JPH07209459A - Fuel assembly, nuclear reactor core, and boiling water reactor operation method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a fuel assembly capable of preventing an increase in pressure loss without deteriorating a thermal margin, increasing a core heat output reduction effect when a flow rate is reduced, and further improving a core stability increasing function. A spectrum shift rod (1) has a coolant inlet (8) for flowing a coolant upward and a coolant outlet (4) for connecting the coolant upward passage (9). The coolant descending flow path 10 is provided to flow downward. The upper end 3 of the coolant ascending flow path 9 is at the same height as the upper end of the effective length of the average fuel rod 12, and the lower end of the coolant outlet 4 is higher than the lower end of the effective length of the average fuel rod 12 and is at the second node. Coolant rising channel 9
The coolant descending flow passage 10 has a uniform horizontal cross-sectional area from the upper end 3 (24th node) to the lower end (second node) of the coolant outlet 4. The average fuel rod 12 has a fissionable material concentration e _H = 4.6% by weight from the midpoint (13th node) between the upper end 3 and the lower end of the coolant outlet 4 to the 24th node, and the 13th node
The concentration from the node to the second node is higher than e _L = 4.4% by weight.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water reactor, and more particularly to a fuel assembly and a core provided with a spectrum shift rod, and a method for operating a boiling water reactor.

[0002]

2. Description of the Related Art Generally, in a light water reactor, high energy neutrons generated by nuclear fission are decelerated by light water, and
By sustaining the fission chain reaction at a rate such that fissionable substances such as 235 and plutonium-239 easily react, heat output is constantly obtained. Therefore, it is important to appropriately distribute the light water serving as a coolant and a moderator, and as a known technique for this purpose, for example, as described in Japanese Patent Laid-Open No. 54-121389, light water flows in as a moderator. A water rod which is a hollow tube arranged to enhance the neutron moderating effect, or a spectrum shift rod for changing the moderator water level in the water rod during operation as described in JP-A-63-73187. Has been proposed to control the neutron moderating effect and control the nuclear reaction in the reactor.

Here, as an advantage of using the spectrum shift rod, in addition to the effect of controlling the nuclear reaction described above, there is an effect of improving the stability of the core at a low flow rate of the coolant. That is, for example, if the coolant flow rate in the core is suddenly reduced due to some failure or accident, the core will be in a thermothermally unstable state. Here, the water level in the spectrum shift rod is generally more than half of the ascending flow path height during rated output operation, but if the coolant flow rate decreases due to a failure or accident, the moderator water level will be immediately increased accordingly. As a result, the neutron multiplication factor can be reduced and the core heat output can be promptly reduced. That is, even if such a failure or accident occurs, the output of the core can be stably converged, so that the margin regarding the stability of the core (hereinafter appropriately referred to as stability margin) can be increased.

[0004]

As described above, the spectrum shift rod has a function of increasing the stability margin of the core (hereinafter, referred to as a stability increasing function). However, to further improve this stability enhancement function,
That is, the conventional spectrum shift rod has not been considered from the viewpoint of converging the core output more quickly and safely when a failure or accident occurs.

As one of the methods for further improving the stability increasing function, it is possible to increase the reduction amount of the moderator in the spectrum shift rod when the flow rate of the coolant is reduced to increase the core heat output reduction effect. Conceivable. As a structure of the spectrum shift rod for this purpose, for example, a structure for increasing the moderator decrease amount when the coolant flow rate is decreased by increasing the horizontal cross-sectional area of the spectrum shift rod particularly in the upper part of the core where the moderator water level changes. However, in this case, the following problems occur.

That is, firstly, increasing the region of the spectrum shift rod in the limited volume of the fuel assembly leads to a decrease in the number of fuel rods which are the other constituent elements, and the amount of heat generation per fuel rod. To increase the thermal margin. Secondly, if the region of the spectrum shift rod is increased without reducing the fuel rods, the coolant flow passage area other than the spectrum shift rod is reduced, but the upper core is a water / steam two-phase flow with a large pressure loss coefficient. Therefore, the decrease in the flow passage area causes a significant increase in pressure loss. This is disclosed in JP-A-63-23
As described in Japanese Patent No. 1292, from the viewpoint of the coolant flow in the fuel assembly, the stability margin may be rather deteriorated.

An object of the present invention is to increase the core heat output reduction effect when the coolant flow rate is reduced and to improve the stability of the core without deteriorating the thermal margin and preventing the pressure loss from increasing. It is another object of the present invention to provide a fuel assembly, a core, and a method of operating a boiling water reactor which can be further improved.

[0008]

To achieve the above object, according to the present invention, a coolant inlet opening at the bottom of a fuel assembly is provided, and the coolant introduced from the coolant inlet flows upward. A coolant descending channel that has a coolant ascending passage and a coolant outlet opening above the coolant inlet and is connected to the coolant ascending passage to flow the coolant downward from the coolant ascending passage. At least one water rod with a flow path;
In a fuel assembly having a plurality of fuel rods filled with fissile material, each of the coolant ascending passage and the coolant descending passage has a horizontal cross-sectional area at least in a portion other than near both ends of the passage. The height of the uppermost end of the coolant ascending channel is equal to or lower than the upper end of the effective length of the plurality of fuel rods, and the height of the lowest end of the coolant outlet of the coolant descending channel is uniform. Is a height equal to or higher than the lower end of the effective length, and a region from the midpoint between the coolant rising passage uppermost end position and the coolant outlet lowermost end position to the coolant rising passage uppermost position. The average concentration of the fuel assemblies of the fissile material present in the above is higher than the average concentration of the fuel assemblies of the fissile material present in the region from the midpoint to the lowest end of the coolant outlet. Provided by a fuel assembly characterized by It is.

Further, in order to achieve the above object, according to the present invention, there is a coolant inlet opening at a lower portion of the fuel assembly, and a coolant ascending passage for causing the coolant taken from the coolant inlet to flow upward. And a coolant descending passage that has a coolant outlet opening above the coolant inlet and is connected to the coolant ascending passage to flow the coolant from the coolant ascending passage downward. In a fuel assembly having one water rod and a plurality of fuel rods filled with fissile material, each of the coolant ascending passage and the coolant descending passage is at least a portion other than the vicinity of both ends of the passage. The horizontal cross-sectional area in the horizontal direction is uniform, and the height of the uppermost end of the coolant ascending passage is equal to or lower than the upper end of the effective length of the plurality of fuel rods, and The height of the bottom of the coolant outlet is below the effective length The fissile material having the above-mentioned height and existing in the region from the midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet to the uppermost end position of the coolant rising passage. The fuel assembly average concentration of the fuel assembly is higher than the fuel assembly average concentration of the fissile material present in the region from the midpoint to the coolant outlet bottom end position. The body is provided.

Preferably, in the fuel assembly, the water rod is such that the water level at the reactor rated output operation determined by the pressure difference between the coolant inlet position and the coolant outlet position is the cooling water. There is provided a fuel assembly having a height which is equal to or higher than a midpoint between the uppermost end position of the material ascending flow path and the lowermost end position of the coolant outlet.

