JP3175996B2 - First loading core of boiling water reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an initially loaded core of a boiling water reactor.

[0002]

2. Description of the Related Art A boiling water reactor (hereinafter referred to as BWR)
The first-loaded core is loaded with a plurality of types of fuel assemblies having different enrichments to improve the burn-out of the first-loaded core. Thus, by replacing the fuel assembly with reduced reactivity with a new fuel assembly every time the operation cycle is updated and continuing the operation, the transition to the equilibrium cycle can be performed quickly. Here, the equilibrium cycle means the following.

That is, the operation with the initially loaded core is referred to as a first cycle. The operation cycle is repeated as the second, third,... While partially replacing the fuel assembly as described above, and the first cycle is repeated. A cycle in which the fuel component of the entire core has become almost constant between adjacent cycles after a considerable period of time is called an equilibrium cycle.

When the equilibrium cycle is reached, the thermal characteristics (maximum linear power density, MCPR, radial power peaking, etc.) of the adjacent cycle, the number of fuel assemblies to be replaced after the cycle is completed, the fuel loading arrangement of the reactor core, and the cycle operation The control rod pattern plan is almost equally stable.

In a nuclear reactor having a core as described above,
At the end of one cycle of operation, the reactor is shut down, the fuel assembly with the lowest reactivity is replaced with a new one, and the next operation cycle is started. The operation of the reactor is continued by 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 In addition, this is a problem in terms of fuel assembly economy.

In view of the integrity of the fuel assembly and the economics of the reactor core and the fuel assembly, the intermediate cycle of the transition from the first cycle to the equilibrium cycle, in other words, the thermal characteristics and cycle in the transition cycle It is desirable that the acquired burnups be similar to those of the equilibrium cycle or converge quickly toward them.

[0007] As a prior art of a boiling water reactor which is excellent in fuel economy in which the thermal characteristics and the obtained burn-up during the transition cycle have little fluctuation in each cycle and which are excellent in fuel economy, for example,
Japanese Patent Publication No. 3-045358 discloses a "boiling water reactor".

In this case, when loading fuel assemblies that stay in the reactor for N cycles in the equilibrium core, N types of fuel assemblies having different average enrichments are loaded in the initially loaded core, and each fuel assembly is loaded. The average enrichment of the body is set to give a neutron infinite multiplication factor that is almost equal to the neutron infinite multiplication factor at the beginning of the equilibrium cycle of each of the N batch fuel assemblies staying in the equilibrium core when no burnable poison is included. Has been proposed. The average enrichment of each fuel assembly was ± 0.2 wt% from the value obtained by the above setting.
It also allows the range of change up and down.

[0009] By the way, the burnout taken out of the initially loaded core using a plurality of types of fuel assemblies is determined by a method of increasing the core average enrichment. Studies have shown that increasing the dispersion parameter can also be increased.

[0010]

(Equation 1)

[0011]

In recent years, the enrichment of the replacement fuel has been increased from 3.2 to 3.6 from the viewpoint of improving the fuel economy by taking out the burn-up from the 13-month operation.
wt%, and the number of fuel batches in the equilibrium cycle has been increasing from about 3 batches to more than 4 batches in the past.

As a result, in order to easily operate the core thermal characteristics from the first cycle to the equilibrium cycle, in particular, the radial power peaking while keeping the thermal limitation (maximum linear power density, MCPR), the prior art is required. As a base, it is necessary to prepare four types of enrichment for the type of initially loaded fuel.

By the way, when the enrichment is evaluated by the above-mentioned enrichment dispersion parameters, the enrichment of the fuel with the maximum enrichment is higher, the number of fuels is larger, and the enrichment of the fuel with the lowest enrichment is selected. The main point is to reduce the number of fuel enrichments so as not to lower the average core enrichment. If possible, after the first cycle, the number of fuels should be approximately the same as that of the fuel to be taken out, and the type of fuel enrichment should be reduced.

As a result, the use of four enrichment types reduces the enrichment dispersion, and the newly introduced minimum enrichment type 4 fuel assembly reduces the core average enrichment and removes and burns. This is disadvantageous in that it is evaluated in terms of a decrease in the degree of production and an increase in the production cost due to an increase in the type of initially loaded fuel.

