JPH06347578A - Firstly loaded core of boiling water reactor - Google Patents

Abstract

PURPOSE:To improve fuel efficiency by classifying fuel assemblies into high, medium and low average concentration levels, letting highly concentrated fuel assemblies include fuel rods containing inflammable toxicants, and dividing medium condensed fuel assemblies into two types according to the amount of inflammable toxicants. CONSTITUTION:Concentration levels of a firstly loaded core are classified into three types; for example, 3.75wt.% (type 1), 2.5wt.% (types 2A and 2B) and 1.3wt.% (type 3'). Fuel assemblies of the type 2A include fewer fuel rods containing inflammable toxicants by more than one rod than those of the type 2B. Fuel assemblies of the type 3' are placed in the outermost circumference of a core, and a control cell C consisting of four fuel assemblies of the type 3' around fuel rods is placed in the central area of the core. Fuel assemblies of the type 1 are placed in all or most of the section in the second and third layers from the outermost circumference. In a control rod cell facing the cell C in the remaining sections, fuel assemblies of the type 2 (2A and 2B) and those of the type 1 are placed alternately. In a control rod cell not facing the cell C, two fuel assemblies of the type 2, one of the type 2A and one of the type 1 are placed. Consequently, the average condensation level in a core is raised.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Boiling water reactor (hereinafter referred to as BWR)
The first-loading core has been put into practical use in which a plurality of types of fuel assemblies having different enrichments are loaded to improve the take-out burnup of the first-loading core. Therefore, every time the operation cycle is updated, the fuel assembly whose reactivity has been lowered is replaced with a new fuel assembly and the operation is continued, whereby the transition to the equilibrium cycle can be performed quickly. The equilibrium cycle means the following.

That is, the operation by the initially loaded core is called the first cycle, and while the fuel assemblies are partially replaced as described above, the operation cycles of the second, third, ... The equilibrium cycle is a cycle in which the fuel component of the entire core becomes almost constant between adjacent cycles after a considerable period of time.

When this equilibrium cycle is reached, the thermal characteristics (maximum linear power density, MCPR, radial power peaking, etc.) of adjacent cycles, the number of fuel assemblies to be replaced after the end of the cycle, the fuel loading arrangement of the core, and the cycle operation The control rod pattern plans, etc. of 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 Also, this is a problem in terms of economical efficiency of the fuel assembly.

From the viewpoint of the integrity of the fuel assembly, the economic 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. It is desirable that the acquired burnup be comparable to those of the equilibrium cycle, or converge rapidly towards them.

[0007] For example, as a prior art of a boiling water reactor excellent in fuel economy, there is little variation in thermal characteristics and acquired burnup during each transition cycle, and the fuel economy is excellent.
No. 3-045358, "Boiling Water Reactor" is disclosed.

Among these, when loading the fuel assemblies which stay in the reactor for N cycles in the equilibrium core, N types of fuel assemblies having different average enrichments are loaded in the initial loading core, and the respective fuel assemblies are loaded. The average enrichment of the body is set so as to give an infinite multiplication factor of neutrons which is almost equal to the infinite multiplication factor of the N batch fuel assemblies staying in the equilibrium core at the beginning of the equilibrium cycle when the burnable poison is not included. Is proposed. The average enrichment of each fuel assembly is ± 0.2 wt% from the value obtained by the above setting.
It also allows a variation range of up and down.

[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]

[0011]

Recently, from the viewpoint of improving the fuel economy by increasing the take-out burnup under the condition that the replacement fuel is enriched for 13 months, 3.2 to 3.6 is adopted.
The number of fuel batches in the equilibrium cycle has conventionally increased from about 3 to more than 4 in the equilibrium cycle.

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, in compliance with the thermal limit (maximum linear power density, MCPR), As a base, it becomes necessary to prepare four types of enrichment for the types of initially loaded fuel.

When the enrichment dispersion parameter is evaluated, the enrichment of the fuel having the maximum enrichment is higher, the number of the bodies is greater, and the enrichment of the fuel having the lowest enrichment is selected in selecting the enrichment. Is lower, and 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 the fuel taken out after the first cycle, and the number of types of fuel enrichment is reduced.

