JPH08129091A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE: To share fuel pellets with a high utilization factor by selecting the number of first type fuel rods, the enrichment and places of borders of the charged pellets in the upper and lower sections of the fuel rods. CONSTITUTION: The axial places of 17/24 from a lower section are used as borders, 3.3wt.% pellets are charged into an upper region and 3.8wt.%, pellets into a lower region, and first type fuel rods, in which the difference of enrichment is formed in upper and lower sections, are employed in fuel rods in a type 2, and twelve fuel rods are used. Second type fuel rods, in which pellets having uniform enrichment in the axial direction are charged with the exception of upper and lower blankets, are employed as each fuel rod in types 1, 3, G1, G2, and forty-two fuel rods in the type 1 having enrichment of 4.5wt.%, four fuel rods in the type 3 having enrichment of 2.6wt.%, four fuel rods in the type G1 containing gadolinia of 4wt.% in enrichment of 3.8wt.% and ten fuel rods in the type G2 containing gadolinia of 5wt.% in enrichment of 3.3wt.% are used in the number of the second type fuel rods.

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 water channel occupying a 3 × 3 lattice position for 9 fuel rods loaded with fuel pellets. The present invention relates to a 9 × 9 square lattice array boiling water reactor fuel assembly.

[0002]

2. Description of the Related Art Core configurations of boiling water reactors (BWRs) are roughly classified into two types, that is, a C lattice and a D lattice. FIG. 4 is a layout diagram around the fuel assembly showing an example of the C-lattice core configuration.
[Fig. 4] is a layout drawing around a fuel assembly showing an example of a D-lattice core configuration.

In the water gap in the adjacent space of the fuel assembly 42 in the C lattice, the width a at the insertion position of the control rod 41 and the width b at the non-insertion position of the control rod 41 are substantially equal to each other. On the other hand, the water gap is symmetrical. On the other hand, in the D lattice, the water gap between the adjacent fuel assemblies 52 is substantially non-uniform because the width a at the insertion position of the control rod 51 is larger than the width b at the non-insertion position of the control rod, The water gap is asymmetric with respect to the fuel assembly. In these drawings, reference numeral 43 or 53 indicates a water rod, and reference numerals 44 or 54 indicate a fuel rod, respectively, and an 8 × 8 square lattice fuel assembly is formed by these.

In the operating nuclear reactor, the cooling water (moderator) in the fuel assembly boils and its density decreases, so that the neutron moderation is predominant in the water gap outside the fuel assembly. Become. Therefore, the distribution of thermal neutrons in the core cross section is biased toward the fuel rods around the water gap and the assembly located in the vicinity, and the fission reaction between thermal neutrons and the fissile material (uranium) in the fuel is The fuel rods in the peripheral portion become more active than the fuel rods in the central portion of the assembly. In this way, the output distribution of each fuel rod is biased, and thus local peaking occurs. Here, the local peaking is a relative value between the output of each fuel rod and the average output of all the fuel rods.

Excessive local peaking impairs the thermal and mechanical integrity of the fuel rods, and therefore it is necessary to select the enrichment of the fuel pellets used for each fuel rod to suppress the local peaking. In the fuel assembly, the fuel rod array is provided with the enrichment distribution in the cross section.

The enrichment referred to here is the weight ratio of the fissile isotope, typically ²³⁵ U, to the weight of uranium contained in the fuel pellet. In the case of the C lattice, the enrichment distribution depends on the water gap. However, in the D lattice, the enrichment distribution becomes asymmetric between the control rod insertion position and the opposite side depending on the asymmetry of the water gap. Therefore, in the D lattice, the pellets used for the fuel assembly compared to the C lattice. There are many types of enrichment.

On the other hand, in recent years, in order to improve the economical efficiency by increasing the burnout energy of the fuel assembly to increase the extraction energy per fuel assembly, the conventional 8 × 8 type fuel assembly There is a tendency to move from the body to 9 × 9 type fuel assemblies, and this has also been confirmed to be effective in suppressing the generation of spent fuel. In transitioning to such a 9 × 9 type fuel assembly, it is necessary to increase the average enrichment of the fuel assembly in order to achieve the original purpose as described above. The average enrichment here is the weight ratio of fissile isotopes, typically ²³⁵ U, to the weight of all uranium contained in the fuel assembly.

