JPH04303796A - Fuel assembly for reactor - Google Patents

Abstract

PURPOSE:To attain a good core characteristics regardless of core with UO2 fuel only or core with mixture of UO2 fuel and MOX fuel. CONSTITUTION:A fuel assembly is constituted of an outer assembly 2 on the periphery of the assembly where thermal neutrons coming from outside can reach and an inner assembly 3 in the center of the assembly where thermal neutrons from outside can rarely reach. In between the outer assembly 2 and inner assembly 3, an internal gap region 8 where single or two-phase water flows, is provided. The width of the gap region 8 is made controllable by changing the size of the channel box 6 of the inner assembly 3.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is directed to BWR or ABW.
Regarding fuel assemblies used in light water reactors such as R, especially UO
2. The present invention relates to a fuel assembly that can obtain good core characteristics in both a single-fuel core and a mixed core of UO2 fuel and MOX fuel.

0002

2. Description of the Related Art Conventionally, as a UO2 fuel assembly used in BWR or ABWR, the one shown in FIG. 12 is known.

This fuel assembly 100 has 60 fuel rods 101 and one water rod 102 arranged regularly as shown in FIGS. 12 and 13. 12 and 15.
Each is fixed by an upper tie plate 105 shown in FIG.
These are housed in a rectangular cylindrical channel box 106.

[0004] The shape and structure of this fuel assembly 100 (hard specifications of the assembly) are determined with the target of a take-off burn-up of 38 Gwd/t, which is higher than the take-off burn-up of 33 Gwd/t one generation ago.
/t, the cross-sectional area of the water rod (a hollow tube through which coolant flows) is about twice as large, and the number of fuel rods is reduced by two accordingly. In this way, in conventional technology, increasing the burn-up of UO2 fuel with the goal of improving economic efficiency is dealt with by increasing the number or size of water rods, and the nuclear properties are affected by increasing enrichment. The effects of changes, such as an increase in the absolute value of the void reactivity coefficient and a decrease in the margin for reactor shutdown, are measured by the ratio of the number of hydrogen atoms to the number of heavy metal fuel atoms during operation (hereinafter referred to as H/HM.
．． It was reduced by increasing the ratio (referred to as the ratio).

[0005] In BWR, fuel assemblies are replaced using a batch method (instead of replacing all fuel assemblies at once, 1/3 to 1/5 of the total number of fuel assemblies are replaced at each periodic inspection). ) is adopted. Figure 16 shows an example of an ABWR core, in which about 1/4 of the 872 loaded fuel assemblies are replaced. Therefore, when adopting a new fuel assembly, the conventional fuel and new fuel will be mixed in the same core during the transient cycle, and if the nuclear properties of the two fuels are significantly different, the core loading The core characteristics may change depending on the ratio, which may cause operational constraints. In addition, the control rods used in the reactor core are approximately one control rod per four assemblies, and 205 control rods are used in the entire reactor core. The fuel cells (called cells) are composed of low-reactivity fuel to prevent thermal limits from being exceeded due to distortions in the axial power distribution associated with control rod operations.

On the other hand, when using MOX fuel in BWR, the same hardware as UO2 fuel assemblies is used, since it is advantageous to use fuel assemblies with the same shape and structure for manufacturing and acquiring thermo-hydraulic data. If an attempt is made to achieve the same extraction burnup as that of a UO2 fuel assembly, the difference in neutron cross section between uranium and plutonium will become noticeable, resulting in a large difference in combustion characteristics.

For example, as shown in FIG. 17, a discrete type M in which the entire fuel assembly is composed of only MOX fuel
In the case of OX fuel, thermonuclear plutonium is converted into uranium-2
Since the thermal neutron cross section is larger than that of 35, the neutron spectrum of the entire assembly becomes hard, and furthermore, the thermal neutrons moderated in the non-boiling water region outside the channel box (hereinafter referred to as the external water gap region) are It is easily absorbed by the fuel rods in the area around the body, and the amount of thermal neutrons that reach the inside of the assembly is smaller than that of UO2 fuel.

