JP3062548B2 - Fuel assembly for boiling water reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly used for a boiling water reactor having a core structure of a D lattice arrangement.

[0002]

2. Description of the Related Art The core structure of a boiling water reactor (hereinafter referred to as BWR) includes a C lattice core structure and a D lattice core structure. The C lattice core structure and the D lattice core structure will be briefly described with reference to FIG. FIG.
(A), the fuel assembly shown in FIG.
Has an 8 × 8 configuration, and has two water rods at the center. In FIG. 6 and all the drawings described below, the same or corresponding parts are denoted by the same reference characters.

As shown in FIG. 6A, a C lattice core structure has a fuel assembly channel box 12 and a fuel assembly channel box 1 adjacent to the fuel assembly.
2, that is, the water gap widths a and b have the same width (ie, a = b) regardless of the arrangement of the control rods 14. The D lattice core structure is shown in FIG. (B)
As shown in the figure, the width a of the water gap in which the control rods 14 are disposed is equal to the width b of the water gap in which the control rods 14 are not disposed.
The structure is arranged so as to be wider (ie, a> b).

Since the distribution of the thermal neutron flux inside the fuel assembly becomes higher on the water gap side, the fission reaction of thermal neutrons and uranium becomes more active on the water gap side closer to the outer periphery than the inner side of the fuel assembly. Therefore, assuming that the enrichment of all fuel rods constituting the fuel assembly, that is, the weight ratio of fissile isotopes (typically ²³⁵ U) to the weight of uranium contained in the fuel pellets is the same, Large peaking (local peaking) occurs in the outermost fuel rods.

[0005] The local peaking is a relative value indicating the actual output of each fuel rod with respect to the average output of the fuel rod. If this value is too large, the thermal and mechanical integrity of the fuel rod is impaired, and in the worst case. In this case, the fuel rod will be damaged.

Therefore, generally, the enrichment of each fuel rod is changed in accordance with the deceleration state of the neutron at the place, and the increase in local peaking is suppressed by giving the enrichment distribution to the fuel assembly.

For example, FIG. 7A shows a fuel assembly having a C lattice core structure having an average enrichment of about 3.7 wt% and having 72 fuel rods and one large-diameter water channel (W in the figure). It is an example of a concentration distribution. In this case, the large diameter water channel is 9 rows and 9
The size of the fuel rods in the column covers the area of three rows and three columns at the center. The enrichment levels of the fuel rods 11 are five levels, with the highest being level 1 and the lowest being level 5.

In this case, since the core has a C lattice structure, the width of the water gap is the same over the entire circumference, and the deceleration state of the neutrons is uniform over the four outermost regions inside the fuel assembly. Therefore, the difference between the maximum enrichment and the minimum enrichment of the fuel rods arranged inside is small.

FIG. 7B shows 72 fuel rods and one fuel rod.
It is an example of the enrichment distribution of a fuel assembly having a D lattice core structure having an average enrichment of about 3.8 wt% and having a large diameter water channel (W in the figure). Also in this case, the large-diameter water channel has 9 rows and 9 rows.
Among the fuel rods in the column, the size of the fuel rod 11 is set to the size of the central three rows and three columns.
The minimum is six levels of level 6.

In this case, since the core has a D lattice structure, the water gap width is wide on the two sides on which the control rods are arranged, and the water gap width is on the other two sides on which the control rods are not arranged. The distribution of thermal neutrons inside the fuel assembly is high on the control rod side and low on the side where no control rods are arranged.

[0011] For this reason, the fuel enrichment distribution is biased such that fuel rods with low enrichment are arranged on the control rod side and fuel rods with high enrichment are arranged on the side where no control rods are arranged. The output for each bar is flattened. Therefore, in the fuel assembly having the D-lattice core structure, the difference between the highest enrichment and the lowest enrichment of the fuel rods disposed inside is larger than that of the fuel assembly for the C-lattice core structure.

[0012] Generally, a nuclear reactor facility is provided with an earthquake detector for ensuring safety in the event of an earthquake. This earthquake detector ensures safety by automatically shutting down the reactor (automatic scram) when a large earthquake occurs. At the same time, the goal is to avoid unnecessary scrum and ensure the reliability of reactor operation in small and medium-sized earthquakes without serious problems.

[0013]

The fuel assembly has a length in the vertical direction of about 4 m, and upper and lower ends thereof are fixed to the core. Therefore, even when a small-to-medium-scale earthquake occurs, the horizontal vibration caused by the earthquake displaces the middle part of the fuel assembly by about several mm in the horizontal direction, and the water gap between the fuel assemblies The width may fluctuate around 1 mm.

