JP3199814B2 - Fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a boiling water reactor (BW).
The present invention relates to a fuel assembly for R), and more particularly to a fuel assembly with improved stability in the event of a control rod falling accident.

[0002]

2. Description of the Related Art One of the accidents assumed in a BWR is a reactivity insertion accident due to a drop of a control rod. This is an event in which when the reactor is at or near the criticality, when the control rod falls, a sharp reactivity is injected into the reactor core. In particular, when the applied reactivity exceeds 1 °, there is a possibility that fuel may be damaged due to an increase in fuel enthalpy due to a sharp rise in core power.

[0003] In the case of a control rod drop accident of this type, the power distribution becomes very local in the radial direction of the core centering on the control rod withdrawal cell. As for the axial direction of the core, as shown in the axial power distribution of the control rod withdrawal cell at the time of the control rod fall accident illustrated in FIG.
An output peak occurs at the node. Therefore, the risk of fuel breakage in these parts is much higher than in other parts.

In order to cope with such a problem, a method of previously setting the control rod withdrawal order so that the control rod value does not become excessive and taking out the control rod in accordance with this order has been adopted. Was.

Incidentally, the fuel assembly is formed by bundling a large number of fuel rods, and the increase in enthalpy at the time of the control rod falling accident described above is caused by a fuel rod having a large output (local peaking) in the cross section of the fuel assembly. Is remarkable. Conventionally,
When determining the enrichment distribution in the cross section of the fuel assembly, the main focus was on reducing local peaking during output operation in order to reduce the linear power density during output operation.

[0006] However, the control rod fall accident is most severe at low temperatures, and tends to be more severe with new fuels that contain a large amount of fissile material and have high fuel assembly outputs. In addition, since local peaking generally increases in the early stage of combustion, it is necessary to reduce local peaking of fresh fuel at low temperatures. However, in the conventional fuel, if local peaking during output operation is reduced, local peaking at low temperatures tends to be reduced at the same time, and therefore, it is not necessary to pay special attention to local peaking at low temperatures.

[0007]

However, in recent years, as shown in FIG. 4, the purpose of the present invention is to improve the critical power ratio and stability and to flatten the axial power distribution by improving the axial water to fuel atomic ratio. A fuel assembly using a so-called partial-length fuel rod (P fuel) in which a fuel rod is cut off in an upper region in the axial direction of the fuel assembly has been considered.

FIG. 4 is a diagram for explaining the enrichment distribution of a conventional fuel assembly. FIG. 4 (a) shows UO ₂ control rods (full length fuel rods) 1 to 4 in a channel box of the fuel assembly.
3 shows the enrichment distribution state in the cross section of the upper region of the 9-by-9 array of the Gd fuel rods G, the partial length fuel rods P, and the water rods W. FIG. 4B shows UO ₂ fuel rods 1 to 3 and G
d is a legend diagram showing the wt% of ²³⁵ U and the wt% of Gd ₂ O ₃ and the number of each of them, and FIG.
3, P, and G show the concentration distribution state in the axial direction.

The thermal neutron flux distribution is plotted along the direction of the arrow from the control rod insertion side to the right side. FIG.
(A) The coordinates in (a) are to be read with numbers 1 to 9 from the lower left to the upper part and from 1 to 9 from the lower left to the right with the lower left corner as the origin.

Further, the number of splits in FIG. 4 (b) is the number of uranium enrichment, and in FIG. 4, 4.6, 3.9, 3.3, 2.5
There are four types. LPF stands for Local Peaking Factor,
The maximum output / average output is shown. Here (5,1)
It shows that the fuel rod at the position of the position has the highest output ratio.
Further, the upper region refers to a region above the partial length fuel rod.

In such a fuel assembly, an area without fuel rods is formed in an axially upper area where an output peak occurs in the control rod withdrawal cell at the time of a control rod drop accident, and water concentrates at this position at low temperatures. Therefore, the density distribution of the water at the time of the output operation and at the time of the low temperature greatly differ.

Therefore, as shown in FIGS. 5 and 6, the thermal neutron flux distribution at the outermost peripheral position adjacent to the position of the partial length fuel rod is greatly different between at the time of output operation and at the time of low temperature.
At a low temperature, the thermal neutron flux distribution has a peak ((1, 5) position in the figure) due to the water concentrated at the position of the partial length fuel rod.

FIG. 5 shows the thermal neutron flux distribution in the lateral direction (arrow position) at the outermost peripheral position of the upper region when the enrichment of all the fuel rods is the same in the fuel assembly shown in FIG. ing. In FIG. 5, a solid line indicates a low temperature, and a broken line indicates an output operation.

