JP3318193B2 - Fuel loading method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of loading a nuclear fuel assembly, and more particularly to a method of loading a fuel suitable for use in a boiling water reactor.

[0002]

2. Description of the Related Art Reactors continue to emit energy through a chain reaction in which neutrons are absorbed by fissile material to cause fission, and neutrons released with energy cause the next fission. The state in which this chain reaction is in equilibrium is called critical, and a reactor operated at a constant output keeps this state. In addition, a state in which the chain reaction increases is called supercritical, and a state in which the chain reaction decreases decreases is called subcritical.

Since a nuclear reactor needs to be operated without refueling for a certain period of time, the core is loaded with more fissile material than required for maintaining criticality. Thus, the reactor will be supercritical without control materials. This excess reactivity is called excess reactivity, and it is important to appropriately control this reactivity throughout the operation period. As a technique for controlling the excess reactivity, a technique of mixing a burnable poison into fuel is well known. The burnable poison is a neutron absorbing material that gradually burns during the operation period and its amount decreases, and gadolinia and the like used in combination with a nuclear fuel material are known.

In general, if the number of fuel rods containing burnable poisons decreases, the infinite multiplication factor at the beginning of combustion increases. If the concentration of the burnable poison mixed is increased, the gadolinia burnout time can be delayed, and as a result, the maximum value of the infinite multiplication factor can be suppressed. By using this effect, the excess reactivity can be appropriately controlled by a combination of the concentration of the burnable poison and the number of fuel rods into which the burnable poison is mixed.

[0005] As the prior art relating to the initially loaded core, Japanese Patent Application Laid-Open No. 5-249270 discloses a high enrichment fuel assembly having an average enrichment of 3.4%, a medium enrichment fuel assembly of 2.3%, and a medium enrichment fuel assembly of 2.3%. A core composed of a 1.1% low enrichment fuel assembly is described. In this publication, furthermore, in order to make effective use of fissile material, it is described that a fuel assembly with a low enrichment is taken out of the core earlier, and a fuel assembly with a high enrichment is loaded into the core for a longer time. ing.

As a prior art for improving the economy of the initially loaded core, Japanese Patent Laid-Open Publication No. 60-13283 discloses an average enrichment of a replacement fuel assembly among the fuel assemblies constituting the initially loaded core. A technique of loading a fuel assembly having a higher average enrichment on the outermost periphery of the core is described.

Further, as another measure for improving the economical efficiency of the initially loaded core, there is a conventional technique described in Japanese Patent Application Laid-Open No. 61-165682. This is because the number of high enrichment fuel assemblies constituting the initially loaded core is made larger than the number of replacement fuel assemblies of the equilibrium core, so that the increase in burnup accompanying the start-up test in the first operation cycle is achieved. Is to supplement.

[0008]

Conventionally, a control rod is inserted or a combustible poison is added to fuel in response to an increase in the excess reactivity caused by increasing the average enrichment of the initially loaded core in order to increase the burnup. Was mixed in. However, in the core aiming at higher burnup, the surplus reactivity further increases.

On the other hand, if control rods are inserted, the number of control rods to be inserted increases, the peaking in the radial direction increases, and the thermal margin decreases. In addition, it is necessary to repeat the adjustment of the control rod insertion amount in order to compensate for a large change in the excess reactivity during the operation period. This causes a reduction in the reactor operation rate, which is not preferable from the viewpoint of fuel economy. The excess reactivity can be suppressed by increasing the amount (concentration) of the burnable poison, but increasing the amount of the burnable poison causes a decrease in the thermal conductivity of the fuel pellet,
There was a problem in terms of fuel integrity. As described above, in realizing high burnup of the initially loaded core, an increase in the excess reactivity of the core was the main factor that hindered this.

[0010] Even if an attempt is made to achieve a high burnup of the initially loaded core by the prior art described in JP-A-60-13283,
A sufficient effect cannot be obtained. This is because the loading position of the high-enrichment fuel assemblies is limited to the outermost periphery of the core, so the number of high-enrichment fuel assemblies loaded in the initially loaded core is limited, This is because there is a limit in increasing the average enrichment of the initially loaded core to improve fuel economy. Further, in the fuel assembly on the outermost periphery of the core, the channel box is liable to be bent due to the difference in irradiation elongation due to the gradient of the neutron flux around the core. Therefore, arranging a high-enrichment fuel assembly having a long fuel life from the first operation cycle to the outermost periphery of the core promotes the subsequent bending of the channel box, thereby limiting the irradiation period and achieving high burnup. It was difficult.

