JP4161486B2 - Initial loading core of boiling water reactor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an initially loaded core of a boiling water reactor and a fuel assembly used for the configuration.
[0002]
[Prior art]
In recent years, reduction of power generation cost has become an important issue in boiling water reactors. As countermeasures to this problem, reduction of fuel cycle cost, increase of output, and extension of operation period are being studied. In order to reduce the fuel cycle cost, it is effective to increase the average take-off burnup of the fuel assembly (high burnup). To increase the burnup, it is necessary to increase the uranium enrichment of the fuel assembly. Further, in order to increase the output and extend the operation period, it is necessary to increase the uranium enrichment of the fuel assembly.
[0003]
In the initial loading core, it has been studied to increase the average take-off burnup of the initially loaded fuel by increasing the average enrichment of the core and extending the period of stay in the reactor.
[0004]
On the other hand, it is necessary for the nuclear reactor to take out the burned-out fuel at regular intervals and replace it with new fuel. However, in the initial loading core, the early removal of the fuel does not advance the burnup, so the concentration is low enough to match the time of removal. However, the fuel with a later extraction time increases the average enrichment of the fuel assembly and increases the extraction burnup, thereby improving the average extraction burnup. Thus, in order to make the enrichment of the fuel assembly suitable for the extraction time, the initial loading core is generally constituted by fuel assemblies having different average enrichments.
[0005]
Due to the increase in the uranium enrichment of the fuel assembly due to the high burnup and the increase in the core average void ratio due to the increase in output, the reactivity difference between the cold core temperature and the power operation increases. In addition, the rate at which thermal neutrons are absorbed by uranium-235 increases, and the rate at which the control rod is absorbed by the control material decreases, thereby reducing the value of the control rod. That is, the reactor shutdown margin tends to decrease by increasing the enrichment of the fuel assembly.
In particular, in an initial loading core composed of a plurality of types of fuel assemblies having different enrichments, it is common to load a low enrichment fuel having a relatively low enrichment to a control cell. Highly enriched fuel with relatively high enrichment is loaded in other areas except for the outermost periphery of the core, but high enriched fuel is mixed with gadolinia etc. as a flammable poison. Rather, the low enrichment fuel is higher than the highly enriched fuel. In such a case, the reactivity value of the control rod inserted into the control cell made of low enriched fuel tends to increase, and the furnace shutdown margin tends to decrease when the control rod is pulled out.
[0006]
In addition, since fuel assemblies having different enrichments are loaded in the furnace, the output difference between the fuel assemblies becomes large, so that the thermal margin tends to decrease.
[0007]
As described above, the problems of increasing the burnup in the initially loaded core include 1) reduction of reactor shutdown margin and 2) reduction of thermal margin.
[0008]
As a countermeasure for 2), for example, in Japanese Patent Laid-Open No. 9-90077, a control cell is configured in an initial loading core having a control cell composed of a cross-shaped control rod in the middle of four fuel assemblies. It is stated that fuel rods containing uranium fuel having a lower isotope abundance ratio of uranium 235 than natural uranium are loaded at the control rod side corner of the fuel assembly. This is a technique for preventing the output on the control rod side from becoming excessive when the control rod is pulled out after the control rod is inserted for a long period of time when the burnup is high.
However, the countermeasure for reducing the furnace stop margin of 1) is not considered in the conventional technology.
[0009]
[Problems to be solved by the invention]
An object of the present invention is to provide a fuel assembly capable of simultaneously improving the reactor shutdown margin and the thermal margin in an initially loaded core in which the average enrichment of the fuel assembly has increased due to high burnup, and an initially loaded core loaded with the same. It is to be.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, (1) in the initial loading core of the boiling water reactor of the present invention, all the fuels positioned on the outermost periphery of the fuel element bundle in at least a partial cross section in the axial direction excluding the upper and lower ends The enrichment of rods is equal to or less than that of natural uranium, and the enrichment of all fuel rods located outside the outermost periphery in the partial cross section is higher than the enrichment of fuel rods located at the outermost periphery. And at least a plurality of first fuels, each of which is a fuel assembly in which the enrichment of the fuel rods located on the outermost periphery is equal to or less than that of natural uranium, at least in the upper half of the effective fuel length. Loaded configuration.
[0011]
(2) In the initially loaded core of the boiling water reactor, the position where the first fuel is loaded is configured as the outermost periphery of the core or the periphery of a control rod inserted into the core during operation.