[0008] As a result, compared to UO2 fuel, this leads to (1) an increase in the absolute value of the void coefficient, (2) a decrease in the value of control rods, and (2) an increase in the local peaking coefficient, resulting in lower core stability, reactor shutdown margin, and
Deteriorates the thermal properties of the reactor core. As an example of alleviating these characteristics, as shown in FIG.
There is an island-type MOX fuel that replaces O2 fuel with UO2 fuel, which softens the average neutron spectrum of the assembly by placing UO2 fuel around the assembly, and at the same time reduces absorption by the fuel around the assembly. ,
It is possible to increase the value of control rods and reduce local peaking coefficients. However, it will be necessary to simultaneously handle MOX fuel and UO2 fuel, which are difficult to manufacture and manage.

One method for solving the above-mentioned problems when using MOX fuel is to allow changes in the hard use of the assemblies and change the H/H. M. There are ways to increase the ratio. For example, as in the case of increasing the burnup of UO2 fuel, there is a method of increasing the number or size of water rods.

0010

[Problems to be Solved by the Invention] Currently, there is a demand for higher burnup of UO2 fuel from the viewpoint of improving fuel economy, and furthermore, as the development trend of fuel cycle plants is fluid, various uses of plutonium are required. It is desired to respond to various configurations (from efficiently promoting plutonium combustion to increasing conversion to plutonium), and it is desirable to develop fuel assemblies and loading them that reduce differences in nuclear properties due to differences in fuel composition. It is necessary to devise a core configuration that will

[0011] In order to increase the burnup of UO2 fuel, H/H. M. It is necessary to increase the ratio. In addition, when reducing the difference in the nuclear characteristics of each fuel assembly in a mixed core of conventional fuel and new fuel, the H/H of each fuel assembly can be reduced. M. It is necessary to adjust the ratio appropriately. However, conventional methods of increasing the number or size of water rods have problems such as decreasing fuel rod loading and increasing linear power density. Another method of expanding the external water gap region has problems such as a change in control rod value and an increase in core size.

On the other hand, when using MOX fuel, the conventional method of using the hardware of a UO2 fuel assembly made exclusively for a certain extraction burnup as MOX fuel is to increase the void coefficient and control the Core characteristics are becoming stricter due to increased rod value and local power peaking. , loading pattern). In addition, island type MOX fuel uses MOX fuel and UO2 fuel, which have problems in terms of radiation exposure management.
Problems remain, such as the need to handle fuel in the same process during fuel production and reprocessing, and the total amount of MOX fuel that can be loaded into the reactor core.

Furthermore, when the size of the assembly is increased in order to reduce the number of control rods and refueling work, if a conventional control cell is used, reactivity is lost as the area of the control cell increases. Also, increasing the size of the aggregate increases the radial output peaking coefficient.

The present invention has been made in consideration of these points, and even in a UO2 fuel-only core, UO2
It is an object of the present invention to provide a fuel assembly for a nuclear reactor that can obtain good core characteristics even in a mixed core of fuel and MOX fuel.

[0015]

[Means for Solving the Problems] As a means for achieving the above object, the present invention provides an external aggregate in a peripheral area of the aggregate where thermal neutrons moderated in a non-boiling water area outside the channel box reach the aggregate; By changing the size of the channel box of the inner aggregate, there is a single-phase or two-phase structure between the outer aggregate and the inner aggregate. It is characterized by being able to form an internal water gap region of arbitrary width through which phase water flows.

[0016]

[Operation] In the nuclear reactor fuel assembly according to the present invention,
By changing the size of the channel box of the inner assembly, an internal water gap region of arbitrary width can be formed between the outer assembly and the inner assembly. For this reason, there is no reduction in fuel loading as would be the case when the number or size of water rods was increased, and there would be no change in control rod value as would be the case when changing the external water gap width. /H. M. The ratio can be adjusted appropriately and differences in fuel characteristics can be reduced. Furthermore, if the internal water gap region is made into a closed structure with small holes at the top and bottom ends, the void ratio can be adjusted by changing the internal flow rate, and it can also be used to control the reactivity of the aggregate.

