JP3067291B2 - Reactor fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear fuel assembly, and more particularly to a nuclear fuel assembly suitable for improving core characteristics of a fast breeder reactor.

[0002]

2. Description of the Related Art The core of a conventional fast breeder reactor of this type is composed of a large number of fuel assemblies having a hexagonal cross section and a long length, and uses plutonium mixed oxide as a fuel. As the fuel, small-diameter cylindrical fuel pellets are used, and a large number of the fuel pellets are stacked in the axial direction and held in a fuel rod. A fuel assembly has a structure in which a large number of small-diameter rod-shaped fuel rods are arranged in a triangular arrangement with wire spacers, and the outer periphery thereof is wrapped by a trumpet tube. The fuel assembly is divided into core fuel and radial blanket fuel.The core fuel is loaded with the above-mentioned plutonium mixed oxide, and the radial blanket fuel is loaded with depleted uranium oxide to propagate plutonium fuel. General.

In this type of fuel assembly for a fast breeder reactor, a number of fuel rods having the same length are arranged in a hexagonal section of a wrapper tube, and a core having a shaft length of about 1 m is provided in the fuel rods. It is common to place a fuel pellet and blanket fuel pellets of about 0.3 m above and below it, and to arrange a gas plenum to accumulate gas generated from fuel whose space is secured by a spring or sleeve above and below it. It was a target.

The power coefficient of the conventional core using the fuel assembly for the fast breeder reactor is always negative, but by making it more negative, the safety of the core can be improved. This power coefficient is composed of the Doppler coefficient of the fuel, the coolant density coefficient, and the like. In the conventional fast breeder reactor, the Doppler coefficient was negative and the coolant density coefficient was slightly positive. Therefore, if the coolant density coefficient can be made more negative, the power coefficient can be made more negative, so that the power control of the core and the safety characteristics of the core are further improved. Therefore, an invention that can make the coolant density coefficient more negative is desired.

The reason why the coolant density coefficient of the core using the fuel assembly for the conventional fast breeder reactor is slightly positive is as follows.
With the increase in the temperature of the coolant passing through the fuel assembly, the density of sodium, which is a coolant, decreases, but the mass of sodium is small, the effect as a moderator for neutrons decreases, and the energy of neutrons increases, Uranium 238,
This is because fission by fast neutrons of a fuel such as plutonium increases. Also, a decrease in the density of sodium due to an increase in the coolant temperature has the effect of increasing the leakage of neutrons to the outside of the fuel, but this has a negative reactivity effect, but has a large reactivity of about 300,000 kW or more. In the case of the core, the fission enhancement effect of fast neutrons was relatively dominant over the neutron leakage effect, and the coolant density coefficient was positive. Therefore, there is a demand for an invention of a fuel assembly for a fast breeder reactor capable of making the coolant density coefficient close to zero even in the case of a large core having an electric output of about 300,000 kW or more.

[0006] In the conventional fast breeder reactor, since the coolant density coefficient is positive, research has been conducted to reduce it. For example, if the core height is reduced, neutron leakage increases when the coolant density decreases when the coolant temperature rises, so that the coolant density coefficient can be reduced.

[0007]

However, if the core height is reduced while the core power and the core diameter are kept constant, the core volume decreases, and the power density and the fuel wire output increase. The volume needs to be preserved. For this,
When decreasing the core height, it was necessary to increase the core diameter. An increase in the core diameter has a drawback that the core construction cost increases. Also, lowering the core height increases neutron leakage during normal operation, which has the disadvantage of increasing the plutonium enrichment of the fuel, increasing the combustion reactivity, and lowering the breeding ratio, leading to the deterioration of core core characteristics. there were.

[0008] Due to the above-mentioned disadvantages in the reduction of the coolant density coefficient in the conventional core, the core without increasing the core size, increasing the plutonium enrichment, increasing the combustion reactivity, and decreasing the breeding ratio. There is a need for an invention of a core and a fuel that can reduce the coolant density coefficient while suppressing fluctuations in nuclear properties as much as possible.

An object of the present invention is to provide a fuel assembly capable of reducing a coolant density coefficient without affecting the core size of a fast breeder reactor as much as possible.

[0010]

According to a first aspect of the present invention, there is provided a fuel assembly for a nuclear reactor having a blanket region for fuel breeding above and below a core fuel.
Of the plurality of fuel rods constituting the fuel assembly, some of the fuel rods are shorter than the other fuel rods, and the shorter fuel rods are made shorter.
The fuel rod is located at the center of the fuel assembly more than the other fuel rods.
The upper ends of the short fuel rods are collectively arranged so that the upper end is at a lower position, and the upper part of the short fuel rods serves as a coolant flow path to the end of the other long fuel rods. A fuel assembly for a nuclear reactor, wherein fuel is used, and the position of the internal blanket fuel is lower than the center in the axial direction of the core.

[0011] Similarly, the second means is that in a fuel assembly for a reactor having a blanket region for fuel breeding above and below a core fuel, some of the plurality of fuel rods constituting the fuel assembly are provided. It was used as a shorter than the other fuel rods, the short
At the center of the fuel assembly,
Collective arrangement so that the upper end is lower than other fuel rods
And the upper part of the short fuel rod extends to the end of the other long fuel rod.
A fuel assembly for a nuclear reactor, characterized in that a coolant passage is provided, and a neutron absorber is contained in an axially upper blanket portion of a fuel rod around the passage .

