, the infinite multiplication factor at the beginning of the first cycle is about 10% Δk higher than that of the replacement fuel assembly.

The type 3 fuel assembly exhibits an infinite multiplication factor of 2.5 cycle burnup in the equilibrium core at the beginning of the first cycle, and thereafter, the infinite multiplication factor decreases with combustion.

Further, as in the type 3 fuel assembly of FIG. 8, by arranging the combustion rods with high enrichment at the fuel rod positions with high neutron importance such as the outermost periphery of the fuel assembly and around the large water rod, low enrichment is achieved. A higher infinite multiplication factor is obtained with a higher degree, resulting in a more economical fuel composition. That is,
This is because all the type 3 fuel assemblies in this embodiment are initially loaded cores arranged at positions other than the control cells.

In the conventional multi-rich enrichment core, the power peaking of the outermost fuel rod is lowered in order to arrange the type 3 fuel assembly in the control cell. Therefore, there is a loss in the infinite multiplication factor, that is, in the combustion economy by using a relatively low enrichment in the cross section. FIG. 9 shows a conventional type 3 combustion assembly.

From the combustion change diagram of infinite multiplication factor for each fuel assembly type in FIG. 10, the same type 1, type 2, and type 3 in the enrichment 3 type core of FIG. 13 (A) in the core of FIG. With the gadolinia design and the same enrichment design,
As the type 3 fuel assembly of the control cell in FIG. 13 (A) is replaced with the type 2C fuel assembly, the core excess reactivity at the end of the cycle increases and the number of the first replacement fuel assemblies decreases. It will be.

Next, the effect of the first embodiment will be described. In the BWR replacement core, in order to make the radial power distribution flat, the fuel assemblies having different infinite multiplication factors are dispersed and arranged at an arbitrary minimum arrangement of 4 during the combustion period of the cycle.
The average infinite multiplication factor of the body is to be arranged almost the same. Further, if the average infinite multiplication factor is gradually increased from the center of the core having high importance toward the outside, the power distribution in the radial direction of the core can be flattened.

In the core having various enrichment types of the present embodiment, the type 2 having the maximum reactivity in the early stage of the first cycle
Since the fuel assembly is surrounded by the less reactive type 3 fuel assembly and type 1 fuel assembly, there is a radial output peaking effect of the type 2 fuel assembly.

Further, instead of the type 3 fuel assembly having a low reactivity, which is arranged in the control cell in the enrichment type 3 core, the enrichment and gadolinia addition amount is larger than that and the type 2C fuel assembly is added at the end of the cycle. Higher than the power type 3 fuel assembly The power mismatch in the core is mitigated, the core power is flattened, and the radial power peaking is improved.

Therefore, since the end of the first cycle can be welcomed with a sufficient margin for the excess reactivity, the number of the first-time replacement fuel assemblies can be greatly reduced as compared with the conventional case. As a result, the average enrichment of the core can be increased by about 0.16 wt% compared with the conventional one, and the take-out burnup of the initially loaded core can be increased. This means that the fuel economy of the initially loaded core is improved.

Further, by adopting the axial enrichment distribution and gadolinia distribution design of each fuel type, the takeout burnup is improved and the type 2 and type 1 fuel assemblies which are not adjacent to the control rod in the control cell furnace The axial power distribution can be stably controlled by the axial reactivity distribution of the fuel assembly, and the core thermal limits such as maximum linear power density and MCPR can be satisfied.

In particular, the characteristic of BWR is that a concentration boundary and a gadolinia amount distribution boundary a are provided at positions L / 3 to L / 2 from the lower part of the effective fuel portion and the reactivity below the boundary is suppressed. It is possible to suppress the lower peak output distribution due to the occurrence of voids and flatten it.

Furthermore, if this boundary is the same between the type 1 fuel assembly and the type 2 fuel assembly, an output peak occurs just above the boundary, so that the reactivity of the type 2 fuel is low and the lower output peak characteristic is weak. By shifting the boundary a by L / 12 or more, it can be alleviated.

Further, a low burnable poisonous substance region is provided at the upper end of the concentration region to reduce the unburned residue of the burnable poisonous substance at the end of the cycle, thereby improving the combustion economy. This time,
Since the type 1 fuel assembly has a large number of internal furnace loading cycles, reducing the enrichment also contributes to improving the reactor shutdown margin in the transition cycle.