On the other hand, when using MOX fuel, the inner assembly is composed of MOX fuel and the outer assembly is composed of UO2 fuel, and since both are bundled with separate spacers, they can be easily separated and manufactured. and UO2 fuel and MOX fuel can be handled separately during reprocessing. Furthermore, compared to when the entire assembly is used as MOX fuel, ■ the amount of plutonium loaded per assembly is reduced, the neutron spectrum becomes softer and the absolute value of the void coefficient becomes smaller; By arranging UO2 with a small thermal neutron cross section in the assembly, the value of the control rods increases, and the amount of thermal neutrons that reach the center of the assembly increases, flattening the power distribution within the assembly, improving core stability,
This leads to improved reactor shutdown margin and core thermal margin. Furthermore, by flattening the power distribution inside the assembly, it is also possible to reduce the number of enrichment types of MOX fuel.

For various forms of utilization of plutonium, in order to promote the combustion of plutonium, it is possible to soften the neutron spectrum by widening the width of the internal water gap and efficiently burn plutonium in the internal region.
To promote the conversion to plutonium, the internal water gap width is reduced and the fuel rods in the internal assembly are arranged in a triangular lattice or other H/H. M. By adopting a configuration with a lower ratio and hardening the neutron spectrum, the conversion rate of the internal assembly can be increased. In this case, since the UO2 fuel is placed in the outer assembly, the conversion rate to plutonium is lower than when the entire assembly is composed of MOX fuel, but the degree of deterioration of the fuel properties is small. The core loading ratio of MOX fuel can be adjusted from 0 to 50% in a mixed core of MOX fuel and UO2 fuel by making the number of fuel rods in the outer assembly and the inner assembly equal to each other.

[0019] Furthermore, if the structure is such that the internal assembly is removable from the external assembly, and partial assemblies can be rearranged, one of the problems associated with increasing the size of the fuel assembly can be solved due to the increase in area of the control cell. Reactivity losses can be addressed by creating control cells with large assemblies with an infinite multiplication factor of the outer assemblies lower than that of the inner assemblies, avoiding excessive concentration of low-reactivity fuel, and reducing radial output power. The increase in the peaking coefficient can be solved by combining subsets with different infinite multiplication factors (number of batches) to create a large aggregate.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained below with reference to the drawings.

FIG. 1 shows a fuel assembly according to a first embodiment of the present invention, and this fuel assembly 1 includes an outer assembly 2 consisting of fuel rods 2a indicated by white circles in the figure, and an outer assembly 2 consisting of fuel rods 2a indicated by white circles in the figure. It is composed of two partial assemblies, an inner assembly 3 consisting of fuel rods 3a indicated by black circles, and both fuel rods 2a, 3a.
are all composed of UO2 fuel, and channel boxes 4, 5, and 6 are arranged on both the inner and outer sides of the outer assembly 2 and on the outside of the inner assembly 3, respectively. Further, a water rod 7 is arranged in the internal assembly 3.

An internal water gap region 8 through which single-phase or two-phase water flows is provided between the two aggregates 2 and 3.
The width of this internal water gap region 8 can be arbitrarily adjusted by changing the size of the channel box 6 of the internal assembly 3. By widening the width of this internal water gap region 8, H/H. M. The ratio increases, making it possible to achieve higher burnup. In addition, by making the internal water gap region 8 a closed structure with small holes at the top and bottom ends, the void ratio can be adjusted by changing the flow rate in the internal water gap region 8, and it can be used for spectrum shift operation. ing.

In this way, by simply changing the size of the channel box 6 of the internal assembly 3, the internal water gap region 8 can be
Since the width of the water rod can be adjusted, there is no reduction in fuel loading as is the case when the number or size of water rods is increased, and there is no change in control rod value as is the case when the external water gap width is changed. H/H depending on the degree. M. The ratio can be adjusted appropriately.

FIG. 2 shows a second embodiment of the present invention, in which the fuel composition is different from that of the first embodiment.