As a result, when the enrichment types are changed to four types, the enrichment dispersion becomes small, and the newly introduced minimum enrichment type 4 fuel assembly lowers the core average enrichment and takes out combustion. However, it is disadvantageous to evaluate because the manufacturing cost increases due to a decrease in the degree of fuel consumption and an increase in the types of initially loaded fuel.

However, similar to the equilibrium core in the first cycle and the second cycle, the same function as the function of flattening the radial power distribution by the distributed arrangement of the fuels in the third and fourth cycles, which has a low output, is concentrated in the first cycle The fuel with the lowest degree of concentration will be borne by the fuel with the next lowest degree of enrichment in the second cycle, and the radial output distribution will be flattened and the maximum line output, M
It is easy to achieve within the limits of CPR.

The present invention has been made in order to solve the above-mentioned problems. A fuel having an enrichment of 3.4 wt% or more, which serves as a replacement fuel for about 4 batches or more in an equilibrium core, is used as a fuel having the highest enrichment of initial loading fuel. To provide the initial loading core of a boiling water reactor that can simultaneously satisfy the thermal characteristics of the first and second cycles, improve fuel economy and reduce the cost of initial loading fuel when used as To do.

[0017]

According to the present invention, in an initially loaded core of a boiling water nuclear reactor which uses a plurality of types of fuel assemblies having different average enrichments, each of the fuel assemblies is typed from the one having the highest average enrichment. 1, type 2, and type 3 fuel assemblies, the type 3 fuel assembly does not include burnable poison-containing fuel rods, and the type 1 fuel assembly and the type 2 fuel assembly include burnable poison-containing fuel rods. The number of burnable poison-bearing fuel rods in the type 1 fuel assembly is greater than that in the type 2 fuel assembly, and the number of burnable poison-bearing fuel rods in the type 2 fuel assembly is 2 or more. It is characterized by having two different types.

[0018]

[Function] When a fuel with a concentration of 3.4 wt% or more, which is a replacement fuel of about 4 batches or more in the equilibrium core, is used as the maximum enrichment fuel of the initial loading fuel, the enrichment is divided into three types and medium enrichment is performed. The fuel assembly is divided into two types according to the amount of burnable poison, and the difference in the number of fuel rods containing burnable poison is two or more.

Further, the enrichment of the fuel assembly having the lowest enrichment is set to such a low extent that it can be arranged at the control cell position. The control cell is used to control the excess reactivity of the core during the cycle operation by the control rod, so that the movement of the control rod reduces the output distribution distortion of the fuel adjacent to the control rod. Fuel assembly in the core in which fuel having a considerably low degree, for example, low enrichment or advanced combustion is arranged around four body control rods-control rod units, which are discretely arranged in the reactor, There are 9 when the number is low and 37 when the number is high.

First, assuming that the maximum enrichment fuel is the same as the replacement fuel having the enrichment of 3.75 wt% in the initial loading enrichment type 4 core, for example, the enrichment types are 3.75 (type 1) and 2.5 (type 2). , 1.6 (Type 3), 0.9
(Type 4) Concentration distribution like wt%.

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

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

In addition, the enrichment design is made common among different types of gadolinia designs of medium-enriched initially loaded fuel, so that the types of enrichment of enriched uranium powder required for initially loaded fuel are not increased. Can handle.

As a result, the enrichment type of the initially loaded core can be reduced to three types, and the radial output distribution of the first cycle can be flattened to the same level as the 4 enrichment type.

Further, a fuel (type 3 ') having an almost intermediate enrichment between the minimum enrichment fuel (type 4) of the 4 enrichment type core and the enrichment fuel (type 3) above it is initially loaded in the present invention. The lowest enrichment fuel (type 3 ') of the core is replaced by the type 2B fuel assembly by the type 4 fuel assembly of the 4 enrichment type core.

Therefore, if the enrichment is 4 type core, the number of type 1 and type 2 fuel assemblies and 3 of the present invention
If the number of type 1 and type 2A fuel assemblies of the type core is the same, the enrichment 3 type core of the present invention can have a higher average core enrichment, and the take-out burnup of the initially loaded core is increased. it can.

[0027]

EXAMPLE FIG. 1 shows an example of fuel arrangement of a 1/4 90 ° rotationally symmetrical initially loaded core according to the present invention. In the present embodiment, three types of fuel assemblies having different average enrichment of fuel assemblies (type 1 fuel assembly of high enrichment fuel, type 2A, 2 of medium enrichment fuel) are used.
B 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.

[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 types of enrichment of the initially loaded core are 3.75 (type-1), 2.5 (type-2A,
Type-2B), 1.3 (Type-3 ') wt% 3
Make a kind.

For 2.5 wt% (Type-2A, Type-2B) fuel, the number of fuel rods containing gadolinia added as a burnable poison is small (Type-2).
A) and a large number (type-2B) are prepared, and the difference in the number of gadolinia-containing fuel rods is 2 or more, and
The enrichment distribution design is common among fuel assemblies with different gadolinia designs of medium-enriched initially loaded fuel.

In this embodiment shown in FIG. 1, a type 3'fuel assembly is arranged at the outermost periphery of the core, and in the central area of the core, four control rod peripheral bodies are all composed of the type 3'fuel assembly. C is arranged. The control cell C is a dedicated control rod cell for performing reactivity control and output distribution control during output operation, and the fuel around the control rod is a low-reactivity fuel assembly.

From the outermost periphery 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 control rod cells facing the control cell C at the other remaining positions, in principle, the type 2 (2A, 2B) fuel assemblies and the type 1 fuel assemblies 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 (2A, 2B) fuel assemblies and one type 1 fuel assembly. At this time, two type 2B fuel assemblies are two type 2B fuel assemblies around the control rod,
One type 2A fuel assembly is arranged.

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

FIG. 3A shows a fuel assembly of 8 rows and 8 columns,
(B) shows a column of a 9-row by 9-column fuel assembly. Figure 4
(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 fuel pellets loaded in a cladding tube 11, and the upper and lower ends of the cladding tube 11 are sealed with an upper end plug 12 and a lower end plug 13. Cladding
A gas plenum 14 and a spring 15 are provided in 11.

This initially loaded fuel has blanket areas (fuel effective areas using natural uranium, depleted uranium or reprocessed recovered uranium) at the upper and lower ends of the fuel effective length "L", and the lengths thereof are L / 24, respectively. ~ 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 region “L _e ” in the type 3 ′ fuel assembly has a uniform enrichment in the axial direction. Is.

Type 1 fuel assemblies and types 2A, 2
B fuel assembly is approximately L / 3 to the lower end of the active fuel length "L".
At the position of L / 2, there is a division boundary a of enrichment, and the enrichment at the top and bottom of the border a is about 0.2 wt% higher than that at the bottom.

The boundary a between the type 1 fuel assembly and the type 2 (both 2A and 2B) fuel assembly may be axially displaced. In the case of shifting, the boundary a of the type 2 (both 2A and 2B) fuel assembly is set to L / 12 or more above that of the type 1 fuel assembly.

Further, the type 1, type 2A and 2B fuel assemblies have burnable poison fuel rods, the number of which is type 2.
The number increases in the order of A, type 2B, and type 1. 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 concentrated region "L _e " in the active fuel length "L", and the burnable poison is uniform or distributed in the region "L _e ". Design is 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 design of the entire fuel assembly, the gadolinia 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 assembly 1 and type 2
(Both 2A and 2B) Either or both of the fuel assemblies have a low burnable poison level of about L / 12 to L / 6 from the upper end of the enrichment region “L _e ” above the boundary a. It has a flammable poison area " _LLG ".

As a means of reducing the amount of combustible poison, the gadolinia concentration is made smaller in the type 1 fuel assembly than in the region immediately below the low combustible poison region " _LLG ". For example, the gadolinia concentration may be set to a low concentration of 1.5 to 4 wt%, one gadolinia-added fuel rod may be reduced, or both of them may be used.