In order to increase the average enrichment, it is necessary to increase the maximum enrichment of the pellets together with the average enrichment of the fuel assembly. The maximum enrichment of the pellets is used in the fuel processing step, transportation and reprocessing. There is a certain limit due to criticality control, and the maximum concentration of pellets due to this limit is conventionally set to 5 wt%.

FIG. 6 is a layout diagram (A) around a fuel assembly showing a typical example of a conventional 9 × 9 square lattice fuel assembly for a boiling water reactor, and an axial enrichment of fuel rods. It is a distribution map (B). The fuel assembly 62 includes one water channel 63 and 72 fuel rods 64, and has an average enrichment of about 3.77 wt%. Water channel 6
3 is arranged at an area position corresponding to the central 3 rows and 3 columns (3 × 3 lattice) in the 9 rows × 9 columns square lattice arrangement. The inside of the water channel 63 is filled with water (moderator) that does not boil even during the operation of the nuclear reactor, and alleviates the deviation of the power distribution among the fuel rods caused by the water gap around the fuel assembly. .

The symbols (1, 2, 3, 4, G1) given to the fuel rods 64 in FIG. 6A correspond to the fuel rod types for each enrichment shown in FIG. 6B, and the symbol NU in FIG. 6B is natural uranium. Shows a blanket. In the 9 × 9 type fuel assembly having such a large diameter water channel 63, the difference between the maximum enrichment and the minimum enrichment can be reduced in the enrichment distribution of the fuel rods in the cross section, and the pellets Even if the maximum enrichment of the fuel is limited to 5 wt%, it is possible to design the enrichment distribution in which the average enrichment of the fuel assembly is increased to about 4 wt% without increasing the local peaking.

[0011]

Similar to the 8 × 8 type fuel assembly, a low enriched uranium dioxide sintered material is used as a fuel material for a 9 × 9 type fuel assembly for the purpose of transition for higher burnup. Pellets are used, but in order to suppress the local peaking of the fuel rods, which is inevitably caused by the core structure of the BWR, pellets of various enrichments are required.

Further, there are two types of BWR core configurations, C lattice and D lattice, and the types of enrichment required for the production of fuel assemblies increase corresponding to these core configurations. In order to make the fuel pellet enrichment common to both core configurations, that is, to share the enrichment, there are the following two types of sharing targets, the first of which is C lattices or D When there are two or more types of enrichment distribution designs that match each operating condition between reactors with the same core configuration such as lattices, when using pellets with a common enrichment for fuel assemblies of these designs Secondly, when there are two or more types of enrichment distribution designs that match the respective operating conditions between reactors with different core configurations, such as the C lattice and the D lattice, common to these fuel assemblies. This is the case when using pellets with a high degree of concentration.

In the conventional manufacturing of fuel assemblies, it has been necessary to manage such various kinds of enrichment levels.
There is a problem that the burden on the manufacturing side is unavoidable and strict enrichment control must be carried out for many kinds in the uranium enrichment arrangement, transportation, re-conversion, pellet and fuel rod manufacturing process, etc. I had left.

For example, taking the 9 × 9 type fuel assembly shown in FIGS. 6A and 6B as an example, to obtain an appropriate enrichment distribution by selecting the enrichment of pellets under the restrictions imposed by the above-mentioned conditions. The problem will be specifically described as follows.

Axial blankets of natural uranium and depleted uranium are provided at the upper and lower ends of the fuel rods, and a small number of fuel rods are mixed with burnable poisons such as gadolinia (Gd ₂ O ₃ ) as a neutron absorbing substance. However, in order to simplify the explanation, here, regarding the enrichment distribution of the fuel assembly, it is considered that the fuel rod is composed of only enriched uranium pellets.

In the fuel assembly shown in FIG. 6, the enrichment distribution is composed of four types of enrichment pellets. The average enrichment of this fuel assembly is given by the following equation excluding natural uranium blanket NU. To be (4.7 x 34 + 3.9 x 12 + 3.4 x 22 + 2.5 x 4) / 72 = 4.047
[wt%]

For this design, the average enrichment is increased by 0.05 wt% by changing the operating conditions of the reactor. In this case, there are the following two methods that do not increase the local peaking.