That is, in this fuel assembly 1,
The fuel rods 2a of the outer assembly 2 are composed of UO2 fuel,
Furthermore, the fuel rods 3a of the internal assembly 3 are made of MOX fuel. In this embodiment, in order to promote the combustion of plutonium, the width of the internal water gap region 8 is widened and the H/H. M. increase the ratio and change the neutron spectrum to UO2
It is as soft as fuel.

In other respects, the configuration is the same as that of the first embodiment.

Thus, by adjusting the width of the internal water gap region 8, the H/H. M. The ratio can be adjusted appropriately, the difference in fuel properties can be reduced, and the outer assembly 2 and the inner assembly 3 can be easily separated, so that UO2 fuel and MOX fuel can be separated during production and reprocessing. can be treated separately.

FIG. 3 shows a third embodiment of the present invention, in which the neutron spectrum is made as hard as possible in order to promote conversion to plutonium.

That is, in this fuel assembly 1, the fuel rods 2a of the outer assembly 2 are composed of UO2 fuel, and the fuel rods 3a of the inner assembly 3 are composed of H/H. M. MO with lower ratio
It is composed of X fuel (the fuel lattice is a triangular lattice, etc.), and no water rod is arranged in the internal assembly 3. Further, the width of the internal water gap region 8 is set to be as narrow as the aggregate characteristics allow.

In other respects, the configuration is the same as that of the first embodiment.

[0031] With this configuration, the neutron spectrum can be hardened and the conversion to plutonium can be promoted. In addition, when the flow path area inside and outside the assembly differs greatly as in this example, it is desirable to adjust the pressure drop inside and outside the assembly from the viewpoint of channel stability. It is conceivable to shorten the length of the fuel rods 3a of the body 3.

4 to 6 respectively show fourth to sixth embodiments of the present invention, and unlike the first to third embodiments, the number of layers of fuel rods in the outer assembly 2 is This increases the number of layers to three instead of two.

In other respects, the structure is the same as that of the first to third embodiments.

In this way, the number of layers of fuel rods 2a in the outer assembly 2 is determined by the difference in relative amounts of thermal neutrons (difference in spectra) between the inner and outer subassemblies 2 and 3, so the inner water gap By widening the width of the region 8 and increasing the amount of thermal neutrons in the outer assembly 2, the number of layers of fuel rods 2a in the outer assembly 2 can be increased to three or more.

FIG. 7 shows a seventh embodiment of the present invention, in which the internal water gap region 8 is removed from the fuel assembly 1 in the third embodiment, and the channel boxes 5 and 6 are removed.
Instead, a structural member 9 is used that separates the inside and outside areas.

In other respects, the configuration is the same as that of the third embodiment.

With this configuration, the conversion rate of the internal assembly 3 can be increased. In the case of this embodiment, the structural material 9 may be thin and have a partially perforated structure in order to reduce neutron absorption by the structural material 9 and eliminate pressure differences between the inside and the outside.

FIG. 8 shows an eighth embodiment of the present invention, in which an external assembly 2 adjacent to the control rod 10 is used instead of the conventional assembly consisting only of low-reactivity fuel assemblies indicated by ■ in the figure. Using a fuel assembly 1 in which the internal assembly 3 is a low-reactivity fuel assembly 4 and the internal assembly 3 is a high-reactivity fuel assembly 2, the control rod 1 is
The infinite multiplication factor of the outer aggregate 2 adjacent to 0 is the inner aggregate 3
It is smaller.

In other respects, the configuration is the same as that of the first embodiment.

In this way, since the infinite multiplication factor of the outer assembly 2 is made smaller than that of the inner assembly 3, even in a core composed of large assemblies, fuel assemblies with low reactivity do not accumulate excessively. , the loss of reactivity can be avoided.

FIG. 9 shows a ninth embodiment of the present invention, in which the inner assembly 3 and the inner water gap region 8 in the first to third embodiments are arranged around the axis relative to the outer assembly 2. It is rotated by 45 degrees so that the internal water gap region 8 has a rhombic shape.

In other respects, the structure is the same as that of the first to third embodiments.

Even with this configuration, the same effects as in the first to third embodiments can be expected.