Such a variation in the water gap width is such that in the fuel assembly having the C lattice core structure, the water gap width outside the fuel assembly is uniform over the entire circumference and is optimally arranged in the core. Therefore, the input reactivity (reactivity change amount) becomes negative with respect to the fluctuation of the water gap width. However, in the D-lattice core structure, the water gap width around the fuel assembly is different between the control rod side and each of the two sides on the opposite side, and the relatively high enrichment fuel is provided on the two sides with the smaller water gap width. In the case of a conventional fuel assembly with rods, the thermal neutron flux at the narrow gap side increases when the narrow water gap widens during an earthquake, despite the fact that the thermal neutron flux at the narrow gap side is originally low. However, there is a problem that the input reactivity is likely to be positive.

Here, for a conventional fuel assembly having a D-lattice core structure, the relationship between the injection reactivity and the displacement of the assembly due to the vibration substantially equivalent to that of a small-to-medium-scale earthquake is shown in terms of the burnup of 0 GWd / t and 20 GWd. FIG. 8 shows the results obtained by examining each of / GW and 40 GWd / t. In FIG. 8, the vertical axis represents the input reactivity (% ΔK), and the horizontal axis represents the aggregate spacing displacement (m).
m).

This analysis result is obtained when the width of the water gap on the two sides opposite to the control rod, that is, the width of the water gap on the side where the control rod is not arranged, changes simultaneously by the same amount. This is due to the fact that the direction of the horizontal vibration of the earthquake is not uniform, and the change in the reactivity becomes the largest, in other words, the water gap width on the two sides opposite to the control rod changes simultaneously by the same amount We analyzed when we did.

Further, since the volume of the entire core is not expected to expand or shrink due to the earthquake, the area of the entire water gap is not changed. In the analysis, when the water gap on the opposite side of the control rod is expanded, the control rod is simultaneously opened. The water gap on the side shrinks by the same amount. That is, the area of the entire water gap did not change.

FIG. 8 shows that regardless of the degree of burnup,
It can be seen that the greater the change in the water gap, the greater the fission reaction and the greater the input reactivity.

The increase in the input reactivity depends on the expansion amount of the water gap width between the fuel assemblies regardless of the number of rows of fuel rods included in the fuel assemblies.
It can be said that this occurs not only in the fuel assemblies having the lattice core structure, but also in all the fuel assemblies having the D lattice core structure.

Such an increase in the input reactivity increases the probability that the reactor is unnecessarily shut down (scrum). For example,
In a reactor operating at the rated power (100%), when a throwing reactivity of about 30 cents is applied by horizontal vibration, the increase in neutron flux is estimated to be about 140% by using the quick jump approximation. Become.

Since this value exceeds the set level of the automatic scram function due to the increase of the neutron flux of the normal reactor, which is 120%, the automatic scram function operates to shut down the reactor. The operation reliability of the vehicle is reduced.

From the above, the present invention reduces the probability of an unnecessary reactor shutdown (scram) by reducing the input reactivity applied by horizontal vibration when a small-to-medium-scale earthquake occurs, It is an object of the present invention to obtain a fuel assembly for a boiling water reactor for a D lattice arrangement with improved operation reliability.

[0023]

In order to achieve the above object,
The invention according to claim 1 is arranged in a lattice shape with a large-diameter water channel.
A plurality of fuel rods, and the large-diameter water channel
Is more controlled than the center of the fuel assembly in the grid array.
Is arranged at a position displaced from the rod far direction, the grid-like distribution
Out of the fuel rods arranged in the outermost peripheral region of the row,
75 of the number of fuel rods arranged on the two sides opposite to the rod
% Or more indicates that the weight fraction of fissile material is
Core structure of D lattice array composed of fuel material of lower value
In a boiling water reactor fuel assembly for use in forming the
Arranged in the first area between the large-diameter water channel and the two sides.
Gadolinia concentration of the fuel rods containing gadolinia
Other two sides of the outermost peripheral area adjacent to the control rod
And in the second region between the large-diameter water channel and
Relative to gadolinia concentration of fuel rods containing gadolinia
It is characterized by being high .

In other words, according to the first aspect of the present invention, the large-diameter water channel is arranged at a position farther from the control rods than the center of the fuel assembly in the lattice-like arrangement, so that the large-diameter water channel can be disposed. The neutron moderation of the region sandwiched between the narrow water gap outside the fuel assembly is enhanced, and the thermal neutrons and the uranium fission reaction are activated.

In addition, among the fuel rods arranged in the outermost peripheral region of the fuel assembly, the fuel rods arranged on the two sides opposite to the control rods collectively account for 75% or more of the number of fuel rods. By setting the enrichment level lower than the maximum enrichment of all fuel rods in the body, even if the width of the water gap on the opposite side of the control rod increases due to horizontal vibration, the increase in the input reactivity due to it will be suppressed low. I am trying to do it.

As a result, the change of the thermal neutron flux due to the fluctuation of the water gap width becomes small especially at the two sides on the side opposite to the control rod, so that the input reactivity can be suppressed to a relatively low level even if the assembly vibrates horizontally. . Therefore, the setting level of the automatic scram function (for example, 1
20%), it is possible to prevent unnecessary scram operation of the nuclear reactor and secure the reliability of operation.

Here, in the fuel assembly according to the first aspect of the present invention, as for the relationship between the displacement of the assembly caused by vibration substantially equivalent to that of a small-to-medium-scale earthquake and the injection reactivity, the burn-up is 0 GW.
FIG. 3 shows the results of investigation on d / t, 20 GWd / t and 40 GWd / t.

It is to be noted that fuel assemblies having different burnups are mixed in the nuclear reactor, and when an average take-up burnup is about 45 GWd / t and one operation cycle is a continuous operation for about one year,
The average core burnup of the equilibrium core at the beginning of the cycle is about 20 GW
d / t, average core burnup at the end of the cycle is about 30 GWd / t
Becomes

3 and 8, a comparison is made between the conventional fuel assembly and the fuel assembly of the present invention by focusing on the range of the average burnup of the core, that is, the range of about 20 GWd / t to 30 GWd / t. It can be seen that the reactivity change in the case of the present invention is suppressed to about 1/2 of the reactivity change in the conventional case.

Further, a new fuel having a low burnup (0 GW in the figure)
d / t), the relative reduction in reactivity is about 2
Although it is not reduced to about 1/2 as in the range of 0 GWd / t to 30 GWd / t, the absolute value of the change in reactivity itself is small at the beginning of combustion because of the gadolinia neutron absorption effect. Therefore, it can be considered that the fuel assembly of the present invention has a reactivity change smaller than about one half of the conventional fuel assembly when the reactivity change is viewed as a core average.

For example, when horizontal vibrations occur in a reactor operating at a rated output (100%) due to a small-to-medium-scale earthquake, the increase in reactivity was about 30 cents in the past, but the present invention Then it is only about 15 cents. Therefore, when the increase of the neutron flux is estimated using the quick jump approximation, the increase of the neutron flux becomes about 118%, which does not exceed 120% of the set level of the automatic scram function by the neutron flux of the normal reactor. Unnecessary shutdown of the reactor is avoided without the automatic scram function working in the dark.

In this case, the amount of increase in the reactivity corresponding to 120% of the set level of the automatic scrum function is approximately 17 cents when calculated by the quick jump approximation. Therefore, the conventional fuel assembly is at a limit where the automatic scram function does not work up to an earthquake of a magnitude that provides an increase in reactivity of 17 cents.

According to the first aspect of the present invention, even in the case of an earthquake of a magnitude giving a reactivity increase of 17 cents, which was the limit where the automatic scram function does not work conventionally, the fuel assembly of the present invention can be used. Gives only half of 8.5 cents of reactivity,
As the non-operational limit range of the substantial automatic scram function has been widened, the reactor will not be operated in the dark in the event of an earthquake of a scale where an unnecessary automatic scram function was conventionally operated, and The reliability of reactor operation can be improved.

As described above, the fuel assembly according to the first aspect of the present invention can provide a fuel assembly having a small area even if the width of the narrow water gap on the opposite side to the control rod is widened due to horizontal vibration when a small-to-medium-scale earthquake occurs. Since the nuclear fission reaction does not increase rapidly, the applied reactivity is suppressed as a result, and unnecessary reactor shutdown (scram) can be avoided, and the reliability of reactor operation can be improved. is there.

Further, according to the first aspect of the present invention , all the fuel rods arranged on the two side portions are made of a fuel material having a weight ratio of fissile material lower than the maximum value in all the fuel rods .
You .

That is, among the fuel rods arranged in the outermost peripheral region of the fuel assembly, the enrichment levels of the fuel rods arranged on the two sides opposite to the control rods are determined by the enrichment level of all the fuel rods of the assembly. By making the level lower than the maximum enrichment, the increase in the input reactivity due to the increase in the water gap width on the side opposite to the control rod is further suppressed.

Therefore, the practical operation of the automatic scrum function is not performed.
The dynamic limit range is widened, and the reliability of reactor operation can be further improved.

In the fuel assembly for a boiling water reactor according to the first aspect of the present invention, the arrangement of the large-diameter water channel is shifted in a direction farther from the control rod than the center of the fuel assembly. Since the moderating power of thermal neutrons in the first region between the water channel and the two side portions is high and the fission reaction of thermal neutrons and uranium becomes active, the gadolinia-containing fuel rods arranged in this region are the outermost. The combustion is promoted more than the gadolinia-containing fuel rod arranged in the second area between the other two sides of the peripheral area adjacent to the control rod and the large-diameter water channel.

Therefore, the present invention further provides the gadolinia concentration of the gadolinia-containing fuel rod arranged in the first region between the large-diameter water channel and the two sides, and The gadolinia concentration of the gadolinia-containing fuel rod disposed in the second region between the other two sides adjacent to the control rod and the large-diameter water channel is relatively high.

Here, in the fuel assembly of the present invention, FIG. 4 is a graph showing the reactivity change with respect to the water gap change when the gadolinia concentration of all the gadolinia-containing fuel rods is the same. As a comparative example, FIG. 9 is a graph showing a change in reactivity with respect to a change in water gap when the gadolinia concentration of all gadolinia-containing fuel rods is the same in a conventional fuel assembly. In both figures, the vertical axis is the input reactivity (% ΔK), the horizontal axis is the burnup (GWd / t), and the water gap width change is -0.5 mm, +0.5 mm, 1.0 m
The four states of m and 1.5 mm were examined.

FIG. 4 shows that in the fuel assembly of the present invention, the reaction is suppressed by the effect of gadolinia up to a burnup of about 10 GWd / t, so that the input reactivity with respect to the change of the water gap width does not change much. Burnup 10 GW
When the burnout exceeds 15 GWd / t, the change in the injection reactivity suddenly decreases a little again. It can be seen that the increase in the charging reactivity with respect to the change in the water gap width over the combustion period is effectively suppressed.

On the other hand, in the conventional fuel assembly, as apparent from FIG. 9, the input reactivity to the change of the water gap from the initial stage of combustion is slightly higher than that of the present invention, and the burnup is about 10 GWd / t. , The input reactivity tends to be the minimum value. However, when the burnup exceeds 10 GWd / t, a sharp increase in the input reactivity with respect to the change in the water gap is observed, which decreases until the burnup reaches about 40 GWd / t. It turns out that it does not.

Here, the infinite doubling rate (absolute value of the reactivity) of the fuel assembly of the present invention and the conventional fuel assembly when all the gadolinia-containing fuel rods have the same gadolinia concentration,
FIG. 5 shows the relationship with the burnup. FIG. 5 shows that the gadolinia effect disappears when the burnup is about 10 GWd / t.

According to FIGS. 4, 9 and 5, near the burnup of 10 GWd / t at which the gadolinia effect disappears, the change in the input reactivity with respect to the change in the water gap amount is as follows.
It can be seen that there is not much difference between the present invention and the conventional case. By the way, in the case of the fuel assembly of the present invention, since the probability of thermal neutrons being decelerated in the first region is higher than in the conventional case, the combustion of the gadolinia-containing fuel rod is promoted by that much, and the gadolinia effect disappears. The residual effect of gadolinia is small at the burnup near. This is apparent from the infinite doubling rate in FIG. 5, that is, the peak value of the reactivity, which is higher in the fuel assembly of the present invention than in the conventional fuel assembly.

Therefore, from FIGS. 5 and 9, it can be seen from FIG. 5 and FIG. 9 that the expansion of the water gap has the effect of suppressing the reaction due to the absorption of thermal neutrons into the residual gadolinia, despite the increased fission of the fuel rods. I understand. From this,
The gadolinia concentration of the gadolinia-containing fuel rods arranged in the first region is made relatively higher than the gadolinia concentration of the gadolinia-containing fuel rods arranged in the second region. This is effective for achieving the effect of suppressing an increase in the input reactivity when the water gap on the side opposite to the control rod increases in the period up to the burnup in which disappears.