However, in the first cycle and the second cycle, the same function as the function of flattening the radial power distribution by the dispersing arrangement of the fuel in the third and fourth cycles having a low output similarly to the equilibrium core is concentrated in the first cycle. In the second cycle, the fuel with the lowest enrichment carries the fuel with the next lowest enrichment, and the radial output distribution is flattened, and the maximum linear output, M
It is easy to achieve within the CPR limit.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is intended to replace a fuel having an enrichment of 3.4 wt% or more, which is a replacement fuel of about four batches or more in an equilibrium core, with a maximum enrichment fuel of an initially loaded fuel. The present invention provides an initial core of a boiling water reactor which can simultaneously satisfy the thermal characteristics of the first and second cycles, improve fuel economy and reduce the cost of producing initially loaded fuel. Is to do.

[0017]

SUMMARY OF THE INVENTION The present invention relates to an initially loaded core of a boiling water reactor using a plurality of types of fuel assemblies having different average enrichments. 1, type 2, and type 3 fuel assemblies, wherein the type 3 fuel assembly does not include a burnable poison-containing fuel rod, and the type 1 fuel assembly and the type 2 fuel assembly include a burnable poison-containing fuel rod. The type 1 fuel assembly has a greater number of burnable poison-containing fuel rods than the type 2 fuel assembly, and the type 2 fuel assembly has two or more burnable poison-containing fuel rods. It is characterized by having two types with a difference.

[0018]

[Function] When a fuel with an enrichment of 3.4 wt% or more, which becomes about 4 batches or more of replacement fuel in the equilibrium core, is used as the maximum enrichment fuel of the initially loaded fuel, the enrichment is divided into three types and the medium enrichment The 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 set to two or more.

The enrichment of the fuel assembly having the lowest enrichment is set to a low enrichment that can be arranged at the control cell position. In addition, the control cell is used to control the excess reactivity of the core during the cycle operation with the control rods, so that the control rod movement reduces the power distribution distortion of the fuel adjacent to the control rods. A fuel assembly-control rod unit in a core in which considerably low fuel, for example, low enrichment or burned fuel is arranged around four control rods, which are discretely arranged in the furnace; There are 9 cases in small cases and 37 cases in large cases.

First, in the initially loaded enrichment type 4 core, if the highest enrichment fuel is the same as the replacement fuel having, for example, 3.75 wt% enrichment, for example, the enrichment types are 3.75 (type 1) and 2.5 (type 2). , 1.6 (type 3), 0.9
(Type 4) A concentration distribution such as wt% is obtained.

On the other hand, according to the present invention, three types such as 3.75 (type 1), 2.5 (type 2) and 1.3 (type 3) wt% are used, and 2.5 wt% (type 2) fuel is used. In contrast, two types of fuel rods containing gadolinia to be added as burnable poisons, one with a small number (type 2A) and one with a large number (type 2B), are prepared.

By setting the difference in the number of gadolinia-containing fuel rods to two or more, the reactivity behavior of the first cycle with the high-gadolinia fuel (type 2B) of the medium enrichment fuel is improved.
The low-enrichment fuel (type 3,
Radial output peaking of a type 1 fuel assembly can be suppressed and controlled to the same extent as in type 4).

Furthermore, the enrichment design is made common among different types of gadolinia designs of the initially loaded fuel of the medium enrichment so that the type of enrichment of the enriched uranium powder required for the initially loaded fuel is not increased. Can respond to.

As a result, the enrichment type of the initially loaded core can be reduced to three types, and the radial power distribution in the first cycle can be flattened to the level of the four enrichment type.

In addition, a fuel (type 3 ') having a substantially intermediate enrichment between the minimum enrichment fuel (type 4) of the 4-enrichment type core and the enrichment fuel (type 3) thereon is used for the first loading of the present invention. The core is the lowest enriched fuel (type 3 '), and the type 4 fuel assembly of the 4 enriched core replaces the type 2B fuel assembly.

Therefore, if the number of type 1 and type 2 fuel assemblies in the enrichment type 4 type core and the
Assuming that the type 1 and type 2A fuel assemblies of the type core have the same number, the enrichment type 3 core of the present invention can increase the average core enrichment and increase the take-out burnup of the initially loaded core. it can.

[0027]

FIG. 1 shows an example of a fuel arrangement of an initially loaded core having a rotationally symmetric 1/4 90 ° according to the present invention. In this embodiment, three types of fuel assemblies (average enrichment type 1 fuel assembly, medium enrichment type 2A,
B fuel assembly, low enriched fuel type 3 'fuel assembly)
Is used. The average enrichment and the number of fuel assemblies are shown in the table below.

