JP2004077287A - Fuel assembly for nuclear reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel assembly realizing high-burnup and improving fuel economy taking the end plug regions into consideration for decreasing fuel cycle costs. <P>SOLUTION: In this fuel assembly for a nuclear reactor with the average enrichment of 4 wt% or higher, a plurality of bar-shaped elements including fuel rods arranged in parallel are loaded in a bundle shape with intervals between each other, and fuel pellets are divided into a predetermined number of nodes in the fuel rod axial direction. Maximum enrichment of the pellets in the fuel assembly is 4.9-5wt%, and at least a plurality of fuel rods contain burnable poison along the axial region including upper end node including the top fuel pellet and/or the lower end node including the bottom fuel pellet. The average enrichment at the cross section in the upper end node and/or the lower end node is larger than 3.0wt% and less than 5.0wt%. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fuel assembly for a nuclear reactor, and more particularly to a fuel assembly for a boiling water reactor intended to increase the burnup.
[0002]
[Prior art]
First, the fuel cycle cost will be described. The fuel cycle cost (FCC) is defined by equation (1) as one per assembly.
FCC (yen / kWhe) = C _total / Q _total (1)
However,
C _total : Total cost of one aggregate (yen)
Q _total : Total power generation of one aggregate (kWhe)
[0003]
On the other hand, the total power generation is calculated by the following equation.
Q _total = E _out · WU · f (2)
However,
E _out : average removal burnup (GWd / t)
WU: Aggregate uranium weight (kg)
f: Thermal efficiency including unit conversion.
[0004]
The total cost C _total is mainly a sum of the following cost components.
(A) natural uranium purchase, (b) conversion, (c) enrichment, (d) molding,
(E) new fuel transportation, (f) reprocessing, (g) spent fuel transportation,
(H) Pu, U credit, (i) Waste disposal
In general, an increase in the average enrichment of the fuel assembly is accompanied by an increase in the reactivity, thereby contributing to an increase in the average withdrawal burnup. When the enrichment is increased, (a) to (c), particularly (c), the enrichment cost increases, but other costs hardly change. Typically, the effect of increasing enrichment costs on the total cost is only around 20%. On the other hand, from the equations (1) and (2), it can be seen that the fuel cycle cost is inversely proportional to the average removal burnup.
[0006]
As a result, if the average removal burnup can be increased more than the cost increase accompanying the increase in the average enrichment, the fuel cycle cost can be reduced.
[0007]
Thus, in the field of fuel assembly design in a nuclear reactor, increasing the average enrichment of fissile material typified by uranium 235 in order to increase the fuel economy reduces the average removal burnup. Measures have been taken to achieve this (higher burnup due to higher enrichment).
[0008]
By the way, in a boiling water reactor, for example, natural uranium or the like is disposed at the upper end and / or lower end of the heat generating effective portion of the fuel assembly to reduce the enrichment and reduce neutron leakage, thereby saving uranium resources. And is widely used. Here, this technology is referred to as blanket technology.
[0009]
However, since the blanket has low enrichment, the use of the blanket tends to lower the average burnup of the aggregate. In particular, when there is an upper limit (at present 5 wt%) in the concentration that can be handled, it is not easy to increase the average concentration when the blanket technology is used.
[0010]
Conventional techniques for solving this are as follows. This situation will be described with reference to FIG. 4, but prior to the description, FIG. 5 is shown.
[0011]
FIG. 5 shows an example of a conventional fuel assembly for a boiling water reactor (conventional example 1). This example is a typical fuel assembly for high burnup in which fuel rods in a 9 × 9 grid arrangement are provided with water rods in which nine fuel rods are replaced. In this example, as a typical blanket, a natural uranium blanket having a length of 1/24 at each of the upper and lower ends is provided. Here, the upper and lower ends are referred to as an end region, and the region excluding the upper and lower ends is referred to as a central region. In the figure, the numerical value described in each fuel rod indicates the enrichment (wt%) in each region of each fuel rod. The shaded area indicates the area to which gadolinia, which is a burnable poison, is added, and the numerical value with “G” indicates the gadolinia concentration (wt%). The enrichment of Conventional Example 1 is 0.71 wt% in the end region, 4.28 wt% in the central region, and 3.98 wt% in the aggregate average. Assuming 13 months of continuous operation at this concentration, the average removal burnup can achieve about 50 GWd / t.
[0012]
The gadolinia distribution of Conventional Example 1 was designed according to the conventional technology from the following viewpoints. First, the reason why gadolinia having a high concentration is used in a part of the lower side is to suppress the output distribution from being excessively distorted in the middle part of the operation cycle and the linear output density from increasing. On the upper side, the gadolinia concentration is reduced in order to minimize the reduction in economic efficiency due to gadolinia remaining at the end of the cycle, assuming that a sufficient stop margin is secured throughout the operation cycle.
[0013]
For this design, consider reducing the fuel cycle cost by increasing the aggregate average enrichment and increasing the average removal burnup. FIG. 4 shows the result of evaluating the relationship between the increase amount of the average enrichment and the fuel cycle cost when Conventional Example 1 is used as a reference (point (a) in the figure). Lines (1) to (3) in the figure correspond to the following conventional methods (1) to (3), respectively. Here, it is also examined whether each method can secure a thermal operation margin with respect to the maximum linear power density, the minimum critical power ratio, and the like (referred to as thermal design criteria). The line in FIG. 4 is indicated by a solid line when the thermal design standard is satisfied, and is indicated by a broken line when the thermal design standard cannot be satisfied.
[0014]
(1) Method of increasing the enrichment in the central region by using a natural uranium blanket In a high-enrichment fuel assembly in which the enrichment in the central region is sufficiently increased, when the enrichment that can be handled has an upper limit of 5 wt%. Unless special measures are taken, the increase in enrichment in the central region will be accompanied by an increase in the fuel rod power distribution in the radial direction. Difficult to do. Now, it is considered that Conventional Example 1 is a fuel assembly with a high enrichment such that the enrichment in the central region cannot be increased without reducing the thermal operation margin. In this case, when the enrichment in the central part is increased, it follows the line (1) in FIG. 4, but the line (1) becomes a broken line when the average enrichment of the aggregate is increased. . That is, it is not possible to maintain the thermal design standard and increase the concentration.
[0015]
(2) Method without using a natural uranium blanket When a natural uranium blanket is not used, the axial output distribution is flattened by an increase in the output sharing in the end region, and the thermal operation margin does not become strict. The fuel cycle cost of a fuel assembly without a blanket shows the trend shown by line (2) in FIG. A fuel assembly without a blanket has a large loss of neutrons in the end region and a loss of reactivity, resulting in a decrease in average withdrawal burnup and a disadvantage in economy. For example, in order for a fuel without a blanket to have the same fuel cycle cost as at point a, it is necessary to further increase the average enrichment by about 0.15 wt% in order to increase the average withdrawal burnup. Point b corresponds to this. However, in this case, since the enrichment of the central section is lower than that of 4.28 wt% of the conventional example 1 (3.98 + 0.15 = 4.13 wt%), the output of the fuel rod in the radial direction is sufficient. Since the distribution does not increase, there is no problem with the thermal design criteria. Further, when the upper and lower blankets are removed from the fuel assembly of Conventional Example 1, point c in FIG. 4 is obtained, and the fuel cycle cost is lower than point a.
[0016]
As described above, in each of the cases at points b and c, the fuel cycle cost can be reduced without being thermally strict as compared with the conventional example 1. However, it is clear that the use of blankets is wasteful from the viewpoint of uranium resource saving. Further, the point c is the limit for the high enrichment within the thermal design standard.
[0017]
(3) A method of using a concentrated blanket in the end region A method has been proposed in which the average concentration of the end region is increased to 1.4 to 3.0 wt% without containing a burnable poison (see Japanese Patent No. 3262022). ). In the case of this method, since the end region does not contain burnable poisons, the reactivity of the end region is larger than that when a natural uranium blanket is used, the axial power distribution is flattened, and the heat such as the linear power density is reduced. The target margin can be increased. On the other hand, since the end area does not contain burnable poisons, the average enrichment of the end area needs to be 3 wt% or less to ensure the subcriticality in the storage pool, and a sufficiently high enrichment In some cases, it may not be possible to improve the accuracy. When applied to Conventional Example 1, the average concentration of the aggregate can be increased only to 4.04 to 4.17 wt%. This is shown by the line (3) in FIG. 4. In this method, the fuel cycle cost is limited at the point d.
[0018]
[Problems to be solved by the invention]
As described above, when the prior arts (1) to (3) are used, if the enrichment is performed stepwise within the thermal design criteria, the points a to d e to e in FIG. The path of the point c will be followed. In particular, between the point d and the point e, no reduction in the fuel cycle cost accompanying the increase in the enrichment can be expected at all, and it can be seen that there is an area having a problem in terms of economy. Further, the conventional technology cannot provide a fuel assembly more economical than point c.
[0019]
The present invention aims at achieving high burnup by aiming for high enrichment, and improving fuel economy, that is, reducing fuel cycle costs, a fuel assembly that realizes this by focusing on the end region The purpose is to provide.