FIG. 10 shows a tenth embodiment of the present invention, in which a flow path resistance 11 is provided in the channel box 5 inside the outer assembly 2 and the channel box 6 of the inner assembly 3. Internal water gap region 8 in insertion direction of 3
This allows the flow rate to be adjusted.

In other respects, the configuration is the same as that of the first embodiment.

[0046] By providing the flow path resistance 11 in this way, spectrum shift operation is possible in which the void ratio of the internal water gap region 8 is high at the early stage of combustion and low at the late stage of combustion.

FIG. 11 shows an eleventh embodiment of the present invention, in which a box-shaped control rod 12 is disposed within the internal water gap region 8, and a normal external water gap that does not affect the control rod value is provided. The region 13 is used as a spectrum shift region by controlling the flow rate.

[0048] In other respects, the configuration is the same as that of the first embodiment.

Even with this configuration, the same effects as in the first embodiment can be expected.

[0050]

As explained above, the present invention divides a fuel assembly into an external assembly and an internal assembly depending on the amount of thermal neutrons arriving from the outside, and between these two assemblies,
By adjusting the size of the channel box in the internal assembly, it is possible to form an internal water gap region of any width, even in a UO2 fuel-only core.
Even in a mixed core of UO2 fuel and MOX fuel, good core characteristics can be obtained.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing a nuclear reactor fuel assembly according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing a nuclear reactor fuel assembly according to a second embodiment of the present invention.

FIG. 3 is a configuration diagram showing a nuclear reactor fuel assembly according to a third embodiment of the present invention.

FIG. 4 is a configuration diagram showing a nuclear reactor fuel assembly according to a fourth embodiment of the present invention.

FIG. 5 is a configuration diagram showing a nuclear reactor fuel assembly according to a fifth embodiment of the present invention.

FIG. 6 is a configuration diagram showing a nuclear reactor fuel assembly according to a sixth embodiment of the present invention.

FIG. 7 is a configuration diagram showing a nuclear reactor fuel assembly according to a seventh embodiment of the present invention.

FIG. 8 is a configuration diagram showing a nuclear reactor fuel assembly according to an eighth embodiment of the present invention in comparison with a conventional fuel assembly.

FIG. 9 is a configuration diagram showing a nuclear reactor fuel assembly according to a ninth embodiment of the present invention.

FIG. 10 is an explanatory diagram showing a nuclear reactor fuel assembly according to a tenth embodiment of the present invention.

FIG. 11 is a configuration diagram showing a nuclear reactor fuel assembly according to an eleventh embodiment of the present invention.

FIG. 12 is a sectional view showing the configuration of a conventional UO2 fuel assembly.

13 is an explanatory diagram showing the fuel arrangement of the fuel assembly of FIG. 12. FIG.

14 is an explanatory diagram showing the configuration of a spacer of the fuel assembly in FIG. 12. FIG.

15 is an explanatory diagram showing the configuration of the upper tie plate of the fuel assembly in FIG. 12. FIG.

FIG. 16 is an explanatory diagram showing an example of a core loading pattern of a conventional nuclear reactor.

[Fig. 17] In a conventional MOX fuel assembly, the entire assembly is
FIG. 2 is a configuration diagram showing a discrete MOX fuel assembly composed of OX fuel rods.

FIG. 18 is a configuration diagram showing an island-type MOX fuel assembly in which the periphery of the assembly is composed of UO2 fuel rods, which is a conventional MOX fuel assembly.

[Explanation of symbols]

1 Fuel assembly 2 External aggregate 3 Internal aggregate 4,5,6...channel box 8 Internal water gap region

Claims (1)

[Claims] 1. An outer aggregate in the peripheral area of the aggregate, where thermal neutrons slowed in the non-boiling water area outside the channel box reach, and an inner aggregate in the central area of the aggregate, where the thermal neutrons hardly reach. By changing the size of the channel box of the internal assembly, it is possible to form an internal water gap region of any width between the external assembly and the internal assembly, through which single-phase or two-phase water flows. A nuclear reactor fuel assembly characterized by: