JP2004144757A - Reactor core for nuclear reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem wherein an excess reactivity is increased in a reactor core of a nuclear reactor having an increased average enrichment of initially loaded fuel in order to enhance remarkably economy by increasing a discharged burnup to use the fuel effectively, and to control the excess reactivity within a proper range. <P>SOLUTION: In this reactor core split concentrically into two radial-directionally to make the number of fuel assemblies included in respective regions substantially equal, in the nuclear reactor loaded with initially loaded fuel assemblies in the first cycle, and loaded with replaced fuel assemblies in the second cycle, the third cycle or thereafter, the initially loaded fuel assemblies arranged in a peripheral area of the reactor core in the first cycle are arranged to get large in a reactor core inside area compared with the peripheral area of the reactor core in the second cycle. <P>COPYRIGHT: (C)2004,JPO

Description

(4) The present invention relates to a reactor core of a boiling water reactor, which is an initially loaded core and has improved fuel economy by increasing the take-out burnup.

核 The nuclear fuel assembly loaded into the core for the first time after the construction of the boiling water reactor is called the first loaded fuel. In the early stage of the development of a nuclear power plant, one type of fuel with a single enrichment was used as such initially loaded fuel. In recent years, however, multiple types of fuel with different enrichments have been used in order to improve the extraction burnup. Stuff came to be used.

However, in any case, the average enrichment of the initially loaded fuel is set so that the excess reactivity of the core at the end of the first cycle becomes almost zero, and is therefore set to 2.1 to 2.5%.

As an example of such an initially loaded core, a 1/4 part of the core of a boiling water reactor with an electric output of 1.35 million kW composed of four types of initially loaded fuels having different enrichments is a fuel arrangement diagram shown in FIG. Shown in In FIG. 20, the squares represent one of the fuel assemblies generally called fuel, and the core 1 is composed of a total of 872 fuel assemblies.

Here, H shown in the square indicates that the enrichment is 3.7%, M is 2.5%, L is 1.6%, and S is the initially loaded fuel assembly having an enrichment of 0.9%. The enrichment of the fuel H is the same as that of the replacement fuel assembly, and the enrichment of the fuels M, L, and S is the reaction after the replacement fuel assembly stays in the first, second, and third cycle furnaces. It is set to simulate the degree.

In FIG. 20, the fuel assembly 2 for fuel H, the fuel assembly 3 for fuel M, and the fuel assembly 4 for fuel L are each 200, and 272 fuel assemblies 5 for fuel S are loaded. The average enrichment of the initially loaded fuel is 2.1%.

The fuel used here is an example of a high burn-up fuel assembly, and is a longitudinal sectional view of FIG. 21 (a) and a sectional view taken along the line bb of FIG. 21 (a) of FIG. 21 (a). 21 (c) and a cross-sectional view taken along the line cc of FIG. 21 (a).

Each of the fuel assemblies 2 to 5 is composed of a long fuel rod 6, a short fuel rod 7, and a large-diameter water rod 8 which are bundled by a spacer 9 in a square grid of 9 rows and 9 columns, and an upper tie plate 10 and a lower tie plate. The fuel rod bundle is fixed to 11 and is surrounded by a channel box 12. The short fuel rods 7 increase the coolant flow path in the upper part of the fuel assemblies 2 to 5 to reduce pressure loss and improve the furnace stop margin.

で は In the core 1, 205 control rods 13 are provided at positions indicated by white circles in FIG. 20, and one of the control rods 13 and four fuel assemblies surrounding the control rods 13 are collectively called one cell. However, fuel that does not constitute a cell exists in a part of the outermost periphery of the core.

In order to control the excess reactivity of the reactor core 1 with the control rod 13 during the operation of the reactor, the enrichment is controlled so that the power distribution distortion of the fuel adjacent to the control rod 13 due to the movement of the control rod 13 is reduced. As a result, the low-reactivity cells 14 (also referred to as control cells) in which four low-reactivity fuels are arranged are discretely arranged in the reactor core 1 based on the result of low combustion or advanced combustion. In the present core 1, there are 21 low-reactivity cells 14 indicated by thick frames, and the fuel assemblies 5 (S) having the lowest enrichment are loaded.

When the operation of the first cycle with the initially loaded core is completed, about 200 of the initially loaded fuel assemblies with reduced reactivity are taken out of the reactor core 1 and a new fuel assembly is loaded and replaced. The operation is performed for two cycles, and thereafter, the operation is repeated in the same manner as the third and fourth cycles.

に 対 し て On the other hand, Patent Document 1 discloses an invention in which the take-up burnup of initially loaded fuel is greatly increased. According to the present invention, the enrichment of all initially loaded fuels is made equal to that of the replacement fuel, so that the second cycle operation is performed without refueling after the completion of the first cycle operation.

This will eliminate the need to replace the fuel with replacement fuel and significantly improve fuel economy compared to the past. However, in the present invention, there is a problem that the excess reactivity of the reactor core does not become zero at the end of the second cycle, and the reactor must be shut down with the remaining operating capacity still remaining.

Further, an invention for solving the above problem is disclosed in Patent Document 2. In the present invention, two or more types of initially loaded fuels having different enrichments are prepared, the enrichment of the high enrichment fuel is made equal to that of the replacement fuel, and the enrichment of the low enrichment fuel and the number of loaded bodies are changed It has been proposed that the operation of the second cycle be performed without refueling after the completion of the operation of the first cycle, and that the excess reactivity of the core be zero at the end of the second cycle.
JP-A-60-119492 JP-A-2-222867 JP-A-63-168589 JP-A-4-58191 JP-A-61-240193

However, the following two problems still remain in the above two inventions.

In a boiling water reactor, in order to control the excess reactivity in the core 1 to an appropriate range of 1-2% Δk, some fuel rods 6 and 7 constituting the fuel assemblies 2 to 5 are generally provided with gadolinia or the like. Contains burnable poisons.

The number of burnable poison-containing fuel rods per one of the fuel assemblies 2 to 5 is set so that the excess reactivity in the early stage of the combustion cycle is set in an appropriate range, and the concentration of the burnable poison is determined by the length of the combustion cycle. Is determined almost in proportion to. In the initially loaded core having an average enrichment of 2.1% shown in FIG. 20, gadolinia having a maximum concentration of 7.5%, which has been conventionally used, is applied to the appropriate number of fuel rods constituting the fuel assemblies H, M and L. The surplus reactivity in the first cycle when it is contained takes a constant value of 1.2 to 2% Δk, and becomes almost zero at the end of the cycle.

However, when the average enrichment of the initially loaded fuel becomes high, even if the operation period of the reactor is constant, the excess reactivity after the middle stage of the operation cycle increases if the conventional concentration of burnable poison is used. . As an example, in FIG. 20, the surplus reactivity of the initially loaded core, in which the average enrichment is increased to 2.7% by adjusting the number of loaded bodies of each initially loaded fuel, exceeds 3% Δk in the latter half of the cycle.

Further, when gadolinia is used as the burnable poison, the reactivity effect per fuel rod containing the burnable poison is 3 to 4% Δk. By adjusting the number of burnable poison-containing fuel rods, the surplus reactivity cannot always be set to an appropriate range.

For example, to increase the average enrichment of the initially loaded fuel by 0.1%, it is necessary to reduce the excess reactivity by about 1% Δk. However, if the number of burnable poison-containing fuel rods is increased by one, the excess reactivity becomes It decreases by 2 to 3% Δk.

It is an object of the present invention to increase the excess reactivity in the core of a nuclear reactor in which the average enrichment of the initially loaded fuel is increased in order to increase the extraction burn-up and greatly improve the economic efficiency through effective use of the fuel. An object of the present invention is to provide a reactor core that can solve the problem and can control the excess reactivity within an appropriate range.

In order to achieve the above object, in the reactor core according to the first aspect of the present invention, an initially loaded fuel assembly is loaded in a first cycle, and a replacement fuel assembly is loaded in a second cycle or a third cycle or later. In the nuclear reactor, the initially loaded fuel assemblies disposed in the core peripheral region in the first cycle are divided into two concentric radially divided cores so that the number of fuel assemblies included in each region is substantially equal. In the second cycle, more cores are arranged in the core inner region than in the core peripheral region.

The core of the nuclear reactor according to the invention according to claim 2, wherein the core of the reactor loaded with two or more types of initially loaded fuel assemblies having different enrichments is located at the outermost periphery of the core in the first cycle and the second cycle. In the first cycle, the first enriched fuel assembly having the highest enrichment and being different from the highest enrichment initially loaded fuel assembly arranged in the outermost periphery of the core in the first cycle is provided in the second cycle. It is characterized by being arranged at the outermost periphery of the core.