[0020]
[Means for Solving the Problems]
A nuclear reactor fuel assembly according to the invention described in claim 1 includes a plurality of rod-shaped elements including fuel rods arranged in parallel and loaded in a bundle at intervals from each other, wherein the fuel rods are fuel rods. In a fuel assembly for a nuclear reactor having an overall average enrichment of 4 wt% or more loaded with fuel pellets divided into a predetermined number of nodes in the axial direction,
The maximum enrichment of the pellets in the fuel assembly is 4.9 wt% or more and 5 wt% or less;
At least some of the fuel rods contain burnable poison over an axial region including an upper end node including the uppermost fuel pellet and / or a lower end node including the lowermost fuel pellet, and the upper end node and / or Alternatively, the average enrichment of the cross section at the lower end node is larger than 3.0 wt% and smaller than 5.0 wt%.
[0021]
In the reactor fuel assembly according to the second aspect of the present invention, the cross section at the upper end node and / or the lower end node according to the first aspect has a cross-sectional area having the highest average enrichment in the cross section of the assembly. Is included.
[0022]
According to a third aspect of the present invention, there is provided a nuclear reactor fuel assembly, wherein the reactor fuel assembly according to the first or second aspect has a large water rod occupying a plurality of fuel rod regions. It has a fuel rod arrangement of 9 grids or more.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, a plurality of rod-shaped elements including fuel rods arranged in parallel are loaded in a bundle at intervals from each other, and the fuel rods are divided into a predetermined number of nodes in the fuel rod axial direction. In a fuel assembly for a nuclear reactor having an overall enrichment of 4 wt% or more loaded with fuel pellets, the maximum enrichment of the pellets in the fuel assembly is 4.9 wt% or more and 5 wt% or less, At least some of the rods contain burnable poison over an axial region including an upper end node including a top fuel pellet and / or a lower end node including a bottom fuel pellet, and the top node and / or The average enrichment of the cross section at the lower end node is larger than 3.0 wt% and smaller than 5.0 wt%. For this reason, the uneconomical property of the enrichment area corresponding to the point d-point e in FIG. 4 can be reduced. By aiming for high enrichment, high burnup can be achieved, and fuel economy can be improved, that is, fuel cycle cost can be reduced.
[0024]
In addition, according to the present invention, by including the burnable poison in the end region, it is not necessary to reduce the concentration in the end region to 3.0 wt% or less in order to secure the subcriticality during storage. Further, it is possible to further increase the concentration in the end region. As a result, the average withdrawal burnup is improved and the fuel cycle cost is further reduced. In this case, the characteristic shown by the line (4) in FIG. 4 is obtained.
[0025]
In addition, in this enrichment range (enrichment of the end region is greater than 3.0 wt%), the infinite multiplication factor of the end region is larger than that of natural uranium even when a burnable poison is mixed. As a result, the axial power distribution does not become severe, so that a fuel assembly satisfying the thermal design criteria can be provided.
[0026]
Further, the presence of the burnable poison at the upper end portion can also avoid the deterioration of the stop margin, which is a concern in the method (3), for example. Furthermore, in the present invention, the number of fuel rods containing burnable poisons can be minimized because the fuel rods to which burnable poisons are added to the end regions are also fuel rods containing burnable poisons also in the central region. it can. Therefore, even when gadolinia is used as the burnable poison, the work efficiency does not decrease for the work of filling the cladding tube with pellets at the time of molding.
[0027]
In the present invention, the cross section at the upper end node and / or the lower end node may be a cross-sectional area having the highest average enrichment in the cross section of the aggregate. That is, in the present invention, since the end regions of the upper end node and the lower end node are not originally regions in which the thermal operation margin such as the maximum linear power density and the minimum limit power ratio becomes severe, the enrichment is further increased, and the fuel in this region is further increased. It is possible to increase the bar output. In the present invention, the enrichment in the end region is higher than in the central region, and therefore the reactivity is increased, which contributes to the flattening of the axial power distribution. That is, the thermal design standard is not impaired. In the case of the present invention, the relationship between the increase amount of the average enrichment and the fuel cycle cost is in a range to the right of the point C of the line (4) in FIG. Aggregates can be provided.
[0028]
The present invention is particularly directed to a boiling water atom suitable for high burnup, in which a fuel assembly for a reactor is provided with a fuel rod array of 9 × 9 grid or more having a large diameter water rod occupying a plurality of fuel rod regions. It is suitable for use in furnaces.
[0029]
【Example】
In the embodiment described below, the present invention is applied based on the conventional fuel assembly 1 for a boiling water reactor shown in FIG.
[0030]
(Example 1)
Example 1 is shown in FIG. As shown in the figure, a natural uranium blanket having a length of 1/24 at each of upper and lower ends is provided. Here, the upper and lower ends are referred to as an end region, and the region excluding the upper and lower ends is referred to as a central region. In the figure, the numerical value described in each fuel rod indicates the enrichment (wt%) in each region of each fuel rod. The shaded area indicates the area to which gadolinia, which is a burnable poison, is added, and the numerical value with “G” indicates the gadolinia concentration (wt%). The same applies to the figures of the other embodiments described below. In the present embodiment, the fuel rods including gadolinia and some fuel rods not including gadolinia are designed so that the upper and lower blankets are not provided. That is, the fuel rod types 2, G1, G2, and G3 have the same enrichment at the upper and lower ends as the enrichment in the central region. As a result, although the cross-sectional average enrichment at the upper and lower ends is 3.14 wt%, which is larger than 3.0 wt%, the reactivity in the initial stage of combustion is suppressed by including gadolinia. Subcriticality of the fuel in the fuel cell.