In the type 1 fuel assembly, the concentration of the portion corresponding to the low-combustible poison region " _LLG " is set to the lowest concentration in the concentration region, or the concentration below the boundary a. May be

[0046] Type 2 (A, B) for the fuel assembly gadolinia added fuel rods of the low burnable poison regions "L _LG" 1
Reduce the gadolinia concentration to a low concentration of 1.5 to 4 wt% at the same time. Further, the concentration of the portion corresponding to the low-burnable poison region " _LLG " is the same as the concentration below it.

The operation of the first embodiment of the present invention will be described with reference to FIGS.
Fig. 13 and Fig. 15 which are examples of a conventional enrichment 4 type core
Will be explained.

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 is changed as if the replacement fuel has been burned for 2.5 cycles in the equilibrium core. Specifically, the infinite multiplication factor of 3% Δk or more (in this example, about 6% Δk by two fuel rods with gadolinia) can be suppressed as compared with the type 2A fuel assembly.

As a result, as can be seen from the combustion change of infinite multiplication factor of each fuel assembly type shown in FIG.
In the fuel assembly, the infinite multiplication factor at the beginning of the first cycle is equivalent to the fuel after 2.5 cycles of combustion in the equilibrium core,
After that, the infinite multiplication factor slowly increases as the combustible poison burns, and at the end of the first cycle, the infinite multiplication factor of the replacement fuel assembly burned in two cycles in the equilibrium core is reached. At this point, the burnable poison is almost burned out. Second
In the cycle, the infinite multiplication factor decreases with subsequent combustion.

On the other hand, the type 2A fuel assembly is the first
The infinite multiplication factor changes little from the beginning to the end of the cycle, and the reactivity is maintained close to the infinite multiplication factor after two-cycle combustion in the equilibrium core, increasing the reactivity due to the burning of combustible poisons and the reactivity due to the burning of U ^235. The decrease shows a balanced transition.

The Type 1 fuel assembly has the same enrichment as the replacement fuel, but has 5-6 less gadolinia-added fuel rods. Therefore, the infinite multiplication factor at the beginning of the first cycle is about 10% Δk higher than that of the replacement fuel.

The gadolinia concentration of the type 1, type 2A, and 2B fuel assemblies was longer than that of the replacement fuel because the first cycle length was longer than that of the replacement core, which was a considerable length (2000 to 3000 MWd / st) due to the start test and the like. Shall be highly concentrated. In the present invention, the gadolinia addition concentration is 7.5 wt% or more. As a result, the maximum infinite multiplication factor of the Type 1 fuel assembly was smaller than that of the replacement fuel, and the peak position was 3000-
It occurs after about 5000MWd / 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 the infinite multiplication factor decreases with subsequent combustion.

From the combustion change diagram of infinite multiplication factor for each fuel type in FIG. 7, the type 1 and type 2 (corresponding to type 2A in FIG. 1) fuel in the enrichment 4 type core in FIG. Using the same gadolinia design of the assemblies, there is no difference in the reactivity in the early stage of combustion of the type 3 fuel assembly of FIG. 13 and the type 3 ′ fuel assembly of FIG.

Therefore, since the type 4 fuel assembly is replaced with the type 2B fuel assembly, the excess reactivity at the beginning of the first cycle is increased 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 fuel assemblies of the control cells are output-controlled by the control rods, and four fuels with low reactivity are collected. Therefore, the output will change to about 0.9 or less.

Next, the effect of the above embodiment will be described. BWR
In the replacement core, in order to flatten the radial power distribution, the fuel assemblies with different infinite multiplication factors are distributed and arranged, and during the combustion period of the cycle, the average infinite multiplication factor of four bodies with arbitrary minimum arrangement is almost the same. It turns out that it is important to arrange. It is also known that the radial power distribution of the core can be flattened by gradually increasing the average infinite multiplication factor from the center of the core with high importance toward the outside.