The reactor core according to the third aspect of the present invention is characterized in that the number of control rods inserted into the reactor core during the operation of the first cycle is larger in the core peripheral region than in the core inner region.

A reactor core according to a fourth aspect of the present invention is a reactor core configured by arranging a large number of cells each composed of one control rod and four fuel assemblies surrounding the control rod in a square lattice. A core having two or more types of initially loaded fuel assemblies having different enrichments and having lower enrichment than the initially enriched fuel assemblies having the highest enrichment in the first cycle and the second cycle. The low-reactivity cells are constituted by the initially loaded fuel assemblies, and the number of the low-reactivity cells into which control rods are inserted during operation is larger in the second cycle than in the first cycle.

FIG. 1 shows a fuel arrangement diagram showing 1/4 of the core. FIG. 1 (a) shows a first cycle core 15a, and FIG. 1 (b) shows a second cycle core 15b. The first loaded fuel assembly is loaded. The fuel assemblies 16 indicated by 1 and the fuel assemblies 17 indicated by 2 are the same high-enrichment initially loaded fuel in the four corners in the figure, and the fuel assembly 18 indicated by L is the low-enrichment initially loaded fuel. Fuel.

One fuel rod having four fuel assemblies 16 to 18 arranged around one control rod 13 is a single cell, and all four fuel assemblies are fuel assemblies 18 (low-enrichment initially loaded fuel). The low-reactivity cell 14 in the portion indicated by the thick frame in which L) is arranged is called the high-reactivity cell 19.

The fuel assemblies loaded in the reactor cores 15a and 15b of the nuclear reactor are each configured by bundling a plurality of fuel rods in a square lattice, and the average enrichment of the initially loaded fuel indicated by 1 and 2 in a square is about The concentration of gadolinia contained in the fuel rods as a burnable poison is 2.7% or more and is higher than 7.5% in at least some of the initially loaded fuel assemblies.

Also, an initially loaded fuel assembly is loaded in the first cycle core 15a shown in FIG. 1 (a), and a replacement fuel assembly is loaded in the second cycle core 15b or the third and subsequent cycles shown in FIG. 1 (b). In the core, the average unloading burnup of the initially loaded fuel assembly is about 0.7 times or more the average unloading burnup of the replacement fuel assembly.

According to the configuration described above, in the initially loaded core in which the average enrichment is significantly increased, the excess reactivity in the first cycle can be set in an appropriate range. The characteristic diagram of FIG. 2 shows the number of loaded bodies using the fuel assemblies 2 (H) and 4 (L) among the four types of initially loaded fuel assemblies used in the conventional example of FIG. It is a graph showing the maximum value of the excess reactivity in the first cycle in various initially loaded cores having different average enrichments constituted by adjusting the ratio.

The fuel assembly 2 (H) has nine gadolinia-containing fuel rods with a concentration of 7.5%, and the fuel assembly 4 (L) has two conventionally used fuel rods. Accordingly, it is necessary to adjust the number of fuel rods containing gadolinia, but since the reactivity value of gadolinia has disappeared near the end of the cycle, the maximum surplus reactivity is not significantly affected by the number of fuel rods containing gadolinia. .

In FIG. 2, a curve 20 indicates the case where the fuel assembly 4 (L) is arranged at the outermost periphery of the core, and a curve 21 indicates a case where the fuel assembly 2 (H) is arranged. When the fuel assemblies 2 are arranged at the outermost periphery of the core, the excess reactivity exceeds 4.5% Δk in the core having an average enrichment of about 2.7% or more. Generally, the low reactivity cells are arranged so as not to be adjacent to each other in the X direction, the Y direction and the diagonal direction. Therefore, in the conventional core 1 shown in FIG. 20, up to 45 low reactivity cells 14 can be arranged.

However, the surplus reactivity that can be controlled by the 45 control rods 13 is about 4.5% Δk. Therefore, as long as gadolinia with a concentration of 7.5% is used, the control of the reactor in the first loaded core with an average enrichment of about 2.7% or more is performed. Can not do.

Therefore, by using gadolinia with a concentration higher than 7.5% based on this configuration, it is possible to construct a first-load core with an appropriate excess reactivity and an average enrichment of about 2.7% or more.

On the other hand, when the high enrichment fuel assembly 16 (1) and the high enrichment fuel assembly 17 (2) are arranged at the outermost periphery of the core, the neutron leakage from the core increases, so that the excess reactivity can be reduced and the average enrichment can be reduced. Surplus reactivity exceeds 4.5% Δk in a core with a degree of about 2.9% or more. Therefore, in a core employing such a fuel arrangement, gadolinia having an average enrichment of about 2.9% or more and a concentration higher than 7.5% may be used.

Curve 22 in FIG. 2 shows the maximum value of the excess reactivity in the first cycle when gadolinia with a concentration of 7.5% is used in an initially loaded core of various enrichment composed of one type of fuel assembly. As can be seen from FIG. 2, in the case of the initially loaded core composed of a plurality of fuels having different enrichments, the excess reactivity is considerably larger than the curve 20 or the curve 21. This is because the higher the enrichment of the fuel, the harder the neutron spectrum becomes, and the more the gadolinia combustion is delayed.

In other words, if the average enrichment is the same, the core is composed of a plurality of fuels having different enrichments, and the highly enriched fuel contains more burnable poisons, thereby extending the reactivity life of the burnable poisons for a long time. And thereby the excess reactivity can be reduced.

By significantly increasing the average enrichment by the above-described configuration, the average unloading burnup of the initially loaded fuel is greatly increased as compared with the conventional case. In the conventional initially loaded core 1 having the average enrichment of 2.1% shown in FIG. The average withdrawal burnup of the first loaded fuel is about 25 GWd / t, which is only 0.56 times the average withdrawal burnup of 45 GWd / t of the replacement fuel with a 3.7% enrichment, but increased the average enrichment to 2.7%. In the first loaded core 15a, the average discharge burnup is about 32 GWd / t, which is 0.71 times the average discharge burnup of the replacement fuel.

Claims 1 to 4 relate to the first invention group. For example, as shown in the fuel arrangement diagram of FIG. 1, an initially loaded fuel assembly is loaded in a first cycle, and a second cycle or a third cycle is performed. The reactor in which the replacement fuel assembly is loaded after the cycle is divided into two concentrically and radially diagonally shaded portions 23 so that the number of fuel assemblies included in each region is substantially equal. The initially loaded fuel assemblies arranged in the core peripheral region in the cycle are arranged and operated in the core inner region more than the core peripheral region in the second cycle.

Further, in the reactor core loaded with two or more types of initially loaded fuel assemblies having different enrichments, the firstly loaded fuel assemblies having the highest enrichment are placed on the outermost periphery of the core in the first cycle and the second cycle. The first enrichment initially loaded fuel assembly, which is different from the highest enrichment initially loaded fuel assembly arranged on the outermost periphery of the core in the first cycle, is arranged and operated on the outermost periphery of the core in the second cycle.

Further, the core is divided into two concentrically and radially diagonally shaded portions 23 so that the number of fuel assemblies included in each region of the core becomes substantially equal, and is inserted into the reactor core during the operation of the first cycle. The operation is performed with a larger number of rods in the core peripheral region than in the core internal region.

作用 As an effect of the first invention group, the excess reactivity in the second cycle can be suppressed, and the operation can be performed for a longer time. In the first cycle shown in the core 15a of FIG. 1 (a), the high-enrichment fuel assemblies 16, 17 of the initially loaded fuel arranged in the core peripheral region have low output, so that combustion does not proceed, so that they are contained therein. Some of the burnable poisons and fissile material remain in the second cycle.

Therefore, by moving the initially loaded fuel assemblies 16 to 18 to the inner region of the core in the second cycle shown in the core 15b of FIG. 1B, the second cycle is performed without excessively increasing the concentration of the burnable poison. Can be controlled in an appropriate range, and the combustion efficiency in the second cycle can be increased.

For this purpose, it is effective to arrange the fuel assemblies 16, 17 of the highest enrichment initially loaded fuel containing a large amount of burnable poisons in the core peripheral region in the first cycle, particularly in the core outermost periphery, Further, by further delaying the combustion of the fuel in the peripheral region of the core by inserting the control rod 13 in the first cycle, the above-mentioned operation can be further emphasized.