[0013] Similarly, a third means is to provide a fuel assembly for a nuclear reactor having a blanket region for fuel breeding in upper and lower portions of a core fuel, wherein a part of the plurality of fuel rods constituting the fuel assembly is provided. It was used as a shorter than the other fuel rods, the short
At the center of the fuel assembly,
Collective arrangement so that the upper end is lower than other fuel rods
And the upper part of the short fuel rod extends to the end of the other long fuel rod.
A neutron absorber arranged as a coolant passage and located in the middle of the passage
Both when the absorbent body is held by support rod which is supported on the upper shield of the fuel assembly, the support rod reactors, characterized in that a material having a large thermal expansion than the other fuel constituent materials such as a fuel cladding tube Fuel assembly.

[0013] Similarly, the fourth means is the third means,
Put the metal blanket fuel and sodium into the short fuel rods, and put the metal core fuel and sodium into the other fuel rods, and make the vertical length of the gas plenum of the short fuel rods longer than the vertical length of the gas plenum of the other fuel rods. Is also shortened.

A fifth means is that in a fuel assembly for a nuclear reactor having a blanket region for fuel breeding above and below the core fuel, some of the plurality of fuel rods constituting the fuel assembly are provided. It was used as a shorter than the other fuel rods, the short
At the center of the fuel assembly,
Collective arrangement so that the upper end is lower than other fuel rods
And the upper part of the short fuel rod extends to the end of the other long fuel rod.
A reactor having a coolant passage, a blanket region for fuel breeding above and below the other fuel rod, and a coolant communication hole formed in a wrapper pipe adjacent to the upper blanket fuel. Fuel assembly.

[0015]

In the first means, one part of the fuel rods constituting the fuel assembly is made shorter than the other fuel rods, and the downstream side thereof is used as a coolant flow path, so that one fuel rod downstream of the fuel is provided. Increase the volume fraction of the coolant in the part, when the density decreases due to the temperature rise of the coolant, let the neutrons leak from the coolant volume fraction increase part,
In addition to the effect of reducing the coolant density coefficient, the neutron flux distribution can be increased in the upper part of the core by the effect of the internal blanket fuel arranged in part of the core fuel of the fuel rod, and when the density decreases due to the temperature rise of the coolant Since the effect of leaking neutrons from the coolant volume ratio increasing portion can be obtained, the coolant density coefficient can be significantly reduced by the synergistic effect of each action.

In the second means, one part of the fuel rods constituting the fuel assembly is made shorter than the other fuel rods, and the downstream side thereof is used as a coolant passage, so that one of the fuel rods downstream of the coolant is provided. In addition to the effect of increasing the volume fraction of the coolant in the part, when the density decreases due to the temperature rise of the coolant, the neutron leaks from the part where the coolant volume ratio increases, and the effect of reducing the coolant density coefficient is obtained. When the density decreases due to the rise, the neutrons increase due to leakage due to the increase in the neutrons in the coolant volume fraction, and a neutron absorber is included around the coolant volume fraction increase portion to increase the density due to the temperature rise of the coolant. At the time of the decrease, the neutron absorber flowing into the coolant volume ratio increasing portion is absorbed by the neutron absorber, so that the coolant density coefficient can be further reduced as compared with the case of the first means. With coolant dense The coefficient can be greatly reduced.

In the third means, a part of the fuel rods constituting the fuel assembly is made shorter than the other fuel rods, and the downstream side thereof is used as a coolant flow path, so that one of the fuel rods on the downstream side of the coolant is provided. In addition to the effect of increasing the volume fraction of the coolant in the part, when the density decreases due to the temperature rise of the coolant, the neutron leaks from the part where the coolant volume ratio increases, and the effect of reducing the coolant density coefficient is obtained. At the time of density decrease due to temperature rise of the coolant, by utilizing the fact that neutrons increase due to leakage when the density decreases due to the rise, the neutron absorber is included in the coolant volume ratio increase part. To increase the neutron absorption, and to move the neutron absorber more toward the center in the core axis direction due to the thermal expansion of the absorber rod containing the neutron absorber supported above the fuel when the temperature of the coolant rises. Increases, since the effect of reducing the coolant density coefficient is obtained, it is possible to significantly reduce the coolant density coefficient at their synergistic effects of each action.

In the fourth means, in addition to the action of the third means, a short fuel rod is filled with a blanket fuel of a metal fuel, the other fuel rods are charged with a core fuel of a metal , and the blanket fuel is Since gas generation is less intense than core fuel, the gas plenum of short fuel rods containing blanket fuel can be made shorter in the vertical direction than fuel rods containing core fuel, and the neutron absorber is deeper in the fuel assembly. The neutron absorber can be lengthened, and the length of the support rod that supports the neutron absorber can be made longer.The neutron absorber can be placed more centrally in the axial direction of the core to increase neutron absorption and cool down. The effect of further reducing the material density coefficient can be obtained.

In the fifth means, one part of the fuel rods constituting the fuel assembly is made shorter than the other fuel rods, and the downstream side thereof is used as a coolant passage, so that one of the fuel rods on the downstream side of the coolant is provided. In addition to increasing the volume fraction of the coolant, when the density decreases due to the temperature rise of the coolant, the neutron leaks from the coolant volume fraction increase part, the effect of reducing the coolant density coefficient is obtained, and the upper blanket fuel Drilling holes for coolant in adjacent wrapper pipes allows leakage of neutrons in the axial direction outside the wrapper pipe by flowing the coolant with increased temperature outside the wrapper pipe when the density decreases due to the temperature rise of the coolant. As a result, the effect of reducing the coolant density coefficient can be obtained, so that the coolant density coefficient can be significantly reduced by a synergistic effect of the respective actions.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have devised the structure of a fuel assembly shown in FIGS. 1 and 2 as a basic structure for reducing a coolant density coefficient in a fast breeder reactor.

FIG. 1 is a longitudinal sectional view of a fuel assembly showing its basic structure. FIG. 2 is a radial sectional view of the core fuel assembly of FIG.

The core fuel assembly shown in FIGS. 1 and 2 has 271 fuel rods, of which 234 are the long fuel rods 1 at the periphery and 37 are the short fuel rods at the center. It is 10. Around the fuel rod, a wrapper tube 2, an upper shield 3, a lower shield 4, and an entrance nozzle 5 serving as a coolant inlet are formed. 1 and 2
Although not shown, the spacing between each fuel rod is maintained by a wire helically wound around the fuel rod, which is conventionally referred to as a wire spacer in fuel. The fuel rod contains a core fuel pellet 6, which is a ceramic obtained by mixing and sintering plutonium oxide and uranium oxide, and blanket fuel pellets 7 of depleted uranium oxide at the upper and lower portions. A gas plenum 8 for storing gas generated from fuel is provided below the fuel rods, and upper and lower ends of the fuel rods are sealed by welding end plugs 9.

As shown in FIGS. 1 and 2, in the example of the basic structure, 37 fuel rods at the center of the fuel are shorter than the other fuel rods, and the volume fraction of sodium coolant increases in the upper part of the core. doing.

The dimensions of the example of the basic structure shown in FIGS. 1 and 2 will be described below.

The diameter of the fuel rod 1 is 8.0 mm, and the inner and outer diameters of the wrapper tube 2 are 150 mm and 158.1 mm, respectively. The core height is 1 m, and there is a 350 mm axial blanket above and below it. The total length of the 234 fuel rods 1 except for the central 37 fuel rods is 3 m.
This book is a short fuel rod 10 in which the upper part in the axial direction is deleted and an end plug is welded in the middle of the core. The length of the short fuel rod 10 is 2.4 m in the example of the basic structure.

The fuel rods 1 and the short fuel rods 10 are fixed in the axial direction at the lower shield 4, and have a structure in which the fuel expands freely toward the upper part in the axial direction. Since the displacement of the fuel rods outside the 37 short fuel rods 10 at the center of the fuel to the center of the fuel is an issue, the 234 fuel rods outside the 37 central fuel rods are the shafts as shown in FIG. The structure is such that it expands freely toward the top in the direction, but its radial position is fixed by a hexagonal fuel radial fixing pipe 11 extending downward from the upper shield.

Although the mixed oxide is used as the core fuel, the present invention can be applied to other metal fuels, nitride fuels and carbide fuels.

In the example of the basic structure, the ratio of the short fuel rods 10 to the fuel rods is about 14%, but the ratio can be changed to adjust the coolant density coefficient.

The principle by which the coolant density coefficient can be reduced in the example of the basic structure shown in FIGS. 1 and 2 will be described below.

The reason why the coolant density coefficient of the core using the fuel assembly for the conventional fast breeder reactor is slightly positive is as follows.
With the increase in the temperature of the coolant passing through the fuel assembly, the density of sodium, which is a coolant, decreases, but the mass of sodium is small, the effect as a moderator for neutrons decreases, and the energy of neutrons increases, Uranium 238,
This is because fission by fast neutrons of a fuel such as plutonium increases. For uranium 238, fission also increases in the core when the coolant temperature rises, but fast neutrons in the blanket outside the core increase relatively more,
Fission increase is remarkable in the axial and radial blankets. Also, an increase in neutron energy due to a decrease in sodium density due to an increase in coolant temperature also has the effect of increasing leakage outside the core, but this has a negative reactivity effect. In the case of a large core having an electric power of about 300,000 KW or more, the fission enhancement effect by fast neutrons was relatively dominant than the neutron leakage effect, and the coolant density coefficient was positive.

Therefore, even if the core has a large electric power of about 300,000 KW or more, if the neutron leakage effect can be relatively increased when the coolant temperature rises, the coolant density coefficient can be shifted to the negative side. Also, if the leakage effect can be reduced at the coolant temperature at the time of the rated output, and if the leakage effect can be increased when the output is further increased, the coolant density coefficient can be shifted to the negative side, and wasteful neutron leakage can be suppressed. .

In view of the above, the basic structure focuses on the fact that the coolant at the rated output also has a neutron shielding effect, and utilizes the fact that the density of the coolant decreases when the temperature rises. On the downstream side, a region of only the coolant is provided from the middle of the core, so that the upward leakage in the axial direction is small at the rated output, and the upward leakage in the axial direction is increased due to the decrease in the density of the coolant when the output from the rated power increases. . As a result,
The coolant density coefficient can be reduced.