In the initially loaded core of the present invention, the type 2A fuel assembly having the maximum reactivity in the early stage of the first cycle is the type 2B fuel assembly having a lower reactivity, the type 1 fuel assembly.
Being surrounded by a fuel assembly or a type 3'fuel assembly, it serves to suppress radial power peaking.

Further, in the present invention, since the type 4 fuel assembly having a low reactivity in the enrichment type 4 core is replaced with the type 2B fuel assembly having a higher reactivity than that of the type 2B fuel assembly, Higher power than Type 4 fuel assemblies The core power flattens out, showing lower radial power peaking as in the Type 2A fuel assemblies of FIG.

Further, at the end of the first cycle, the reactivity of the conventional type 4 fuel assembly decreases rapidly, whereas in the present invention, the type 2B fuel assembly increases conversely and then decreases slightly. Since the end of the first cycle is reached, it ends with an infinite multiplication factor that is about 20% Δk higher than that of the type 4 fuel assembly. As a result, the end of the first cycle can be accommodated 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 less than that of the conventional type 4 fuel assembly, the output mismatch in the reactor is alleviated and the radial output distribution is flat. As a result, in the core of the present invention, radial power peaking at the end of the first cycle is suppressed more than before as shown in FIG.

Further, in the arrangement of the fuel assemblies of each enrichment type in the core, the control rod cells facing the control cells C are, as a rule, alternating between the type 2 (2A, 2B) fuel assemblies and the type 1 fuel assemblies. Almost all of them are arranged on the checkerboard, and 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.

In principle, the control rod cells not facing the control cell C are provided with three type 2 (2A, 2B) fuel assemblies and a type 1 fuel assembly. At this time, since two type 2B fuel assemblies and one type 2A fuel assembly are arranged around the control rod, the average infinite multiplication factor of four cells in this cell is at the beginning of the first cycle. Suppresses the higher infinite multiplication factor of 1 type 2A fuel assembly with 1 type fuel assembly and 2 type 2B fuel assemblies, and increases the higher infinity of 1 type 1 fuel assembly at the end of the first cycle. The multiplication factor can be suppressed by the other three type 2 fuel assemblies.

As a result, no type 3'fuel assembly was used in the center of the core other than the control cell C. As a result, the average core enrichment was increased by about 0.17 wt. Can increase. This means that the fuel economy of the initially loaded core is improved.

In addition, in the initial loading enrichment type multi-core, the adjacent fuels have a large neutron flux ratio in the low enrichment fuel and conversely small in the high enrichment fuel due to the difference in the neutron spectrum between the fuel assemblies. There is a problem of calculation error due to the difference between the neutron flow between the aggregates and the thermal neutron flow, especially due to the difference with the calculation result in the infinite system.

This has the greatest effect on the local output and reactivity of the highly enriched fuel adjacent to the low enriched fuel. The core of the present invention can greatly alleviate this problem due to the adjacent type 4 fuel assembly and type 1 fuel assembly in the conventional 4 type core.

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 rods in the control cell core are provided. The axial power distribution of can be stably controlled by the axial reactivity distribution of the fuel, and the thermal limits of the core such as maximum linear power density and MCPR can be satisfied.

Particularly, from the lower part of the fuel effective portion, L / 3 to L /
The distribution boundary a of the enrichment and the gadolinia amount is provided at the position of 2, and the reactivity below the boundary is suppressed, so that the BW
It is possible to suppress the lower peak output distribution due to the occurrence of voids, which is a characteristic of R, and flatten it.

Further, 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 is low and the type 2 fuel assembly has a weak lower output peak characteristic. By shifting the boundary a of the body upward by L / 12 or more, it can be alleviated.

Further, the fuel economy is improved by providing the low burnable poison region at the upper end of the enriched region to reduce the unburned residue of the burnable poison at the end of the cycle. At 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.

FIG. 10 and FIG. 12 show examples of the conventional enrichment 3 type 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 assembly does not contain burnable poisons.