Further, preferably, in the fuel assembly, the water rod has a water level at the reactor rated output operation determined by a pressure difference between the coolant inlet position and the coolant outlet position. Provided is a fuel assembly having a height near the uppermost end of the coolant rising passage.

[0012] More preferably, in the fuel assembly, the height of the uppermost end of the coolant ascending passage is the same as the height of the upper ends of the effective lengths of the plurality of fuel rods, and the cooling of the coolant descending passage is performed. The height of the lowest end of the material outlet is higher than the height of the lower end of the effective length, and the water rod is a reactor rating determined by the pressure difference between the coolant inlet position and the coolant outlet position. Provided is a fuel assembly, wherein the water level during output operation is a height that is equal to or higher than a midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet.

Further preferably, in the fuel assembly, the height of the uppermost end of the coolant ascending passage is lower than the height of the upper ends of the effective lengths of the plurality of fuel rods, and the coolant in the coolant descending passage is formed. The height of the bottom end of the outlet is higher than the height of the bottom end of the effective length, and the water rod has a reactor rated output determined by the pressure difference between the coolant inlet position and the coolant outlet position. There is provided a fuel assembly characterized in that the water level during operation is at least as high as the midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet.

More preferably, in the fuel assembly, the height of the uppermost end of the coolant rising passage is lower than the height of the upper ends of the effective lengths of the plurality of fuel rods, and the coolant outlet of the coolant descending passage. The height of the lowermost end is higher than the height of the lower end of the effective length, and the water rod has a reactor rated output operation determined by a pressure difference between the coolant inlet position and the coolant outlet position. There is provided a fuel assembly, wherein the water level at the time is a height near the uppermost end of the coolant ascending flow path.

Further, preferably, in the fuel assembly, a part of the plurality of fuel rods is a fuel rod whose axial length is shorter than that of the other fuel rods, and the height of the upper end of these short fuel rods is high. Is higher than the height of the midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet.

More preferably, in the fuel assembly, the water level during reactor rated output operation, which is determined by the pressure difference between the coolant inlet position and the coolant outlet position, is the coolant rising passage. There is provided a fuel assembly characterized by having at least two water rods having a height which is equal to or higher than a midpoint between the uppermost position and the coolant outlet lowermost position and which are different from each other. .

Further preferably, in the fuel assembly, each of the coolant ascending passage and the coolant descending passage is 80% or more of a region from an uppermost end of the ascending passage to a lowermost end of the coolant outlet. There is provided a fuel assembly characterized by having a uniform horizontal cross-sectional area in the section.

Further preferably, in the fuel assembly, there is provided a fuel assembly characterized in that the fissile material comprises a uranium-plutonium mixed oxide.

Further, in order to achieve the above object, a core of a boiling water reactor loaded with the fuel assembly is provided.

Further, in order to achieve the above object, in a method of operating a boiling water nuclear reactor having a core in which at least one fuel assembly is loaded in a row other than the outermost periphery, in the coolant inlet position of the water rod. And the coolant level during the reactor rated output operation determined by the pressure difference between the coolant outlet position and the height of the middle point between the coolant ascending flow path uppermost end position and the coolant outlet lower end position. There is provided a method of operating a boiling water reactor characterized by performing a rated output operation in a core flow rate range having the above height.

[0021]

In general, during rated operation of the nuclear reactor, the water level in the spectrum shift rod is located above the midpoint between the uppermost end position of the coolant ascending passage and the lowermost end position of the coolant outlet. Immediately after the flow rate of the coolant decreases, the area near the water level changes from the water-filled state to the steam void, but the degree of decrease in the neutron multiplication factor when the water changes to the steam void in this way. Is higher the higher the uranium enrichment (ie the higher the concentration of fissile material). In the present invention, the average concentration of the fissile material in the upper half from the midpoint of the coolant rising flow path uppermost end position and the coolant outlet lowermost end position is higher than the lower half,
The water level during the rated operation exists in the high concentration region, and immediately after the coolant flow rate decreases, the water level decreases from this high concentration region.Therefore, compared to the conventional case where the average concentration of fissile material is uniform. The neutron multiplication factor sharply decreases, and the core heat output can be rapidly reduced. That is, the core heat output reduction effect can be increased. At this time, since the horizontal cross-sectional area of the coolant ascending channel and the coolant descending channel is uniform except for the vicinity of both ends, the spectrum shift rod region increases and the heat generation amount per fuel rod increases. The thermal margin does not deteriorate, and the coolant flow passage area does not decrease and pressure loss does not increase.
Further, the core heat output reduction effect is larger than that of the prior art in which the horizontal cross-sectional area of the coolant ascending passage is increased as it goes upward and the horizontal cross-sectional area of the coolant descending passage becomes smaller as it goes upward.

Further, the water level of the water rod is higher than the midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet, or near the uppermost end of the coolant rising passage. The height of the core makes it possible to reliably lower the water level from the high-concentration region immediately after the reduction of the coolant flow rate, sharply reduce the neutron multiplication factor, and quickly reduce the core heat output.

Further, as the structure of the water rod and the fuel rod, for example, the uppermost end of the coolant ascending passage is at the same height as the upper end of the effective length of the fuel rod, and the lowermost end of the coolant outlet is lower than the lower end of the effective length. It is high and the water level of the water rod is higher than the midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet, or the uppermost end of the coolant rising passage is the effective length of the fuel rod. Is lower than the upper end of the coolant, the lower end of the coolant outlet is higher than the lower end of the effective length, and the water level of the water rod is higher than the midpoint between the uppermost position of the coolant rising passage and the lowermost end of the coolant outlet. Or the uppermost end of the coolant ascending passage is lower than the upper end of the effective length of the fuel rod, the lowermost end of the coolant outlet is higher than the lower end of the effective length, and the water level of the water rod is There is a configuration that is the height near the upper end. Among these, in the configuration in which the uppermost end of the coolant ascending passage is lower than the upper end of the effective length of the fuel rod, the axial length of the entire water rod becomes short, and the coolant passage area above the fuel assembly becomes wide. Therefore, the pressure loss can be reduced, and in the configuration where the water level is near the uppermost end of the coolant ascending flow path, even if the coolant flow rate decreases from the rated output operating state where the flow rate is low at the beginning of the operation cycle, etc. There is a large amount of reduction in the water level of the moderator inside the rod. Further, even when a short fuel rod is included in a part of the plurality of fuel rods, the upper end of the short fuel rod is higher than the midpoint between the coolant upflow passage uppermost end position and the coolant outlet lowermost end position. As a result, both the short fuel rods and the other long fuel rods are present in the region where the water level begins to drop, and a large amount of fissile material is present, so that the core heat output can be promptly reduced.

Further, the water levels of at least two water rods are higher than the midpoint between the uppermost end position of the coolant ascending passage and the lowermost end position of the coolant outlet, and are different from each other. As a result, the water level of each rod can be made different even in the same coolant flow rate state. Generally, in a fuel assembly using a spectrum shift rod, the heat generation density is significantly different above and below the water level.However, by shifting the water level of multiple spectrum shift rods in this way, the moderator amount can be reduced in the entire fuel assembly. The heat generation density can be gradually changed by decreasing the heat generation stepwise.