Further, even if the prior art described in JP-A-61-165682 is used, a sufficient effect cannot be obtained. In this technique, even if the number of high enrichment fuel assemblies constituting the initially loaded core is large, the number is only about 20 to 30% of the total number of fuel assemblies. Due to limitations, fuel economy was not sufficiently improved. Further, when the proportion of the highly enriched fuel assembly in the initially loaded core is increased, not only the first operation cycle but also the excess reactivity in the second operation cycle and the third operation cycle are increased. However, no consideration is given to the fuel loading position for suppressing the excess reactivity that increases in the second and third operation cycles, and this is a problem in increasing the take-out burnup.

An object of the present invention is to provide a fuel loading method and a nuclear reactor core capable of increasing the take-out burnup while suppressing the excess reactivity even when the average enrichment of the initially loaded core is increased.

[0013]

In order to achieve the above object, according to the present invention, at the time of initial loading, the first fuel assembly having the highest average enrichment is placed in the third layer from the outermost periphery of the core. A second fuel assembly having the lowest average enrichment is loaded in the outer layer, and after the first operation cycle, the first fuel assembly in the third layer is moved to the outermost layer, and the outermost layer is moved to the outermost layer. (2) The fuel assembly is moved from the outermost periphery of the core to a layer inside the second layer, and after the second operation cycle, the first outermost layer is moved to the first layer.
The entire fuel assembly is moved, and the entire third layer is filled with the first fuel.
In the charge aggregate, the third layer which did not fit in the third layer
When the first fuel assembly is moved to the fourth layer from the outermost periphery of the core, the second fuel assembly moved to the inner layer is moved to the outermost layer or taken out of the core.

Preferably, the fuel assembly moved from the outermost layer to the third layer includes a channel box which has been quenched into a β phase of a crystal lattice (β quenched). .

Preferably, the first fuel assembly to be moved to the outermost layer after the end of the first operation cycle has a minimum gadolinia during manufacture among the first fuel assemblies.

Here, the outermost layer of the core refers to at least one of the four outer surfaces when the fuel assembly is loaded.
Where the two outer surfaces face the outer region of the core. In addition, the second layer from the outermost periphery indicates a position where at least one of the four outer surfaces faces the outermost layer of the core when the fuel assembly is loaded, and the third layer from the outermost periphery. Indicates a position where at least one of the four outer surfaces faces the second layer when the fuel assembly is loaded. Hereinafter, similarly, it is referred to as a fourth layer, a fifth layer, and the like.

According to the present invention, the first fuel assembly having the highest average enrichment (high enrichment fuel assembly) is moved from the inner layer of the core to the outermost layer after the end of the first operation cycle, Since the high enrichment fuel assembly moved to the outermost layer can be delayed in burning more than the other high enrichment fuel assemblies, even if the average enrichment of the initially loaded core is increased, the extraction burnup is increased. be able to. Further, as the high-enrichment fuel assembly moves to the outermost layer, the low-enrichment fuel assembly previously loaded on the outermost layer is moved to the inner layer of the core. Uranium 235 inside
Can be reduced, and the excess reactivity can be suppressed.

Further, by subjecting the channel box to β quenching, the crystal orientation of the zircaloy material as the material can be randomized. Thereby, irradiation growth which is a cause of irradiation bending can be suppressed, and irradiation bending can be reduced. This is for the following reason. The irradiation bending of the channel box is caused by a difference in irradiation extension between the opposing surfaces. Since the magnitude of the irradiation elongation is determined by the crystal lattice of the zircaloy material (hexagonal lattice), by improving the crystal orientation by β-quenching, the irradiation bending of the channel box can be reduced. Therefore, the restriction on the irradiation period based on the irradiation bending can be eased, which also contributes to an increase in the take-out burnup.

FIG. 9 is a Zr-Sn system diagram described on page 522 of "Reactor Materials Handbook" (published by Nikkan Kogyo Shimbun). The Zircaloy material is, as shown in FIG.
As the temperature rises, the phase changes from the α phase to the α + β and β phases, and the crystal structure also changes from a hexagonal dense lattice (hexagonal) to a body-centered cubic lattice.

As described above, the irradiation elongation is caused by the crystal orientation being oriented in a specific direction. Therefore, if the crystal orientation is completely randomized, the irradiation elongation disappears. For this purpose, a method of heating the zirconium alloy member to a temperature range of the β phase (980 ° C. or higher) to grow β zirconium crystal grains and then quenching is effective. By such a heat treatment, the orientation of crystal grains is in a randomized state similar to that of cubic β-zirconium.