For the control rod cells not facing the control cell C in the central region of the core, one type 1 fuel assembly and one type 2 fuel assembly were used.
Two fuel assemblies and one type 3 fuel assembly are arranged to suppress the average value of the infinite multiplication factors of the control rod cells to control the radial output peaking. Here again, the arrangement of the type 3 fuel assemblies is a bottleneck in increasing the average core enrichment, but it is necessary for controlling the radial power distribution.

Also in this case, as in the present invention, instead of the type 3 fuel assembly of the control rod cells excluding the control cell C in the central region of the core, a type 2 fuel assembly having a large number of gadolinia-containing fuel rods is introduced. By doing so, it is possible to obtain the effect of improving the take-out burnup of the initially loaded core while maintaining the flattening of the radial power distribution similar to the present invention.

The type 1, type 2A, 2B and type 3'fuel in the case of the fuel assembly having the partial length fuel rods shown by P in FIGS. 5 and 6 in FIGS. 3 (C) and 3 (D) An example of the axial concentration distribution of the aggregate and the burnable poison axial distribution is shown.

FIG. 5 shows the low flammability poison region "L" in FIG.
" _LG " is the entire upper part of the fuel rod that is not the fuel rod effective part of the partial length fuel rod.
_PLR "is an example.

The structure in the axial direction is almost the same as in FIG.
For Type 1 and Type 2 (2A, 2B) fuel assemblies, the enrichment is the same as or slightly lower than that of the lower region, considering that the fuel loading amount in the region "L _PLR " is smaller than that in the lower region. In addition, since the output peaking is likely to occur in the lower part in the axial direction where the amount of fuel uranium in the lower part of the fuel is large, further flattening is required.

For example, at the boundary of a, the output in the axial direction is more likely to be flattened at a position of about L / 3, and it is also effective to increase the difference in enrichment above and below the boundary a.

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

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

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

FIG. 9 shows 1/4 90 of the second embodiment according to the present invention.
° An example of the fuel arrangement of the initially loaded core with rotational symmetry is shown. 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 types 2A and 2B fuel assembly, low enrichment fuel type 3) are used. 'Fuel assembly) is used. 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 this embodiment, for example, when the average enrichment of the replacement fuel is 3.75 wt%, the types of enrichment 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), prepare two types of fuel rods containing gadolinia (Type-2A) and many (Type-2B) with few gadolinia as burnable poisons. 2 or more, and the enrichment distribution design is made common among fuel assemblies having different gadolinia designs of the initially loaded fuel of medium enrichment.

In the second embodiment of the present invention shown in FIG. 9, type 1 fuel assemblies are arranged at the outermost periphery of the core, and in the central region of the core, four control rod peripheral bodies are all composed of type 3'fuel assemblies. A control cell C (a control rod cell dedicated to performing reactivity control and output distribution control during output operation, in which fuel around the control rod is a low-reactivity fuel assembly) is arranged. .

From the outermost circumference to the second layer, only the type 1 fuel assembly having the highest enrichment is arranged, or most of the type 1 fuel assemblies are arranged as type 1.
Fuel assembly. The control rod cells facing the control cell C at the other remaining positions are basically type 2 (2A, 2
B) Fuel assemblies and type 1 fuel assemblies 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 (2A, 2B) fuel assemblies and one type 1 fuel assembly. At this time, two type 2B fuel assemblies are two type 2B fuel assemblies around the control rod,
One type 2A fuel assembly is arranged. The axial design of the fuel assembly of this embodiment may be any of those shown in FIGS. 2, 5 and 6.

In this embodiment, a highly reactive type 1 is provided on the outermost circumference.
Since the fuel assemblies are arranged, the radial power distribution is further flattened, and the MCPR and maximum linear power density characteristics are the first.
It can be improved more than the embodiment. In addition, the type-1 fuel assembly in the outermost periphery is approximately 50% of the fuel in the central region of the core.
Since the output is of a degree and the combustion in the first cycle does not proceed, the reactivity brought to the second cycle is large.

As a result, the number of refueling elements for the second cycle can be reduced, and the average enrichment of the initially loaded core is also increased, which contributes to the increase in the burn-up burnup of the initially loaded core.