[0012]
In the fuel assembly of the present invention, (3) a plurality of types of fuels having different average enrichments among the fuel assemblies constituting the initial loading core of the boiling water reactor described in (1) and (2) above In addition to the assembly, at least one fuel assembly other than the highest enrichment is configured as the first fuel.
[0013]
(4) In the initial loading core of the boiling water reactor described in (3) above, the fuel assembly having the lowest enrichment is configured as the first fuel .
[0017]
The above configuration operates as follows.
[0018]
Nuclear characteristics such as the reactivity difference during cold operation and output operation depend on the enrichment of the fuel and tend to increase as the enrichment increases.
[0019]
The outermost circumference of the fuel element bundle faces the water gap between the fuel assemblies where the thermal neutron flux increases, so the neutron importance is high, and the nuclear characteristics in this region have an important effect on the nuclear characteristics of the fuel assembly. Effect. That is, if the average enrichment is constant, the lower the enrichment of the outermost fuel rod, the lower the difference in reactivity during cold operation and output operation.
[0020]
FIG. 2 shows a fuel element bundle in a fuel assembly of 1.7% having the lowest average enrichment among the fuel assemblies constituting the initial loading core of the boiling water reactor of the embodiment described later. The horizontal axis represents the ratio between the average enrichment of the fuel rods located at the outermost periphery and the average enrichment of the other fuel rods, and the vertical axis represents the difference in reactivity difference during cold-power operation. .
[0021]
From FIG. 2, it can be seen that if the enrichment ratio is less than 0.8, the difference in reactivity during cold operation and output operation is reduced compared to the case where the enrichment is uniform (concentration ratio 1.0). .
[0022]
In the prior art (Japanese Patent Laid-Open No. 9-90077), the fuel rods located at the corners of the fuel element bundle are depleted uranium. However, the enrichment ratio is about 0.8, and the reaction at the time of cold-output operation is performed. A sufficient effect cannot be obtained with respect to reduction of the degree difference.
[0023]
Further, in a boiling water reactor fuel assembly, a technique for reducing the enrichment of the outermost fuel rod is common. This is intended to reduce local output peaking from the viewpoint of securing thermal margin. The reason why the conventional technology does not consider reducing the enrichment ratio by using natural uranium or deteriorated uranium for most fuel rods as in the configuration of the present invention is that the local output peaking increases. (See FIG. 2).
[0024]
As described above, in the configuration of the present invention, the local output peaking coefficient is not necessarily minimized, but the axial position of the cross-section where the enrichment of the outermost periphery of the fuel element bundle is lowered is relatively high in the output in the axial direction. Or the fuel assembly to which the present invention is applied has a relatively low average enrichment, or the loading position is limited to a position where the output is not relatively high such as the outermost periphery or the control cell. A sufficient margin can be secured.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0026]
FIG. 1 shows an embodiment of the present invention. In the fuel assembly of FIG. 1, natural uranium is loaded on the upper and lower ends of the fuel rods 1 to 3. Since this is the same for the other embodiments, only the concentration distribution in the region excluding the upper and lower ends will be described below.
[0027]
The fuel rod 1 has a concentration of 2.8% and the fuel rod 2 has a concentration of 1.7%. The fuel rod 3 located at the outermost periphery of the fuel assembly has a concentration of natural uranium (about 0%). 0.7%).
[0028]
A water rod 11 is disposed near the center of the fuel element bundle, and the fuel element bundle is covered with a channel box 21. A control rod 22 is inserted on the outer periphery.
[0029]
In this embodiment, the ratio of the average enrichment of the fuel rods located on the outermost periphery to the average enrichment of the other fuel rods is about 0.22, and the difference in reactivity during cold operation / output operation is one. It is reduced by about 3% Δk compared to that of such a concentration distribution (see FIG. 2).
[0030]
FIG. 3 shows the fuel assembly of the second embodiment.
[0031]
In FIG. 3, the enrichment distribution of fuel rod 1 is uniform at 2.4%, the enrichment of fuel rod 2 is 2.0% at the bottom, 1.6% at the top, and the enrichment distribution of fuel rod 3 is at the bottom. 1.6% at the top, natural uranium at the top, and the enrichment distribution of fuel 4 is 1.4% at the bottom and natural uranium at the top. Here, the fuel rod type P is a partial-length fuel rod whose effective fuel length is shorter than other fuel rods, and its enrichment is 1.6%.