FIG. 6 is a similar view of a fuel assembly in which the width of the water region between the fuel assembly and the fuel assembly is different between the control rod insertion side and the non-insertion side of the control rod. Due to the influence of water from the side, the lateral thermal neutron flux distribution is biased. In the drawing, a solid line indicates a low temperature, and a broken line indicates an output operation.

Generally, the output of each fuel rod is determined by the product of the enrichment and the thermal neutron flux. Therefore, if an enrichment distribution that reduces local peaking during output operation as in the prior art is adopted, the output during operation will be reduced. The enrichment at fuel rod positions having the same thermal neutron flux size has an enrichment distribution such that the same one is used.
In this case, for example, the enrichment distribution as shown in FIG. 4, that is, the enrichment at the (1, 5) position adjacent to the position of the partial length fuel rod P and the positions ((1, 4) and ( 1,
6) The enrichment of the position) becomes an enrichment distribution using the same type of enrichment.

However, if such an enrichment distribution is adopted in the axially upper region where an output peak occurs in the control rod withdrawal cell in the event of a control rod fall accident, when the temperature is low, it is adjacent to the position of the partial length fuel rod P. Since the thermal neutron flux of the fuel rod at the outermost position of the fuel assembly increases, the local output at that position tends to increase. In other words, from the viewpoint of reducing local peaking at low temperatures, it may be preferable to set the enrichment distribution in which the enrichment of the outermost fuel rod of the fuel assembly adjacent to the position of the partial length fuel rod P is reduced in advance. is there.

As described above, in the enrichment distribution of the fuel assembly which emphasizes the reduction of the local peaking at the time of the output operation which is the conventional design method, there is a problem that the local peaking at the low temperature is not sufficiently reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. In a fuel assembly using a partial length fuel rod,
An object of the present invention is to provide a fuel assembly having an enrichment distribution that is effective for reducing local peaking at low temperatures so as to improve safety in the event of a control rod falling accident.

[0019]

SUMMARY OF THE INVENTION The present invention is directed to a full length fuel rod.
That the fuel rods of the first group is constructed by arranging fuel rods in the second group axial length is shorter partial length fuel rods than the fuel rods of the first group in a lattice fuel assembly Gd _{2 at} outermost position
In the fuel assembly in which the first group of fuel rods not including O ₃ are arranged, at least a control rod is inserted in a fuel assembly cross section at an axial position where the second group of fuel rods is missing. enrichment of the fuel rods a of all of the first group disposed on the fuel assembly outermost position adjacent to the fuel rod positions of the second group that side of the outermost position adjacent to the fuel rods a two said first group of fuel rods B, among and C, the fuel rods a position closest corner and are low rather set Ri by enrichment of the fuel rods B of the first group located on the opposite side from the It is characterized by that.

[0020]

FIG. 1A shows a sectional view of a second group of fuel rods, that is, a sectional view of an axial fuel assembly without a partial length fuel rod .
The enrichment of the fuel rod at the outermost position A ₁ (hereinafter referred to as A) of the fuel assembly adjacent to the position of the partial length fuel rod is
Outermost positions B ₁ (hereinafter referred to as B) and C ₁ ( hereinafter referred to as
At least position A of the two fuel rods
The enrichment distribution of the fuel assembly, which is lower than the enrichment of the fuel rod at the position B on the side opposite to the corner closest to the fuel cell, is adopted.
As a result, the local output at the low temperature at the position A can be suppressed, and the local peaking during the output operation can be reduced without deteriorating the local peaking at the low temperature.

Here, the fuel rods to be compared with the enrichment of the fuel rods at the outermost peripheral position A of the fuel assembly adjacent to the position of the partial length fuel rods are the two fuel rods at the outermost peripheral positions adjacent to the position A. The reason why the fuel rod at the position B opposite to the corner closest to the position A is B is as follows.

In other words, when the position A of interest approaches the corner of the fuel assembly, the influence of the water region between the fuel assembly and the fuel assembly is smaller than the influence of the water at the position where the partial length fuel rod is lost. Is superior, and the enrichment of the fuel rod at the position C on the same side as the nearest corner from the position A is lower than the enrichment at the position A among the two outermost fuel rods adjacent to the position A. This is because it may be.

Therefore, the enrichment of the fuel rod at the position A is
The enrichment of the fuel rod at position B, opposite the corner closest to position A and opposite position B, should be compared to the fuel rod at position C, not on the same side as the corner closest to position A.

[0024]

An embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows a first embodiment of a fuel assembly according to the present invention. FIG. 1A shows an enrichment distribution in a cross section of an axially upper region where an output peak occurs in a control rod withdrawal cell at the time of a control rod falling accident, and FIG. 1B shows a fuel rod of FIG. 1A. 1
-4 and the enrichment of the Gd fuel rod G. The enrichment distribution in which the enrichment of all the fuel rods (indicated by ○) at the outermost peripheral position of the fuel assembly adjacent to the position P of the partial length fuel rod P is reduced is adopted.