Claim 4 belongs to the first invention group and also belongs to the second invention group. For example, as shown in the fuel arrangement configuration diagram of FIG. 1, one control rod 13 and four fuel assemblies surrounding the control rod 13 In the core of a nuclear reactor configured by arranging a large number of cells composed of the bodies 16 to 18 in a square lattice, two or more types of initially loaded fuel assemblies having different enrichments are loaded, and the first cycle and In the second cycle, the low-reactivity cell 14 is constituted by four of the initially-loaded fuel assemblies 18 having a lower enrichment than the fuel assemblies 16 and 17 of the initially-enriched fuel having the highest enrichment. The operation is performed with the number of the low-reactivity cells 14 into which the control rod 13 is inserted being larger in the second cycle than in the first cycle.

The operation of the second invention group is achieved by inserting more control rods 13 than in the first cycle into the core when the surplus regression of the second cycle cannot be sufficiently suppressed by the first invention group. , The excess reactivity in the second cycle can be controlled.

Generally, the reactivity value of one control rod 13 decreases with the combustion of the fuel surrounding the control rod 13 because the reactivity of the fuel decreases with the combustion. As a result, when the initially loaded fuel having the same enrichment is used for the low-reactivity cell 14 in the first and second cycles, the second cycle is more effective even if the excess reactivity is equal in both cycles. Many control rods 13 must be inserted.

However, according to the second invention group, such a situation can be dealt with, and the reactor can be operated by using an appropriate number of control rods 13 in both the first and second cycles. .

Next, in a third invention group, the core of the nuclear reactor is constituted by arranging a large number of cells each composed of one control rod and four fuel assemblies surrounding the control rod in a square lattice. . This core is loaded with two or more types of initially loaded fuel assemblies 16 to 18 having different enrichments, and the initially enriched fuel assemblies 16 and 17 having the highest enrichment are further flammable to at least most except the upper and lower ends. It is composed of two or more types of fuel rods containing different toxic poisons.

Further, in the core radially divided into two concentric circles so that the number of fuel assemblies included in each region is substantially equal, the highest enrichment initially loaded fuel assembly is divided into a first fuel assembly and a first fuel assembly. And a second fuel assembly having a greater number of burnable poison-containing fuel rods than the fuel assembly described above, and the second fuel assembly is disposed more in the core inner region than in the core peripheral region.

{Circle around (2)} The second fuel assembly is arranged on the outermost periphery of the core, and the first fuel assembly is arranged on at least one of the second and third layers from the outermost periphery of the core.

Also, the high-reactivity cell 19 is constituted by the four fuel assemblies 16 and 17 of the highest enrichment initially loaded, and four fuel assemblies of lower enrichment than the fuel assemblies 16 and 17 of the highest enrichment initially loaded are formed. The high-reactivity cell 19, which does not face the low-reactivity cell 14 composed of the initially loaded fuel assembly 18 in any of the X and Y directions, has the low-reactivity cell 14 at least in the X and Y directions. The second fuel assembly is loaded more than the high-reactivity cell 19 facing at either one.

As a function of the third invention group, in the initially loaded core 15a in which a large amount of highly enriched initially loaded fuel is loaded, two or more types of initially enriched fuel assemblies 16 having different numbers of burnable poison-containing fuel rods 16 are provided. , 17, and adjusting the number ratio of the loaded bodies, the excess reactivity in the first and second cycles can be set in an appropriate range.

Furthermore, by arranging a large number of fuel assemblies 17 with high enrichment initially loaded with a large number of burnable poison-containing fuel rods in the core inner region, the core radial power distribution can be flattened, so that thermal margin is provided. Increase. In particular, the highly enriched initially loaded fuel 16 having a small number of burnable poison-containing fuel rods is arranged at the outermost periphery of the core, and the highly enriched initially loaded fuel having a different number of burnable poison-containing fuel rods inside the outermost periphery of the core is used. It is good to arrange them almost equally.

Alternatively, a high-enrichment initially loaded fuel assembly 17 having a large number of burnable poison-containing fuel rods is arranged on the outermost periphery of the core, and a high-enrichment initially loaded fuel assembly 16 having a small number of burnable poison-containing fuel rods is located at the core outermost. By arranging the second layer or the second and third layers from the outer periphery, the power distribution in the core radial direction in the first cycle can be flattened.

In this case, further, based on the first invention group, after the end of the first cycle, the highly enriched initially loaded fuel assembly 17 having a large number of burnable poison-containing fuel rods loaded on the outermost periphery of the core is moved to the core inner region. By replacing with a fuel assembly, the excess reactivity of the second cycle can be sufficiently reduced.

On the other hand, when the fuel assembly 16 with a high enrichment initially loaded and having a small number of burnable poison-containing fuel rods is arranged on the outermost periphery of the core, the fuel is replaced based on the first invention group. The excess reactivity of the cycle does not decrease much. Therefore, it is sufficient to select the initially loaded fuel to be loaded on the outermost periphery of the core according to the excess reactivity in the second cycle.

In addition, since the reactivity value of the control rod 13 is large in a cell that is not adjacent to the low reactivity cell 14, it is necessary to load more fuel assemblies with a high enrichment initially loaded with a large number of burnable poison-containing fuel rods in this cell. Thereby, the reactor shutdown margin can be improved without lowering the average enrichment of the initially loaded fuel.

Next, in the fourth invention group, as shown in the sectional view of the fuel rod arrangement in FIG. 4, FIGS. 4A and 4B show the fuel assemblies 25 and 26 of the prior art, and FIGS. The fuel assemblies 27 and 28 of d) show the fuel assemblies with the fuel rod arrangement based on the fourth invention group. In a fuel assembly formed by bundling a plurality of fuel rods in a square lattice and in a core of a nuclear reactor loaded with the fuel assembly, at least one burnable poison-containing fuel rod 29 constituting the fuel assembly is provided. Face the other burnable poison-containing fuel rod 24 in four directions of the X and Y directions. The fuel rods marked with G in the figure indicate the fuel rods 24 and 29 containing gadolinia.

Further, in the core of a nuclear reactor loaded with two or more types of initially-loaded fuel assemblies having different enrichments, the initially-enriched fuel assembly having the highest enrichment has the at least one burnable poison-containing fuel rod having X and X. A fuel assembly facing another burnable poison-containing fuel rod in four directions in the Y direction, or a double tube structure comprising an outer tube and an inner tube, formed of an outer tube and an inner tube; A fuel assembly having at least one water rod provided with a burnable poison in the annular portion, and the first-load fuel assembly having a lower enrichment than the highest-enrichment initially-loaded fuel assembly removes the burnable poison. It is configured not to contain, or to contain a burnable poison having a shorter reactivity life than the burnable poison contained in the fuel assembly with the highest enrichment initially loaded.

作用 As a function of the fourth invention group, the reactivity value of the burnable poison can be maintained for a long time. FIG. 4 shows various arrangements of gadolinia-containing fuel rods, which are burnable poisons. In FIG. 4A shows a fuel assembly 25 having a conventional fuel rod arrangement, and FIG. 4B shows a fuel assembly 26 having an arrangement based on the invention disclosed in Patent Document 3, and FIG. 4) and FIG. 4 (d) are fuel assemblies having a fuel rod arrangement based on the fourth invention group.

で は In these four fuel assemblies 25 to 28, the number of gadolinia-containing fuel rods 24 and 29 is adjusted so that the infinite multiplication factor at the beginning of combustion becomes equal. The infinite multiplication factors of the fuel assemblies 25 to 28 in the case of using gadolinia having a concentration of 7.5% are shown by curves 25a to 28a in the characteristic diagram of FIG.

In the fourth invention group, the burnable poison-containing fuel rod 29 surrounded in four directions of the X and Y directions by the burnable poison-containing fuel rod 24 prevents neutrons from being incident by the burnable poison surrounding the fuel rod. Therefore, the burnable poison of the surrounding fuel rods 24 containing burnable poison does not act as a burnable poison until the burnable poison is burned and disappears, and the burnable poison disappears and also acts as a burnable poison.

Accordingly, even a low-concentration burnable poison acts as a burnable poison for a sufficiently long period of time, so that the excess reactivity in the first and second cycles can be controlled. Further, by using the fuel assembly of the fourth invention group, the combustion change of the surplus reactivity in the second cycle can be made flat, so that the operation of the control rod is simplified and the operation of the reactor is facilitated. Become.

Generally, the low-enrichment fuel assembly 18 has a softer neutron spectrum than the high-enrichment fuel assemblies 16 and 17, and thus burns burnable poisons faster. Therefore, if the burnable poison concentration is set so that it disappears at the end of the second cycle in the high-enrichment initially loaded fuel, and this is also used for the low-enrichment initially loaded fuel, the flammability of the low-enrichment initially loaded fuel is reduced. Since the toxic poison disappears at the end of the first cycle or in the middle of the second cycle, the infinite multiplication factor decreases with combustion after the middle stage of the second cycle.

Therefore, in order to flatten the combustion change of the excess reactivity in the second cycle, the infinite multiplication factor of the high-enrichment initially loaded fuel in the second cycle must increase with the combustion. That is, it is desirable that the combustion change of the high-enrichment initially loaded fuel at the infinite multiplication factor be flat in the first cycle and increase in the second cycle. However, the neutral absorption effect of a burnable poisoned fuel rod generally decreases almost linearly because the surface area decreases with combustion.

However, in the fuel assembly according to the fourth invention group, the neutron absorption effect of the burnable poison surrounding the four directions decreases with combustion, and the neutron absorption effect of the central burnable poison-containing fuel rod increases. Therefore, the neutron absorption effect of the burnable poison as a whole is maintained at a constant value for a relatively long time. Therefore, by appropriately setting the number of gadolinia-containing fuel rods 24 and 29, which are burnable poisons, the above-described desirable infinite multiplication factor can be achieved.

Further, the infinite multiplication factor similar to that described above is a water rod having a double pipe structure composed of an outer pipe and an inner pipe, in which an annular portion formed between the outer pipe and the inner pipe is filled with a burnable poison. Can be realized in a fuel assembly having at least one fuel assembly. When the burnable poison has an annular structure, the surface area of the area occupied by the burnable poison hardly changes with combustion, and its density decreases.

However, in the case of a substance with a large neutron absorption cross-section, such as gadolinium, the neutron absorption effect is determined by the surface area alone regardless of the density if the density is a certain level or more. Maintains a constant value and then decreases rapidly. As a result, the desired infinite multiplication factor described above is obtained.

As described above, according to the present invention, in the reactor core in which the average enrichment of the initially loaded fuel is significantly increased, the excess reactivity in the first cycle and the second cycle is controlled to an appropriate range, and the reactor shutdown margin is sufficient. As a result, the removal burn-up of the initially loaded fuel is increased, and the fuel economy is greatly improved.

(4) Since refueling is not performed after the end of the first cycle, the periodic inspection process of the nuclear reactor can be simplified, and the period of the periodic inspection in the nuclear power plant can be shortened. Further, there is an effect that the soundness of the fuel can be maintained by avoiding the excessive concentration of the burnable poison.

An embodiment of the present invention will be described with reference to the drawings. Note that the same components as those of the above-described conventional technology are denoted by the same reference numerals, and detailed description thereof will be omitted.

炉 The core of the first embodiment is shown in the fuel arrangement diagram of FIG. FIG. 6 shows a quarter of the core. FIG. 6A shows a first cycle core 30a, and FIG. 6B shows a second cycle core 30b, in which three types of initially loaded fuel are loaded. I have. In the figure, the fuel assemblies 31 indicated by 1 in the four corners and the fuel assembly 32 indicated by 2 are high-enrichment initially loaded fuels having the same enrichment of 3.7% as the replacement fuel, and the fuel indicated by L The assembly 33 is a low-enrichment initially loaded fuel with an enrichment of 1.6%.

A portion in which four fuel assemblies 31 to 33 are arranged around one control rod 13 is called a cell, and the fuel assemblies 33, which are low-enrichment initially loaded fuels, are provided in all four fuel assemblies. The arranged portion is indicated by a bold frame in the low reactivity cell 14.

In the core 30a of the first cycle, 372 fuel assemblies 31 (1) of high enrichment fuel and 316 fuel assemblies 32 (2) of a total of 668, and fuel assembly 33 of low enrichment fuel are used. In (L), a total of 872 bodies of 184 bodies are loaded, and the average enrichment of this initially loaded core is 3.3%.

7 shows the enrichment of each of the initially loaded fuel assemblies 31 to 33 and the replacement fuel assembly 34 and the configuration of gadolinia-containing fuel rods according to the fuel configuration diagram of FIG. For example, “3.9e, 10G 7.0” in each of the regions 35 to 42 in FIG. 7 indicates “having ten enriched 3.9% fuel rods containing gadolinia with a concentration of 7.0%”. "4.2e, 10G 9.0" indicates that "the enrichment is 4.2% and there are 10 fuel rods containing 9.0% gadolinia containing gadolinia."

Further, each of these fuels is a fuel for high burn-up having a shape similar to that of the fuel assembly shown in FIG. 21 described above. Is provided.

This high enrichment fuel has two types of fuel rods containing gadolinia only in the lower part and has a different number of fuel rods, and is different from the first fuel assembly A high enrichment fuel assembly 31 shown in FIG. The B enrichment fuel assembly 32, which is the second fuel assembly shown in (b), has one more fuel rod containing gadolinia only in the lower region 39.

Among the gadolinia-containing fuel rods in the A and B high-enrichment fuel assemblies 31 and 32, ten fuel rods containing gadolinia over the entire length excluding the natural uranium regions at the upper and lower ends are shown in the sectional view of FIG. The fuel assemblies 31 and 32 are formed by arranging two sets of five fuel rods adjacent to each other like a gadolinia-containing fuel rod 43 indicated by G in a cross shape.

ガ In addition, gadolinia with a maximum concentration of 10% is used, and the excess reactivity in the first and second cycles is flattened together with the arrangement of gadolinia-containing fuel rods. The low enrichment fuel assembly 33 shown in FIG. 7C has two gadolinia-containing fuel rods except for the natural uranium regions at the upper and lower ends.

Next, a reference invention related to the low enrichment fuel assembly will be described. In this reference invention, the core of a nuclear reactor is constituted by arranging a large number of cells, each of which comprises a single control rod and four fuel assemblies surrounding the control rod, in a square lattice. Are loaded with two or more different fuel assemblies.

The core may be such that almost all of the cells except the outermost periphery thereof include at least one low enrichment initially loaded fuel assembly having a lower enrichment than the highest enrichment initially loaded fuel assembly, or the X, The low-enrichment initially loaded fuel assembly faces at least two of the low-enrichment initially-loaded fuel assemblies on any surface in the Y direction, or the diluent direction of the low-enrichment initially-loaded fuel assembly at at least three of the four corners of the cell. Or at least one of four corners of the cell facing the low-enrichment initially loaded fuel assembly at one of the X and Y directions of the cell and at least one of the four corners of the cell. The fuel assembly is either diagonally adjacent to the fuel assembly.

A high-reactivity cell 19 composed of four high-enrichment initially-loaded fuel assemblies 16 and 17 and a low-reactivity cell 14 composed of four low-enrichment initially-loaded fuel assemblies 18 are also provided. In almost all of the cells except for the outermost periphery of the core, the high-reactivity cell 19 faces the low-reactivity cell 14 in at least one of the X and Y directions, and the low-reactivity cell 14 At least one low-enrichment initially loaded fuel assembly 18 is loaded in the cell not facing in either the X or Y direction.

(4) As an effect of the reference invention, a sufficient reactor stop margin can be secured in a core having a significantly increased average enrichment. That is, as shown in the main part fuel arrangement diagram of FIG. 3 (a), in the fuel arrangement in the 6-row, 6-column portion of the core, the control rod value of the control rod 13a at the black circle position is such that the control rod 13a surrounds the control rod 13a. It greatly depends on the reactivity of the position A of the body fuel, the position B of the eight fuels further surrounding them, and the position C of the four fuels.

The characteristic diagram of FIG. 3B is based on the control rod value of the control rod 13 when all the fuel is the high enrichment fuel assembly 16 (1) and the high enrichment fuel assembly 17 (2). This shows that the loading of the low enrichment fuel assembly 18 (L) improves the furnace stop margin.

The effect of replacing one of the high enrichment fuel assemblies 16 and 17 with one of the low enrichment fuel assemblies 18 is greatest at the fuel position A, and decreases in the order of position B and position C. From FIG. 3B, one low enrichment fuel assembly 18 is placed at the position A, two at the position B, three at the position C, or one at the positions B and C. It can be seen that the arrangement can improve the furnace stop margin to the same extent.