According to a second aspect of the present invention, there is provided the fuel assembly for a boiling water reactor according to the first aspect , comprising a fuel rod arrangement of 9 rows and 9 columns and at least one large-diameter water channel. The large-diameter water channel is disposed in a region of a plurality of fuel rods, which is shifted by one or more rows and one row in a direction farther from the control rod than the center of the fuel assembly.

In any of the above-mentioned inventions, the fissile substance to be used is not limited to uranium. For example, the present invention has a similar effect even with a mixed oxide fuel in which plutonium and uranium are mixed (a so-called MOX fuel). In this case, the enrichment ( ²³⁵ U weight ratio) in the above description may be read as the enrichment which is the weight ratio of fissile plutonium isotopes ( ²³⁹ Pu and ²⁴¹ Pu).

Similarly, the cross-sectional shape of the large-diameter water channel is not particularly limited as long as it is a large-diameter water channel. .

[0049]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in more detail. In all the drawings, the same or corresponding portions are denoted by the same reference numerals.

FIG. 1 is an explanatory view showing one embodiment of a fuel assembly for a boiling water reactor according to the present invention. The fuel assembly shown in FIG. 1 has a fuel rod arrangement of 9 rows and 9 columns, and has an average enrichment of about 3.8 wt% having 72 fuel rods 1 and one large-diameter water channel W. is there.

This large-diameter water channel W has a rectangular cross-section having a size of three rows and three columns, and is shifted away from the control rod 4 by one row and one column from the center of the lattice arrangement. Is located in the position. The enrichment levels of the fuel rods 1 are six levels, with the highest being level 1 and the lowest being level 6.

Since the neutron deceleration is predominant on the water gap side around the entire circumference of the channel box 2 and around the large diameter water channel W in the fuel assembly, in this embodiment, the center 3 of the grid-like arrangement is used. The control rod 4 is compared with the fuel assembly in which the large-diameter water channel is arranged in the region of three rows.
The neutron deceleration is further promoted in a region D between the two sides 2a and 2b of the water gap where no is disposed and the large-diameter water channel W.

In the present embodiment, since the large-diameter water channel W extends along the two sides 2a and 2b over a relatively wide range, the thermal neutrons and uranium over a wide range with respect to the two sides 2a and 2b. Promotes the fission reaction. for that reason,
In this example, the enrichment level of the fuel rods 1 arranged along the two sides 2a and 2b in the region D is set to level 2 or less.

As a result, the change of the thermal neutron flux due to the fluctuation of the water gap width becomes small particularly at the two sides of the anti-control rod side. Even if it expands, the thermal neutron flux does not increase much in that part, and conversely, the water gap on the control rod side becomes narrower and the reaction in that part decreases, so the neutron flux as a whole is at the set level of the automatic scram function. This does not exceed 120%, so that the automatic scram function does not work in the dark and unnecessary scram operation of the reactor can be prevented.

FIG. 2 is an explanatory view showing another embodiment of the fuel assembly for a boiling water reactor according to the present invention. The fuel assembly shown in FIG. 2 has a fuel rod arrangement of 9 rows and 9 columns,
It has one fuel rod 1 and two large diameter water rods W1 and W2 and has an average enrichment of about 3.8 wt%.

Each of these large-diameter water rods W1 and W2 has a diameter substantially equal to the diameter of two fuel rods in the fuel assembly, and has one end closest to the control rod of the fuel assembly and the furthest from the control rod. The two rods are arranged side by side on a diagonal line connecting the corners, and the center position of the water rod when the two rods are set as a set is shifted from the center of the assembly in a direction away from the control rod 4 by one row and one column. Are located. The enrichment levels of the fuel rods 1 are six levels, with the highest being level 1 and the lowest being level 6.

In this example, since the cross-sectional shape of the large-diameter water channel is circular, the peripheral surface of the rod does not face the two sides 2a and 2b in parallel, and is not as large as the case shown in FIG. Although the influence is not exerted over a wide range of the two sides 2a and 2b, the fission reaction of thermal neutrons and uranium is promoted in the region D as compared with the conventional case where the rod is disposed at the center. I have.

Further, in this example, the enrichment level of the fuel rods 1 arranged along the two sides 2a, 2b is lower than the highest enrichment level of all the fuel rods in the assembly. Is selected and arranged so as to be 75% or more of the above.
That is, it is assumed that the fuel rods having the highest enrichment among the fuel rods arranged along the two sides 2a and 2b are selectively arranged so that the number thereof is 25% or less.

The present invention is not limited to the fuel assembly as shown in FIGS. 1 and 2, and for example, the cross section of the large diameter water rod may be elliptical, rectangular or any other irregular cross section. It goes without saying that the present invention can be applied to a fuel assembly having another configuration.