[0031]
Further, the average enrichment of the aggregate is 4.18 wt%, which is about 0.2 wt% larger than that of Conventional Example 1. The fuel cycle cost of this fuel assembly is as shown in FIG. Although the fuel cycle cost is slightly disadvantageous as compared with the point d which can be achieved by the conventional method (3), the design of a fuel rod containing gadolinia, which is particularly toxic and may deteriorate the pellet filling work efficiency during molding, is required. The simplification contributes to a reduction in the cost of processing the fuel, so that there is not actually a cycle cost difference as large as the evaluation value in FIG.
[0032]
Further, the present invention includes gadolinia at the upper and lower ends. For this reason, in the upper cross section where the reactivity sharing is large and the reactor shutdown margin characteristic is likely to be affected, the reactivity can be reduced in the early stage of combustion in the case of the fuel assembly of the present invention. Therefore, from the viewpoint of the stop margin, the fact that the fuel assembly of the present invention can increase the stop margin is more advantageous in terms of safety than the assembly constituted by the conventional method (3).
[0033]
(Example 2)
Example 2 is shown in FIG. Concentrated pellets of 2.4 wt% in natural uranium were used in Example 1. As a result, the average concentration of the aggregate can be increased to 4.23 wt%, which is 0.25 wt% larger than that of Conventional Example 1. The fuel cycle cost of this fuel assembly is as shown in FIG. The fuel cycle cost can be further reduced by 0.4% as compared with the point d which can be achieved by the conventional method (3).
[0034]
(Example 3)
FIG. 3 shows Example 3 in which a higher concentration than Example 2 was aimed. Except for the corner part where the output is most likely to be largest, pellets of 4.9 wt%, which is the highest enrichment, were all arranged on the upper and lower ends of the fuel rods not containing gadolinia. As a result, the cross-sectional average enrichment at the upper and lower ends is 4.61 wt%, which is the highest in the aggregate. The average concentration of the aggregate can be increased to 4.31 wt%, which is 0.33 wt% larger than that of Conventional Example 1. The fuel cycle cost of this fuel assembly is as shown in FIG. The fuel cycle cost can be further reduced by 0.5% compared to the second embodiment.
[0035]
Further, as a result of analyzing the equilibrium core characteristics, it was confirmed that the maximum linear power density and the minimum critical power ratio were not much different from those of the conventional example 1 or that the operation margin was slightly increased in each of the examples.
[0036]
【The invention's effect】
As described above, the present invention achieves high burnup by aiming for high enrichment, and realizes this by focusing on the end region when improving fuel economy, that is, reducing fuel cycle cost. There is an effect that a fuel assembly can be provided.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a configuration of an embodiment of a fuel assembly of the present invention.
FIG. 2 is an explanatory view showing the configuration of another embodiment of the fuel assembly of the present invention.
FIG. 3 is an explanatory view showing a configuration of still another embodiment of the fuel assembly of the present invention.
FIG. 4 is an explanatory diagram showing the results of evaluating the relationship between the increase amount of the average enrichment and the fuel cycle cost.
FIG. 5 is an explanatory diagram showing a configuration of a conventional example of a fuel assembly for a boiling water reactor.

Claims (3)

A plurality of rod-shaped elements including fuel rods arranged in parallel are loaded in a bundle at intervals from each other, and the fuel rods are divided into a predetermined number of nodes in the fuel rod axial direction and loaded with fuel pellets. In the reactor fuel assembly having a total average enrichment of 4 wt% or more,
The maximum enrichment of the pellets in the fuel assembly is 4.9 wt% or more and 5 wt% or less;
At least some of the fuel rods contain burnable poison over an axial region including an upper end node including the uppermost fuel pellet and / or a lower end node including the lowermost fuel pellet, and the upper end node and / or Alternatively, a fuel assembly for a nuclear reactor, wherein an average enrichment of a cross section at a lower end node is larger than 3.0 wt% and smaller than 5.0 wt%. 2. The fuel assembly for a nuclear reactor according to claim 1, wherein a cross-sectional area having the highest average enrichment in the cross-section of the assembly is included in the cross-section at the upper end node and / or the lower end node. 3. 3. The fuel assembly for a nuclear reactor according to claim 1, further comprising a fuel rod array having a 9 × 9 grid or more having a large diameter water rod occupying a plurality of fuel rod regions.

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