Further, the first fuel assembly moved to the outermost layer after the end of the first operation cycle is such that the amount of gadolinia at the time of manufacture among the first fuel assemblies is the smallest.
Since the amount of gadolinia remaining in the core can be increased as compared with a case where the remaining fuel of the first fuel assembly is moved to the outermost layer, it is possible to suppress excess reactivity more effectively.

[0022]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a method for loading fuel into a reactor core according to the present invention. In the first step of the method, the reactor core is loaded with a new fuel assembly to form an initially loaded core.
FIG. 2 shows the initially loaded core configured in the first step. As shown in FIG. 2, the initially loaded core has a high-enrichment fuel assembly having an average enrichment of 664 bodies of about 4.1 wt%,
A low-enrichment fuel assembly having an average enrichment of about 1.5 wt% of 08 fuel assemblies has a total of 872 fuel assemblies.

The highly enriched fuel assembly is composed of three types of fuel assemblies having different amounts of gadolinia, which is a burnable poison.
That is, 356 high-enrichment fuel assemblies (high Gd) 2 and 1
It comprises 28 high enrichment fuel assemblies (low Gd) 3 and 180 high enrichment fuel assemblies (peripheral Gd) 4. Less than,
Low enrichment fuel assembly, high enrichment fuel assembly (high Gd),
The high enrichment fuel assembly (low Gd) and the high enrichment fuel assembly (peripheral Gd) are referred to as a low enrichment fuel, a high Gd fuel,
They are called low Gd fuel and peripheral Gd fuel.

The high Gd fuel, the low Gd fuel and the peripheral Gd fuel are fuel rods containing gadolinia (hereinafter referred to as Gd fuel) in the fuel assembly.
(Referred to as fuel rods) have substantially the same gadolinia concentration and differ in the number of Gd fuel rods. That is, the number of Gd fuel rods of the high Gd fuel, the low Gd fuel, and the peripheral Gd fuel is 13 and 1, respectively.
The number of gadolinia is two, and the amount of gadolinia is maximum for high Gd fuel and minimum for peripheral Gd fuel.

The low-enrichment fuel 1 comprises a control cell 5 composed of four members surrounding the outermost layer of the core and control rods inside the core.
(29). By arranging the low-enrichment fuel 1 in the control cell 5 in this manner, even when the control rod is pulled out of the core in the first operation cycle, the output of the fuel assembly adjacent to the control rod does not increase significantly. Therefore, the heat load on the fuel can be reduced.

The fuel assemblies surrounding the control cells 5 are composed of eight high Gd fuels 2 adjacent in plane to one control cell 5.
And four low Gd fuels 3 that are in contact with one control cell 5 in a diagonal direction. As described above, since the arrangement of the low-enrichment fuel 1 constituting the control cell 5 and the fuel assembly surrounding the low-enrichment fuel 1 is symmetric, the power distribution of the fuel rods inside the fuel assembly is also a regular distribution with good symmetry. Become. Also, by improving the symmetry of the power distribution, the maximum linear power density of the fuel rods can be reduced.
The linear power density of the fuel rods in the first operation cycle can be reduced.

The peripheral Gd fuel 4 is mainly loaded on the second layer (hereinafter, referred to as a second layer) and the third layer (hereinafter, referred to as a third layer) from the outermost periphery of the reactor core, whereby the first operation is performed. Radial output peaking at the end of the cycle is reduced. Hereinafter, the fourth and fifth layers from the outermost periphery of the reactor core
Layer, fifth layer, etc.

After operating the first operating cycle with the core configuration of FIG. 2, the first fuel transfer is performed in the second step of FIG. In the first fuel transfer, as shown in FIG. 3, the peripheral Gd fuel 4 mainly loaded in the third layer is moved (shuffled) to the outermost layer of the core indicated by 4a. Further, as shown in FIG. 4, the low-enrichment fuel 1 loaded on the outermost layer of the core
Is moved to the region of the third to fifth layers indicated by 1a. The region 1a is located substantially concentrically around the center of the core. In the first fuel transfer, the initially loaded fuel assembly (hereinafter, referred to as initially loaded fuel) is not exchanged for a new fuel assembly (hereinafter, referred to as new fuel), and all the initially loaded fuel is not replaced. The second operation cycle also continues to burn.