(Second Embodiment) Next, a second embodiment of the core of the nuclear reactor according to the present invention will be described. 11, 12, and 3
(C) Type 1, type 2, 2C, type 3 using a fuel assembly having partial length fuel rods indicated by P in (D)
An example of the axial enrichment distribution of the fuel assembly and the burnable poison axial distribution is shown.

FIG. 11 shows the low-combustible poisonous substance region L in FIG.
_{This is an example in which LG} is a standard length fuel rod, and the entire region corresponding to the region above the fuel rod effective portion of the partial length fuel rod is L _PLR .

The axial configuration is almost the same as in FIG.
For Type 1 and Type 2 (2,2C) fuel assemblies, the enrichment is the same as or slightly lower than that of the lower part in consideration of the fact that the amount of fuel loaded in the region, L _PLR, is smaller than that in the lower region. The amount of fuel uranium in the lower part of the fuel is large, and output peaking is likely to occur in the axial direction, so a more flattening reaction is required.

For example, in the boundary of a, the output in the axial direction tends to be flattened at a position of about L / 3. It is also effective to increase the difference in enrichment above and below the boundary a. With such an axial design, it is possible to flatten the axial distribution of the initially loaded core of this embodiment using the fuel assembly having the partial length combustion rods.

FIG. 12 is intended to simplify the axial design of FIG. Except for Type 1 fuel assemblies, the enrichment and gadolinia axial design of the lower region (the effective region of the partial length fuel rod) is uniform. The Type 1 fuel assembly has a gadolinia amount boundary a at a position of about L / 3 as shown by the solid line, and has no limit of enrichment.

In this case, in the gadolinia design of the type 1 fuel assembly, the number of gadolinia-containing fuel rods may be increased as one or two partial gadolinia-added fuel rods only in the lower region as shown by the dotted line. In addition, type 2 (2, 2
C), the axial design of the Type 3 fuel assembly and FIG. 12 Type 1 fuel assembly may be combined.

FIGS. 14 (A) and 14 (B) show an example of fuel arrangement in the 1/4 90 ° rotationally symmetric initially loaded core according to the second embodiment of the present invention. In this embodiment, three types of fuel assemblies having different average enrichment of fuel assemblies (high enrichment fuel type 1 fuel assembly, medium enrichment fuel type 2, 2C fuel assembly, low enrichment fuel type 3 fuel) are used. Aggregate) is used. The average enrichment and number of fuel assemblies are shown in the table below. An example of a conventional enriched type 3 initially loaded core corresponding to the second embodiment is shown in FIG.
5 (A) and (B).

[0109]

[Table 4]

[0110]

[Table 5]

In the core of the present invention, for example, when the average enrichment of the replacement fuel assembly is 3.5 wt%, the enrichment type of the initially loaded core is 3.5 (type 1 fuel assembly), 2.3 (type 2, type 2). 2C fuel assemblies), 1.3 (type 3 fuel assemblies) wt%, and 2.3 wt% (type 2 and type 2C fuel assemblies) added as a burnable poison to fuel assemblies. Prepare two types of fuel rods with gadolinia (type 2 fuel assembly) and those with many gadolinia fuel rods (type 2C fuel assembly), and make the difference in the number of gadolinia-containing fuel rods two or more. Moreover, the design of the enrichment requirement amount is common among fuel assemblies having different gadolinia designs of the initially loaded fuel assemblies of medium enrichment.

In the second embodiment of the present invention shown in FIGS. 14A and 14B, the type 1 fuel assembly 1 _P is located at the outermost periphery of the core (here, the same as the type 1 fuel assembly arranged in the center of the core). Refueling from the first cycle to the second cycle, even in the case of design,
The type 1 fuel assemblies at the outermost periphery of the core are marked with 1 _P so that the movement is easy to understand. ) Is arranged, and in the central region of the core, all four control rod peripheral bodies are control cells C composed of type 2C fuel assemblies (dedicated control for performing reactivity control and power distribution control during power operation). In the rod cell, the fuel assembly around the control rod is arranged with a low reactivity fuel assembly.).

From the outermost periphery to the second layer, only the type 1 fuel assemblies having the highest enrichment are arranged, or most of them are type 1 fuel assemblies.