The first method is to measure all pellet enrichments.
(4.047 + 0.05) /4.047 = 1.012 It is a method to uniformly increase the value. This method has a drawback in that when fuel assemblies having different average enrichments are used for nuclear reactors having different operating conditions, pellet enrichments cannot be shared by the fuel assemblies, so that the enrichment is high. Inevitable categorization.

The second method is a method of preparing a new pellet enrichment. In this case, it should be noted that the measurement of the enrichment of the fuel rod, which is a product, in the manufacturing process of the fuel assembly. Since it is essential and the resolution of the inspection device for that is limited, unless the enrichment split of the pellet has a relative enrichment span of 10% or more, the determination accuracy of the enrichment based on the measurement results It is not reliable.

For example, when the four types of enrichment shown in FIG. 6B are used as they are and one kind of pellet of a new enrichment is prepared, the enrichment between the concentrations of 3.9 wt% and 4.7 wt% is performed. The degree difference is 0.8 wt%, that is, it corresponds to a span of 20.5% of 3.9 wt%, but if a new enrichment of, for example, 4.28 wt% is set in the middle, the enrichment between this new enrichment and 3.9 wt% is The difference of 0.38 wt% is only about 9.74% of 3.9 wt%, and
Concentration difference between 4.7 wt% and 0.42 wt% is about 9.81 of 4.28 wt%
%, And as a result, when measuring the enrichment of the fuel rods during the fuel assembly manufacturing process, the determination of these pellet enrichments by the inspection device becomes uncertain due to the addition of a new enrichment. become.

For the same reason, there is only a span of about 15% between 3.9 wt% and 3.4 wt% anyway, so it is not possible to newly select an intermediate enrichment between them. Also, in order to increase the average enrichment of the assembly by 0.05 wt% with four fuel rods having a enrichment of 2.5 wt%, the number of fuel rods must be small.
Since it has to be raised to 3.4 wt%, local peaking becomes excessive. Furthermore, for a concentration of 4.7 wt%, if a 10% increase in pellet concentration is set on the premise of a concentration inspection, it will be 5.17 wt%, so the maximum concentration of pellets used will be limited to 5 wt% or less. become unable.

A first object of the present invention is to have a large diameter water channel 9 on the premise that the pellet concentration degree is shared.
It is to provide a configuration having a high degree of freedom in designing the enrichment distribution in a × 9 type fuel assembly.

A second object of the present invention is, in addition to the first object, of measuring the enrichment of the fuel rods in the manufacturing process of the fuel assembly with sufficient accuracy under the limitation of the resolution of the inspection device. It is an object of the present invention to provide the 9 × 9 type fuel assembly in which the local peaking is suppressed by the pellet having the smallest possible enrichment based on the enrichment split that can be discriminated.

The third object of the present invention is, in addition to the first and second objects, the average enrichment of the fuel assembly is, for example, 0.01 wt% under the limitation that the maximum enrichment of pellets is 5 wt% or less.
It is possible to select relatively accurately with high accuracy, and to have a structure capable of sufficiently satisfying the required reactor operating conditions, that is, the conditions such as the average take-out burnup and the number of refueling lines. To provide a × 9 type fuel assembly.

The fourth object of the present invention is, in addition to the first to third objects, in the fuel assembly manufacturing process from the arrangement of raw material enriched uranium to reconversion, production of pellets, and processing up to the final product. By providing the 9 × 9 type fuel assembly having a structure that enables the process to be performed with a small number of types of enrichment and is suitable for sharing the enrichment with the difference in the core configuration and operating conditions of the reactor. is there.

[0026]

A fuel assembly for a boiling water reactor according to the present invention comprises water channels occupying a 3 × 3 lattice position corresponding to nine fuel rods loaded with fuel pellets. It also has a 9 × 9 square lattice array.

The fuel pellets are composed of seven or less kinds of fuel pellets having different enrichment levels of fissionable materials in enrichment spans of 10% or more, and the maximum enrichment of these fuel pellets is 5 wt% or less.

The number of fuel rods constituting the square lattice array is 4 to 12.
The first type fuel rods and the remaining second type fuel rods of the book, and the low enrichment corresponding to natural uranium or depleted uranium at the upper and lower ends of the first type fuel rods and the second type fuel rods, respectively. An axial blanket of degrees is provided.

Fuel rods of the first type are loaded with fuel pellets having different enrichment levels of fissionable material in the upper and lower portions with a predetermined boundary in the axial direction as a boundary, while the fuel pellets of the second type are loaded. The fuel rods are loaded with fuel pellets having substantially the same fissionable material enrichment in the axial direction.

The position of the boundary in the first type fuel rod is selected so that the enrichment average of the fuel assembly has a predetermined value, and is located above the boundary of the first type fuel rod. Is loaded with relatively low enrichment fuel pellets, and the lower portion is loaded with relatively high enrichment fuel pellets, and the enrichment difference between these upper and lower fuel pellets corresponds to one step of the span. Have been chosen to do.

According to a second aspect of the invention, in the fuel assembly for a boiling water reactor according to the first aspect, the boiling of a C-lattice core structure in which the width of the water gap surrounding the fuel assembly is substantially equal in four rounds. A fuel assembly loaded in a water reactor, characterized in that it is composed of five or less types of fuel pellets having different enrichment spans of 10% or more. .

According to a third aspect of the present invention, in the fuel assembly for a boiling water reactor according to the first aspect, the water gap surrounding the fuel assembly has a D-lattice core structure in which the widths of the water gaps are four circles and are substantially uneven. A fuel assembly to be loaded into a boiling water reactor, characterized in that it is composed of seven or less types of fuel pellets having different enrichment spans of 10% or more. is there.

[0033]

In the invention of claim 1, the fuel pellets are made of fissile material, typically 7 kinds or less of enrichment of ²³⁵ U differing from each other in enrichment span of 10% or more.
Here, when the enrichment span of 10% or less is A, B, C ... in order from the lowest enrichment, the enrichment B is the enrichment A.
More than 10% (110% of A), enrichment C is enrichment B
Of 10% of B (110% of B) or more. Therefore, the pellets of these enrichments can be discriminated with sufficient accuracy by the inspection device when measuring the enrichment of the fuel rods.

The kind of enrichment is determined in consideration of the fact that the smaller the kind in terms of manufacturing, the more the effect of reducing the manufacturing cost, and the higher the kind in terms of nuclear characteristics, the more optimal the nuclear design can be made. Further, in consideration of the output peaking characteristics, there are five types of enrichment in the C lattice and 7 or less in the D lattice.
It is appropriate that the number is less than or equal to the type.

In the fuel assembly of the present invention, 4 to 12 first-type fuel rods and the remaining number of second-type fuel rods are constituted by such pellets of enrichment of 7 types or less, These fuel rods are 9 × 9 with one water channel occupying the position of the 9 × 3 × 3 grid.
Form a square lattice array.

Four to twelve fuel rods of the first type are loaded with fuel pellets having different enrichment levels of the fissile material at the upper and lower sides of a predetermined boundary in the axial direction. . In contrast, the remaining second type fuel rods are loaded with fuel pellets having substantially the same enrichment of fissile material in the axial direction. 4-1
By selecting the axial position of the boundary in the two first type fuel rods, the average enrichment of the fuel assembly is set to a predetermined value.

In this case, not only the case where the boundary positions of the first type fuel rods are all the same, but it is also possible to design so as to have different boundaries.

As described above, in the present invention, the features of the 9 × 9 type fuel assembly described above with reference to FIG. 6 are utilized, and the fuel assembly in which the local peaking is suppressed with a small enrichment type of 7 types or less. The number of fuel rods of the first type, the enrichment of the loading pellets above and below it, and the position of the boundary can be selected so that the fuel assembly can be constructed according to the enrichment span and the number of enrichment types. Average enrichment is 0.01
It is possible to set various values with wt% accuracy, which makes it possible to adapt with high accuracy to differences in operating conditions such as the average take-out burnup of the reactor and the number of refueling lines, as well as the core. It becomes possible to share fuel pellets of a fuel assembly between reactors having different configurations or operating conditions at a high utilization rate.

According to the invention of claim 1, the maximum enrichment of the fuel pellets is limited to 5 wt% or less. Therefore, in addition to the above-mentioned characteristics, the criticality control in the fuel processing process, transportation and reprocessing is conventionally performed. It can be performed in the same way as above, and no new measures are required.

According to the invention of claim 1, the fuel pellets loaded in the lower portion of the fuel rod of the first type have a relatively higher enrichment than the fuel pellets loaded in the upper portion of the boundary. The difference in enrichment between the upper and lower fuel pellets corresponds to one step of the enrichment span. In the channel in the BWR core, the thermal neutron utilization rate of the upper part, where the vapor volume is large due to boiling of cooling water, is lower than that of the lower part, so that the upper part has a low enrichment and the lower part has a high enrichment as in the present invention. As a result, the uranium utilization rate of the fuel is improved, the number of fuel rods of the first type is limited to 4 to 12, and the enrichment difference above and below that is limited to one enrichment span. Therefore, the difference in the average enrichment of the fuel assembly can be suppressed to about 17% of the enrichment span at the maximum in the case of 72 fuel rods, for example, and excessive output peaking at the lower part can be prevented. It is possible to prevent.

According to the second aspect of the present invention, a fuel assembly for a C-lattice core structure can be provided, and in this case, the enrichment of the fuel pellets is 5 types. On the other hand, according to the invention of claim 3, a fuel assembly for a D-lattice core configuration can be provided, and the enrichment of the fuel pellets in this case is seven types. It goes without saying that the enrichment of each fuel pellet of the fuel assembly for the C lattice and the fuel assembly for the D lattice can be shared with each other.

[0042]

FIG. 1 is a cross-sectional layout view (A) showing an embodiment of a 9 × 9 type fuel assembly for a C lattice according to the present invention and an axial enrichment distribution map (B) of a fuel rod. Here, the average enrichment of the aggregate is selected to be 3.77 wt%.

In FIG. 1, the fuel assembly 12 is surrounded by a water gap, and the water gap in contact with the adjacent side on one side is the insertion position of the control rod 11. The fuel assembly 12 includes one water channel 13 and 72 fuel rods 14, and has an average enrichment of about 3.77 wt%.
The water channel 13 is arranged so as to occupy the area corresponding to the position of the central 3 × 3 lattice in the square lattice array of 9 rows and 9 columns, that is, the area corresponding to 3 rows and 3 columns. The inside of the water channel 13 is filled with water (moderator) that does not boil even during the operation of the nuclear reactor, and alleviates the deviation of the power distribution among the fuel rods caused by the water gap around the fuel assembly. .

The reference numerals (1, 2, 3, G1, G2) given to the fuel rods 14 in FIG. 1A correspond to the fuel rod types for each enrichment shown in FIG. 1B, and the reference numeral NU in FIG. 1B is 1 /. 24 shows a natural uranium blanket of 24 axial lengths. The types of enrichment of the pellets constituting each of these types of fuel rods 14 are 2.6 wt%, 3.3 wt%, 3.
There are four types, 8 wt% and 4.5 wt%.

Type 2 fuel rods are 17/24 from the bottom
With 3.3 wt% pellets in the upper region and 3.8 wt% pellets in the lower region with the axial position of
It is the first type of fuel rod with the enrichment difference at the top and bottom, and the number is 12.

The type 1, 3, G1, and G2 fuel rods are the second type fuel rods loaded with pellets of uniform enrichment in the axial direction except for the upper and lower blankets, and the number of them is 4. 42 type 1 fuel rods with 5wt% enrichment
4 fuel rods of type 3 with 2.6 wt% enrichment and 4 fuel rods of type G1 containing 4 wt% gadolinia with 3.8 wt% enrichment and 3.3 wt% 5 in degrees
10 type G2 fuel rods containing wt% gadolinia
It is a book.

FIG. 2 is a cross-sectional view showing an embodiment of the 9 × 9 type fuel assembly for the D lattice according to the present invention when the pellet enrichment is shared with the embodiment for the C lattice of FIG. It is a surface layout drawing (A) and an axial enrichment distribution map of a fuel rod (B), where the average enrichment of the assembly is selected to be 3.86 wt%.