[0032]
In the present embodiment, natural uranium is loaded on the fuel rods 3 and 4 positioned on the outermost periphery of the fuel element bundle only in the upper portion of the fuel, and the same effect as in the first embodiment can be obtained. Because the local output peaking coefficient is reduced, the reactivity difference during cold operation and output operation can be reduced without reducing the thermal margin.
[0033]
In this embodiment, the ratio of the average enrichment of the fuel rods located on the outermost periphery to the average enrichment of the other fuel rods is about 0.32 at the upper portion of the fuel assembly, and the reactivity at the time of cold-power operation There is a reduction effect of about 2% Δk with respect to the difference (see FIG. 2).
[0034]
FIG. 4 shows a third embodiment of the present invention in combination with a flammable poison. The fuel assembly shown in FIG. 4 uses fuel pellets in which the enrichment distribution of the fuel rod 1 is 2.8% and the enrichment distribution of the fuel rod 2 is 2.4%. The enrichment distribution of 3 is natural uranium. Here, fuel rod type P is a short fuel rod with a concentration distribution of 2.2%, G is a fuel rod containing a flammable poison, the concentration of the fuel rod is 2.8%, and the concentration of gadolinia which is a flammable poison Is 8.0%. Further, 11 is a water rod, 21 is a channel box, and 22 is a control rod. The present embodiment can obtain the same effect as that of the first embodiment and also can suppress the decrease in the value of the control rod by arranging the combustible poison-containing fuel rod on the side opposite to the control rod insertion side. In this embodiment, the ratio of the average enrichment of the fuel rods located at the outermost periphery to the average enrichment of the other fuel rods is about 0.21 at the upper part of the fuel assembly and about 0.23 at the lower part of the fuel assembly. Yes, the difference in reactivity during cold operation and output operation is reduced by about 3% Δk compared to that with a uniform concentration distribution (see FIG. 2).
[0035]
FIG. 5 shows a fourth embodiment of the present invention. The fuel assembly in FIG. 5 uses 2.8% fuel pellets for the enrichment distribution of the fuel rod 1, and the enrichment distribution of the fuel rod 2 located on the outermost periphery of the fuel assembly is natural uranium. Here, the fuel rod type P is a short fuel rod and the enrichment distribution is 1.7%. Further, 12 is a water box, 21 is a channel box, and 22 is a control rod. In the present embodiment, the same effect as in the first embodiment can be obtained. In this embodiment, the ratio of the average enrichment of the fuel rods located at the outermost periphery to the average enrichment of the other fuel rods is about 0.20 at the upper part of the fuel assembly and about 0.22 at the lower part of the fuel assembly. Yes, the difference in reactivity during cold operation and output operation is reduced by about 3% Δk compared to that with a uniform concentration distribution (see FIG. 2).
[0036]
FIG. 6 shows a fifth embodiment of the present invention. The fuel assembly in FIG. 6 uses 2.8% fuel pellets for the enrichment distribution of the fuel rod 1, and the enrichment distribution of the fuel rod 2 located on the outermost periphery of the fuel assembly is natural uranium. Further, 13 is a water cross, 21 is a channel box, and 22 is a control rod. In the present embodiment, the same effect as in the first embodiment can be obtained. In this embodiment, the ratio of the average enrichment of the fuel rods located on the outermost periphery to the average enrichment of the other fuel rods is about 0.20, and the difference in reactivity during cold-power operation is uniform. It is reduced by about 3% Δk compared to that of a high concentration distribution (see FIG. 2).
[0037]
An example of an initial loading core loaded with the above fuel assembly is also conceivable. Hereinafter, an embodiment of the initial loading core will be described.
[0038]
FIG. 7 shows an example of the fuel loading pattern of the initial loading core in which the fuel assembly shown in FIG. 1 is loaded on the core as the low enrichment fuel 23 excluding the outer periphery of the core. The low enrichment fuel 23 is loaded in a place called a control cell 26 where control rods are often inserted to adjust the reactivity during reactor operation. Of the remaining region, the region other than the outer periphery of the core is loaded with highly enriched fuel 25 whose enrichment distribution is shown in FIG. On the outer periphery of the core, a low enrichment fuel (for outermost periphery loading) 24 whose enrichment distribution is shown in FIG. 10 is loaded.