In particular, the point that the enrichment of the fuel rods at the (1,5) position and its symmetrical position is reduced is different from the conventional enrichment distribution as shown in FIG. Thus, as shown in FIGS. 1 and 4, under the same conditions of the average enrichment, the number of Gd, and the number of enrichment types, the LPF at low temperature is 1.40 to 1.40.
It was reduced by about 3% to 36.

In this embodiment, the enrichment of the fuel rods at the (1,2) position and the (1,8) position adjacent to the partial length fuel rod position and at the symmetrical position (No. 3) thereof are reduced. This is due to the influence of the water at the position where the partial length fuel rod is lost and the influence of the water region between the fuel assemblies. That is, by symbol A ₁ in FIG. 1 (a) (1,2) position the fuel rod is adjacent to the position A _1, the position of the same side as the nearest corner from A ₁ C _1, on the opposite side when positioned to the B _1, it has a enrichment of the fuel rods in position a ₁ lower than the enrichment of the fuel rods in position B _1. FIG.
By symbol A ₂ in (a) (5,1) with respect to the fuel rod positions, the positions and the two corners (1,1), (9,
The distance to 1) is equal. Therefore, for convenience, adjacent to the position A _2, the position on the same side as the nearest corner from A ₂ C ₂
As the (4,1) position, choose the position (6,1) as the position B ₂ of the opposite side to the enrichment of the fuel rods in position A ₂ lower than the enrichment of the fuel rods in position B ₂ . This means that (2,1) position as C _{2 and} (4,1) as B ₂
The same is true even if the position is selected. In other words, A ₂ Der
That (5,1) enrichment of the fuel rods in position, the B ₂
Fuel rod at either (4,1) or (6,1) position
Is set lower than the enrichment.

Next, a second embodiment of the fuel assembly according to the present invention will be described with reference to FIG. FIG. 2A shows UO ₂ fuel rods (full length fuel rods) 1 to 5, Gd fuel rods G _{1 to} G ₂ , partial length fuel rods P and water rods W in the channel box.
2 shows the distribution of concentration in the cross section of the upper region of FIG.
2 (b) shows the fuel rods 1 to 5, P, G ₁ in FIG.
It is shown as legend view uranium enrichment distribution in the axial direction of ~G _2. That is, by symbol A ₁ in FIG. 2 (a) (1,2) fuel rod position is adjacent to the position A _1, the position of the same side as the nearest corner from A ₁ C _1, on the opposite side when positioned to the B _1, it has a enrichment of the fuel rods in position a ₁ lower than the enrichment of the fuel rods in position B _1. FIG.
By symbol A ₂ in (a) (5,1) with respect to the fuel rod positions, the positions and the two corners (1,1), (9,
The distance to 1) is equal. Therefore, for convenience, adjacent to the position A _2, the position on the same side as the nearest corner from A ₂ C ₂
As the (4,1) position, choose the position (6,1) as the position B ₂ of the opposite side to the enrichment of the fuel rods in position A ₂ lower than the enrichment of the fuel rods in position B ₂ . This means that (2,1) position as C _{2 and} (4,1) as B ₂
The same is true even if the position is selected. In other words, A ₂ Der
That (5,1) enrichment of the fuel rods in position, the B ₂
Fuel rod at either (4,1) or (6,1) position
Is set lower than the enrichment.

This embodiment, as seen in recent designs, provides regions of different enrichment in the axial direction for flattening the power distribution in the axial direction. The point that the enrichment of the fuel rods at the (1,5) position in the upper region and the symmetric position thereof is reduced is different from the enrichment distribution of the related art.

Next, a third embodiment of the fuel assembly according to the present invention will be described with reference to FIG. FIG. 3A shows UO ₂ fuel rods (full length fuel rods) 1 to 7, Gd fuel rods G _{1 to} G ₃ , partial length fuel rods P and water rods W in the channel box.
3 shows the state of concentration distribution in the cross section of the upper region of FIG.
FIG. 3B shows the fuel rods 1 to 7, P, G ₁ in FIG.
It is shown as legend view uranium enrichment distribution in the axial direction of ~G _3. That is, the fuel rod at the position (1, 2) denoted by the reference symbol A ₁ in FIG. 3A is adjacent to the position A ₁ , the position on the same side as the corner closest to A ₁ on the same side as C ₁ , when positioned to the B _1, it has a enrichment of the fuel rods in position a ₁ lower than the enrichment of the fuel rods in position B _1. FIG.
By symbol A ₂ in (a) (5,1) with respect to the fuel rod positions, the positions and the two corners (1,1), (9,
The distance to 1) is equal. Therefore, for convenience, adjacent to the position A _2, the position on the same side as the nearest corner from A ₂ C ₂
As the (4,1) position, choose the position (6,1) as the position B ₂ of the opposite side to the enrichment of the fuel rods in position A ₂ lower than the enrichment of the fuel rods in position B ₂ . This means that (2,1) position as C _{2 and} (4,1) as B ₂
The same is true even if the position is selected. In other words, A ₂ Der
That (5,1) enrichment of the fuel rods in position, the B ₂
Fuel rod at either (4,1) or (6,1) position
Is set lower than the enrichment.