In FIG. 2, the periphery of the fuel assembly 22 is surrounded by a water gap, and the water gap in contact with the adjacent side on one side is the insertion position of the control rod 21. The fuel assembly 22 has one water channel 23 and 72 fuel rods 24, and has an average enrichment of about 3.86 wt%.
The water channel 23 is arranged so as to occupy an area corresponding to the position of the central 3 × 3 lattice in the square lattice array of 9 rows and 9 columns, that is, 3 rows and 3 columns. The inside of the water channel 23 is filled with water (moderator) that does not boil even during the operation of the reactor, and alleviates the deviation of the power distribution among the fuel rods caused by the water gap around the fuel assembly. .

The reference numerals (1, 2, 3, 4, 5, 6, G1, G2) given to the fuel rods 24 in FIG. 2A correspond to the fuel rod types for each enrichment shown in FIG. 2B. The reference numeral NU in FIG. 1 denotes a natural uranium blanket having an axial length of 1/24. The types of enrichment of the pellets constituting each of these types of fuel rods 24 are 1.8 wt%, 2.6
wt%, 3.0 wt%, 3.3 wt%, 3.8 wt%, 4.5 wt
% And 4.95 wt%.

The type 5 fuel rod is 17/24 from the bottom.
With 2.6 wt% pellets in the upper region and 3.0 wt% pellets in the lower region with the axial position of
It is the first type of fuel rod with the enrichment difference at the top and bottom, and the number is six.

The type 1, 2, 3, 4, 6, G1 and G2 fuel rods are second type fuel rods loaded with pellets of uniform enrichment in the axial direction except for the upper and lower blankets. Is the type 1 with a concentration of 4.95 wt%
22 fuel rods, 9 type 2 fuel rods with 4.5 wt% enrichment, 16 type 3 fuel rods with 3.8 wt% enrichment, 3.3 wt% enrichment type 4 fuel rods 4
, One type 6 fuel rod with 1.8 wt% enrichment, and eight type G1 fuel rods containing 4 wt% gadolinia with 4.5 wt% enrichment, 3.3 wt% enrichment 5 in degrees
There are six type G2 fuel rods containing wt% gadolinia.

As is apparent from FIGS. 1 and 2, the four types of enrichment of the pellet raw materials used for the types 1 to 3, G1 and G2 in the fuel assembly for C lattice (FIG. 1) have different average enrichments. In the fuel assembly for the D-lattice of FIG. 2 (FIG. 2), the enrichment and complete sharing of the pellets used in the upper regions of types 2 to 4 and type 5 and type G2 have been achieved. As a result, the processes of arranging, transporting, reconverting and pelletizing the uranium raw materials relating to these four types of enrichment can be shared by the fuel assemblies for the C lattice and the D lattice.

FIG. 3 shows a case where the pellet enrichment is shared with the embodiment for D lattice of FIG. 2 including the highest enrichment of 4.95 wt%, and 9 × 9 for C lattice according to the present invention. FIG. 5 is a cross-sectional layout view (A) and an axial enrichment distribution map (B) showing another embodiment of the fuel assembly, where the average enrichment of the assembly is 3 as in FIG. It is selected to be 77% by weight.

In FIG. 3, the fuel assembly 32 is surrounded by a water gap, and the water gap in contact with the adjacent side on one side is the insertion position of the control rod 31. The fuel assembly 32 includes one water channel 33 and 72 fuel rods 34, and the water channel 33 has 9 rows and 9 rows.
It is arranged so as to occupy the position of the central 3 × 3 grid in the square grid array of columns, that is, the area corresponding to 3 rows and 3 columns.
The inside of the water channel 33 is filled with water (moderator) that does not boil even during the operation of the reactor, and alleviates the deviation of the power distribution among the fuel rods caused by the water gap around the fuel assembly. .