[0028]

[Table 1]

[0029]

[Table 2]

In the core of the present invention, for example, when the average enrichment of the replacement fuel is 3.75 wt%, the enrichment types of the initially loaded core are 3.75 (type-1), 2.5 (type-2A,
3 such as type-2B), 1.3 (type-3 ') wt%
Type.

For 2.5 wt% (type-2A, type-2B) fuel, the number of fuel rods containing gadolinia added as burnable poison is small (type-2).
A) and two types (type-2B) are prepared, and the difference in the number of gadolinia-containing fuel rods is set to two or more, and
The enrichment distribution design will be shared between fuel assemblies with different gadolinia designs of initially loaded fuel with medium enrichment.

In the present embodiment shown in FIG. 1, a type 3 'fuel assembly is arranged at the outermost periphery of the core, and a control cell comprising all four control rods around the control rod in the center region of the core is composed of the type 3' fuel assembly. C is arranged. The control cell C is a dedicated control rod cell for performing the reactivity control and the power distribution control during the output operation, and the fuel around the control rod is provided with a fuel assembly having a low reactivity.

In the second and third layers from the outermost periphery, only the type 1 fuel assemblies having the highest enrichment are arranged, or the majority are type 1 fuel assemblies.

The control rod cells facing the control cell C at the other remaining positions are, in principle, arranged in a substantially checkerboard-like manner in which type 2 (2A, 2B) fuel assemblies and type 1 fuel assemblies are alternately arranged.

The control rod cells not facing the control cell C are basically provided with three type 2 (2A, 2B) fuel assemblies and one type 1 fuel assembly. At this time, the type 2 fuel assemblies around the control rods are two type 2B fuel assemblies,
One type 2A fuel assembly is arranged.

FIG. 2 shows axial type enrichment distributions of type 1, type 2A, 2B and type 3 'fuel assemblies when the effective length of the fuel rods included in the fuel assembly is the same at least in the enrichment region. An example of the burnable poison axial distribution is shown. As such a fuel assembly, the fuel effective portion as shown in FIG. 4A is constituted only by fuel rods having a standard length.
There are examples of fuel assemblies such as (A) and (B).

FIG. 3A shows a fuel assembly having eight rows and eight columns.
(B) shows a fuel assembly column of 9 rows and 9 columns. FIG.
(A) is an elevation view of the standard length fuel rod 2, and (B) is an elevation view of the partial length fuel rod 3. In FIG. 4, reference numeral 10 denotes a fuel pellet loaded in the cladding tube 11, and upper and lower ends of the cladding tube 11 are sealed by an upper end plug 12 and a lower end plug 13. Cladding tube
Inside 11 is provided a gas plenum 14 and a spring 15.

This initially loaded fuel has blanket regions (effective fuel regions using natural uranium, depleted uranium or reprocessed uranium) at the upper and lower ends of the active fuel length "L", each having a length of L / 24. ~ L / 12. In the type 1, type 2A, and 2B fuel assemblies, the enrichment region “L _e ” has an axial distribution of enrichment, and the enrichment in the enrichment region “L _e ” of the type 3 ′ fuel assembly is uniform in the axial direction. It is.

Type 1 fuel assembly and type 2A, 2
B fuel assembly is approximately L / 3 ~ from the lower end of active fuel length "L".
At the position of L / 2, there is a division boundary a of the enrichment, and the enrichment at the upper part above and below the boundary a is about 0.2 wt% higher than at the lower part.

The boundary a between the type 1 fuel assembly and the type 2 (both 2A and 2B) fuel assemblies may be shifted in the axial direction. In the case of shifting, the boundary a of the type 2 (both 2A and 2B) fuel assemblies is set to be at least L / 12 higher than that of the type 1 fuel assembly.

The type 1, type 2A and type 2B fuel assemblies have burnable poison fuel rods, the number of which is
A, type 2B, and type 1 increase in this order. Here, as the burnable poison, a form in which gadolinia is added to fuel pellets is considered. The axial distribution design of the burnable poison is such that the burnable poison is added to the enrichment area “L _e ” within the active fuel length “L” and has a uniform or distribution in the area “L _e ”. Design is possible.