Further, if the horizontal cross-sectional areas of the coolant ascending passage and the coolant descending passage are uniform, the spectrum shift rod region increases, the heat generation amount per fuel rod increases, and the thermal margin deteriorates. And prevent the coolant flow passage area from decreasing and increasing pressure loss, but in practice
From the viewpoint of manufacturing convenience and mechanical strength such as earthquake resistance, it is difficult to make the horizontal section uniform in all parts of the coolant ascending passage and the coolant descending passage. However, 80 out of the region from the top of the ascending flow path to the bottom of the coolant outlet
Since the horizontal cross-section is uniform in the portion of not less than%, it is possible to prevent deterioration of the thermal margin and increase of pressure loss.

Further, since the fissile material has a uranium-plutonium mixed oxide, it can be used not only in uranium fuel but also in a nuclear reactor using mixed oxide fuel.

Further, in the present invention, in the core loaded with the above fuel assembly, the water level is higher than the midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet. By operating the rated output in the flow rate range, the water level will drop from this high concentration area immediately after the coolant flow rate decreases, so the neutron multiplication factor will be higher than that in the case where the average concentration of fissile material is uniform. Rapidly decreases, and the core heat output can be promptly reduced. That is, the core heat output reduction effect can be increased. At this time, generally, the coolant flow rate is extremely narrowed at the outermost periphery of the core, and its water level is different from the water level in the fuel assembly loaded inside the core, but the fuel assemblies arranged outside this outermost periphery Based on the above, a proper operation can be performed, and the core heat output reduction effect can be obtained.

[0028]

Embodiments of the present invention will be described below with reference to FIGS. A first embodiment of the present invention will be described with reference to FIGS. The structure of the fuel assembly of this example is shown in FIG. In FIG. 2, the fuel assembly 11 includes a plurality of fuel rods 2 filled with fissile material, at least one (two in this embodiment) spectrum shift rod 1, a fuel rod 2, and a spectrum shift rod 1. It has an upper tie plate 16 and a lower tie plate 17 that support the upper and lower ends of and, and these are covered with a rectangular tubular channel box 18.

A horizontal cross section of the fuel assembly 11 is shown in FIG. In FIG. 3, in the fuel assembly 11, two spectrum shift rods 1 are arranged in the center of a channel box 18, and 74 fuel rods 2 are arranged 9 × 9 in the periphery thereof.
Are arranged in a square lattice.

FIG. 1 shows a schematic configuration of the spectrum shift rod 1 and the fuel rods 5, which are the main parts of this embodiment. The right side of FIG. 1 shows the structure of the spectrum shift rod 1 arranged in the fuel assembly 11, and the left side of FIG. 1 shows the distribution of the fissionable substance concentration of all the 74 fuel rods 2 arranged in the fuel assembly 11. The fuel distribution average concentration distribution of the fissile material calculated by summing the above is conceptually expressed as one fuel rod corresponding to the height position of the spectrum shift rod 1 on the right side (hereinafter, appropriately, the average Fuel rod).

In FIG. 1, the spectrum shift rod 1
Is a coolant ascending passage 9 which has a coolant inlet 8 which is an inlet for a coolant (that is, light water) opened at the bottom of the fuel assembly 11 and which causes the coolant taken in from the coolant inlet 8 to flow upward. Coolant outlet 4 opened above the coolant inlet 8
Is connected to the coolant rising passage 9 and has a coolant rising passage 9
And a coolant descending flow path 10 for causing the coolant from the above to flow downward. At this time, the ratio of the horizontal cross-sectional area of the coolant ascending passage 9 to the horizontal cross-sectional area of the coolant descending passage 10 is 29: 1 uniformly in the axial direction. Spectrum shift rod 1
Means that the height of the upper end 3 of the coolant rising flow passage 9 is the average fuel rod 12
The height of the lower end of the coolant outlet 4 is higher than the height of the lower end of the effective length of the average fuel rod 12, and the position of 2/24 of the active fuel length from the lower end of the active fuel length ( From the bottom of the effective fuel length to n / 24 of the effective fuel length
Is called the n-th node). Further, each of the coolant upflow passage 9 and the coolant downflow passage 10 of the spectrum shift rod 1 has an upper end 3 (24th coolant) of the coolant upflow passage 9.
Between the node) and the lower end of the coolant outlet 4 (second node), that is, in the portion other than the lower end of the coolant ascending flow path 9 (near the 0th node to the 2nd node), the horizontal cross-sectional area is equal to one. It is like.

The average fuel rods 12 are arranged from the 13th node, which is the midpoint between the position of the upper end 3 of the coolant rising passage 9 (the 24th node) and the position of the lower end of the coolant outlet 4 (the 2nd node). Two
Fuel assembly average fissile material concentration up to 4 nodes e _H
= 4.6% by weight, which is higher than the concentration e _L = 4.4% by weight from the 13th node to the 2nd node. In the above configuration, the ascending channel 9 of the spectrum shift rod 1
The height of the water level 5 inside is determined by the pressure difference between the coolant inlet 8 and the coolant outlet 4. This pressure difference is determined by the pressure loss of the coolant flow in the fuel assembly 11, and if the coolant flow rate is the same, for example, the coolant inlet 8 and the coolant outlet 4
If the difference in height between the two is increased, the pressure difference is increased and the water level 5 rises (see the third embodiment described later). Further, this pressure difference is substantially proportional to the square of the flow rate of the coolant flowing into the fuel assembly 11. Therefore, by adjusting the flow rate of the coolant in the furnace,
Can be controlled. That is, for example, if the coolant flow rate at the beginning of the operation cycle is made low and the coolant flow rate at the end of the operation cycle is made high by adjusting the recirculation pump speed, the water level in the spectrum shift rod 1 becomes 5 at the beginning of the operation cycle.
, The neutron multiplication factor is kept low, and the neutron multiplication factor is increased because the water level 5 is high at the end of the operation cycle. This makes it possible to significantly control the reactivity only by controlling the flow rate, and suppress the excess reactivity at the beginning of the operation cycle without using control rods. Further, in this case, since the deceleration of neutrons is suppressed in the first half of the operation cycle, the neutron capture reaction by uranium-238 is promoted, and more plutonium-239, which is a new fissile material, is generated, and the fuel economy is improved.

Next, the operation of this embodiment will be described with reference to FIGS. First, the principle of this embodiment will be described with reference to FIG. In general, the position of the coolant outlet of the spectrum shift rod is usually determined so that the inside of the spectrum shift rod is filled with water at a time even during the operation cycle. For example, if the core flow rate control range of the rated output operation is 80% to 105% of the rated flow rate, the flow rate is 1
The water level is just full at 05% and the water level is 60 at 80%.
When designed to be%, the maximum reactivity control range is obtained. Current BWR or improved BWR (ABW
In the core flow rate control range in R), the water level in the spectrum shift rod during the rated output operation is located above the midpoint between the upper end of the coolant ascending passage and the coolant outlet height of the descending passage. Here, also in the fuel assembly 11 of the present embodiment, it is assumed that the core is operated at the rated output operation in the flow rate range of 90% rated flow rate to 111% rated flow rate, and the coolant inlet 8 The water level in the reactor rated operation, which is determined by the pressure difference between the cooling medium outlet 4 and the coolant outlet 4, is near the 16th node at 90% rated flow rate and at the 24th node (full water) at 111% rated flow rate. Height of midpoint (13th node) of the position of the upper end 3 of the ascending channel 9 (24th node) and the position of the lower end of the coolant outlet 4 (2nd node)
Is always higher than. Here, the region near this water level will change from a state filled with water to a steam void immediately after the flow rate of the coolant decreases, and the moderating action due to water disappears in that region and the neutron multiplication factor decreases. However, the degree of decrease in the neutron multiplication factor is greater as the uranium enrichment in that region is higher (that is, the concentration of fissile material is higher), and the core heat output reduction effect is increased.
This is shown in FIG. Figure 4 shows the change in infinite neutron multiplication factor in the cross section of the aggregate when the spectrum shift rod is filled with water and becomes a vapor void, with the horizontal axis representing burnup. It is shown by comparing the high concentration and the low concentration when the average concentration of the substance is about 4% by weight.

It can be seen from FIG. 4 that the neutron infinite multiplication factor decreases more largely in the almost entire region from the early stage to the latter stage of combustion, as the uranium enrichment is higher (that is, the fissile material concentration is higher). In particular, this tendency is remarkable in the region of burnup of 10 GWd / t or more, which corresponds to a combustion period of about 1 year.

In the present embodiment, based on the principle of the decreasing behavior of the infinite neutron multiplication factor, the fuel concentration in the region near the water level in the spectrum shift rod is relatively increased to promptly increase the effective neutron multiplication factor in the core. To increase the core heat output reduction effect. Hereinafter, this will be specifically described using a comparative example.

The fuel assembly 11 of the first comparative example of this embodiment
1 is shown in FIG. This comparative example is a comparative example relating to the structure of the average fuel rod, and parts corresponding to parts of the fuel assembly 11 of the present example are indicated by the same numbers in the 100s. 5, the fuel assembly 111 is different from the fuel assembly 11 of the present embodiment in that the lower end position of the coolant outlet 104 (second node)
From the position of the upper end 103 of the coolant ascending flow path 109 (24th
Node), the average fuel rod 112 concentration e
_M = 4.5% by weight, which means a uniform concentration.

FIG. 6 shows a change in effective neutron multiplication factor for each core when the coolant flow rate decreases and the water level in the spectrum shift rod decreases. FIG. 6 shows the changes in the effective neutron multiplication factor along the horizontal axis when the water level of the spectrum shift rod drops from the state of full water (that is, when the flow rate of the coolant decreases from the state of 111% rated flow rate). It represents the coolant core flow rate,
3 shows a comparison between the core loaded with the fuel assembly 11 of the present example and the core loaded with the fuel assembly 111 of the first comparative example.

In FIG. 6, the effective multiplication factor decreases as the core flow rate decreases from the 111% rated flow rate. However, the core loaded with the fuel assembly 11 of the present example is the same as that of the comparative example. Fuel assembly 111 with uniform enrichment distribution
It can be seen that the effective multiplication factor decreases faster than in the case of.
That is, for example, when the core flow rate drops to 80% of the rated flow rate (the water level at this time is almost the 13th node), the effective multiplication factor of the core of the present embodiment is about 0. 1% Δk smaller.

That is, in this embodiment, from the midpoint (thirteenth node) of the upper end 3 (24th node) of the coolant ascending passage 9 and the lower end (second node) of the coolant outlet 4 to the upper half ( The average concentration of the fissile material in the thirteenth node to the twenty-fourth node is higher than the average concentration of the lower half (the zeroth node to the thirteenth node), and the water level of the spectrum shift rod 1 is as described above. Since the height is higher than the midpoint (13th node), the water level 5 at the time of rated operation always exists in the high concentration area, and immediately after the decrease of the coolant flow rate, the water level 5 is surely released from this high concentration area. Therefore, based on the principle described with reference to FIG. 4, the effective neutron multiplication factor decreases more rapidly than in the case of the above-mentioned comparative example in which the average concentration is uniform, and the core thermal power is rapidly increased. To reduce Door can be. Therefore, the core heat output reduction effect can be increased.

A fuel assembly 611 of the second comparative example of this embodiment is shown in FIG. This comparative example is a comparative example relating to the structure of the spectrum shift rod.
It is based on the fuel assembly described in Japanese Patent No. 187. Parts corresponding to the parts of the fuel assembly 11 of the present embodiment are indicated by the same numbers in the 600s. In FIG. 7, the fuel assembly 611 differs from the fuel assembly 11 of the present embodiment in that in the spectrum shift 611, the coolant ascending flow path 609.
The horizontal cross-sectional area of the cooling medium descending passage 605 is increased and the horizontal cross-sectional area of the coolant descending flow passage 605 is decreased upward, and the horizontal cross-sectional area of the both is uniform in the axial direction. That is. Here, in the spectrum shift rod 601 of the fuel assembly 611 of this comparative example, for comparison with the present embodiment shown in FIG. 1, the horizontal cross-sectional area of the coolant ascending passage 609 and the coolant descending flow. Road 6
The ratio of 10 to the horizontal cross-sectional area at the upper end is 29:
1, which is the same as the ratio in the spectrum shift rod 1 of the present embodiment, but is 15:15 at the second node at the position of the coolant outlet 604. The height of the coolant outlet 604 is the same as that in this embodiment, and the water level is the same under the same flow rate condition. Other points are the fuel assembly 1 of the present embodiment.
1 is substantially the same as that of Example 1. For example, the total area of the horizontal cross-sectional area of the coolant ascending passage and the horizontal cross-sectional area of the coolant descending passage is the same as that of the present embodiment.

In the above-mentioned structure, the above-mentioned known technology pays attention to the moderator distribution when the spectrum shift rod is full, and the horizontal cross-sectional area of the coolant ascending passage is increased toward the upper part of the core and the fissile property is increased toward the upper part of the core. Increasing the material concentration to flatten the axial power distribution of the core, that is, to focus on the static characteristics of the core power, and to increase the heat output reduction effect when the core flow rate decreases as in the present embodiment,
That is, it does not focus on the dynamic characteristics. Therefore, when comparing the heat output reduction effect obtained in this comparative example and the heat output reduction effect obtained in this embodiment, the horizontal cross-sectional area of the coolant ascending flow path 609 is equal to that of this embodiment at the upper end 603. Is smaller as it goes downward, the moderator reduction amount in the spectrum shift rod when the water level 605 is decreased by the same height is smaller than that in the present embodiment.

However, in this embodiment, between the upper end 3 (24th node) of the coolant rising passage 9 and the lower end (second node) of the coolant outlet 4, that is, the lower end of the coolant rising passage 9. Neighborhood (near 0th node to 2nd node)
If the water level 5 drops from the full state (24th node) to the vicinity of the coolant outlet 4 (2nd node) due to the horizontal cross-sectional area being uniform in other parts, the second comparison The moderator is reduced by about 4/3 times as much as in the example. Therefore, the infinite neutron multiplication factor is reduced to a greater extent than in the second comparative example, and the core heat output can be reduced promptly. Therefore, the core heat output reduction effect can be increased. Further, as described above, in the portion other than the lower end of the coolant rising passage 9 (near the 0th node to the 2nd node), the horizontal cross-sectional area is uniform, so that the spectrum shift rod region increases. As a result, the calorific value per fuel rod does not increase and the thermal margin does not deteriorate, and the coolant passage area does not decrease and pressure loss does not increase.

As described above, according to this embodiment,
Fissionability from the midpoint (13th node) of the upper end 3 (24th node) of the coolant upflow channel 9 and the lower end (2nd node) of the coolant outlet 4 to the upper half (13th to 24th nodes) Since the average concentration of the substance is higher than the lower half (0th node to 13th node) and the water level of the spectrum shift rod 1 is higher than the midpoint (13th node), the coolant flow rate Immediately after the decrease, the water level 5 surely decreases from the high-concentration region, and the core heat output reduction effect can be increased as compared with the conventional case. At this time, the coolant ascending channel 9 and the coolant descending channel 4 are connected to the coolant ascending channel 9
Since the horizontal cross-sectional area is uniform except for the vicinity of the lower end (near the 0th node to the 2nd node), the spectrum shift rod region increases, the heat generation amount per fuel rod increases, and the thermal margin deteriorates. In addition, the coolant flow passage area does not decrease and the pressure loss does not increase, and the horizontal cross-sectional area of the coolant ascending flow passage becomes larger as it goes upward. The effect of reducing the core heat output can be increased as compared with the conventional technology in which the area is decreased upward. In addition, in the above-described embodiment, the horizontal cross-sectional area was uniform in the portion other than the vicinity of the lower end (near the 0th node to the 2nd node) of the coolant rising flow passage 9. Considering the case where it is difficult to make the horizontal cross section uniform for all parts of the coolant ascending passage and the coolant descending passage from the viewpoint of manufacturing convenience and mechanical strength such as earthquake resistance, The exception of the cross-sectional uniformity is not limited to the vicinity of the lower end of the coolant rising flow passage 9 and may be, for example, the vicinity of the lower end of the coolant descending flow passage 10 or the upper end 3 of the coolant rising flow passage 9. Further, the horizontal cross-sectional area may be uniform in 80% or more of the region from the upper end 3 of the ascending passage 9 of the coolant descending passage 10 to the lower end of the coolant outlet 4. Also in this case, it is possible to obtain substantially the same effect of preventing the deterioration of the thermal margin and preventing the increase of the pressure loss as in the above embodiment.

A second embodiment of the present invention will be described with reference to FIG. FIG. 8 shows a schematic configuration of the spectrum shift rod and the fuel rod of the fuel assembly of this embodiment. In FIG. 8, the fuel assembly 211 of this embodiment is the fuel assembly 1 of the first embodiment.
The difference from No. 1 is that the axial length of the spectrum shift rod 201 is shortened and the upper end 203 of the coolant ascending flow path 209 is reduced.
Is the height of the upper end of the effective length of the average fuel rod 212 (24th
Node 13 which is considerably lower than the node) and the height of the lower end of the coolant outlet 204 of the coolant descending flow path 210 is higher than the height of the effective length lower end of the average fuel rod 212 (node 0). That is, the first node is lower than the first embodiment (second node). Here, the water level 205 during the reactor rated output operation is the upper end 203 of the coolant ascending flow path 209.
It is similar to the first embodiment in that it is at a height higher than the midpoint between the bottom of the coolant outlet 204 and the bottom of the coolant outlet 204, but the middle point between the top 203 and the bottom of the coolant outlet 204 is the seventh node. Therefore, the water level 205 exists in the seventh node and above.

In a form corresponding to this, the average fuel rod 21
2 is the average fissile material concentration e _H = 4.6 wt% of the fuel assembly from the 7th node to the 13th node, which is the midpoint,
Concentration e _L = 4.4 wt% from the 7th node to the 1st node
Is higher. The other points are almost the same as in the first embodiment.

According to this embodiment, in addition to the effect obtained in the first embodiment, the upper end 20 of the coolant ascending passage 209 is increased.
3 (the 13th node) is lower than the average fuel rod 212 effective length upper end (the 24th node) and the spectrum shift rod 211
Since the overall axial length is shortened, the coolant passage area above the fuel assembly 211 is increased, and the pressure loss can be reduced.

A third embodiment of the present invention will be described with reference to FIG. FIG. 9 shows a schematic configuration of the spectrum shift rod and the fuel rod of the fuel assembly of this embodiment. In FIG. 9, the fuel assembly 311 of this embodiment is the fuel assembly 1 of the first embodiment.
1 is that the height of the upper end 303 of the coolant rising passage 309 of the spectrum shift rod 301 is the average fuel rod 31.
The height of the lower end of the coolant outlet 304 of the coolant descending passage 310 is lower than that of the effective length of the average fuel rod 312. It is located at the sixth node, which is considerably higher than the height, and the water level 305 during the reactor rated output operation is near the upper end 303 (22nd node) of the coolant ascending flow path 309.

Here, the average fuel rod 312 is the average fissionable material concentration e _{H of the} upper half of the fuel assembly from the midpoint between the position of the upper end 303 of the coolant rising passage 309 and the lower end position of the coolant outlet 304. However, it is similar to the first embodiment in that it is higher than the concentration e _L in the middle half to the lower half, but the positions of the upper end 303 of the coolant ascending flow path 309 and the lower end of the coolant outlet 304 are as described above. Therefore, in a form corresponding to this, it is the midpoint between the 22nd node (the position of the upper end 303 of the coolant rising passage 309) and the 6th node (the position of the lower end of the coolant outlet 304). Average fuel assembly fissile material concentration from node 14 to node 22 e _H = 4.6% by weight, 1st
4 node has a density e _L = 4.4 wt% up to the sixth node. The other points are almost the same as in the first embodiment.

According to the present embodiment, in addition to the effect obtained in the first embodiment, the water level 305 changes the coolant ascending passage 30.
Since the height is near the upper end 303 of No. 9, the water level can be surely lowered from the high-concentration region immediately after the coolant flow rate is reduced, and the core heat output reduction effect can be increased. Further, even when the flow rate of the coolant decreases from the rated output operation state where the flow rate is low at the beginning of the operation cycle or the like, the water level of the moderator within the spectrum shift greatly decreases. Therefore, the core heat output can be greatly reduced over the entire operation cycle.

A fourth embodiment of the present invention will be described with reference to FIGS. A horizontal cross section of the fuel assembly 411 of this embodiment is shown in FIG. Parts common to the first embodiment are designated by common numbers. In FIG. 10, in the fuel assembly 411, one spectrum shift rod 401 is arranged in the center of the channel box 18, 64 normal fuel rods 2 are arranged around the spectrum shift rod 401, and the pressure in the upper part of the fuel assembly 411 is increased. Eight short fuel rods 406 having a shorter axial length than the fuel rods 2 provided for reducing the loss (15/24 = 5/8 of the fuel rods 2) and a 9 × 9 square lattice shape. It is arranged.

FIG. 11 shows a schematic structure of the spectrum shift rod and the fuel rod of the fuel assembly 411 of this embodiment. 11, the fuel assembly 411 of the present embodiment is different from the fuel assembly 11 of the first embodiment in that the coolant ascending flow path 4 is provided.
The height of the upper end 403 of 09 is the 22nd node, which is slightly lower than the height of the effective length upper end of the average fuel rod 412 (the 24th node), and for the convenience of manufacturing, 0.5 node of the upper end and the outlet. That is, the coolant ascending flow path 409 is narrower than the other portions by one node of the portion. However, at this time, 9 out of the upper end 403 (second node) of the coolant ascending flow path 409 to the lower end (second node) of the coolant outlet 404.
It can be said that the cross-section is uniform in the area of 2.5%, and the cross-sectional area is substantially uniform.

The first point is that the water level 405 during the reactor rated output operation is higher than the midpoint between the upper end 403 of the coolant rising passage 409 and the lower end of the coolant outlet 404. Similar to the embodiment, but with the upper end 403 as described above.
Since the height is changed, the midpoint between the upper end 403 (22nd node) and the lower end of the coolant outlet 404 (2nd node) is the 12th node. The core flow rate control range for rated output operation is 80% to 105% of the rated flow rate. Therefore, if the reactor is designed to reach full water (water level 22nd node) at the 105% rated flow rate, the 80% rated flow rate The water level at is about 13 nodes, and the water level is above the 12th node at the midpoint.

On the other hand, the average fuel rod 412 is the fuel assembly 4
Since 11 includes eight short fuel rods 406, as shown in the drawing, above the 15th node (from the 15th node to the 2nd node).
It is different from the average fuel rod 12 of the first embodiment in that the width is represented by 4 nodes). The upper end (15th node) of the short fuel rod 406 is higher than the midpoint (12th node) of the upper end of the spectrum shift rod (22nd node) and the lower end of the coolant outlet 404 (2nd node), and the reactor. It is higher than the water level 405 during rated output operation. In addition, from the 8th node to the 22nd node (the upper end 40 of the coolant rising passage 409)
Fuel assembly average fissile material concentration up to 3) e _H = 4.
Concentration e _L from the 8th node to the 2nd node at 7% by weight
= Higher than 4.5% by weight. At this time, in the average fuel rod 412, the upper end 403 of the coolant ascending flow path 409.
From the 12th node, which is the midpoint between the position (the 22nd node) and the position of the lower end of the coolant outlet 404 (the 2nd node).
Fuel assembly average fissile material concentration up to 2 nodes e _H
= 4.70 wt%, which is higher than the concentration e _L = 4.58 wt% from the twelfth node to the second node as in the first embodiment. The configuration other than these is almost the same as that of the first embodiment.

Next, the operation of this embodiment will be described with reference to FIG.

FIG. 12 shows a fuel assembly of a comparative example of this example. This comparative example is a comparative example relating to the structure of the average fuel rod, and the parts corresponding to the parts of the fuel assembly 411 of the present example are indicated by the numbers in the 900s, including the 500s.

In FIG. 12, the fuel assembly 911 is different from the fuel assembly 411 of the present embodiment in that the upper end position of the short fuel rod is at the 11th node, which is lower than the water level during the reactor rated output operation, and is cooled. Upper end 40 of material ascending flow path 409
It is lower than the 12th node, which is the midpoint between the position 3 (the 22nd node) and the position of the lower end of the coolant outlet 404 (the 2nd node).

On the other hand, the fuel assembly 411 of this embodiment is used.
Then, the upper end position of the short fuel rod 406 is the 15th node, which is higher than the water level during the reactor rated output operation. That is, comparing the number of fuel rods at the 12th node, which is the lower limit of the water level, the fuel assembly 411 of this embodiment has 72 fuel rods, whereas the fuel assembly 911 of the comparative example has 64 fuel rods and 8 fuel rods less. . In these two fuel assembly cross sections at the twelfth node, the decrease amount of the infinite neutron multiplication factor when the inside of the spectrum shift rod changes from full water to void is similar to the tendency shown in FIG. 4 in the first embodiment. There is. That is, the fuel assembly 411 of this embodiment, which has a large number of fuel rods and a large amount of fissile material near the water level, corresponds to the high concentration shown in FIG. The case of corresponds to the case of low concentration in FIG. Therefore, the fuel assembly 411 of the present embodiment has a larger amount of decrease in reactivity when the flow rate is decreased than the fuel assembly 911 of the comparative example. In the present example and the comparative example, the upper and lower ends of the spectrum shift rod 401 are partially narrowed, but the effect on the neutron multiplication factor reducing effect is negligible and can be ignored.

As described above, according to this embodiment,
In addition to the effect obtained in the first embodiment, the coolant ascending flow path 4
09 upper end 403 (22nd node) and coolant outlet 404
Since the water level 405 of the spectrum shift rod 401 is higher than the midpoint (12th node) with the lower end (2nd node) of the, and the upper end (15th node) of the short fuel rod 406 is higher than that water level, the water level drops. In the starting region, both short fuel rods and other ordinary fuel rods are present and a large amount of fissile material is present, so that the core heat output can be promptly reduced.

The fifth embodiment of the present invention is shown in FIGS. 13 and 14.
Will be described. A horizontal cross section of the fuel assembly 511 of this embodiment is shown in FIG. In FIG. 13, the fuel assembly 51
In FIG. 1, one spectrum shift rod 551 is arranged in the center of the channel box 518, and eight spectrum shift rods 501 are arranged around it, and 160 fuel rods 502 of 14 × 14 are arranged around them. They are arranged in a square lattice.

FIG. 14 shows a schematic structure of the spectrum shift rod and the fuel rod of the fuel assembly 511 of this embodiment. In FIG. 14, the fuel assembly 511 of the present embodiment is different from the fuel assembly 11 of the first embodiment in that the height positions of the spectrum shift rod 501 and the spectrum shift rod 551 are different, that is, the spectrum. In the shift rod 501, the upper end 50 of the coolant rising passage 509 is
3 is the 22nd node and the lower end of the coolant outlet 504 is the second node, and in the spectrum shift rod 551, the upper end 553 of the coolant ascending channel 559 is the 24th node and the lower end of the coolant outlet 554 is the 5th node. There is. Here, among the upper ends 509 and 559 of the coolant ascending channels 503 and 553 of the two spectrum shift rods 501 and 551, the highest uppermost end is the upper end 559 of the coolant ascending channel 553.
(24th node), the lowest bottom of the lower ends of the coolant outlets 504 and 554 is the lower end (second node) of the coolant outlet 504, and the midpoint thereof is the 13th node. Water level 5 of the spectrum shift rod 501 and the spectrum shift rod 551 during the reactor rated output operation
05 and 555 are the same as those in the first embodiment in that they are higher than the height of the midpoint (13th node), but the spectrum shift rod 501 and the spectrum shift rod 55
Due to the difference in structure from 1, the spectrum shift rod 5
It differs from the first embodiment in that the water level 555 of 51 is always higher than the water level 505 of the spectrum shift rod 501.
Further, the average fissile material concentration e _H = 5.8 wt% in the fuel assembly from the midpoint (the 13th node) to the 24th node, and the concentration e _L = 5. 5 from the 13th node to the 2nd node.
It is similar to the first embodiment in that it is larger than 5% by weight.
The configuration other than these is almost the same as that of the first embodiment.

According to this embodiment, in addition to the effect obtained by the first embodiment, two spectrum shift rods 50 are provided.
The water levels 505 and 555 of 1,551 are the uppermost ends of the coolant ascending channels 509 and 559, respectively.
(24th node) and respective coolant outlets 504, 55
4 is lower than the lowest point (second node) of the coolant outlet 504, which is the lowest point among the lower ends of No. 4, and is different from each other. As described in Kaihei 1-195392, the water level of each rod can be made different even in the same coolant flow rate state.
Generally, in a fuel assembly using a spectrum shift rod, the heat generation density is significantly different above and below the water level.However, by shifting the water level of multiple spectrum shift rods in this way, the moderator amount can be reduced in the entire fuel assembly. The heat generation density can be gradually changed by decreasing the heat generation stepwise. Therefore, the change in the heat generation density near the water level becomes gentle when the water level drops, and the thermal load on the fuel rods can be reduced.

Although uranium was used as the nuclear fuel material in the first to fifth embodiments, the present invention is not limited to this, and all or part of the nuclear fuel material may be a uranium-plutonium mixed oxide. In the case, the same effect is obtained. Further, in the above-mentioned first to third embodiments, the number of spectrum shift rods is plural, but the number of spectrum shift rods is not limited to this, and in the fourth embodiment there is only one spectrum shift rod. However, a plurality of them may be provided, and similar effects are obtained in these cases.

Further, although the first to fifth embodiments are examples relating to fuel assemblies, similar effects can be obtained even when these fuel assemblies are loaded in the core.

[0064]

According to the present invention, the average concentration of the fissile material in the upper half from the midpoint between the uppermost position of the coolant ascending channel and the lowermost position of the coolant outlet is higher than that in the lower half. Immediately after the coolant flow rate decreases, the water level decreases from the high-concentration region, and the core heat output reduction effect can be increased compared to the conventional case. Further, at this time, since the horizontal cross-sectional area of the coolant ascending channel and the coolant descending channel is uniform except for the vicinity of both ends, the spectrum shift rod region increases and the calorific value per fuel rod increases. The thermal margin is not deteriorated, the pressure drop is not increased by decreasing the coolant flow passage area, and the coolant descending flow is larger as the horizontal cross-sectional area of the coolant ascending flow passage goes up. The effect of reducing the core heat output can be increased as compared with the prior art in which the horizontal cross-sectional area of the passage is decreased upward.

The water level of the water rod is higher than the midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet, or near the uppermost end of the coolant rising passage. Therefore, the water level can be surely lowered from the high-concentration region immediately after the coolant flow rate is reduced, and the core heat output reduction effect can be increased. Further, since the uppermost end of the coolant ascending passage is lower than the upper end of the effective length of the fuel rod, the coolant passage area in the upper part of the fuel assembly is widened to reduce the pressure loss,
In addition, since the water level is near the uppermost end of the coolant ascending flow path, even if the coolant flow rate decreases from the rated output operation state where the flow rate is low, it is possible to increase the water level reduction amount of the moderator within the spectrum shift. The core heat output can be greatly reduced over the entire cycle. Further, even when a short fuel rod is included in a part of the plurality of fuel rods, the upper end of the short fuel rod is higher than the midpoint between the coolant upflow passage uppermost end position and the coolant outlet lowermost end position. Therefore, a large amount of fissile material is present in the region where the water level begins to drop, and the core heat output can be promptly reduced. Further, since the water levels of at least two water rods are different from each other at a height higher than the midpoint between the uppermost end position of the coolant ascending passage and the lowermost end position of the coolant outlet, the same coolant flow rate is obtained. Even in the state, the water levels of the rods can be made different, and the amount of moderator can be gradually reduced from the bottom to the top and the heat generation density can be gradually changed in the whole fuel assembly. Therefore, the change in the heat generation density near the water level becomes gentle when the water level drops, and the thermal load on the fuel rods can be reduced. Also,
Since 80% or more of the region from the uppermost end of the ascending flow path to the lowermost end of the coolant outlet has a uniform horizontal cross-section, practical convenience and mechanical strength such as seismic resistance should be taken into consideration. It is possible to prevent deterioration of the thermal margin and increase of pressure loss while taking into consideration. Furthermore, since the fissile material has a uranium-plutonium mixed oxide, it can be used not only in uranium fuel but also in a reactor using mixed oxide fuel.

Further, according to the present invention, in the core loaded with the above fuel assembly, the water level is higher than the midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet. Since the rated output operation is performed within the core flow rate range, immediately after the coolant flow rate is reduced, the water level is lowered from this high concentration region, and the core heat output can be promptly reduced. That is, the core heat output reduction effect can be increased. At this time, generally, in the outermost periphery of the core, the coolant flow rate is extremely narrowed and the water level is different from the others, but proper operation is performed with reference to at least one fuel assembly arranged outside the outermost periphery. The core heat output reduction effect can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a spectrum shift rod and a fuel rod according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a structure of a fuel assembly.

FIG. 3 is a horizontal cross-sectional view of a fuel assembly.

FIG. 4 is a diagram showing a change in infinite neutron multiplication factor when the inside of the spectrum shift rod changes from full water to a void.

FIG. 5 is a schematic configuration diagram of a spectrum shift rod and a fuel rod according to a first comparative example of the first embodiment.

FIG. 6 is a diagram showing a change in effective neutron multiplication factor in terms of core when the water level in the spectrum shift rod is lowered.

FIG. 7 is a schematic configuration diagram of a spectrum shift rod and a fuel rod according to a second comparative example of the first embodiment.

FIG. 8 is a schematic configuration diagram of a spectrum shift rod and a fuel rod according to a second embodiment of the present invention.

FIG. 9 is a schematic configuration diagram of a spectrum shift rod and a fuel rod according to a third embodiment of the present invention.

FIG. 10 is a horizontal cross-sectional view of a fuel assembly according to a fourth embodiment of the present invention.

FIG. 11 is a schematic configuration diagram of a spectrum shift rod and a fuel rod.

FIG. 12 is a schematic configuration diagram of a spectrum shift rod and a fuel rod according to a comparative example of the fourth embodiment.

FIG. 13 is a horizontal cross-sectional view of a fuel assembly according to a fifth embodiment of the present invention.

FIG. 14 is a schematic configuration diagram of a spectrum shift rod and a fuel rod.