As a result, the configuration of the core in the second operation cycle is as shown in FIG. Since the number of control cells 5 in this core is 37, which is larger than that in the first operation cycle, the increase in excess reactivity due to burnout of gadolinia, which is a burnable poison in the second operation cycle, is determined by the number of inserted control rods. Can be suppressed by increasing the

In the first operation cycle, the peripheral Gd which is loaded on the third layer, which is the peripheral region of the core, and whose combustion is relatively delayed,
By loading fuel 4 on the outermost layer of the core in the second operation cycle, it is possible to further retard the combustion in the second operation cycle, so that the fissionable uranium in these fuel assemblies at the end of the second operation cycle. 235 can be increased. Therefore, the amount of uranium 235 in the initially loaded fuel carried over to the third operation cycle can be increased. This is effective for burning the initially loaded fuel for a long period of time, which results in an increase in the average withdrawal burnup of the entire initially loaded fuel.

Further, the high-enrichment fuel assembly moved to the outermost layer in the first fuel transfer is defined as the peripheral Gd fuel 4 having the smallest gadolinia amount during manufacture in the high-enrichment fuel assembly. Since the amount of gadolinia remaining in the core can be increased as compared with the case where the rest of the high enrichment fuel assembly is moved to the outermost layer, the excess reactivity can be more effectively suppressed.

Next, after operating the second operation cycle with the core configuration of FIG. 5, the second fuel transfer is performed in the third step of FIG. In the second fuel transfer, as shown in FIG. 6, the peripheral Gd fuel 4 loaded on the outermost layer of the core is moved mainly to the third layer indicated by 4b. Also, as shown in FIG.
Most of the low-enrichment fuel 1 transferred to the third to fifth layers by the first fuel transfer is moved to the outermost layer of the core indicated by 1b. At this time, the remaining low-enrichment fuel 1 is taken out of the reactor core and replaced with new fuel (fuel replacement). Assuming that the length of the third operation cycle is 13 months, 136 low-enrichment fuels 1 are taken out of the core by this refueling, and
This is about half of the low-enrichment fuel 1 initially loaded.

As a result, the configuration of the core in the third operation cycle is as shown in FIG. In the figure, the high-enrichment fuel 6 does not need to be distinguished between the high Gd fuel, the low Gd fuel and the peripheral Gd fuel.
It is represented by In the present core, the outermost layer of the core is mainly composed of low-reactivity, low-enrichment fuel 1, so that neutron leakage from the core can be reduced. Therefore, the number of neutrons used for uranium fission is relatively small. To increase.

Since the power of the reactor is proportional to the product of the number of neutrons used for fission and the amount of uranium 235, if the number of neutrons used for fission can be increased under a constant power condition, the amount of uranium 235 is reduced. Can be reduced. Therefore, in the present embodiment, it is possible to reduce the number of replacement fuel assemblies in the second fuel transfer and increase the number of initially loaded fuels (low-enrichment fuel 1 and high-enrichment fuel 6) that continue to burn. it can. As a result, it is possible to increase the average withdrawal burnup of the entire initially loaded fuel. All the fuel assemblies removed from the core in the second fuel transfer are low-enrichment fuels 1 and all the high-enrichment fuels 6 are continuously burned in the third operation cycle without being removed from the core. I do. This also contributes to an increase in the removal burnup.

As described above, the reason why the low-enrichment fuel 1 can be left in the core even in the third operation cycle is that the initially loaded fuel is composed of two types of low-enrichment fuel and high-enrichment fuel, This is a result of increasing the average enrichment of the fuel to more than 4 wt% and reducing the number of low-enrichment fuels to about 1/4 of the entire fuel assembly.

The irradiation of the channel box on the outermost layer of the core causes bending of the irradiation. When this is moved to the inside of the core and used, it is necessary to pay attention to the irradiation bending amount so that the operability of inserting the control rod does not deteriorate. The irradiation bending of the channel box is caused by a difference in irradiation elongation between the opposing surfaces, and the magnitude of the irradiation elongation is determined by a crystal lattice of a zircaloy material (hexagonal lattice) as a material. Therefore, by improving this crystal orientation, the irradiation bending of the channel box can be reduced. That is, irradiation elongation is caused by the fact that the crystal orientation is oriented in a specific direction. Therefore, if the crystal orientation is completely randomized, the irradiation elongation disappears. For this purpose, a method of heating the zirconium alloy member to a temperature range of the β phase (980 ° C. or higher) to grow β zirconium crystal grains and then quenching is effective. By such a heat treatment, the orientation of crystal grains is in a randomized state similar to that of cubic β-zirconium.
Therefore, when the fuel assembly once loaded on the outermost layer of the core is moved to the inside of the core, the channel direction of the zircaloy material is changed by performing β quenching in advance in the channel box of the fuel assembly. Can be randomized. Thereby, irradiation growth, which is the cause of irradiation bending, can be suppressed, and irradiation bending can be reduced.