[0039]
The number of control cells 26 is 37 in the example of the fuel loading pattern shown in FIG. 7, but may be increased or decreased depending on the operating conditions.
[0040]
In this embodiment, the furnace stop margin can be improved by loading the fuel assembly shown in FIG. 1 as the low enrichment fuel (for control cell loading) 23.
[0041]
FIG. 8 shows an embodiment of the core shown in FIG. 7 in which the fuel loaded on the outermost periphery is a medium enrichment fuel 27 having an average enrichment of about 2.3%. As shown in FIG. 11, the enrichment distribution of the intermediate enrichment fuel 27 has natural uranium arranged on the outer periphery in the same manner as the low enrichment fuel. In the core of the present embodiment, a margin for stopping the reactor can be ensured even if the enrichment on the outermost periphery is increased as compared with the core embodiment described above. Therefore, higher burnup can be achieved.
[0042]
【The invention's effect】
According to the present invention, it is possible to ensure a reactor stop margin in a fuel assembly whose average enrichment has increased with an increase in burnup of the initially loaded core.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of fuel enrichment and gadolinia distribution according to a first embodiment.
FIG. 2 shows the relationship between the average enrichment of the fuel rods located on the outermost periphery, the ratio of the mean enrichment of the other fuel rods, the difference in reactivity difference during cold-power operation, and the local output peaking coefficient. FIG.
FIG. 3 is a diagram showing an example of fuel enrichment and gadolinia distribution according to the second embodiment.
FIG. 4 is a diagram showing an example of fuel enrichment and gadolinia distribution according to a third embodiment.
FIG. 5 is a diagram showing an example of fuel enrichment and gadolinia distribution according to a fourth embodiment.
FIG. 6 is a diagram showing an example of fuel enrichment and gadolinia distribution according to a fifth embodiment.
FIG. 7 is a diagram showing an example of a fuel loading pattern of an initial loading core in which the fuel of the first embodiment is loaded on the core as a low enrichment fuel.
FIG. 8 is a diagram showing a fuel loading pattern example of an initial loading core in which the fuel of the first embodiment is loaded as a low enrichment fuel (an example in which the present invention is applied to the middle enrichment fuel 27 at the outermost periphery); .
9 is a view showing an example of highly enriched fuel constituting the initially loaded core shown in FIG. 7. FIG.
10 is a view showing an example of a low-concentration fuel for the outer periphery of the core that constitutes the initially loaded core shown in FIG. 7;
11 is a view showing an example of a medium enrichment fuel constituting the initially loaded core shown in FIG. 8. FIG.
[Explanation of symbols]
1-5 fuel rod type, 11 ... water rod, 12 ... water cross, 21 ... channel box, 22 ... control rod, 23 ... low enrichment fuel, 24 ... low enrichment fuel (for core periphery), 25 ... highly enriched Fuel, 26 ... control cell, P ... short fuel rod, G ... fuel rod containing flammable poison.

Claims (4)

In a fuel assembly of a boiling water reactor in which a fuel element bundle in which a plurality of fuel rods filled with a fuel containing a fissile material are bundled in a square lattice is covered with a rectangular tube box, In at least a part of the cross section in the axial direction excluding the end, the enrichment of all fuel rods located at the outermost periphery of the fuel element bundle is equal to or less than that of natural uranium, and other than the outermost periphery in the partial cross section. The enrichment of all the fuel rods located is higher than the enrichment of the fuel rods located at the outermost periphery, and the enrichment of the fuel rods located at the outermost periphery is equal to or less than that of natural uranium. An initial loading core of a boiling water reactor, wherein at least a plurality of first fuels, which are fuel assemblies provided at least at the upper half of the effective fuel length, are loaded.   The initial loading core of the boiling water reactor according to claim 1, wherein a position where the first fuel is loaded is an outermost periphery of the core or a periphery of a control rod inserted into the core during operation. The first loaded core of a boiling water reactor.   The initial loading core of the boiling water reactor according to claim 1 or 2, wherein the core is composed of a plurality of types of fuel assemblies having different average enrichments, and at least one fuel assembly other than the one having the highest enrichment. Is a first loaded core of a boiling water nuclear reactor, characterized in that the first fuel is used.   4. The initial loading core of a boiling water reactor according to claim 3, wherein the first fuel is a fuel assembly having the lowest enrichment.

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