The effects of the example of the basic structure will be summarized below.

Table 1 shows the coolant density coefficient of the core portion when a 1,000,000 kW electric power core is formed by using the fuel assembly according to the example of the basic structure in comparison with the conventional core. In the core
There are 421 core fuel assemblies and 78 radial blanket fuels.

As a result, as shown in Table 1, it was found that the core using the fuel assemblies shown in FIGS. 1 and 2 can reduce the coolant density coefficient by about 47% as compared with the conventional fast reactor core. In addition, the effect on the breeding ratio was confirmed because the shaft blanket was slightly removed, but as shown in Table 1, the effect was slight and it was confirmed that the breeding reactor was sufficiently established.

Further, in the example of the basic structure, the outer diameter of the fuel rod is increased from 7.6 mm to 8.0 mm despite the fact that the inner diameter of the wrapper tube and the number of fuel rods are the same as those of the conventional core example. And the core pressure loss can be reduced to below the conventional core. This is because the flow path of the coolant is increased on the downstream side of the short fuel, so that the pressure loss at that portion is reduced, and as a result, the diameter of the fuel rod can be increased under the same pressure loss condition. . As a result, the fuel volume was increased in the core, so that the breeding ratio could be hardly changed even though the upper blanket was slightly reduced.

In this example of the basic structure, the number of short fuel rods in all the fuel assemblies of the core is 37. As a modification of the example of the basic structure of the homogeneous core in the two radial directions, the center of the core of the outer core is used. By increasing the number of short fuel rods of the fuel assembly having a larger output, it is possible to further reduce the coolant density coefficient. For example, when the number of short fuel rods in the fuel assembly having a large output is 61, Table 1
Can be reduced to about 0.4% Δk / k / Δρ / ρ.

Further, as a modification of the example of the basic structure, a part of the fuel assembly is not formed as a short fuel rod, but in some fuel assemblies, all the fuel rods are formed as short fuel rods. In some cases, a sodium region is provided, and other fuel assemblies may be a normal long fuel rod. In this case, it is difficult to increase the diameter of the normal long fuel rod, but it is possible to reduce the coolant density coefficient with a simple structure.

[0039]

[Table 1]

FIG. 3 is a longitudinal sectional view of a fuel assembly showing an embodiment of a core fuel assembly according to an embodiment of the present invention in which the basic structure is improved.

The core fuel assembly shown in FIG. 3 has the same fuel rods and wire spacers as those of the core fuel assembly shown in FIG. 1, a wrapper tube for holding the periphery thereof, upper and lower shield portions, and an entrance nozzle which is a coolant inlet. Unit. The fuel radial direction fixed pipe 11 is the same as that in FIGS. 1 and 2, but has a slightly shorter vertical dimension. The fuel arrangement in the fuel rod is changed from FIG.

Also in this embodiment, 37 fuel rods at the center of the fuel are shorter than the other fuel rods, and the volume fraction of sodium coolant is increased in the upper part of the core. The core fuel portion of the short fuel rod 10 is uranium / plutonium mixed oxide fuel, while the core fuel of the other long fuel rods is plutonium mixed oxide fuel, but part of the axial direction is depleted uranium oxide. Uses internal blanket fuel.

The dimensions of the embodiment of FIG. 3 will be described below.

The diameter of the fuel rod, the inner and outer diameters of the wrapper tube, the core height, and the dimensions of the axial blanket are the same as those of the fuel shown in FIG. The lengths of 234 long fuel rods and 37 short fuel rods at the center are the same as those of the fuel shown in FIG.

The method of fixing the long fuel rods outside the 37 short fuel rods at the center of the fuel in the radial direction is the same as the fuel shown in FIG.

The principle by which the coolant density coefficient of this embodiment can be reduced is the same as that of FIG.

The effects of the embodiment of the present invention will be summarized below.

Table 2 shows the coolant density coefficient in the case where a 1,000,000 kW electric power core is formed using the fuel of the embodiment of the present invention in comparison with the conventional core. The core includes 421 core fuel assemblies and 78 radial blanket fuels.

As a result, as shown in Table 2, the effect of the core using the fuel assembly shown in FIG. 3 is greater than that of the conventional fast reactor as shown in Table 1, and the coolant density coefficient is about 82%. % Can be reduced. In this embodiment, the effect of reducing the coolant density coefficient is greater than that of the embodiment of Table 1 because the core is made non-homogeneous in the axial direction and the blanket inside the core is made lower than the center in the axial direction of the core. Increased, and the neutron leakage in the axially upward direction when the coolant density was reduced could be further increased.

[0050]

[Table 2]

FIG. 4 is a longitudinal sectional view of a fuel assembly showing another embodiment of the core fuel assembly of the present invention.

FIG. 5 is a radial sectional view of the core fuel assembly shown in FIG.

The core fuel assembly shown in FIG. 4 has the same fuel rods and wire spacers as the core fuel assembly shown in FIG. 1, a wrapper tube for holding the periphery thereof, upper and lower shield portions, and an entrance nozzle which is a coolant inlet. Unit. The fuel arrangement in the fuel rod is changed from FIG. Further, in this embodiment, the neutron absorber is arranged at the uppermost portion of the fuel rod adjacent to the upper shield portion of the short fuel and the fuel radial fixing tube above the short fuel. In the upper shield part, the neutron absorber stores the neutron absorber in a circular tube,
The neutron absorber is a boron carbide pellet. The neutron absorber at the top of the fuel rod is also a boron carbide pellet, and is disposed above the fuel pellet.

In this embodiment, the 61 fuel rods at the center of the fuel are shorter than the other fuel rods, and the volume fraction of sodium coolant is increased in the upper part of the core. The core fuel portion of the short fuel rod 10 is uranium-plutonium mixed oxide fuel, while the core fuel of the other long fuel rods is plutonium mixed oxide fuel.

The dimensions of the embodiment of FIG. 4 will be described below.

The diameter of the fuel rod, the inner and outer diameters of the wrapper tube, the core height, and the dimensions of the axial blanket are the same as those of the fuel shown in FIG. The lengths of the 210 long fuel rods and the 61 short fuel rods at the center are the same as those of the fuel shown in FIG.

The method of fixing the long fuel rods outside the 61 short fuel rods at the center of the fuel in the radial direction is the same as the fuel shown in FIG.

The principle by which the coolant density coefficient of this embodiment can be reduced is the same as that of FIG. 1, but in this embodiment,
The neutron leakage to the upper part in the axial direction when the coolant density is reduced is increased, and the neutrons flowing to the upper part in the axial direction are absorbed by the neutron absorber located in that part, so that the coolant is more effectively Reduction of the density coefficient can be performed.

The effects of the embodiment of the present invention will be summarized below.

Table 3 shows the coolant density coefficient in the case where a 1,000,000 kW electric power core is formed using the fuel of the embodiment of the present invention in comparison with the conventional core. The core includes 421 core fuel assemblies and 78 radial blanket fuels.

As a result, as shown in Table 3, it was found that the core using the fuel assemblies shown in FIGS. 4 and 5 can reduce the coolant density coefficient by about 94% as compared with the conventional fast reactor core. .

[0062]

[Table 3]

FIG. 6 is a longitudinal sectional view of a fuel assembly showing another embodiment of the core fuel assembly of the present invention.

The core fuel assembly shown in FIG. 6 has the same fuel rods and wire spacers as the core fuel assembly shown in FIG. 1, a wrapper tube holding the periphery thereof, upper and lower shield portions, and an entrance nozzle serving as a coolant inlet. Unit. The upper structure of the fuel assembly is changed from that of FIG. 1, and in the present embodiment, a neutron absorber fixed to a top shield by a support rod is arranged above the short fuel. The neutron absorber fixed by the support rod is a boron carbide pellet held in a circular tube. The support rod is made of a metal having a higher coefficient of thermal expansion than other wrapper tubes and fuel cladding tubes. The material of each part of the fuel assembly of this embodiment is such that the core fuel pellet 6 is a uranium-plutonium mixed oxide fuel, and the blanket fuel pellet 7 is a depleted uranium oxide. The fuel rod 1, the wrapper tube 2, and the short fuel rod 10 are made of ferritic stainless steel, and the support rod 13 is made of austenitic stainless steel. The core outlet sodium temperature of the fast breeder reactor is about 500 ° C., and the thermal expansion coefficient at this temperature is about 13 × 10 −6 / ° C. for ferritic stainless steel and about 21 × for austenitic stainless steel. 10 ^-6 /
° C.

In this embodiment, the 61 fuel rods at the center of the fuel are shorter than the other fuel rods, and the volume fraction of sodium coolant is increased in the upper part of the core.

The dimensions of the embodiment of FIG. 6 will be described below.

The diameter of the fuel rod, the inner and outer diameters of the wrapper tube, the core height, and the dimensions of the axial blanket are the same as those of the fuel shown in FIG. The lengths of the 210 long fuel rods and the 61 short fuel rods at the center are the same as those of the fuel shown in FIG.

The method of fixing the long fuel rods outside the 61 short fuel rods at the center of the fuel in the radial direction is the same as the fuel shown in FIG.

The principle of the present embodiment that enables the reduction of the coolant density coefficient is the same as that of FIG. 1, but in the present embodiment,
The neutron leakage to the upper part in the axial direction when the coolant density is reduced is increased, and the neutrons flowing to the upper part in the axial direction are absorbed by the neutron absorber located in that part, so that the coolant is more effectively Reduction of the density coefficient can be performed.

Further, in this embodiment, since the support rod is made of a metal having a large thermal expansion, the support rod expands when the temperature of the coolant rises, and the neutron absorber moves to the core side. It is equivalent to
Due to the insertion effect of the neutron absorber due to the thermal expansion of the support rod,
Negative reactivity can be inserted into the core.

The effects of the embodiment of the present invention will be summarized below.

Table 4 shows the coolant temperature coefficient of the core portion when the 1,000,000 kW electric power core is formed using the fuel of the embodiment of the present invention in comparison with the conventional core. The core includes 421 core fuel assemblies and 78 radial blanket fuels.

As a result, as shown in Table 4, in the core using the fuel assembly shown in FIG. 6, the coolant density coefficient can be reduced by about 117% as compared with the conventional fast reactor core.

[0074]

[Table 4]

FIG. 7 is a longitudinal sectional view of a fuel assembly showing another embodiment of the core fuel assembly of the present invention.