This embodiment is a case of a fuel assembly in which the width of the water region between the fuel assemblies is different between the control rod insertion side and the control rod insertion side. Among the fuel rods at the outermost peripheral position of the fuel assembly adjacent to the position of the partial length fuel rod P, the fuel rods ((2, 1),
(1,2), (1,5), (5,1), (1,7),
(7, 1)) and the control rod non-insertion side where the width of the water area is narrow ((2, 9), (9, 2), (8, 9), (9, 8))
An enrichment distribution that reduces the enrichment at a position (indicated by a circle) is employed. In particular, the enrichment distribution that reduces the enrichment of the fuel rods at the (1,5) and (5,1) positions is different from the conventional enrichment distribution.

In the present embodiment, since the enrichment of the fuel rods at the positions (5, 9) and (9, 5) on the side where the width of the water region between the assemblies is small, the influence of the water region is originally small. Even if the maximum enrichment (4.9 wt%) used in the conventional design is used, there is no need to reduce the local output because it does not increase.

[0032]

According to the present invention, there is provided a fuel assembly in which a full length fuel rod of the first group of fuel rods and a partial length fuel rod of the second group of fuel rods are arranged in a lattice pattern. In the cross section of the fuel assembly at the upper part in the axial direction where the fuel rod lacks a part length fuel rod that is likely to increase in the event of a rod drop accident, local peaking at low temperatures is reduced, and the fuel rod Since the increase in enthalpy is reduced, safety is greatly improved.

[Brief description of the drawings]

FIG. 1A is a distribution diagram showing the arrangement of fuel rods and water rods in a first embodiment of a fuel assembly according to the present invention, and FIG. 1B is a legend diagram in FIG.

2A is a distribution diagram showing the arrangement of fuel rods and water rods in a second embodiment of the fuel assembly according to the present invention, and FIG. 2B is the enrichment of fuel rods in the axial direction in FIG. Distribution map.

FIG. 3A is a distribution diagram showing the arrangement of fuel rods and water rods in a third embodiment of the fuel assembly according to the present invention, and FIG. 3B is a legend diagram in FIG.

FIG. 4A is a distribution diagram showing the arrangement of fuel rods and water rods in a conventional fuel assembly, and FIG. 4B is a distribution diagram of FIG.
FIG. 3C is an enrichment distribution diagram in the axial direction of the fuel rod in FIG.

FIG. 5 is a characteristic curve diagram showing a thermal neutron distribution in a lateral direction at an outermost peripheral position of the fuel assembly in FIG. 4;

FIG. 6 is a characteristic curve diagram showing a lateral thermal neutron distribution at the outermost peripheral position of a fuel assembly in which the width of a water region between the fuel assembly and the fuel rod is different between the control rod insertion side and the non-insertion side.

FIG. 7 is a characteristic curve diagram showing an example of an axial output distribution of a control rod withdrawn cell when a control rod falls in a conventional fuel assembly.

[Explanation of symbols]

1,2,3,4,5,6,7 ... UO ₂ fuel rod, G,
G ₁ , G ₂ , G ₃ … Gd fuel rod, ○: low-enrichment fuel rod according to the present invention, P… partial length fuel rod, W… water rod,
NU: Natural uranium blanket, e: ²³⁵ U (wt%),
_{G ... Gd 2 O 2 (wt} %).

Claims (1)

(57) [Claims] 1. A fuel rods in the first group are full-length fuel rods, the first group of fuel rods axial length shorter partial length fuel rods than
And the second group of fuel rods are arranged in a grid pattern,
In the fuel assembly in which the first group of fuel rods containing no Gd ₂ O ₃ is arranged at the outermost position of the fuel assembly, a cross section of the fuel assembly at an axial position where the second group of fuel rods is missing an inner, second side at least a control rod is inserted
Located at the outermost position of the fuel assembly adjacent to the group of fuel rods
The enrichment of all the first group of fuel rods A is determined by the position of the fuel rod A among the two first group of fuel rods B and C at the outermost position adjacent to the fuel rod A. fuel assembly, characterized that you have been enrichment <br/> by Ri low rather setting the nearest corner and fuel rods B of the first group located on the opposite side from.