Therefore, based on this reference invention, by loading the low-enrichment fuel assembly 18 preferentially from a location that is effective for improving the reactor shutdown margin, the number of loaded members of the low-enrichment fuel assembly 18 can be minimized. Therefore, the number of loaded bodies of the high enrichment fuel assemblies 16 and 17 increases by that much, and the average enrichment of the initially loaded fuel can be greatly increased.

6) In the core 30a of the first cycle shown in FIG. 6 (a), 21 low-reactivity cells 14 composed of low-enrichment fuel assemblies 33 (L) are provided in all cores. The low-enrichment fuel assembly 33 is not only loaded in the low-reactivity cell 14, but also loaded almost uniformly in the core according to the reference invention.

At all other positions, high enrichment fuel assemblies 31 (1) and high enrichment fuel assemblies 32 (2) are loaded, and a part of the outermost core cell and the low reactivity cell 14 have X and Most of the cells adjacent to each other in one of the Y directions are the high-reactivity cells 19 constituted only by the high-enrichment fuel assemblies 31 and 32.

As a result of this fuel arrangement, a constant surplus reactivity of about 1.5% Δk and a sufficient reactor shutdown margin of 1% Δk or more are secured in the initially loaded core where the average enrichment of initially loaded fuel is increased to 3.3%. We were able to.

The core 30b of the second cycle shown in FIG. 6 (b) does not replace the initially loaded fuel with the replacement fuel assembly 34 after the end of the first cycle, but instead forms a diagonal line 23 that divides the core 30b into two concentric circles. At the boundary, the fuel assembly in the peripheral region and the fuel assembly in the internal region are simply replaced.

Furthermore, the low-enrichment fuel assembly 33 loaded in the low-reactivity cell 14 in the first cycle is replaced with the low-enrichment fuel assembly 33 loaded in other than the low-reactivity cell 14. Since the combustion of the fuel loaded in the low-reactivity cell 14 does not proceed, the fuel transfer can make the combustion of the low-enrichment fuel assembly 33 uniform.

Further, in the low-enrichment fuel assembly 33 loaded in the low-reactivity cell 14, since the combustion of the fuel rod near the control rod insertion side does not particularly proceed, when the control rod 13 is pulled out at the end of the cycle, this fuel rod 13 can cause excessive output peaks.

This tendency becomes more remarkable as the period of time in which the low-reactivity cells 14 are loaded is longer. Therefore, as in the first embodiment, by replacing the low-enrichment fuel assemblies 33 that are loaded in the low-reactivity cells 14, An excessive output peak can be prevented from occurring at the end of the second cycle.

The characteristic diagram of FIG. 9 compares the surplus reactivity of the second cycle shown by the curve 44 in the first embodiment with the surplus reactivity shown by the curve 45 when no fuel transfer is performed after the end of the first cycle. It was done. The reactivity at the end of the second cycle is about 0.7% Δk short if no fuel transfer is performed, and in order to operate the reactor until the end of the second cycle, the average enrichment of the initially loaded fuel must be determined in this example. About 0.1% more than that. However, in this case, the excess reactivity becomes higher than the curve 45 in FIG. 9 by about 0.7% Δk, and becomes a large excess reactivity of 2.5% Δk at the maximum.

However, in the first embodiment, the excess reactivity is smaller in the second cycle than in the first cycle, but the number of low reactivity cells 14 is larger in the second cycle than in the first cycle. This is because the same low-enrichment initially-loaded fuel assembly 33 is loaded in the low-reactivity cell 14 in both the first cycle and the second cycle, and as the combustion progresses and the fuel reactivity decreases, the control rod 13 decreases. Due to a decrease in the reactivity value of

Further, in the first cycle, the core peripheral region is lower than the core inner region with respect to the diagonal line 23 where the core 30a is concentrically divided into two so that the number of fuel assemblies included in each region becomes substantially equal. The number of reactivity cells 14 is large. This suppresses the combustion of the fuel loaded in the core peripheral region and moves it to the core internal region in the second cycle, thereby helping to achieve the effect shown in FIG. 9.

After the second cycle, all the 184 low-enrichment initially loaded fuel assemblies 33 are removed from the core 30b, and the 3.7% enriched replacement fuel assemblies 34 shown in FIG. 7D are loaded into the core. Thus, the operation of the third cycle is performed. In the first and second cycles of the first embodiment, the high-enrichment initially loaded fuel assemblies 31 and 32 are loaded on the outermost periphery of the core, which not only contributes to the reduction of the excess reactivity but also reduces the second reactivity. By disposing the low enrichment fuel assembly 33 taken out of the core 30b after the cycle is completed in the core for a long period of time, the taken-out burnup is increased.

On the other hand, in the core after the third cycle, the fuel with the most advanced combustion and reduced reactivity is loaded on the outermost circumference of the core, and the leakage of neutrons from the core is reduced to increase the reactivity of the core. Improves combustion efficiency.

As a result of repeating such refueling, the average withdrawal burnup of all 872 initially loaded fuels according to the first embodiment is about 38 GWd / t, and the average withdrawal burnup of the replacement fuel 34 with an enrichment of 3.7% is 45 GWd / t. t reached 0.84 times. Further, the average unloading burnup of the conventional initially loaded fuel having an average enrichment of 2.1% is about 25 GWd / t. As a result, the average unloading burnup of the initially loaded fuel is greatly increased by the present invention. It could be increased by about 50%.

The enrichment and gadolinia of each initially loaded fuel loaded in the core are distributed in the axial direction as shown in FIG. 7, and the change in combustion of each fuel is represented by the infinite multiplication factor shown in the characteristic diagram of FIG. The operation will be described below. Note that the infinite multiplication factor of each region in the axial direction in FIG. 10 is represented by the same reference numeral as in each region shown in FIG.

First, in each of the two types of A and B high enrichment fuel assemblies 31 and 32, based on the invention described in Patent Document 4, the number of gadolinia-containing fuel rods below the upper end of the short fuel rod 7 is determined. Have many. In the high burn-up fuel used in the present embodiment, the characteristics of the infinite multiplication factor are significantly different between the upper and lower parts due to the presence of the short fuel rods 7, and the upper part has less fuel rods and more moderator than the lower part. The neutron absorption effect of gadolinia is larger at the top than at the bottom.

Therefore, if the number of gadolinia-containing fuel rods is the same at the top and bottom, the infinite multiplication factor at the beginning of combustion is smaller at the upper part than at the lower part, and the axial power distribution has a lower peak. Therefore, as in the first embodiment, as a result of increasing the number of gadolinia-containing fuel rods below the upper end of the short fuel rod 7 above, the infinite multiplication factor is such that the upper part is lower as shown by the curves 36a and 37a in FIG. And the axial power distribution is flattened. In the B high-enrichment fuel assembly 32, one fuel rod containing gadolinia is added in the lower region 38, and the axial power distribution is further flattened.

The characteristic diagram of FIG. 11 shows that the curve 46 of the axial power distribution in the middle stage of the first cycle in the first embodiment is used as the highly enriched fuel loaded in the first cycle of the core 30a shown in FIG. A comparison is made between a curve 47 when only the high enrichment fuel assembly 31 shown in FIG. 7A is used and a curve 48 when only the high enrichment fuel 32 shown in FIG. 7B is used. It was done. However, for comparison, just above the natural uranium region at the lower end of the A high enrichment fuel assembly 31 shown in FIG. 7A, the B enrichment fuel assembly 32 shown in FIG. Area 40 to be used.

As can be seen from FIG. 11, the lower peak is large in the curve 47, and the number of gadolinia-containing fuel rods in the lower region is increased by one from the upper region at the upper end of the short fuel rod 7 (arrow in FIG. 7). By itself, the downward distortion of the power distribution cannot be sufficiently corrected. On the other hand, as indicated by the curve 48, when the number of lower gadolinia-containing fuel rods is further increased by one in all the high enrichment fuel assemblies, the lower output becomes too low.

Therefore, as in the present invention, an appropriate axial power distribution is achieved by preparing two types of high-enrichment fuels having different numbers of fuel rods containing gadolinia only in the lower region and adjusting the ratio of the number of loaded bodies. can do.

In each of the high-enrichment fuel assemblies 31 and 32, the gadolinia concentration in the lower regions 38 and 39 is higher than that in the upper region. Generally, in a boiling water reactor, the power is distorted downward, so that the combustion in the lower region proceeds more than in the upper region.

However, since the infinite multiplication factor of the highly enriched fuel increases with combustion in the second cycle, when the upper and lower gadolinia concentrations are the same, the infinite multiplication factor of the lower burning portion is larger than that of the upper portion. As a result, the lower peak of the output will be further emphasized. Therefore, by making the gadolinia density of the lower region higher than that of the upper region, the axial power distribution can be flattened.

In the first embodiment, in addition to the gadolinia distribution, the enrichment of the lower regions 38, 39, and 40 is lower than that of the upper regions 36 and 37, which also flattens the axial power distribution. However, in the low enrichment fuel assembly 33 shown in FIG. 7 (c), the gadolinia content was made uniform vertically. As can be seen from the curves 41a and 42a in FIG. 10, when the number of gadolinia-containing fuel rods is small, the reactivity of gadolinia above and below the upper end of the short fuel rod 7 (arrow in FIG. 7). This is because the difference in value has a small effect on the axial power distribution.

However, in each fuel rod, the enrichment is uniform in the axial direction except for the upper and lower end natural uranium regions in the hatched portion, and the enrichment of the short fuel rod 7 is the lowest enrichment used in the fuel assembly. This substantially gives the axial enrichment distribution.

Here, the low enrichment fuel assembly 33 uses gadolinia with a concentration of 7.5%, and its reactivity value has almost disappeared in the first cycle. As a result, the infinite multiplication factor is substantially constant in the first cycle, and decreases with combustion in the second cycle. By combining this infinite multiplication factor with the infinite multiplication factor of the highly enriched fuel, which is substantially constant in the first cycle and increases with combustion in the second cycle, the combustion change of the excess reactivity in the first and second cycles is reduced. Can be flat.

In the first embodiment, in any high enrichment fuel, the gadolinia concentration is reduced in the region 34 immediately below the natural uranium at the upper end to increase the infinite multiplication factor, and the output in this region is increased to increase the axial power distribution. Flattened. In this core, most of the fuel is highly enriched fuel, and its infinite multiplication factor is almost constant with the combustion in the first cycle and rises with the combustion in the second cycle, as shown by a curve 36a in FIG.

Therefore, if the gadolinia concentration is the same, in the second cycle, the infinite multiplication factor is lower in the region 35 near the upper end where the combustion does not proceed as compared with the region 36 where the combustion proceeds. As a result, in the third cycle, the infinite multiplication factor of the region 35 where combustion has not progressed is larger than the region 36 where combustion has advanced and has already exceeded the peak of the infinite multiplication factor, and an output peak occurs in this region 35. become. Therefore, in the present embodiment, the gadolinia concentration in the region 35 is reduced, and in particular, the infinite multiplication factor is increased in the second cycle to promote combustion.

The characteristic diagram of FIG. 12 shows the axial output distribution at the end of the third cycle. The curve 49 of the first embodiment has a more power distribution than the curve 50 when the gadolinia concentration in the region 34 is the same as that in the region 36. It has been flattened. Further, as a result of reducing the gadolinia concentration, the unburned gadolinia at the end of the second cycle, which causes the reactivity loss, can be reduced, and the reactivity gain is obtained.

In the replacement fuel assembly 34, Patent Document 5 discloses an invention in which the burnable poison concentration immediately below the natural uranium region at the upper end is lower than the burnable poison concentration below the natural uranium region. On the other hand, in the present invention, the difference between the burnable poison concentration in the region immediately below the natural uranium at the upper end and the burnable poison concentration below that is set larger than the replacement fuel assembly 34 in the initially loaded fuel. It is characteristic.

The gadolinia concentration difference in the first embodiment is 1% for the replacement fuel 34, and 2% for the B-rich enrichment initially loaded fuel assembly 32. In this embodiment, the gadolinia concentration at the two nodes is reduced as the region 35. However, if the gadolinia concentration is reduced over three or more nodes, an output peak occurs in this region at the end of the first cycle. Is appropriate.

In the first embodiment, the enrichment of the region 35 is made lower than that of the region 36 to avoid deterioration of the furnace stop margin due to an increase in the infinite multiplication factor of the region 35 at the end of the second cycle. However, the output peak in this region when the furnace is stopped is not extremely large in the second cycle in which combustion has not progressed so much as in the equilibrium cycle. Therefore, it is possible to satisfy the design criteria without lowering the enrichment.

Further, since the output in the region immediately above the natural uranium at the lower end is also low, the number and concentration of gadolinia-containing fuel rods in this region 40 in the B high-enrichment fuel assembly 32 are reduced infinitely by making them smaller than the region 39 thereabove. The magnification is increased and the output of the region 40 is increased to flatten the axial power distribution.

FIG. 13 is a characteristic diagram showing the axial output distribution in the middle stage of the third cycle. The curve 51 of the first embodiment is more output than the curve 52 when the gadolinia content of the region 40 is the same as that of the region 39. It can be seen that the distribution is flattened. In the first embodiment, ten gadolinia-containing fuel rods of the B high-enrichment fuel assembly 32 are used. However, ten fuel rods are used for the A high-enrichment fuel assembly 31 and 11 are used for the B high-enrichment fuel assembly 32. Good as a book.

The second embodiment employs the fuel assembly shown in the sectional view of FIG. This fuel assembly 53 has a double pipe structure consisting of an outer pipe 54 and an inner pipe 55 at substantially the center of a plurality of bundled fuel rods, and a cermet of gadolinia and zirconium in an annular portion 56 sandwiched between them, or The water rod 57 is provided with two burnable poisons such as gadolinium metal.

However, five fuel rods 43 containing gadolinia indicated by G are used in combination in the fuel rods, and one of them includes gadolinia only below the upper end of the short fuel rod 7.

As an example of the above configuration, a cermet containing gadolinia, which is a 40% concentration of burnable poison, is used in the annular portion 56 of the water rod 57 containing burnable poison, and corresponds to the lower region 37 in FIG. 7A. FIG. 15 is a characteristic diagram showing the infinite multiplication factor of the region in which the color shift occurs.

The infinite multiplication factor in this fuel assembly 53 is such that the gadolinia contained in the water rod 57 is relatively long as shown by the curve 59, compared to the infinite multiplication curve 58 when no burnable poison is contained. Maintains a constant neutron absorption effect for a while, then decreases rapidly. However, since five fuel rods 43 containing gadolinia having a concentration of 7% are arranged, a good infinite multiplication factor as shown by a curve 60 is achieved.

In the fuel assembly 53, when the gadolinia concentration is distributed in the axial direction as in the first embodiment, the gadolinia concentration is provided not on the gadolinia contained in the fuel rod 43 but on the annular portion 56 of the water rod 57. What is necessary is just to distribute gadolinia density.

Thereby, the combustion change of the infinite multiplication factor of the high-enrichment initially loaded fuel which is desirable for flattening the combustion change of the excess reactivity of the first cycle and the second cycle in the reactor core, that is, the combustion change of the first cycle is flat. In the second embodiment, a large number of gadolinia-containing fuel rods 43 are used in a cross-like arrangement as shown in FIG. 8 in order to achieve an infinite multiplication factor that increases in the second cycle. In the embodiment, this number can be reduced.

は The third embodiment employs the fuel assembly shown in the fuel configuration diagram of FIG. That is, the initially loaded high-enrichment fuel assembly loaded in the cores 30a and 30b shown in FIG. 6 is replaced by the first highly-enriched fuel assembly 61 which is the first fuel assembly shown in FIG. It is assumed that the fuel assembly 62 is a D-rich enrichment initially loaded fuel assembly 62 which is the second fuel assembly of FIG.

In this fuel assembly, in the first embodiment, the natural uranium regions at the upper and lower ends of the A and B high enrichment fuel assemblies 31 and 32 shown in FIGS. It is an extension of the area, with an average enrichment of 4.1%.

When increasing the average enrichment in the fuel assembly, the enrichment of the internal enrichment region may be increased while the natural uranium regions arranged at the upper and lower ends are not changed. Therefore, the axial output distribution can be easily flattened.

(4) The length of the natural uranium region at the upper and lower ends may be shorter than that of the replacement fuel assembly 34, so that the enrichment of the internal enrichment region may be slightly increased. In the C and D high enrichment fuel assemblies 61 and 62 shown in FIG. 16, if the burnable poison content at the upper and lower ends is further reduced, the axial power distribution can be further flattened.

Alternatively, in the core 30a of the first cycle shown in FIG. 7 (a), only the high enrichment initially loaded fuel arranged on the outermost periphery of the core is transferred to the C high enrichment fuel assembly 61 of FIG. 16 (a). It may be replaced.