FIG. 14 shows an example of a conventional enrichment type 4 type initially loaded core corresponding to the second embodiment, and FIG. 11 shows an example of a conventional enrichment type 3 type initially loaded core.

Also 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 cells excluding the control cell C in the central region of the core. If you introduce a type,
While maintaining the flattening of the radial output distribution similar to the present invention,
It is possible to obtain the effect of improving the take-out burnup of the initially loaded core.

In the above embodiments, the outermost fuel is the type 1 fuel assembly or the type 3'fuel assembly, but a type 2 fuel assembly may be arranged as a modification of the present invention. 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 intermediate effects.

When the core of the present invention is shifted to the second cycle, the type 3'fuel assembly with advanced combustion is preferentially taken out, and the control cell C has type 2A with relatively advanced combustion. 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 it is reduced to 21 to 29 control cells in the second cycle.

At the outermost periphery of the core, type 3'and type 2 (2A, 2B) fuel assemblies with advanced combustion and low reactivity are arranged.

Therefore, for the second cycle, the type 2 fuel assemblies are for control cells → 84 to 116 for the outermost periphery → 92 out of 92, the type 3'fuel assemblies are not enough, and the output in the central radial direction of the core is flat. For chemical use → The remaining number of bodies is required, but according to the present invention, the first replacement fuel is about 100 bodies, which can be sufficiently covered. Therefore, it is possible to easily shift to the second cycle and realize flattening of the radial power distribution, and to configure a control core and a low neutron leakage core for the second cycle.

By constructing the low neutron leakage core from the second cycle onward, the take-out burnup of the initially loaded core is further improved.

[0097]

EFFECTS OF THE INVENTION According to the present invention, except for the control cell and the outermost position, only the high-enrichment fuel and the medium-enrichment fuel can be used in the central region of the core to control the radial power distribution and the initial loading It is possible to improve the burnup of the core taken out.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram for explaining an example of axial enrichment and combustible poison distribution of each fuel type of the initially loaded core in FIG.

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

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

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

FIG. 6 is an explanatory view for explaining an example of axial enrichment of each fuel type and combustible poison distribution in an initially loaded core (another example in the case of a fuel assembly having partial length fuel rods) according to the present invention. .

FIG. 7 is a characteristic diagram showing an example of transition of combustion with 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 in the first embodiment according to the present invention.

FIG. 9 is a quarter 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 enrichment 3 type initially loaded core (in the case of outermost peripheral low enrichment fuel arrangement).

FIG. 11 is a quarter core layout diagram of a fuel assembly of a conventional enrichment 3 type initially loaded core (in the case of outermost peripheral low enrichment fuel arrangement).

FIG. 12 is a characteristic diagram showing an example of a transition of combustion with infinite multiplication factor of each fuel type that constitutes the conventional enrichment 3 type initially loaded core.

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

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

FIG. 15 is a characteristic diagram showing an example of transition of combustion at infinite multiplication factor of each fuel type constituting a conventional enrichment 4 type initially loaded core.

[Explanation of symbols]

2 ... Standard length fuel rod, 3 ... Partial length fuel rod, 10 ... Fuel pellet, 11 ... Cladding tube, 12 ... Upper end plug, 13 ... Lower end plug, 14 ... Gas plenum, 15 ... Spring.

Claims (4)

[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 type 1, type 2, and type 3 from the one having the highest average enrichment, respectively. As a fuel assembly, 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 number of rods in the type 1 fuel assembly is larger than that in the type 2 fuel assembly, and the type 2 fuel assembly has two types in which the number of burnable poison-containing fuel rods is different by 2 or more. Initially loaded core of boiling water reactor characterized by 2. The first boiling water reactor according to claim 1, wherein the type 3 fuel assembly is arranged in a control cell set for controlling excess reactivity during operation of the reactor in principle. Loading core. 3. The initially loaded core of a boiling water reactor according to claim 1, wherein the type 3 fuel assembly is arranged on 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 in the outermost periphery of the core or the outermost periphery of the core and the second layer.