The reference numerals (1, 2, 3, 4, 5, G1, G2) given to the fuel rods 34 in FIG. 3A correspond to the fuel rod types for each enrichment shown in FIG. 3B, and the reference numerals in FIG. 3B. NU indicates a natural uranium blanket with an axial length of 1/24. The types of enrichment of the pellets constituting each of these types of fuel rods 34 are 2.6 wt% and 3.3 wt%.
%, 3.8 wt%, 4.5 wt% and 4.95 wt%.

Type 3 fuel rods are 17/24 from the bottom
With 3.3 wt% pellets in the upper region and 3.8 wt% pellets in the lower region with the axial position of
It is the first type of fuel rod with the enrichment difference at the top and bottom, and the number is four.

The type 1, 2, 4, 5, G1, and G2 fuel rods are the second type fuel rods loaded with pellets of uniform enrichment in the axial direction except for the upper and lower blankets. Are 4 type 1 fuel rods with 4.95 wt% enrichment, 38 type 2 fuel rods with 4.5 wt% enrichment, and type 4 fuel rods with 3.3 wt% enrichment. 8
, 4 type 5 fuel rods with 2.6 wt% enrichment, and 6 type G1 fuel rods with 4 wt% gadolinia with 3.8 wt% enrichment, 3.3 wt% enrichment 5 in degrees
There are 8 type G2 fuel rods containing wt% gadolinia.

As is apparent from FIGS. 2 and 3, the five types of enrichment of the pellet raw materials used for types 1 to 5 and G1 and G2 in the fuel assembly for C lattice (FIG. 3) have different average enrichments. Type 1 to 4 and type 5 upper regions, types G1 and G2 in the fuel assembly for the D lattice of FIG.
The degree of enrichment and complete sharing of each pellet used in the above have been achieved. As a result, the steps from arrangement, transportation, reconversion, and pellet production of the uranium raw materials relating to these five types of enrichment can be shared by the fuel assemblies for the C lattice and the D lattice.

In any of the embodiments described above,
Since each enrichment is split with an enrichment span of 10% or more, it is possible to inspect with sufficient discrimination accuracy in measuring the enrichment of fuel rods.

Further, in addition to the selection of the enrichment of the second type fuel rod having the uniform enrichment in the axial direction, in the embodiment of the C-lattice core configuration of FIG. 1, 12 rods and the D-lattice core configuration of FIG. In one embodiment, six fuel rods, and in another embodiment having the C-lattice core structure shown in FIG. 3, four fuel rods of the first type (type 2, 5 or type 3) are used to determine the enrichment of the fuel pellets in the upper and lower regions. By selecting various enrichment spans for one step and various axial positions of the boundaries between the upper and lower regions, the average enrichment of the fuel assembly can be increased with a sufficiently fine span even with a small enrichment of 7 or less. It can be variably designed with high accuracy, and local peaking can be suppressed.

The average enrichment of the fuel assembly is, for example, 4-1.
When the difference in absolute value of the enrichment between the two fuel pellets in the upper and lower regions of the first type is 0.5 wt%, the maximum of 0.08 wt% (0.5 wt% ×
It is possible to adjust continuously with a span of up to 12/72). This is about 1000 MWd at the average burnup of the fuel assembly.
This corresponds to a cycle length of about 200 MWd / t (about one week), assuming 20% replacement. Being able to make such adjustments has an advantage that the enriched uranium of the fuel assembly can be effectively used according to the reactor operation plan.

Regarding the fuel rods of the first type, it is preferable that the difference between the upper and lower enrichments is low at the top and high at the bottom, which is the BWR cooling water flow channel in the fuel assembly. Because of the boiling of cooling water in the interior, the utilization rate of thermal neutrons in the upper region where the vapor volume is relatively large is lower than that in the lower region.Therefore, it is better to make the upper region low enrichment and the lower region high enrichment. This is because the utilization rate of is improved. In this case, in order to avoid excessive output peaking in the lower part, the upper limit of the number of the first type fuel rods is limited to 12, and the difference in enrichment between the upper and lower sides of the first type fuel rods is one stage. Since the enrichment span is limited, the difference between the upper and lower sides of the average enrichment of the fuel assembly is 0.
When it is 5 wt%, it is suppressed to about 0.08 wt%.