When high-enrichment initially loaded fuel with high enrichment is loaded into the core, there is a concern that the radial output peaking will increase and the reactor shutdown margin will deteriorate.However, loading the high-enrichment fuel assembly on the outermost periphery of the core However, such a problem does not occur, but rather has the favorable result that the radial output peaking is flattened.

According to the third embodiment, the average unloading burnup of the initially loaded fuel in the first embodiment is about 38 GWd / t, of which the low enrichment initially loaded fuel assembly 33 is about 20 GWd / t, A, B high enrichment initially loaded fuel assemblies 16, 17 were about 42 GWd / t. That is, the average withdrawal burnup of the A and B high enrichment initially loaded fuel assemblies 16 and 17 is smaller than the average withdrawal burnup of 45 GWd / t of the replacement fuel assembly 34 having the same enrichment.

This means that all replacement fuel assemblies 34 are loaded into the core during the fourth or fifth cycle, while some high enrichment initially loaded fuel is removed from the core after three cycles. Because.

In consideration of the fact that the soundness of the fuel depends not on the enrichment but on the removal burn-up, the enrichment of the C and D high enrichment initially loaded fuel assemblies 61 and 62 of the third embodiment is further increased. By increasing the removal burn-up to the same level as the replacement fuel assembly 34, the economic efficiency can be further improved.

を The fourth embodiment is shown in the fuel arrangement diagram of the core in FIG. In the first cycle, the core 63 is loaded with three kinds of initially loaded fuels. The fuel assemblies indicated by 1 or 2 in the four corners in the figure are the fuel configuration diagrams (a) and ( The fuel assemblies 64 and 65 with high enrichment initially loaded in E and F shown in b) (the same enrichment as the replacement fuel assembly 34), and the fuel assembly indicated by L are also shown in FIG. 7C. This is a low-enrichment initially loaded fuel assembly 33 with an enrichment of 1.6%.

The core 63 has 384 E high-enrichment fuel assemblies 64 (1), and the F high-enrichment fuel assembly 65
There are a total of 684 low-enrichment fuel assemblies 33 (L), with a total of 872 of the low-enrichment fuel assemblies 33 (L). In the third embodiment, forty-five low-reactivity cells 14 are provided, and 180 low-enrichment fuel assemblies 33 are loaded in these low-reactivity cells 14 and eight low-reactivity fuel assemblies 33 are provided in the peripheral region of the core.

There are two types of high-enrichment fuels having different numbers of gadolinia-containing fuel rods. The first E-rich enrichment fuel assembly 64 shown in FIG. There are two more fuel assemblies than the F high enrichment fuel assembly 65 which is the second fuel assembly shown in b).

In the above-described first embodiment, two types of A and B high-enrichment fuel assemblies 31 and 32 having the same number of gadolinia-containing fuel rods except for a part of the lower portion are used. Reactivity could be achieved.

However, as can be seen from a comparison between the curves 39a and 40a in FIG. 10, for example, the reactivity value of one fuel rod containing gadolinia is 3 to 4% Δk. When the excess reactivity must be adjusted, it is difficult to cope with only one type of high enrichment fuel.

In such a case, as in the fourth embodiment, two types of E, F high-enrichment initially loaded fuel assemblies 64, 65 having different fuel rods containing gadolinia are prepared, and the number of the loaded bodies is adjusted. By doing so, it becomes possible to easily set the surplus reactivity in an appropriate range.

Further, in the fourth embodiment, since the cell 66 not adjacent to the low reactivity cell 14 has a larger control rod value than the adjacent cell 67, the cell 66 not adjacent to the low reactivity cell 14 is replaced by the number of fuel rods containing gadolinia. The high E enrichment fuel assembly 64 (1) having a large number of fuel assemblies 64 (1) and the F high enrichment fuel assembly 65 (2) having a small number of gadolinia-containing fuel rods are constituted by one assembly to improve the margin for stopping the furnace. .

In addition, the E enriched fuel assembly 65 having a large number of gadolinia-containing fuel rods at the outermost periphery of the core
In addition to loading (1), the second layer from the outermost periphery of the core is loaded with an F high enrichment fuel assembly 65 (2) having a small number of gadolinia-containing fuel rods to reduce core radial output peaking.

In the fourth embodiment, the E high enrichment fuel assembly 64 having a large number of gadolinia-containing fuel rods loaded on the outermost periphery of the core is replaced with a fuel assembly in the core inner region after the first cycle. The excess reactivity in the second cycle is sufficiently controlled. In addition, when the fuel is loaded at the same position in the reactor core, the fuel with a smaller number of gadolinia-containing fuel rods tends to increase the axial output peaking. Therefore, in the fourth embodiment, only the lower portion contains gadolinia. One more fuel rod is provided in the F high enrichment fuel assembly 65.

However, in addition to the F high enrichment fuel assembly 65, the E high enrichment fuel assembly 64 loaded on the outermost periphery of the core in the first cycle is increased by one fuel rod containing gadolinia only at the lower portion. Well. In this case, by the fuel movement after the end of the first cycle, the excess reactivity can be sufficiently reduced in the second cycle, and the axial power distribution can be flattened.

In the fourth embodiment, 45 low-reactivity cells 14 are provided, and the core 63 is divided into two concentric circles by a diagonal line 23, and the area around the core is lower than the low-reactivity cells 14 in the core inner area. In the low-reactivity cell 14, the first cycle operation is performed by inserting more control rods 13.

(4) After the completion of the first cycle, the high-enrichment fuel assembly in the core peripheral region and the high-enrichment fuel assembly in the core inner region are replaced to constitute the second cycle core. Further, in the second cycle, the control rod 13 is inserted into the low-reactivity cell 14 composed of the low-enrichment initially-loaded fuel assembly 33 which has been loaded in the low-reactivity cell 14 in which the control rod 13 has not been inserted in the first cycle. And drive.

As a result, the control of the excess reactivity in the second cycle and the combustion efficiency are improved, and the output peaking of the fuel rod 13 on the control rod insertion side of the low-enrichment fuel assembly 33 at the end of the second cycle is increased. can avoid.

In the fifth embodiment, as shown in the fuel arrangement configuration diagram of FIG. 19, the fuel arrangement of the core 68 in the first cycle has three types of initial loadings having different enrichments indicated by H, M, and L in four corners, respectively. Each of the fuel assemblies is composed of 480 high-enrichment fuel assemblies 69 (H) with an average concentration of 4.1%, and 208 enrichments with medium-enrichment fuel assemblies 70 (M) with an average concentration of 2.8%. There are 184 low-enrichment fuel assemblies 33 (L) with a 1.6% degree, and a total of 872 bodies are loaded. The average enrichment of the initially loaded fuel is 3.3%.

In the above-described first embodiment and the like, two types of initially loaded fuels of the high enrichment fuel assemblies 31 and 32 and the low enrichment fuel assembly 33 are used in the first embodiment as an example. All enriched fuel assemblies 33 are removed after the second cycle, while high enriched fuel assemblies 31, 32 are removed in the third through sixth cycles.

Therefore, even if the enrichment of some of the high enrichment fuel assemblies 61 and 62 is increased as in the third embodiment, the fuel removed from the core after the end of the third cycle still has a large amount of fissile material. Will remain.

Therefore, in the fifth embodiment, 216 bodies taken out of the core after the end of the third cycle have an intermediate enrichment.

Thereby, the remaining amount of fissile material at the time of removal can be reduced, so that nuclear fuel resources can be effectively used. However, since the average enrichment of the initially loaded fuel is reduced as it is, the high enrichment fuel assembly 69 in the fifth embodiment.
The enrichment of (H) is set to 4.1%, which is higher than 3.7% for the replacement fuel assembly 34.

The low-enrichment fuel assembly 33 (L) is loaded not only in the 37 low-reactivity cells 14 but also in the area around the core to improve the reactor shutdown margin. The medium-enrichment fuel assembly 70 (M) has a low-reactivity cell 14 and one cell in each of the cells 71 except for a part of the cells around the core. It is also located in.

As already mentioned, the lower the enrichment, the softer the neutron spectrum and the shorter the reactivity life of the burnable poison, so gadolinia contained in the medium enrichment fuel assembly 70 is Disappears faster than gadolinia contained in body 69. Therefore, in the fifth embodiment, the middle enrichment fuel assembly 70 is disposed at the outermost periphery of the core to suppress its combustion, thereby reducing the excess reactivity in the second cycle.