As an example having a distribution, as shown in FIG. 2, the gadolinia concentration of the burnable poisoned fuel rod in the region is uniform or the amount of burnable poison at the same position as the boundary a of the enrichment class. There is a difference, and as a gadolinia axial design of the entire fuel assembly, gadolinia is small above the boundary a and large below it as shown in FIG.

Further, type 1 fuel assembly 1 and type 2
(Both 2A and 2B) Either or both of the fuel assemblies have a low burnable poison length of about L / 12 to L / 6 from the upper end of the enrichment area “L _e ” above the boundary a. It has a burnable poison area “L _LG ”.

As a means for reducing the amount of burnable poisons, the concentration of gadolinia in the type 1 fuel assembly is made smaller than that in the region immediately below the low burnable poison region "L _LG ". For example, there is a method of reducing the gadolinia concentration to a low concentration of 1.5 to 4% by weight, or reducing one gadolinia-added fuel rod, or both.

In the type 1 fuel assembly, the enrichment of the portion corresponding to the low burnable poison region “L _LG ” is set to the lowest enrichment in the enrichment region, or the enrichment below the boundary a. It may be.

For the type 2 (A, B) fuel assembly, one gadolinia-added fuel rod in the low burnable poison region “L _LG ” is used.
３3 wires, and at the same time, reduce the gadolinia concentration to a low concentration of 1.5 to 4 wt%. The enrichment of the portion corresponding to the low burnable poison region “L _LG ” is the same as the enrichment below.

FIGS. 7 and 8 show the operation of the first embodiment of the present invention.
13 and 15, which are examples of a conventional 4-type enrichment core.
This will be described with reference to FIG.

According to the first embodiment of the present invention, at the beginning of the first cycle, the output of the type 2B fuel assembly is suppressed, and the reactivity of the fuel is increased as if the replacement fuel had been burned for 2.5 cycles in the equilibrium core. More specifically, the infinite multiplication factor can be suppressed by 3% Δk or more (about 6% Δk by two gadolinia-containing fuel rods in this example) compared to the type 2A fuel assembly.

As a result, as can be seen from the infinite multiplication factor combustion change of each fuel assembly type shown in FIG.
In the fuel assembly, the infinite multiplication factor at the beginning of the first cycle corresponds to the fuel after 2.5 cycles of combustion in the equilibrium core,
Thereafter, the infinite multiplication factor gradually increases as the burnable poison burns, and reaches the infinite multiplication factor of the replacement fuel assembly burned in the equilibrium core in two cycles at the end of the first cycle. At this point, the burnable poison has almost burned out. Second
In the cycle, the infinite multiplication factor then decreases with combustion.

On the other hand, the type 2A fuel assembly is the first
Less change in the infinite multiplication factor toward the end of the initial cycle, maintaining the reactivity near the infinite multiplication factor of two cycles after combustion in approximately equilibrium core, the reaction degree of combustion of the reactivity increases and U ²³⁵ from the combustion of the burnable poison The decline shows a balanced transition.

The Type 1 fuel assembly has the same enrichment as the replacement fuel, but has fewer than five or six gadolinia-added fuel rods. Therefore, the infinite multiplication factor at the beginning of the first cycle is about 10% Δk higher than the replacement fuel.

The gadolinia concentration of the fuel assemblies of type 1 and type 2A and 2B is such that the first cycle length is longer than the replacement core by a considerable amount (2000 to 3000 MWd / st) by a start-up test or the like. Also have a high concentration. In the present invention, the gadolinia addition concentration is 7.5 wt% or more. As a result, the maximum value of the infinite multiplication factor of the type 1 fuel assembly is smaller than that of the replacement fuel, and the peak position is 3000 ~
It occurs after about 5000 MWd / st.

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

From the combustion change diagram of the infinite multiplication factor for each fuel type shown in FIG. 7, it can be seen from the core of FIG. 1 that the type 1 and type 2 (corresponding to type 2A in FIG. 1) fuel of the enrichment type 4 core of FIG. Using the same gadolinia design of the assemblies, there is no difference in the reactivity of the type 3 fuel assembly of FIG. 13 and the type 3 ′ fuel assembly of FIG. 1 at the beginning of combustion.

Accordingly, the replacement of the type 4 fuel assembly with the type 2B fuel assembly increases the excess reactivity at the beginning of the first cycle by about 1.5% Δk. This can be easily dealt with by increasing the number of gadolinia fuel rods of the type 1 fuel assembly by one or two or by increasing the number of gadolinia-containing fuel rods of the type 2 (2A, 2B) fuel assembly by about one.