[0063]

As described above, according to the present invention,
Used as a fuel assembly for boiling water reactors with different core configurations and operating conditions It is possible to share the enrichment of the raw material of fuel pellets, for example, between the fuel assemblies for the C-lattice core and the D-lattice core and the average extraction. For fuel cores with different average enrichments for cores with different operating conditions such as burnup and number of fuel replacements, fuel materials and fuel materials up to the process of arranging, transporting, reconverting enriched uranium, manufacturing pellets and fuel rods Since sharing of related equipment and work can be achieved,
It has an effect of greatly contributing to the reduction of manufacturing cost. In addition,
The fuel assembly design for C-lattice cores according to the present invention can also be applied to other types of nuclear reactors, such as ABWR, which have a core configuration with symmetry in the width of the water gap.

[Brief description of drawings]

FIG. 1 is a cross-sectional layout diagram (A) and an axial enrichment distribution diagram (B) of a 9 × 9 type fuel assembly for a C lattice according to the present invention.

FIG. 2 is 9 × for the D-lattice according to the present invention in the case of sharing the pellet enrichment with the example for the C-lattice of FIG.
It is a cross-sectional layout diagram (A) and an axial enrichment distribution diagram (B) showing an example of a 9-type fuel assembly.

FIG. 3 is 9 × for the C lattice according to the present invention when the concentration of the pellet is shared with the embodiment for the D lattice of FIG.
Cross-sectional layout diagram (A) showing another embodiment of the 9-type fuel assembly
FIG. 3B is a distribution diagram (B) of axial enrichment of fuel rods.

FIG. 4 is a layout diagram around a fuel assembly showing an example of a conventional C-lattice core configuration.

FIG. 5 is a layout diagram around a fuel assembly showing an example of a conventional D-lattice core configuration.

FIG. 6 is a layout diagram around a fuel assembly showing an example of a conventional 9 × 9 square lattice fuel assembly for a boiling water reactor (A).
FIG. 3B is a distribution diagram (B) of axial enrichment of fuel rods.

[Explanation of symbols]

 11, 21, 31: Control rod 12, 22, 32: Fuel assembly 13, 23, 33: Water channel 14, 24, 34: Fuel rod

Claims (3)

[Claims] 1. A fuel assembly for a boiling water nuclear reactor in a 9 × 9 square lattice array having water channels occupying positions of 3 × 3 lattices corresponding to nine fuel rods loaded with fuel pellets. Is composed of 7 or less types of fuel pellets with different enrichment spans of 10% or more, and the maximum enrichment of fuel pellets is 5 wt% or less. It is composed of 4 to 12 first type fuel rods and the remaining second type fuel rods, and the upper and lower ends of the first type fuel rods and the second type fuel rods correspond to natural uranium or depleted uranium. A low-concentration axial blanket is provided, and the first type fuel rods have fuel pellets having different enrichment levels of fissile material at the upper and lower sides of a predetermined axial boundary. Fuel rods of the second type are loaded with fuel pellets having substantially the same enrichment of fissionable material in the axial direction, so that the average enrichment of the fuel assembly has a predetermined value. The position of the boundary of the type 1 fuel rod is selected, and fuel pellets of relatively low enrichment are loaded above the boundary of the type 1 fuel rod and relatively high enrichment of the lower part thereof. The fuel assembly for a boiling water reactor characterized in that the fuel pellets are loaded, and the difference in enrichment between the upper and lower fuel pellets corresponds to one step of the span. 2. A fuel assembly to be loaded into a boiling water reactor having a C-lattice core structure in which the width of a water gap surrounding the fuel assembly is substantially equal on all four sides, the enrichment of fissile material being The fuel assembly for a boiling water reactor according to claim 1, wherein the fuel assembly is composed of five or less kinds of fuel pellets different from each other in enrichment span of 10% or more. 3. A fuel assembly to be loaded into a boiling water reactor having a D-lattice core configuration, in which the width of the water gap surrounding the fuel assembly is four and substantially non-uniform, and the enrichment of fissionable material is provided. 2. The fuel assembly for a boiling water reactor according to claim 1, wherein the fuel assembly is composed of seven or less kinds of fuel pellets different from each other in enrichment span of 10% or more.