The core fuel assembly shown in FIG. 7 has 331 fuel rods, of which 270 peripheral fuel rods are long fuel rods 1 and 61 central fuel rods are short fuel rods 10. . Also,
Around the fuel rod, a trumpet tube 2, an upper shield 3, a lower shield 4, and an entrance nozzle 5 serving as a coolant inlet are formed. Although not shown in FIG. 7, the intervals between the fuel rods are held by wires spirally wound around the fuel rods. Further, inside the fuel rod, a metal core fuel which is an alloy of uranium, plutonium, and zirconium is stored. A metal blanket fuel, which is an alloy of uranium and zirconium, is stored in the upper and lower portions of the metal core fuel. Further, the inside of the fuel rod is filled with sodium between the metal fuel and the cladding tube. A gas plenum 8 for storing gas generated from the fuel is provided above the fuel rod, and the upper and lower ends of the fuel rod are sealed by welding an end plug 9.

In this embodiment, as in the embodiment of FIG. 6, a neutron absorber fixed to a top shield by a support rod is arranged above a short fuel. The neutron absorber fixed by the support rod is a boron carbide pellet held in a circular tube. The support rod is made of a metal having a higher coefficient of thermal expansion than other wrapper tubes and fuel cladding tubes. The difference between the embodiment of FIG. 6 and the present embodiment is that the present embodiment uses a metal fuel as the fuel and uses a metal blanket fuel as the fuel in the short fuel rod. Metallic sodium is filled between the fuel of the metal fuel and the cladding tube to improve the thermal conductivity. As a result, during operation of the reactor, metallic sodium is in a liquid state due to ambient heat, and the gas generated by fission flies upward and accumulates. The gas plenum, where the gas is stored, is located above the fuel.

When an oxide fuel is employed as shown in FIG. 6, a neutron absorber is arranged by a support bar at a position corresponding to a blanket fuel in the upper part of the core, and in this case, the length of the support bar is about 0.4m. On the other hand, when the metal fuel is employed as shown in FIG. 7, the metal fuel of the short fuel rod is a blanket fuel, and the blanket fuel generates less fission and generates less gas than the core fuel. Is shorter than the gas plenum of the fuel rods packed with core fuel. Therefore, the neutron absorber can be disposed deep in the axial direction at the center of the fuel assembly, and the length of the support rod supporting the neutron absorber is also increased to about 1.2 m.

In the present embodiment, the 61 fuel rods at the center of the fuel are shorter than the other fuel rods, and the volume fraction of sodium coolant is increased in the upper part of the core.

The dimensions of the embodiment of FIG. 7 will be described below.

The diameter of the fuel rod 1 is 7.1 mm, and the inner and outer diameters of the wrapper tube 2 are 150 mm and 158.1 mm, respectively. The core height is 0.6 m. No axial blanket will be installed. 27 excluding 61 at the center of fuel
The total length of the zero fuel rod 1 is 2.0 m, but the center 61
This book is a short fuel rod 10 in which the upper part in the axial direction is deleted and an end plug is welded in the middle of the core. The length of the short fuel rod 10 is 1.2 m in this embodiment.

The material of each part of the fuel assembly of this embodiment is such that the core fuel pellet 6 is made of uranium, plutonium, or zirconium alloy metal fuel.
The short fuel rod 10 is a ferritic stainless steel,
The support rod 13 is austenitic stainless steel.

The fuel rods 1 and the short fuel rods 10 are fixed in the axial direction at the lower shield 4 so that the fuel rods 1 and the fuel rods 10 expand freely toward the upper part in the axial direction. Since the displacement of the fuel rods outside the 61 short fuel rods 10 at the center of the fuel to the center of the fuel is a problem, the 270 fuel rods outside the 61 central fuel rods are shafts as shown in FIG. The structure is such that it expands freely toward the top in the direction, but its radial position is fixed by a hexagonal fuel radial fixing pipe 11 extending downward from the upper shield.

The principle by which the coolant density coefficient of this embodiment can be reduced is the same as that of FIG. 1, but in this embodiment,
The neutron leakage to the upper part in the axial direction when the coolant density is reduced is increased, and the neutrons flowing to the upper part in the axial direction are absorbed by the neutron absorber located in that part, so that the coolant is more effectively Reduction of the density coefficient can be performed.

Further, in this embodiment, since the support rod is made of a metal having a large thermal expansion as in the embodiment of FIG. 6, the support rod expands when the temperature of the coolant rises, and the neutron absorber moves to the core side. Therefore, the insertion of the control rod is equivalent to the insertion of the control rod, and in addition to the above-described effects, the negative reactivity can be inserted into the reactor core by the insertion effect of the neutron absorber due to the thermal expansion of the support rod.

The effects of the embodiment of the present invention will be summarized below.

Table 5 shows the coolant density coefficient when a 1,000,000 kW electric power core is formed using the fuel of the present invention in comparison with a conventional metal fuel core. The core includes 421 core fuel assemblies and 78 radial blanket fuels.