In the other remaining positions, the type 1 fuel assemblies are in principle distributed so as to face the type 2 or type 3 fuel assemblies. For example, in the control rod cell facing the control cell C, in principle, the type 2 or type 3 fuel assembly and the type 1 fuel assembly are alternately arranged in a checkerboard shape. In principle, the control rod cells not facing the control cell C are provided with three type 2 and type 3 fuel assemblies and one type 1 fuel assembly.

As the axial design of the fuel assembly of this embodiment, any of the above-mentioned FIG. 2, FIG. 11 and FIG. 12 is possible. In this embodiment, since the high-reactivity type 1 fuel assembly is arranged on the outermost circumference, the radial power distribution is further flattened, and the characteristics of MCPR and maximum linear power density are higher than those of the first embodiment. You can improve more.

Further, the type 1 fuel assembly in the outermost periphery has an output of about 50% as compared with the fuel assembly in the central region of the core, and since combustion in the first cycle does not proceed, it is carried over to the second cycle. The degree of reaction is high. as a result,
The number of exchangers of the fuel assembly for the second cycle can be reduced.
In addition, the average enrichment of the initially loaded core also increases, which contributes to the increase in the burn-up burnup of the initially loaded core.

In the above embodiments, the outermost fuel assembly is the type 1 fuel assembly or the type 3 fuel assembly. However, as a modification of this embodiment, the type 2 fuel assembly is arranged. Alternatively, the type 1 and type 2 fuel assemblies may be mixed, or the type 1 and type 3 fuel assemblies may be mixed. Its properties have intermediate effects.

The cores of the first and second embodiments of the present invention are
When shifting to the cycle, the type 3 fuel assembly with advanced combustion is preferentially taken out, and the type 2 fuel assembly with relatively advanced combustion is arranged in the control cell C. At this time, the number of control cells is reduced from the first cycle. For example, the present invention uses 29 controls in the first cycle, but reduces to 21 to 29 control cells in the second cycle. Alternatively, type 3 and type 2 fuel assemblies with advanced combustion and low reactivity are arranged on the outermost periphery of the core.

Therefore, for the second cycle, type 2
(2,2C) fuel assemblies are for control cells: 84 to 116 outermost: 92 out of 92 type 3 fuel assemblies are lacking Central core radial output flattening: The number of remaining bodies is required.

According to the present embodiment, the number of the first-time replacement fuel assemblies is approximately 100, and the type 2C fuel assemblies are arranged in the second cycle at positions other than the control cells in the central region of the core. Can be satisfied. Therefore, it is possible to easily shift to the second cycle and realize the flattening of the radial power distribution, and to configure the control cell core and the low neutron leakage core of the second cycle.

Further, the U ²³⁵ carried over the type 2C fuel assembly in the second cycle is arranged at a position other than the control cell in the central region of the core having a high importance, and the economical operation of the second cycle can be performed. By forming a low neutron leakage core after the second cycle, the take-out burnup of the initially loaded core is further improved.

In the above description of the embodiments, the maximum enrichment of the fuel assembly of the initially loaded core is 3.5 wt%, but it can be applied to the case where a higher enrichment is used. Further, the cross-sectional structure of the fuel assembly is not limited to the 9 × 9 fuel rod lattice 3 × 3 fuel rod cell water rod in which the enrichment distribution is specifically illustrated.

Further, although the example of the enrichment type 3 core is shown, in the enrichment type 4 core, the control cell is not the lowest enrichment fuel assembly, and the enrichment is 1.5 wt% or more with higher enrichment. It can also be applied to the case of enrichment type fuel assembly.

(Third Embodiment) FIG. 16 shows the arrangement of the fuel assemblies in the first cycle of the reactor core according to the third embodiment of the present invention. In this embodiment, the BWR is the same as that of the conventional example shown in FIG. 24, and FIG. 16 shows its 1/4 core plane. Table 6 shows the average enrichment and the number of loaded bodies of the initially loaded fuel loaded in the core. In FIG. 16, unlike FIG. 24, the fuel assembly L having the second lowest enrichment is arranged at the outermost periphery of the core.

The core average enrichment of this core is almost the same as that of the conventional example shown in FIG. 24, but the fuel assembly enrichment of S, M, and L is slightly lowered to make the core more concentrated than that of the conventional example of FIG. Adjustments are made by reducing the number of S fuel assemblies. Also,
The number of control cells is increased to 29 to increase the flexibility of controlling the excess reactivity.