Although the behavior of radial peaking of the type 3 and type 3 'fuel assemblies is omitted, the output of the fuel assembly of the control cell is controlled by control rods and four fuels with low reactivity are collected. The output is about 0.9 or lower.

Next, the effects of the above embodiment will be described. BWR
In the replacement core, fuel assemblies with different infinite multiplication factors are dispersed and arranged to flatten the radial power distribution, and during the combustion period of the cycle, the average infinite multiplication factor of the four units with the minimum arrangement is almost the same. Has proven to be important. It has also been found that the power distribution in the radial direction of the core can be flattened by gradually increasing the average infinite multiplication factor from the center of the core having high importance toward the outside from the center.

In the initially loaded core of the present invention, the type 2A fuel assembly having the highest reactivity at the beginning of the first cycle is a type 2B fuel assembly having a lower reactivity, type 1
Since it is surrounded by the fuel assembly or the type 3 'fuel assembly, it functions to suppress the radial output peaking.

In the present invention, the type 2B fuel assembly having a higher reactivity is replaced with the type 2B fuel assembly having a higher reactivity instead of the type 4 fuel assembly having a lower reactivity in the enrichment type 4 core. The core power, which is higher in power than the Type 4 fuel assembly, is flattened, indicating lower radial power peaking as in the Type 2A fuel assembly of FIG.

Further, at the end of the first cycle, the reactivity of the conventional type 4 fuel assembly rapidly decreases, while the type 2B fuel assembly of the present invention increases and then slightly decreases. Since the end of the first cycle is reached, the infinite multiplication factor is higher by about 20% Δk than that of the type 4 fuel assembly. As a result, the end of the first cycle is greeted with a sufficient margin for the excess reactivity.

Further, since the distribution of the infinite multiplication factor of the fuel at the end of the first cycle is smaller than that of the conventional type, there is less power mismatch in the furnace and the radial power distribution is flat. As a result, in the core of the present invention, as shown in FIG. 8, the radial output peaking at the end of the first cycle is suppressed as compared with the related art.

Further, in the arrangement of the fuel assemblies of each enrichment type in the core, the control rod cells facing the control cell C alternate between type 2 (2A, 2B) fuel assemblies and type 1 fuel assemblies in principle. Almost on the checkerboard, one of the two type 1 fuel assemblies faces the control cell C, so that the output is suppressed even at the end of the cycle.

The control rod cells not facing the control cell C are basically provided with three type 2 (2A, 2B) fuel assemblies and a type 1 fuel assembly. At this time, the type 2 fuel assembly is composed of two type 2B fuel assemblies and one type 2A fuel assembly around the control rods. Suppresses the higher infinite multiplication factor of one type 2A fuel assembly with one type fuel assembly and two type 2B fuel assemblies, and at the end of the first cycle, the higher infinity of one type 1 fuel assembly. The multiplication factor can be suppressed with the other three type-2 fuel assemblies.

As a result, at the center of the core, the type 3 'fuel assembly was not used except for the control cell C, and as a result, the average enrichment of the core was increased by about 0.17 wt% as compared with the conventional one, and the burn-out of the initially loaded core was increased. Can increase. This means that the fuel economy of the initially loaded core is improved.

In addition, in the first-load multi-enrichment core, in the low-enrichment fuel, the ratio of the thermal neutron flux is high, and in the high-enrichment fuel, the ratio is low. There is a problem of a calculation error due to a difference from a calculation result in an infinite system due to a neutron flow, particularly a thermal neutron flow, between the assemblies.

This has the greatest effect on the local output and reactivity of the highly enriched fuel adjacent to the low enriched fuel. In the core of the present invention, this problem caused by the proximity of the type 4 fuel assembly and the type 1 fuel assembly in the conventional four type core can be greatly reduced.

Further, by designing the enrichment distribution in the axial direction of each fuel type and the gadolinia distribution design, the take-out burnup is improved and the type 2 and 1 type fuel assemblies which are not adjacent to the control rods in the control cell core. Can be controlled stably by the axial reactivity distribution of the fuel, and the thermal limits of the core such as the maximum linear power density and MCPR can be satisfied.

In particular, L / 3 to L /
By providing a distribution boundary a of the enrichment and gadolinia amount at the position 2 and suppressing the reactivity below the boundary, the BW
The lower peak output distribution due to void generation, which is a feature of R, can be suppressed and flattened.