As a result, as shown in Table 5, the core using the fuel assembly shown in FIG. 7 can reduce the coolant density coefficient by about 94% as compared with the conventional fast reactor metal fuel core. Further, in this embodiment, since there is an effect that the pressure loss of the coolant in the gas plenum portion can be greatly reduced, the fuel pin diameter can be made larger and larger if the same pressure loss is used. In this embodiment, the outer diameter of the conventional fuel is 7.1 mm, compared with 6.4 mm. As a result, a large amount of uranium 238, which is a fuel parent substance, can be loaded into the core, so that the breeding ratio can be increased as shown in Table 5. That is, in the embodiment of the present invention, it is possible to reduce the coolant density coefficient and increase the breeding ratio.

[0089]

[Table 5]

FIG. 8 is a longitudinal sectional view of a fuel assembly showing another embodiment of the core fuel assembly of the present invention.

The core fuel assembly shown in FIG. 8 has the same fuel rods and wire spacers as those of the core fuel assembly shown in FIG. 1, a wrapper tube for holding the periphery thereof, upper and lower shield portions, and an entrance nozzle as a coolant inlet. Unit. The use of a short fuel is the same as that of FIG. 1, but the present embodiment is different from that of FIG. 1 in that a communication hole for coolant is further provided in a wrapper tube adjacent to the upper blanket in the axial direction. The thickness of the portion where the communication hole is formed is increased in order to secure the strength of the trumpet tube.

The dimensions of the embodiment shown in FIG. 8 will be described below.

The diameter of the fuel rod, the inner and outer diameters of the wrapper tube, the core height, and the dimensions of the axial blanket are the same as those of the fuel shown in FIG. The lengths of the 234 long fuel rods and the 37 short fuel rods are also the same as the fuel of FIG.

Although the principle of the present embodiment that enables the reduction of the coolant density coefficient is the same as that of FIG. 1, in this embodiment, when the coolant density further decreases, the increase in the neutron leakage in the axial direction is reduced by the wrapper tube. The effect can be increased by the effect that the sodium density between the wrapper tubes is reduced by the opened communication hole of the coolant.

The effects of the embodiment of the present invention will be summarized below.

Table 6 shows the coolant density coefficient in the case where a 1,000,000 kW electric power core is formed using the fuel of the embodiment of the present invention in comparison with the conventional core. The core includes 421 core fuel assemblies and 78 radial blanket fuels.

As a result, as shown in Table 6, it was found that the core using the fuel assemblies shown in FIG. 8 can reduce the coolant density coefficient by about 70% as compared with the conventional fast reactor core.

In this embodiment, while using a short fuel,
The coolant density factor can be significantly reduced by a simple method of drilling coolant communication holes in the wrapper tube adjacent to the axially upper blanket.

[0099]

[Table 6]

[0100]

According to the first aspect of the present invention, for some or all of the fuel assemblies arranged in the core, a part of the fuel rods constituting the fuel assembly is made shorter than the other fuel rods. By making the coolant flow path on the downstream side, the volume fraction of the coolant on the downstream side of the coolant of the fuel is increased, and when the density decreases due to the temperature rise of the coolant, the neutrons from the coolant volume ratio increase part In addition, due to the effect of the internal blanket fuel located in part of the core fuel of the fuel rods, the distribution of neutron flux is enhanced at the upper part of the core, and when the density decreases due to the temperature rise of the coolant, the coolant volume ratio increases. Since the neutrons are leaked from the portion, the effect of significantly reducing the coolant density coefficient is obtained.

According to the second aspect of the present invention, for some or all of the fuel assemblies arranged in the core, a part of the fuel rods constituting the fuel assembly is made shorter than the other fuel rods, and the downstream portion thereof is provided. By using the coolant flow path on the side, the volume fraction of the coolant is increased on the part of the fuel downstream of the coolant, and when the density decreases due to the temperature rise of the coolant, neutrons leak from the coolant volume fraction increase part. In addition, at the time of the density decrease due to the temperature rise of the coolant, utilizing the fact that the neutrons increase by leakage due to the increase in the coolant volume fraction, the neutron absorber is included around the coolant volume fraction increase portion. Accordingly, when the density of the coolant decreases due to a rise in the temperature of the coolant, the neutron flowing into the coolant volume ratio increasing portion is absorbed by the neutron absorber, so that an effect of significantly reducing the coolant density coefficient can be obtained.

According to the third aspect of the present invention, for some or all of the fuel assemblies arranged in the core, a part of the fuel rods constituting the fuel assembly is made shorter than the other fuel rods, and the downstream portion thereof is provided. By using the coolant flow path on the side, the volume fraction of the coolant is increased on the part of the fuel downstream of the coolant, and when the density decreases due to the temperature rise of the coolant, neutrons leak from the coolant volume fraction increase part. Further, when the temperature of the coolant rises, the thermal expansion of the absorber rod containing the neutron absorber supported on the upper part of the fuel increases the neutron absorption to move the neutron absorber more toward the center in the core axis direction, The effect of significantly reducing the coolant density coefficient is obtained.

According to the invention of claim 4, in addition to the effect of the invention of claim 3, since the neutron absorber can be placed more centrally in the core axis direction, the absorption of neutrons is increased and the coolant density is increased. The coefficient can be further reduced than in the case of claim 3.

According to the fifth aspect of the present invention, for some or all of the fuel assemblies arranged in the core, a part of the fuel rods constituting the fuel assembly is made shorter than the other fuel rods, and the downstream portion thereof is provided. By using the coolant flow path on the side, the volume fraction of the coolant is increased on the part of the fuel downstream of the coolant, and when the density decreases due to the temperature rise of the coolant, neutrons leak from the coolant volume fraction increase part. Further, by forming a coolant communication hole in the wrapper pipe adjacent to the upper blanket fuel, when the temperature of the coolant decreases, the coolant with the increased temperature flows outside the wrapper pipe when the density decreases. The neutrons leak in the axial direction even outside the, and the effect of significantly reducing the coolant density coefficient can be obtained.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a core fuel assembly according to an example of a basic structure proposed by the present inventors.

FIG. 2 is a radial cross-sectional view of the fuel assembly of FIG.

FIG. 3 is a longitudinal sectional view of a core fuel assembly according to an embodiment of the present invention.

FIG. 4 is a longitudinal sectional view of a core fuel assembly according to another embodiment of the present invention.

FIG. 5 is a radial cross-sectional view of the fuel assembly of FIG.

FIG. 6 is a longitudinal sectional view of a core fuel assembly according to still another embodiment of the present invention.

FIG. 7 is a longitudinal sectional view of a core fuel assembly according to still another embodiment of the present invention.

FIG. 8 is a longitudinal sectional view of a core fuel assembly according to still another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel rod, 2 ... Wrapper tube, 3 ... Upper shield, 4 ... Lower shield, 5 ... Entrance nozzle, 6 ... Core fuel pellet, 7 ... Blanket fuel pellet, 8 ... Gas plenum, 9 ... End plug, 10 ... Short fuel rod, 11: fuel radial direction fixed tube, 12: neutron absorber, 13: support rod, 14: coolant communication hole.

──────────────────────────────────────────────────続 き Continuing on the front page (51) Int.Cl. ⁷ Identification symbol FIG21C 3/30 Y 3/32 G (72) Inventor Kuniwazu Kano 3-1-1, Kochicho, Hitachi-shi, Ibaraki Hitachi, Ltd. Inside Hitachi, Ltd. (72) Inventor, Mitsuo Wataru 3-1-1, Komachi, Hitachi, Ibaraki Pref. Hitachi, Ltd. Inside Hitachi, Ltd. (56) References JP-A-4-252994 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) G21C 3/30 G21C 3/32 G21C 5/18

Claims (5)

(57) [Claims] In a fuel assembly for a nuclear reactor having a blanket region for fuel breeding in upper and lower portions of a core fuel, a part of a plurality of fuel rods constituting a fuel assembly is replaced with another fuel rod. Shorter fuel rod, and the shorter fuel rod
At the center of the fuel assembly, the upper end is higher than the other fuel rods.
Collectively arranged so as to be at a lower position, the upper part of the short fuel rod is used as a coolant flow path to the end of the other long fuel rod, and the core fuel of the fuel rod is used as an internal blanket fuel with a part in the axial direction A fuel assembly for a nuclear reactor, wherein the position of the internal blanket fuel is lower than the center in the axial direction of the reactor core. 2. A fuel assembly for a nuclear reactor having a blanket region for fuel breeding in upper and lower portions of a core fuel, wherein some of the plurality of fuel rods constituting the fuel assembly are replaced with other fuel rods. Shorter fuel rod, and the shorter fuel rod
At the center of the fuel assembly, the upper end is higher than the other fuel rods.
Collectively arrange so that it is at a low position and above the short fuel rod
The part is a coolant flow path to the end of another long fuel rod,
A fuel assembly for a nuclear reactor, comprising a neutron absorber in an axially upper blanket portion of a fuel rod around a flow path . 3. A fuel assembly for a reactor having a blanket region for fuel breeding in upper and lower portions of a core fuel, wherein a part of the plurality of fuel rods constituting the fuel assembly is replaced with another fuel rod. Shorter fuel rod, and the shorter fuel rod
At the center of the fuel assembly, the upper end is higher than the other fuel rods.
Collectively arrange so that it is at a low position and above the short fuel rod
The part is a coolant flow path to the end of another long fuel rod,
The neutron absorber placed in the middle of the flow path is
Both when held by support rod which is supported on the upper shield, before
Serial support rod be a material having a large thermal expansion fuel assembly for a nuclear reactor, characterized in than other fuel constituent materials such as a fuel cladding tube. 4. The fuel assembly for a nuclear reactor according to claim 3, wherein
Put the metal blanket fuel and sodium into the short fuel rods, and put the metal core fuel and sodium into the other fuel rods, and make the vertical length of the gas plenum of the short fuel rods longer than the vertical length of the gas plenum of the other fuel rods. A fuel assembly for a nuclear reactor, characterized in that the fuel assembly is also shortened. 5. A fuel assembly for a nuclear reactor having a blanket region for fuel breeding in upper and lower portions of a core fuel, wherein some of the plurality of fuel rods constituting the fuel assembly are replaced with other fuel rods. Shorter fuel rod, and the shorter fuel rod
At the center of the fuel assembly, the upper end is higher than the other fuel rods.
Collectively arrange so that it is at a low position and above the short fuel rod
The part is a coolant flow path to the end of another long fuel rod,
A fuel assembly for a nuclear reactor, comprising: a fuel breeding blanket region at upper and lower portions of another fuel rod; and a coolant communication hole formed in a wrapper tube adjacent to the upper blanket fuel.