[0126]

[Table 6]

The fuel assembly S having the lowest average enrichment is arranged only at the positions other than the control cell 16 and the outermost periphery of the core. Combustion proceeds sufficiently in the first cycle, and after the completion of the first cycle, all of the core is removed. Taken out. The number of the loaded fuel assemblies is the same as that of the first-time replacement fuel assemblies, so that other initially loaded fuel assemblies having a high residual enrichment and a high average enrichment are not taken out at the end of the first cycle. .

Further, in order to promote the combustion of the fuel assemblies S, in this embodiment, most of the fuel assemblies S are arranged in the cells not adjacent to the control cells, and the fuel assemblies S are not adjacent to each other. ing. By arranging in this way, it is possible to keep away from the control cell having a low output, and the fuel assemblies H or M having a high average enrichment surround the fuel assembly S on all sides.

Therefore, excess neutrons generated in the high enrichment fuel assembly can flow into the low enrichment fuel assembly S and burn the fuel assembly S more than expected from the enrichment.

Further, according to such an arrangement, since many high enrichment fuel assemblies H and M are adjacent to the low enrichment fuel assemblies S or L, the output of the high enrichment fuel assembly is high. Is suppressed, and the reactor shutdown margin and thermal margin are improved.

FIG. 17 shows the axial enrichment and gadolinia distribution of each initially loaded fuel assembly used in this example. In each of the fuel assemblies, natural uranium blankets are provided at the upper end 2 nodes and the lower end 1 node, and further, with the upper end of the short fuel rod as a boundary, the upper part has a cross-sectional average enrichment of about 0.2 or more. wt% higher.

In the BWR core, since the coolant boils, the moderator is insufficient in the upper part of the core than in the lower part of the core. Therefore, the output distribution is likely to have a downward peak, and this is compensated for by increasing the enrichment of the upper part to flatten the axial output distribution.

In this embodiment, fuel rods having a uniform enrichment in the axial direction except for the natural uranium blanket portions at the upper and lower ends are used, and the enrichment of the short fuel rods is made lower than the average enrichment of the long fuel rods. By doing so, a vertical distribution of enrichment is provided. Further, as another embodiment, a specific long fuel rod may have an enrichment distribution vertically.

By the way, in the high burnup fuel assembly shown in FIG. 21, the characteristics of the infinite multiplication factor greatly differ from each other due to the existence of the short fuel rods 3. That is, when comparing the upper part with the same void ratio as the lower part, there are many fuel rods and many moderators, so if the concentration is the same, the infinite multiplication factor is large, and this itself has the effect of flattening the axial power distribution. Have.

However, since the upper part of gadolinia also has a higher reactivity than the lower part, if the number of fuel rods with gadolinia is the same in the upper and lower parts, the infinite multiplication factor in the initial stage of combustion is suppressed in the upper part more than in the lower part. The directional output distribution has a downward peak.

This tendency becomes more remarkable as the number of fuel rods containing gadolinia increases. This is the fuel assembly H shown in FIG.
And the infinite multiplication factor of the fuel assembly L will be described. Generally, in order to prevent the output of a specific fuel assembly from becoming excessive, the fuel rods with higher average enrichment should contain more gadolinia-containing fuel rods, and the infinite multiplication factor of each fuel assembly should be set to an appropriate value. Keep it at.

In this example, in the fuel assembly H having an average enrichment of 3.7 wt%, about 9 fuel rods with gadolinia were used.
The fuel assembly L having an average enrichment of 1.6 wt% uses two gadolinia-containing fuel rods. As shown in FIG. 18, when the number of gadolinia-containing fuel rods is the same in the upper and lower parts, the fuel assembly L maintains a moderate upper and lower infinite multiplication factor difference. The infinite multiplication factor is smaller than in the lower part, and the lower peak of the output distribution becomes noticeable.

In this embodiment, in order to improve this point and flatten the axial power distribution, a gadolinia design as shown in FIG. 17 is performed. FIG. 17 shows the axial enrichment and gadolinia distribution state of the initially loaded fuel assembly used in this example. In FIG. 17 (A), the initially loaded fuel assembly S has an average enrichment of 0.9 wt%, (B) is the average enrichment of the initially loaded fuel assemblies M, 2.5 wt%, (C) is the average enrichment of the initially loaded fuel assemblies L
1.6wt%, (D) is the initially loaded fuel assembly H with an average enrichment of 3.7w
t%.