In FIG. 19, the middle enrichment fuel assembly 70 is arranged on the outermost periphery of the core, but the low enrichment fuel assembly 33 or the high enrichment fuel assembly 69 may be arranged. Further, as the types of enrichment of the initially loaded fuel, two types are used in the first to fourth embodiments, and three types are used in the fifth embodiment. On the other hand, as shown in FIG. It is also possible to use four types of enrichment initially loaded fuel assemblies of the high enrichment fuel assemblies 2 to 4 and the low enrichment fuel assembly 5.

In this case, as the low-reactivity cell 14 and the fuel to be loaded to improve the margin for shutting down the furnace, the lowest enrichment fuel assembly 5 or the second lowest enrichment initially loaded fuel assembly 4 can be used. . In the first cycle, the initially loaded fuel disposed at the outermost periphery of the core includes the lowest enrichment fuel assembly 5, the second lowest enrichment fuel assembly 4, and the third lowest enrichment fuel assembly 3 , The highest enrichment fuel assembly 2 or a combination thereof.

In the sixth embodiment, as the fuel to be loaded for improving the low reactivity cell 14 and the furnace shutdown margin, any fuel having low reactivity may be used. For example, the low enrichment shown in FIG. Instead of the high fuel assembly 33, the third gadolinia-rich fuel rod having a higher gadolinia concentration and a higher gadolinia concentration than the A and B high enrichment fuel assemblies 31 and 32 shown in FIGS. Using a high enrichment fuel assembly.

In other words, in the above-described first to fifth embodiments, the initially loaded fuel assemblies with low enrichment are arranged for the low-reactivity cell 14 and for improving the margin for stopping the furnace. However, the fuel loaded for the low reactivity cell 14 and the furnace stop margin improvement fuel may be a fuel with low reactivity, and need not necessarily be the low enrichment fuel assembly 33.

Therefore, the use of the third high-enrichment fuel assembly makes it possible to reduce the time from the beginning of the first cycle to the end of the second cycle as compared with the A-rich fuel assembly 31 and the B-rich fuel assembly 32. Since the reactivity is sufficiently low during the combustion period, it can be used as a low-reactivity cell 14 and a low-reactivity fuel assembly for improving the shutdown margin of the furnace.

In this case, since the average enrichment of the initially loaded fuel can be further increased as compared with the first embodiment using the low enrichment fuel assembly 33, the take-out burnup of the initially loaded fuel is increased and the economy is increased. The effect that the property is improved is obtained.

1A is a first cycle and FIG. 2B is a second cycle of a fuel arrangement of a quarter core according to an embodiment of the present invention. FIG. 4 is a characteristic diagram showing the average enrichment of the initially loaded fuel and the maximum value of the excess reactivity in the first cycle of the embodiment according to the present invention. FIG. 1A is a configuration diagram of a control rod and a fuel arrangement of a core part of a core according to an embodiment of the present invention, and FIG. FIG. FIGS. 4A and 4B are cross-sectional views showing the arrangement of various gadolinia-containing fuel rods in a fuel assembly, wherein FIGS. 4A and 4B show prior art examples, and FIGS. 4C and 4D show one embodiment of the present invention. FIG. 4 is a characteristic diagram showing a combustion change at an infinite multiplication factor with respect to the arrangement of gadolinia-containing fuel rods in a fuel assembly. 1A is a first cycle, and FIG. 2B is a second cycle, showing a fuel arrangement of a 1/4 core according to the first embodiment of the present invention. FIG. 3 is a fuel configuration diagram of the axially enriched fuel and the gadolinia distribution of the initially loaded fuel assembly according to the first embodiment of the present invention, wherein (a) is an A high enriched fuel assembly, and (b) is a B highly enriched fuel assembly. (C) shows a low-enrichment fuel assembly, and (d) shows a replacement fuel assembly. FIG. 2 is a cross-sectional view showing a fuel rod arrangement of the fuel assembly according to the first embodiment of the present invention. FIG. 4 is a characteristic diagram showing surplus reactivity in a second cycle in the core of the first embodiment according to the present invention. FIG. 4 is a characteristic diagram showing a combustion change at an infinite multiplication factor in each axial section of the initially loaded fuel assembly according to the first embodiment of the present invention. FIG. 4 is an axial power distribution characteristic diagram of the core of the first embodiment according to the present invention in the middle of the first cycle. FIG. 4 is an axial power distribution characteristic diagram at the end of a third cycle in the core of the first embodiment according to the present invention. FIG. 4 is an axial power distribution characteristic diagram in the middle stage of the third cycle in the core of the first embodiment according to the present invention. FIG. 6 is a sectional view of a fuel rod arrangement of a fuel assembly according to a second embodiment of the present invention. FIG. 8 is a characteristic diagram showing a combustion change of an infinite multiplication factor of the fuel assembly according to the second embodiment of the present invention. FIG. 8 is a fuel configuration diagram of an axially enriched fuel and a gadolinia distribution of an initially loaded fuel assembly according to a third embodiment of the present invention, wherein (a) is a C enriched fuel assembly, and (b) is a D enriched fuel assembly. Show body. FIG. 11 is a view showing a fuel arrangement configuration of a first cycle in a 1/4 core of a fourth embodiment according to the present invention. FIG. 9 is a fuel configuration diagram of an axially enriched fuel and a gadolinia distribution of an initially loaded fuel assembly according to a fourth embodiment of the present invention, wherein (a) is an E enriched fuel assembly, and (b) is an F highly enriched fuel assembly. Show body. FIG. 13 is a diagram showing a fuel arrangement in a first cycle in a 1/4 core of a fifth embodiment according to the present invention. FIG. 4 is a fuel arrangement diagram of a first cycle in a quarter core of a conventional nuclear reactor. It is sectional drawing of the fuel assembly which has a single fuel rod, (a) is a longitudinal cross section, (b) is the bb arrow cross section of (a), (c) is cc arrow cross section of (a). Is shown.

Explanation of reference numerals

1,63,68… core, 2,16,17,25-28,31,32,53,61,62,64,65,69… high burnup fuel assembly, 3,70… medium burnup fuel assembly Body, 4, 5, 18, 33 ... low burnup fuel assembly, 6 ... long fuel rod, 7 ... short fuel rod, 8 ... water rod, 9 ... spacer, 10 ... upper tie plate, 11 ... lower type Rate, 12: channel box, 13, 13a: control rod, 14: low-reactivity cell, 15a, 30a: first-cycle core, 15b, 30b: second-cycle core, 19: high-reactivity cell, 20 to 22, 25a to 28a, 35a to 42a, 44 to 52, 58 to 60: curves, 23: diagonal lines (boundaries), 24, 29, 43: fuel rods containing gadolinia, 34: replacement fuel assemblies, 35 to 42 ... Combustion zone, 54: outer tube, 55: inner tube, 56: annular portion, 57: water rod containing burnable poison, 66: cell not adjacent to low-reactivity cell, 67: cell adjacent to low-reactivity cell, 71 …cell.

Claims (4)

Concentric circles so that the number of fuel assemblies included in each region is substantially equal in the reactor in which the first loaded fuel assembly is loaded in the first cycle and the replacement fuel assembly is loaded in the second or third cycle or later. In the core divided radially into two, the initially loaded fuel assemblies arranged in the core peripheral region in the first cycle are arranged more in the core inner region than in the core peripheral region in the second cycle. Reactor core. In the core of a reactor loaded with two or more types of initially loaded fuel assemblies having different enrichments, the firstly loaded fuel assemblies having the highest enrichment are arranged on the outermost periphery of the core in the first cycle and the second cycle. 2. The maximum enrichment initially loaded fuel assembly, which is different from the maximum enrichment initially loaded fuel assembly arranged on the outermost periphery of the core in the first cycle, is arranged on the outermost periphery of the core in the second cycle. 3. The reactor core of the described reactor. 3. The reactor core according to claim 1, wherein the number of control rods inserted into the reactor core during the operation of the first cycle is larger in the core peripheral region than in the core inner region. A nuclear reactor core comprising a large number of cells each composed of one control rod and four fuel assemblies surrounding the control rod arranged in a square lattice, wherein two or more types of cells having different enrichments are provided. A low-reactivity cell is configured by loading an initially loaded fuel assembly and four of the initially loaded fuel assemblies having lower enrichment than the initially enriched fuel assembly having the highest enrichment in the first cycle and the second cycle. The reactor core according to claim 1, wherein the number of the low-reactivity cells into which the control rods are inserted during the operation is larger in the second cycle than in the first cycle.

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