Further, if this boundary is the same for the type 1 fuel assembly and the type 2 fuel assembly, an output peak is generated immediately above the boundary, so that the type 2 fuel assembly having low reactivity and weak lower output peak characteristics is obtained. By shifting the boundary a of the body upward by L / 12 or more, it can be alleviated.

Further, by providing a low burnable poison region at the upper end of the enrichment region and reducing the unburned residue of the burnable poison at the end of the cycle, fuel economy is improved. At this time,
Since the type 1 fuel assembly has a large number of furnace interior loading cycles, a reduction in enrichment contributes to an improvement in the furnace shutdown margin in the transition cycle.

FIGS. 10 and 12 show an example of a conventional three-type enrichment core when the replacement fuel in the conventional equilibrium core is 3.5 wt%. In this case, the type 1 and 2 fuel assemblies contain gadolinia-containing fuel rods, and the type 3 fuel assemblies do not contain burnable poisons.

The control rod cells which do not face the control cell C in the core central region include one type 1 fuel assembly,
Two fuel assemblies and one type 3 fuel assembly are arranged to control the average value of the infinite multiplication factor of the control rod cell to control the radial output peaking. Also here, the arrangement of the type 3 fuel assemblies is a bottleneck in increasing the average core enrichment, but is necessary for controlling the radial power distribution.

Also in this case, as in the present invention, a type 2 fuel assembly having a large number of gadolinia-containing fuel rods is introduced into the type 2 fuel assembly instead of the type 3 fuel assembly of the control rod cells except for the control cell C in the central region of the core. In this case, it is possible to obtain the effect of improving the burnout of the initially loaded core while maintaining the same level of radial power distribution as in the present invention.

Type 1, Type 2A, 2B and Type 3 'fuel in the case of a fuel assembly having a partial length fuel rod indicated by P in FIGS. 3 (C) and 3 (D). An example of the axial concentration distribution and the burnable poison axial distribution of the aggregate is shown.

FIG. 5 shows the low burnable poison region “L” in FIG.
_LG ”is the whole upper part“ L ”that is not the fuel rod effective part of the partial length fuel rod.
_PLR ".

The structure in the axial direction is almost the same as in FIG.
For the type 1 and type 2 (2A, 2B) fuel assemblies, the enrichment is the same as or slightly lower than that in the lower region, taking into account that the fuel loading in the region "L _PLR " is smaller than that in the lower region. The output peaking tends to occur in the lower part in the axial direction by the amount of the fuel uranium in the lower part of the fuel, so that more flattening is required.

For example, the output of the axial direction is more easily flattened at the position of about L / 3 at the boundary of a, and it is also effective to increase the difference in enrichment between the upper and lower sides of the boundary a.

With such an axial design, the axial power distribution of the initially loaded core of the present invention using a fuel assembly having a fuel rod having a partial length can be flattened.

FIG. 6 simplifies the axial design of FIG. Except for the type 1 fuel assembly, the enrichment of the lower region (effective region of the partial length fuel rod) and the axial design of the gadolinia are uniform. At this time, the axial design of this type 1 fuel assembly also has only the gadolinia distribution in this region.

The type 1 fuel assembly has a capacity of about L /
There is a boundary a for the gadolinia amount at the position 3 and no boundary for the enrichment. At this time, the gadolinia design of the type 1 fuel assembly may be such that the number of gadolinia-added fuel rods is increased as one or two partial gadolinia-added fuel rods only in this lower region as shown by a dotted line. Also, type 2 and type 3 'in FIG.
The axial design of the fuel assembly and the type 1 fuel assembly of FIG. 5 may be combined.

FIG. 9 shows a quarter of the second embodiment according to the present invention.
An example of the fuel arrangement in a first-load core with rotational symmetry is shown. In the present embodiment, three types of fuel assemblies having different average enrichments of the fuel assemblies (high-enrichment fuel type 1 fuel assembly, medium-enrichment fuel types 2A and 2B fuel assemblies, and low-enrichment fuel type 3 'Fuel assembly). The average enrichment and the number of fuel assemblies are shown in the table below.