That is, the fuel rods containing gadolinia are not contained in the fuel assembly S, the fuel assembly L has two upper and lower fuel rods, the fuel assembly M has three upper fuel rods, and four lower fuel rods.
Further, the fuel assembly H contains 9 rods in the upper portion and 11 rods in the lower portion.

As a result, as shown in FIG. 18, the upper and lower infinite multiplication factors of each fuel assembly are kept at an appropriate difference, and the axial power distribution can be made flat. In the fuel assembly L, the vertical difference in infinite multiplication factor is too large, and thus the number of gadolinia-containing fuel rods may be four at the top and bottom.

In all the fuel assemblies in this embodiment, the gadolinia concentration in the portion excluding the upper and lower natural uranium portions is 7.5 wt%. However, an embodiment in which the upper portion is made thinner than the lower portion or the upper portion is made thicker than the lower portion in order to appropriately control the axial power distribution in the latter half of the operation cycle is also conceivable.

Further, in this embodiment, the enrichment and the number of fuel rods with gadolinia were distributed with the upper end of the short fuel rod as a boundary, but the positions of these upper and lower boundaries are from the lower end of the fuel assembly to 1 / of its total length. It may be between 3 and 2/3, and the upper and lower boundaries of the enrichment and the upper and lower boundaries of the number of fuel rods with gadolinia may be different.

(Fourth Embodiment) FIG. 19 (A) shows the fuel arrangement in the first cycle of the core of the nuclear reactor according to the fourth embodiment of the present invention. This embodiment is the same BWR as the third embodiment shown in FIG. 16, and the enrichment and gadolinia distribution of each fuel assembly are the same as those used in the third embodiment shown in FIG. Is.

Table 7 shows the average enrichment and the number of loaded bodies of the initially loaded fuel assemblies loaded in the core. Figure 19 (A)
Indicates the initial enrichment fuel H of the highest enrichment arranged at the outermost periphery of the core by P. In this core, since the core average enrichment is increased and the excess reactivity is increased as compared with the third embodiment shown in FIG. 16, the number of control cells is increased to 37.

[0145]

[Table 7]

In this embodiment, according to Japanese Patent Publication No. 5-27075, a fuel assembly H (indicated by P) having the highest average enrichment is arranged at the outermost periphery of the core, and its combustion is performed in the first cycle. It's suppressed. Therefore, in the fuel assembly H, as shown in FIG.
Since more uranium 235 is carried over after the second cycle as compared with the conventional example and the third example shown in FIG. 4, these initial-loaded fuel assemblies are actively burned after the second cycle. Therefore, the take-out burnup of the initially loaded fuel assembly is increased.

In the first cycle, the initially loaded fuel assembly to be arranged on the outermost periphery of the core may be any fuel assembly other than the fuel assembly S taken out after the end of the first cycle. For example, the fuel assembly having the second highest enrichment from the top. The body M may be used, but the fuel assembly H that stays in the core for the longest time is most effective for increasing the take-out burnup of the initially loaded core.

Initially loaded fuel assembly S having the lowest average enrichment
The effect of the present embodiment in which is arranged only at a position other than the control cell is shown by the burnup of the fuel assemblies S and L at the end of the first cycle. In this embodiment, the burnup of each fuel assembly is 11.0 GWd / t and 11.2 GWd / t.

On the other hand, as a conventional example, in FIG.
When the fuel assemblies S are 100 and the fuel assemblies L are 148 and the fuel assemblies S are arranged in the control cells, the burnups of the fuel assemblies S and L are 8.9 GWd / t and 12.
It was 6 GWd / t.

Therefore, in this embodiment, as compared with the above-mentioned conventional example,
The burnup of the fuel assembly S taken out after the completion of the first cycle is increased by 24%, and the burnup of the fuel assembly L staying in the reactor by the second cycle is suppressed by 12% and the reactivity of about 1.5% Δk is reached. Gain has been obtained. As a result, in the present invention, about 8 replacement fuel assemblies can be saved and the economical efficiency of the initially loaded fuel assembly can be improved.

FIG. 19 (B) is a layout view of the fuel assemblies of the second cycle after the fuel exchange is performed after the operation of the first cycle of this embodiment shown in FIG. 19 (A). Since the fuel assemblies S having the lowest average enrichment are all taken out from the core after the end of the first cycle, they are not loaded in the core in the second cycle.

The initially loaded fuel assemblies M having the third lowest average enrichment are arranged in the control cells, while the fuel assemblies L having the lowest reactivity or the first loaded fuel assemblies M are arranged in all the cells other than the control cells. At least one of the initially loaded fuel assembly P or the replacement fuel assembly 1 having the highest enrichment, which was arranged at the outermost periphery of the core in the cycle, is arranged.

The replacement fuel assembly 1 contains gadolinia, and the initially loaded fuel assembly P of the highest enrichment is also the first.
The gadolinia still remains because the combustion has not progressed so much in the cycle, and the reactivity is low at the beginning of the operation cycle. Therefore, it is possible to work together with the fuel assembly L having the lowest reactivity to suppress the output, thereby improving the reactor shutdown margin and the thermal margin in the second cycle.

(Fifth Embodiment) FIG. 20 shows the arrangement of the fuel assemblies in the first cycle of the reactor core according to the fifth embodiment of the present invention. In the present embodiment, the third shown in FIG.
The same BWR as in the example of FIG. 17, and the enrichment and gadolinia distribution of each fuel assembly are the third and fourth shown in FIG.
Is the same as that used in the above example.

In the fifth embodiment, the initially loaded fuel assembly S having the lowest average enrichment is the third and the fourth in the core.
Although it is arranged at a position other than the control cell as in the third embodiment, it is also arranged at the outermost periphery of the core unlike the third and fourth embodiments. The fuel assemblies S arranged on the outermost periphery of the core do not burn sufficiently in the first cycle.
It is not taken out after the completion of the first cycle, and the second cycle is also placed as it is at the outermost periphery of the core and taken out of the core after the completion of the second cycle.

Therefore, in the fifth embodiment, the fuel arranged inside the core is prevented so that the initially loaded fuel assembly L having the second lowest average enrichment is not taken out from the core after the end of the first cycle. It is important to keep the number of aggregates S equal to or greater than the number of first replacement bodies.

[0157]

According to the present invention, since the lowest enrichment fuel assemblies are arranged only in the central region of the core other than the control cells (or only in the outermost position and the central region of the core other than the control cells) The number of enriched fuel assemblies can be reduced. Further, since a core having a higher average core enrichment that can satisfy the thermal limitation of the initially loaded core can be configured, the take-out burnup of the initially loaded core can be significantly improved.

When the initially loaded fuel assembly is arranged according to the present invention, the initially loaded fuel assembly having the lowest average enrichment is sufficiently burned and then taken out from the core, so that the burnup is increased and the fuel economy is improved. Is improved. Further, the reactor shutdown margin and the thermal margin in the first cycle are improved.

Further, since the initially loaded fuel assemblies having a high average enrichment, which were arranged in the control cell or the outermost periphery of the core in the first cycle, did not burn much in the first cycle, more uranium 235 was added to the first cycle. You can carry up to 2 cycles. Therefore, the number of new fuel assemblies required in the second cycle can be reduced, which further improves the economical efficiency.

Further, when a high burnup fuel assembly having short fuel rods is used, the axial power distribution can be sufficiently flattened by setting the gadolinia-containing fuel rod according to the present invention. .

[Brief description of drawings]

FIG. 1A is a 1/4 core plan view showing the fuel assembly arrangement in the first cycle of the core in the first embodiment of the nuclear reactor according to the present invention, and FIG. 1B is the same second cycle. 1/4 core plan view at.

FIG. 2 is a schematic diagram for explaining axial enrichment and burnable poison distribution of each type of fuel assembly of the initially loaded core according to the present invention.

FIG. 3 is a cross-sectional view schematically showing a fuel assembly, (A)
Shows a first example, (B) shows a second example, (C) shows a third example, and (D) shows a fourth example.

FIG. 4A is a type 2 used in the core according to the present invention.
The schematic cross-sectional view of a fuel assembly, (B) is a legend figure of the rod number corresponding to (A).

5A is a schematic cross-sectional view of a type 2C fuel assembly similar to FIG. 4, and FIG. 5B is a legend diagram of rod numbers corresponding to FIG.