In the above embodiment, the gadolinia concentrations of the Gd fuel rods in the high Gd fuel, the low Gd fuel, and the peripheral Gd fuel are made equal, and the number thereof is changed to adjust the gadolinia amount. However, if a similar magnitude relationship is established for the amount of gadolinia in each high-enrichment fuel, either the gadolinia concentration of the Gd fuel rods or the number of Gd fuel rods may be changed, and the same effect is achieved in this case as well. be able to.

[0039]

According to the present invention, even when the average enrichment of the initially loaded core is increased, it is possible to increase the discharge burnup while suppressing the excess reactivity.

[Brief description of the drawings]

FIG. 1 is a diagram showing a method of loading fuel into a reactor core according to the present invention.

FIG. 2 is a configuration diagram of an initially loaded core according to the present invention.

FIG. 3 is an explanatory diagram of a first fuel movement.

FIG. 4 is an explanatory diagram of a first fuel movement.

FIG. 5 is a configuration diagram of a core in a second operation cycle according to the present invention.

FIG. 6 is an explanatory diagram of a second fuel movement.

FIG. 7 is an explanatory diagram of a second fuel transfer.

FIG. 8 is a configuration diagram of a reactor core in a third operation cycle according to the present invention.

FIG. 9 is a phase diagram of Zircaloy.

[Explanation of symbols]

1: low enrichment fuel, 2: high Gd fuel, 3: low Gd fuel,
4: peripheral Gd fuel, 5: control cell, 6: high enrichment fuel,
7 1st replacement fuel.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shigetada Tanabe 3-2-1 Sachicho, Hitachi-shi, Ibaraki Hitachi Engineering Co., Ltd. (72) Inventor Shinichi Kirihara 3-2-1 Sachicho, Hitachi-shi, Ibaraki No. Hitachi Engineering Co., Ltd. (72) Inventor Akihiro Yamanaka 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Hitachi, Ltd. Inside Hitachi Plant (72) Inventor Akiko Kanda 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Hitachi, Ltd. Hitachi Plant (72) Inventor Katsumasa Narikawa 3-1-1, Komachi, Hitachi-shi, Ibaraki Pref. Hitachi, Ltd. Hitachi Plant (72) Inventor Junichi Yamashita 3-Chome, Kochi-machi, Hitachi, Ibaraki No. 1-1 Co., Ltd. Hitachi, Ltd. Hitachi Plant (56) References JP-A-4-22894 (JP, A) JP-A-5-323070 (J , A) (58) investigated the field (Int.Cl. ^7, DB name) G21C 5/00 G21C 3/30

Claims (6)

(57) [Claims] 1. A fuel loading method for loading at least two types of fuel assemblies, which are filled with nuclear fuel materials and have different average enrichments, into a reactor core, comprising: Is loaded with the first fuel assembly having the highest average and the second fuel assembly having the lowest average enrichment in the outermost layer of the core. After the first operation cycle, the first fuel assembly of the third layer is loaded. Moving to the outermost layer and moving the second fuel assembly of the outermost layer to a layer inside the second layer from the outermost periphery of the core, and after the end of the second operation cycle, the first fuel assembly of the outermost layer And all the third layers are moved to the first fuel assembly.
The first fuel collection that did not fit in the third layer
A fuel loading method comprising: moving the united fuel to the fourth layer from the outermost periphery of the core; and moving the second fuel assembly moved to the inner layer to the outermost layer or removing the second fuel assembly from the core. 2. The fuel loading method according to claim 1, wherein after the second operation cycle is completed, the second fuel assembly loaded in the control cell is taken out of the core. 3. The fuel loading method according to claim 2, wherein the remainder of the second fuel assembly that has not been taken out of the core after the end of the second operation cycle is taken out of the core after the end of the third operation cycle. 4. The fuel assembly according to claim 1, wherein the first fuel assembly moved from the outermost layer after the end of the second operation cycle has a channel box in which the crystal lattice is quenched into the β phase. A fuel loading method characterized by the above-mentioned. 5. The fuel cell according to claim 1, wherein the second fuel assembly moved from the outermost layer to the inner layer after the end of the first operation cycle includes a channel box that has been quenched into a β phase of a crystal lattice. A fuel loading method comprising: 6. The fuel assembly according to claim 1, wherein the first fuel assembly comprises a plurality of fuel assemblies having different gadolinia amounts, and the first fuel assembly moved to the outermost layer after the end of the first operation cycle. The fuel loading method according to claim 1, wherein the amount of gadolinia in the first fuel assembly during manufacture is the smallest.