[0060]

As described above, according to the present invention, since the change of the thermal neutron flux due to the fluctuation of the water gap width is small especially at the two sides on the side opposite to the control rod, the fuel assembly is vibrated by a small-scale earthquake. , The input reactivity can be kept low. As a result, the neutron flux does not increase so much that the neutron flux does not exceed the set level of the automatic scram function (for example, 120%). Can be prevented.

Further, the non-operation limit range of the substantial automatic scram function can be widened, and the reactor can operate the scram in a dark state even in the case of an earthquake of a scale where an unnecessary automatic scram function has conventionally been operated. And the reliability of the operation of the reactor can be improved.

[Brief description of the drawings]

FIG. 1 is an example of an embodiment of the present invention, in which 72 fuel rods and one large-diameter water channel W arranged at a position farther from a control rod than a center of a fuel assembly are combined. It is explanatory drawing which shows the structure of the fuel assembly used for the 9-row 9-column D lattice core structure.

FIG. 2 shows another example of the embodiment of the present invention, in which 74 large fuel channels and two large-diameter water channels W are arranged in positions farther from the control rods than the center of the fuel assembly. FIG. 3 is an explanatory diagram showing a configuration of a fuel assembly used in a 9-row, 9-column D lattice core structure having

FIG. 3 is a diagram showing the relationship between the amount of displacement of a fuel assembly and the increase in reactivity (input reactivity) due to vibration substantially equivalent to that of a small-to-medium-scale earthquake for a fuel assembly according to the present invention.

FIG. 4 is a diagram showing a change in reactivity of a fuel assembly according to the present invention with respect to a change in water gap when the gadolinia concentration of all gadolinia-containing fuel rods is the same.

FIG. 5 shows the relationship between the infinite doubling rate (absolute value of the reactivity) and the burnup of the fuel assembly according to the present invention and the conventional fuel assembly when the gadolinia concentration of all gadolinia-containing fuel rods is the same. FIG.

FIG. 6 shows eight rows and eight rows having 62 fuel rods and two large-diameter water channels W symmetrically arranged in the central region of the fuel assembly.
It is explanatory drawing which shows the structure of the principal part of the C lattice core structure (a) and the D lattice core structure (b) which used the fuel assembly of a row.

FIG. 7 shows a 9-row 9 having 72 fuel rods and two large-diameter water channels W symmetrically arranged in the central region of the fuel assembly.
FIG. 7 is an explanatory diagram showing enrichment distributions of fuel rods of a conventional fuel assembly in a row when used in a C-lattice core structure (FIG. 7A) and when used in a D-lattice core structure (FIG. 7B). is there.

FIG. 8 is a diagram showing the relationship between the amount of displacement of a fuel assembly and the increase in reactivity (input reactivity) due to vibration substantially equivalent to a small-to-medium-scale earthquake, for a conventional fuel assembly.

FIG. 9 is a diagram showing a change in reactivity of a conventional fuel assembly with respect to a change in water gap when all gadolinia-containing fuel rods have the same gadolinia concentration.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fuel rod 2 Channel box 3 Fuel rod containing gadolinia 4 Control rod W, W1, W2 Large diameter water channel

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G21C 3/30

Claims (2)

(57) [Claims] 1. A large-diameter water channel and are arranged in a grid.
A plurality of fuel rods, wherein the large-diameter water channel includes
Positioned farther away from the control rod than the center of
Is location, the fuel rods disposed in the outermost peripheral edge region of said lattice array
That is, fuel rods disposed on two sides opposite to the control rod
More than 75% of the number of fissile
D grid consisting of fuel material with a value lower than the maximum value in the rod
For boiling water reactor fuel assemblies used in array core structures
A first region between the large-diameter water channel and the two sides.
Concentration of gadolinia-filled fuel rods located at
Is the other outermost peripheral region adjacent to the control rod.
Arranged in a second area between a side portion and the large-diameter water channel
Gadolinia concentration of the fuel rod containing gadolinia
A fuel assembly for a boiling water reactor, which is relatively high . 2. A 9 × 9 fuel rod arrangement with at least one
And a large-diameter water channel, wherein the large-diameter water channel is located closer to the center of the fuel assembly.
Multiple fuel rods that are shifted more than one row and one row away from the control rods
2. The fuel assembly for a boiling water reactor according to claim 1, wherein the fuel assembly is disposed in a main area .