6A is a schematic cross-sectional view showing a modified example of the type 2C fuel assembly of FIG. 5, and FIG. 6B is a legend diagram of rod numbers corresponding to FIG.

7A is a schematic cross-sectional view of a type 1 fuel assembly similar to FIG. 4, and FIG. 7B is a legend diagram of rod numbers corresponding to FIG.

8A is a schematic cross-sectional view of a type 3 fuel assembly similar to FIG. 4, and FIG. 8B is a legend diagram of rod numbers corresponding to FIG. 8A.

9A is a schematic cross-sectional view of a conventional type 3 fuel assembly, and FIG. 9B is a legend diagram of rod numbers corresponding to FIG. 9A.

FIG. 10 is a characteristic diagram showing the transition of combustion at infinite multiplication factor of each type of fuel assembly that constitutes the initially loaded core according to the present invention.

FIG. 11 is a schematic diagram for explaining an example of the axial enrichment and burnable poison distribution of each fuel type of the initially loaded core of the present invention in the case of having a short (partial length) fuel rod.

FIG. 12 is a schematic diagram for explaining another example in FIG.

FIG. 13 (A) shows a layout of the first cycle of the conventional enrichment type 3 initial loading core showing the arrangement of the outermost peripheral low enrichment fuel assemblies.
/ 4 core plan view, (B) is a 1/4 core plan view of the second cycle core fuel assembly arrangement in (A).

FIG. 14 (A) is a 1/4 core plan view of the initially loaded core fuel assembly arrangement in the second embodiment of the present invention, and FIG. 14 (B) is the arrangement of the second cycle core fuel assembly in (A). 1
/ 4 Core plan view.

FIG. 15 (A) is a 1/4 core plan view of the initially loaded core fuel assembly arrangement for explaining the other example of FIG. 13, and (B) is the second cycle fuel assembly arrangement of FIG. 1/4 core plan view.

FIG. 16 is a 1/4 core plan view showing the fuel assembly arrangement of the first cycle in the third embodiment of the reactor core according to the present invention.

FIG. 17 is an axial concentration and gadolinia distribution chart of the initially loaded fuel assembly used in the example of the present invention, where (A) shows an average enrichment of 0.9 wt% and (B) shows the same 2.5 wt%. C) shows 1.6 wt% and (D) shows 3.7 wt%.

FIG. 18 is a curve diagram showing the burnup change of the descaling multiplication factor when the void ratio is 40% in the upper and lower cross sections of the initially loaded fuel assembly used in the example of the present invention.

FIG. 19 (A) shows a first-cycle fuel assembly arrangement in a fourth embodiment of the core of a nuclear reactor according to the present invention 1
/ 4 core plan view, (B) is a 1/4 core plan view showing the fuel assembly arrangement for the second cycle in (A).

FIG. 20 is a 1/4 core plan view showing the first-cycle fuel assembly arrangement in the fifth embodiment of the reactor core according to the present invention.

FIG. 21 (A) is a vertical cross-sectional view showing a high burnup fuel assembly used in a conventional nuclear reactor core, and FIG. 21 (B) is a transverse cross section taken along line BB of FIG. 21 (A). FIG. 3C is a cross-sectional view taken along the line C-C of FIG.

FIG. 22 (A) is an elevation view showing a part of a long fuel rod of the fuel assembly in FIG. 21, and FIG. 22 (B) is an elevation view showing a part of a short fuel rod of the same fuel assembly.

FIG. 23 is a 1/4 core plan view showing a fuel assembly arrangement in a balanced cycle in which a conventional high-concentration fuel assembly is loaded.

FIG. 24 is a 1/4 core plan view showing the fuel assembly arrangement in the first cycle of the conventional BWR core.

[Explanation of symbols]

1 ... Fuel assembly, 2 ... Long fuel rod, 3 ... Short fuel rod, 4
... Upper tie plate, 5 ... Lower tie plate, 6 ... Water rod, 7 ... Channel box, 8 ... Spacer,
9 ... Reactor core, 10 ... Fuel pellet, 11 ... Cladding tube, 12 ...
Upper end plug, 13 ... Lower end plug, 14 ... Plenum, 15 ... Spring, 16 ... Control cell.

Claims (10)

[Claims] 1. In an initially loaded core of a boiling water reactor that uses a plurality of types of fuel assemblies having different average enrichments, the fuel assemblies are respectively type 1 from the one having the highest average enrichment,
As Type 2 and Type 3 fuel assemblies, the Type 3 fuel assemblies do not include combustible poison-containing fuel rods, and the Type 1 and Type 2 fuel assemblies include combustible poison-containing fuel rods. The number of fuel rods contained in the type 1 fuel assembly is larger than that in the type 2 fuel rod assembly, and the type 2 fuel assembly has two types in which the number of burnable poison-containing fuel rods differs by 2 or more. And the type 2
Type 2 with many burnable poison-containing fuel rods among fuel assemblies
A reactor core, in which a fuel assembly is arranged in a control cell provided for controlling reactivity during a reactor operation. 2. The type 2 fuel assembly having many burnable poison-containing rods among the type 2 fuel assemblies, wherein the burnable poison-containing fuel rods are arranged from the center of the cross section of the fuel assembly to the control rod side. The core of a nuclear reactor according to claim 1, which is composed of a fuel assembly that is offset and has an average enrichment of 1.5 to 2.8 wt%. 3. The core of a boiling water reactor according to claim 1, wherein the type 2 fuel assembly is arranged in a control cell, and the type 3 fuel assembly is arranged at the outermost periphery of the core. 4. The boiling according to claim 1, wherein the type 2 fuel assembly is arranged in a control cell, and the type 1 fuel assembly is arranged in the outermost core or the outermost core and the second layer. The core of a water reactor. 5. A reactor core having a plurality of cells, each of which is composed of one control rod and four fuel assemblies surrounding the control rod, arranged at different average enrichment levels. Using multiple types of loaded fuel assemblies, in the first cycle, the initial loaded fuel assembly with the lowest average enrichment is located only at the position other than the control cell provided for the reactivity control during reactor operation. The core of a nuclear reactor characterized by: 6. In the first cycle, the initially loaded fuel assemblies having the lowest average enrichment are arranged only at positions other than the outermost periphery of the core, or the first loaded fuel assemblies having the lowest average enrichment are located at the outermost periphery of the core. The initially loaded fuel assembly in which the loaded fuel assemblies are arranged and which is arranged in the outermost periphery of the core and has the lowest average enrichment is also arranged in the outermost periphery of the core in the second cycle as well. Item 5. A nuclear reactor core according to item 5. 7. In the first cycle, the number of loaded bodies of the initially loaded fuel assemblies arranged at positions other than the outermost periphery of the core and having the lowest average enrichment is first taken out from the core after completion of the first cycle. At least equal to the number of loaded fuel assemblies, or in all cells other than the core outermost periphery that are not adjacent to the control cells, at least one initially loaded fuel assembly having the lowest average enrichment is arranged so as not to be adjacent to each other. The core of a nuclear reactor according to claim 5, wherein the core of the nuclear reactor is provided. 8. In the second cycle, all cells except the control cell and the outermost core of the core are provided with an initially loaded fuel assembly having the second lowest average enrichment or the outermost core of the core in the first cycle. At least one of the initially loaded fuel assembly or the replacement fuel assembly is arranged, and in all cells other than the outermost core of the core which are not adjacent to the control cell, the initially loaded fuel assembly having the second lowest average enrichment. 8. The core of a nuclear reactor according to claim 5, wherein at least one body is arranged so as not to be adjacent to each other. 9. In the first cycle, a control cell is provided with an initially loaded fuel assembly having the second lowest average enrichment, and in the second cycle, the control cell is provided with an initial loading having the third lowest average enrichment. 9. A fuel assembly is arranged, and the fuel assembly is arranged.
The core of the described reactor. 10. A core of a nuclear reactor loaded with a fuel assembly configured by bundling a plurality of long fuel rods and a plurality of short fuel rods in a grid pattern, wherein initially loaded fuel assemblies having different average enrichments are provided. The first-loaded fuel assemblies that use multiple types and have the lowest average enrichment do not include burnable poison-bearing fuel rods, and the number of burnable poison-bearing fuel rods and the number of burnable poison-bearing fuel rods per fuel assembly are above and below. The core of a reactor characterized in that the difference is greatest in the initially loaded fuel assemblies with the highest average enrichment and decreases with decreasing average enrichment.