[Explanation of symbols]

1 Spectral shift rod 2 Fuel rod 3 Coolant ascending channel upper end 4 Coolant outlet 5 Water level 8 Coolant inlet 9 Coolant ascending channel 10 Coolant descending channel 11 Fuel assembly 12 Average fuel rod 201 Spectral shift rod 203 Cooling Material ascending channel upper end 204 Coolant outlet 205 Water level 208 Coolant inlet 209 Coolant ascending channel 210 Coolant descending channel 211 Fuel assembly 212 Average fuel rod 301 Spectrum shift rod 303 Coolant ascending channel upper end 304 Coolant outlet 305 Water level 308 Coolant inlet 309 Coolant upflow channel 310 Coolant downflow channel 311 Fuel assembly 312 Average fuel rod 401 Spectral shift rod 403 Coolant upflow channel upper end 404 Coolant outlet 405 Water level 406 Short fuel rod 408 Coolant Inlet 409 Coolant upflow channel 410 Coolant Downflow channel 411 Fuel assembly 412 Average fuel rod 501 Spectral shift rod 502 Fuel rod 503 Coolant upflow channel upper end 504 Coolant outlet 505 Water level 508 Coolant inlet 509 Coolant upflow channel 510 Coolant downflow channel 511 Fuel assembly Body 512 Average fuel rod 551 Spectral shift rod 553 Coolant ascending channel upper end 554 Coolant outlet 555 Water level 558 Coolant inlet 559 Coolant ascending channel 560 Coolant descending channel e _H Fuel assembly average fissile material concentration e _L fuel assembly average fissile material concentration

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G21C 5/12 8908-2G G21C 3/32 GDB E

Claims (13)

[Claims] 1. A coolant ascending passage having a coolant inlet opening at a lower portion of a fuel assembly and allowing a coolant taken from the coolant inlet to flow upward, and a cooling opening above the coolant inlet. Filling with fissile material, at least one water rod having a material outlet and provided with a coolant descending channel connected to the coolant ascending channel and causing the coolant from the coolant ascending channel to flow downward In a fuel assembly having a plurality of fuel rods, each of the coolant upflow passage and the coolant downflow passage,
The horizontal cross-sectional area is uniform in at least a portion other than the vicinity of both ends of the flow path, and the height of the uppermost end of the coolant ascending flow path is equal to or lower than the upper end of the effective length of the plurality of fuel rods. There is a height of the coolant outlet lowermost end of the coolant descending flow path is higher than the lower end of the effective length, and,
The concentration of the fuel assembly average of the fissile material existing in the region from the midpoint of the coolant rising passage uppermost position and the coolant outlet lowermost position to the coolant rising passage uppermost position is, A fuel assembly, wherein the concentration is higher than an average concentration of the fuel assembly of the fissile material present in a region from the midpoint to the coolant outlet lowermost position. 2. A coolant ascending passage having a coolant inlet opening at a lower portion of the fuel assembly and allowing a coolant taken from the coolant inlet to flow upward, and a cooling opening above the coolant inlet. Filled with a fissionable material, one water rod having a material outlet and provided with a coolant descending channel connected to the coolant ascending channel and causing the coolant from the coolant ascending channel to flow downward In a fuel assembly having a plurality of fuel rods, each of the coolant upflow passage and the coolant downflow passage,
The horizontal cross-sectional area is uniform in at least a portion other than the vicinity of both ends of the flow path, and the height of the uppermost end of the coolant ascending flow path is equal to or lower than the upper end of the effective length of the plurality of fuel rods. There is a height of the coolant outlet lowermost end of the coolant descending flow path is higher than the lower end of the effective length, and,
The concentration of the fuel assembly average of the fissile material existing in the region from the midpoint of the coolant rising passage uppermost position and the coolant outlet lowermost position to the coolant rising passage uppermost position is, A fuel assembly, wherein the concentration is higher than an average concentration of the fuel assembly of the fissile material present in a region from the midpoint to the coolant outlet lowermost position. 3. The fuel assembly according to claim 1, wherein the water rod is in a reactor rated output operation determined by a pressure difference between the coolant inlet position and the coolant outlet position. The fuel assembly is characterized in that the water level is higher than a midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet. 4. The fuel assembly according to claim 1 or 2, wherein the water rod is in a reactor rated output operation determined by a pressure difference between the coolant inlet position and the coolant outlet position. A fuel assembly, wherein a water level is a height near the uppermost end of the coolant ascending channel. 5. The fuel assembly according to claim 1, wherein the height of the uppermost end of the coolant ascending passage is the same as the height of the upper ends of the effective lengths of the plurality of fuel rods, and the coolant descends. The height of the coolant outlet lowest end of the flow path is higher than the height of the lower end of the effective length, and the water rod is determined by the pressure difference between the coolant inlet position and the coolant outlet position. A fuel assembly in which a water level during a reactor rated output operation is higher than a midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet. 6. The fuel assembly according to claim 1, wherein the height of the uppermost end of the coolant ascending flow path is lower than the height of the upper ends of the effective lengths of the plurality of fuel rods, and the coolant descending flow is achieved. The height of the coolant outlet lowest end of the passage is higher than the height of the effective length lower end, and the water rod is determined by the pressure difference between the coolant inlet position and the coolant outlet position. A fuel assembly in which a water level during a reactor rated output operation is a height equal to or higher than a midpoint between the uppermost end position of the coolant ascending passage and the lowermost end position of the coolant outlet. 7. The fuel assembly according to claim 1, wherein the height of the uppermost end of the coolant ascending passage is lower than the height of the upper ends of the effective lengths of the plurality of fuel rods, and the coolant descending passage is provided. The height of the coolant outlet lowermost end is higher than the height of the lower end of the effective length, and the water rod is an atom determined by the pressure difference between the coolant inlet position and the coolant outlet position. A fuel assembly, wherein a water level during a reactor rated output operation is a height near the uppermost end of the coolant rising passage. 8. The fuel assembly according to claim 1 or 2, wherein a part of the plurality of fuel rods is a fuel rod whose axial length is shorter than that of the other fuel rods. The height of the upper end is higher than the height of the midpoint between the uppermost end position of the coolant rising passage and the lowermost end position of the coolant outlet. 9. The fuel assembly according to claim 1, wherein the water level during reactor rated output operation, which is determined by the pressure difference between the coolant inlet position and the coolant outlet position, is the coolant rise. A fuel assembly comprising: at least two water rods having a height which is equal to or higher than a midpoint between a flow path uppermost position and the coolant outlet lowermost position and which are different from each other. 10. The fuel assembly according to claim 1, wherein each of the coolant ascending passage and the coolant descending passage has a region from a top end of the ascending passage to a bottom end of the coolant outlet. A fuel assembly having a uniform horizontal cross-sectional area in 80% or more thereof. 11. The fuel assembly according to claim 1 or 2, wherein the fissile material comprises a uranium-plutonium mixed oxide. 12. A core of a boiling water reactor loaded with the fuel assembly according to any one of claims 1 to 4. 13. A method of operating a boiling water nuclear reactor having a core in which at least one fuel assembly according to any one of claims 1 to 4 is loaded in a row other than the outermost periphery, the method comprising: The water level during reactor rated output operation, which is determined by the pressure difference between the coolant outlet position, is equal to or higher than the midpoint between the coolant ascending flow path uppermost end position and the coolant outlet lower end position. A method for operating a boiling water nuclear reactor, comprising operating at a rated output in a core flow rate range that is the height of