[0082]

[Table 3]

[0083]

[Table 4]

In the core of the present embodiment, for example, when the average enrichment of the replacement fuel is 3.75 wt%, the enrichment types of the initially loaded core are 3.75 (type-1) and 2.5 (type-2).
A, type 2B), 1.3 (type-3 ') wt%, and 2.5 wt% (type-2A, type-2)
For the fuel of B), two types of fuel rods containing gadolinia containing a small amount of gadolinia (type-2A) and a large number (type-2B) of fuel rods added as burnable poison are prepared, and the difference in the number of fuel rods containing gadolinia is prepared. And two or more fuel assemblies having different gadolinia designs of initially loaded fuel of medium enrichment have a common enrichment distribution design.

In the second embodiment of the present invention shown in FIG. 9, a type 1 fuel assembly is disposed at the outermost periphery of the core, and all the control rod surroundings are constituted by type 3 'fuel assemblies in the central region of the core. Control cell C (a control rod cell dedicated to performing reactivity control and power distribution control during output operation, and the fuel around the control rod is provided with a fuel assembly with low reactivity). .

In the second layer from the outermost periphery, only the type 1 fuel assemblies having the highest enrichment are arranged, or the majority is type 1 fuel assemblies.
Fuel assembly. Control rod cells facing the control cell C at the other remaining positions are in principle type 2 (2A, 2
B) The fuel assemblies and the type 1 fuel assemblies are alternately arranged in a substantially checkerboard shape.

The control rod cells not facing the control cell C are basically provided with three type 2 (2A, 2B) fuel assemblies and one type 1 fuel assembly. At this time, the type 2 fuel assemblies around the control rods are two type 2B fuel assemblies,
One type 2A fuel assembly is arranged. As the axial design of the fuel assembly according to the present embodiment, any of FIGS. 2, 5, and 6 can be used.

In this embodiment, the type 1 having high reactivity is provided on the outermost periphery.
Since the fuel assemblies are arranged, the radial power distribution is further flattened, and the characteristics of MCPR and maximum linear power density are the first.
Can be further improved as compared with the embodiment. In addition, the outermost type 1 fuel assembly is about 50% of the fuel in the central area of the core.
Since the output is of the order and the combustion in the first cycle does not proceed, the reactivity carried to the second cycle is large.

As a result, the number of refueling bodies for the second cycle can be reduced, and the average enrichment of the initially loaded core also increases, thereby contributing to an increase in the burnup of the initially loaded core.

FIG. 14 shows an example of a conventional four-concentration type initial loading core corresponding to the second embodiment, and FIG. 11 shows an example of a conventional three-concentration type initial loading core.

In the conventional core shown in FIG. 11, as in the present invention, the number of fuel rods containing gadolinia is large in the type 2 fuel assembly instead of the type 3 fuel assembly of the control rod cell except for the control cell C in the central area of the core. If you introduce a type,
While maintaining the flattening of the radial power distribution similar to the present invention,
The effect of improving the burnout of the initially loaded core can be obtained.

In the above embodiments, the outermost fuel is a type 1 fuel assembly or a type 3 'fuel assembly. However, as a modification of the present invention, a type 2 fuel assembly may be provided. The type 1 fuel assembly and the type 2 fuel assembly may be mixed, or the type 1 fuel assembly and the type 3 'fuel assembly may be mixed. Its properties have an intermediate effect.

When the core of the present invention is shifted to the second cycle, the type 3 'fuel assembly is preferentially taken out of the advanced burned fuel assembly, and the control cell C is provided with the type 2A relatively advanced burned fuel. Place the aggregate. At this time, the number of control cells is reduced from the first cycle. For example, in the present invention, 29 control cells are used in the first cycle, but the number is reduced to 21 to 29 control cells in the second cycle.

At the outermost periphery of the core, a type 3 ', type 2 (2A, 2B) fuel assembly having a low reactivity with advanced combustion is disposed.

Therefore, for the second cycle, the type 2 fuel assembly is used for the control cell → 84 to 116 for the outermost circumference → 92 out of 92, the type 3 ′ fuel assembly is insufficient, and the output in the center radial direction of the core is flat. Although it is necessary to change the number of fuels to be used, the number of remaining fuels is required. However, according to the present invention, the first replacement fuel is about 100, which is sufficient. Therefore, it is possible to easily shift to the second cycle and realize the flattening of the radial power distribution, and the control core and the low neutron leakage core of the second cycle can be configured.

After the second cycle, by assembling a low neutron leakage core, the take-out burnup of the initially loaded core is further improved.

[0097]

According to the present invention, except for the control cell and the outermost peripheral position, the power distribution in the radial direction can be controlled using only the high-enrichment fuel and the medium-enrichment fuel in the central region of the core, and the initial loading can be performed. The burnout of the core can be improved.

[Brief description of the drawings]

FIG. 1 is a 1/4 core layout diagram of a fuel assembly in one embodiment of an initially loaded core of a boiling water reactor according to the present invention.

FIG. 2 is an explanatory diagram for describing an example of axial enrichment and burnable poison distribution of each fuel type of the initially loaded core in FIG. 1;

3A is a cross-sectional view showing a fuel assembly having 8 rows and 8 columns, FIG. 3B is a cross-sectional view showing a fuel assembly having 9 rows and 9 columns,
(C) is a cross-sectional view showing a 9 × 9 fuel assembly having partial fuel rods, and (D) is a cross-sectional view showing another example of (C).

4A is an elevation view showing a standard length fuel rod in a fuel assembly, and FIG. 4B is an elevation view showing a partial length fuel rod as in FIG.

FIG. 5 shows the axial enrichment of each fuel type of the initially loaded core (in the case of a fuel assembly having a partial length fuel rod) according to the present invention;
Explanatory drawing for demonstrating the example of a burnable poison distribution.

FIG. 6 is an explanatory diagram for explaining an example of an axial enrichment and a distribution example of burnable poisons of each fuel type in an initially loaded core (another example of a fuel assembly having a partial length fuel rod) according to the present invention. .

FIG. 7 is a characteristic diagram showing an example of a transition of combustion at an infinite multiplication factor of each fuel type constituting the initially loaded core according to the present invention.

FIG. 8 is a characteristic diagram for explaining the effect of reducing radial output peaking of the first embodiment according to the present invention.

FIG. 9 is a 1/4 core layout diagram showing an initially loaded core fuel assembly according to a second embodiment of the present invention.

FIG. 10 is a 1/4 core layout diagram of a fuel assembly of a conventional three-enrichment type initially loaded core (in the case of an outermost peripheral low enrichment fuel configuration).

FIG. 11 is a 1/4 core arrangement diagram of a fuel assembly of a conventional three-enrichment type initially loaded core (in the case of an outermost peripheral low enrichment fuel arrangement).

FIG. 12 is a characteristic diagram showing an example of an infinite multiplication factor combustion transition of each fuel type constituting a conventional three-enrichment type initially loaded core.

FIG. 13 is a 1/4 core arrangement diagram of a fuel assembly of a conventional 4-core enrichment initially loaded core (in the case of an outermost peripheral low enrichment fuel arrangement).

FIG. 14 is a 1/4 core layout diagram of a fuel assembly of a conventional four-enrichment type initially loaded core (in the case of an outermost peripheral low enrichment fuel arrangement).

FIG. 15 is a characteristic diagram showing an example of a combustion transition at an infinite multiplication factor of each fuel type constituting a conventional initially loaded core with four enrichments.

[Explanation of symbols]

2. Standard fuel rods, 3 partial fuel rods, 10 fuel pellets, 11 cladding tubes, 12 upper end plugs, 13 lower end plugs, 14 gas plenums, 15 springs.

Continued on the front page (56) References JP-A-62-80585 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G21C 5/00 G21C 5/12 G21C 5/18 G21C 3 / 30

Claims (4)

(57) [Claims] In a first loading core of a boiling water reactor using a plurality of types of fuel assemblies having different average enrichments, the fuel assemblies are classified into types 1, 2, and 3 in order from the one with the highest average enrichment. A fuel assembly, wherein the type 3 fuel assembly does not include a burnable poison-bearing fuel rod, and the type 1 fuel assembly and the type 2 fuel assembly include a burnable poison-bearing fuel rod; The number of rods is greater in the type 1 fuel assembly than in the type 2 fuel assembly, and the type 2 fuel assembly has two types of burnable poison-containing fuel rods having a difference of 2 or more. The first loading core of a boiling water reactor. 2. The boiling water reactor according to claim 1, wherein the type 3 fuel assembly is disposed in principle in a control cell set for controlling excess reactivity during operation of the reactor. Loading core. 3. The initially loaded core of a boiling water reactor according to claim 1, wherein the type 3 fuel assembly is disposed at the outermost periphery of the core. 4. The initially loaded core of a boiling water reactor according to claim 1, wherein the type 1 fuel assembly is arranged on the outermost periphery of the core or the outermost periphery of the core and the second layer.