JPH0527067A - Fuel assembly for nuclear reactor - Google Patents

Abstract

PURPOSE:To obtain a fuel assembly for suppressing the reactivity in a reactor core when the density of the cooling material lowers, as for the fuel assembly for nuclear reactor which has a blanket region for fuel breeding in the upper and lower parts of the reactor core fuel. CONSTITUTION:As for a fuel assembly for nuclear reactor having a blanket region for fuel breeding in the upper and lower parts of a reactor core fuel, a part of the fuel rods among a plurality of fuel rods which constitute the fuel assembly is made shorter than others, and the upper part of the short fuel rod 10 is formed to a flow passage for cooling material up to the edge part at other long fuel rods. Accordingly, the cooling material density coefficient and cooling material void reactivity can be reduced without increasing the dimension of a reactor core.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear reactor fuel assembly, and more particularly to a nuclear reactor 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 long fuel assemblies each having a hexagonal cross section 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 axially stacked and held in the fuel rods. In addition, a structure of a large number of small-diameter rod-shaped fuel rods arranged in a triangular arrangement by wire spacers, and the outer circumference thereof is wrapped with a trumpet tube is a type of fuel assembly. The fuel assembly is divided into core fuel and radial blanket fuel.The core fuel may be loaded with the above plutonium mixed oxide, and the radial blanket fuel may be loaded with depleted uranium oxide for breeding plutonium fuel. It is common.

Further, in this type of fuel assembly for a fast breeder reactor, a large number of fuel rods having the same length are arranged in a trumpet tube having a hexagonal cross section, and a core having an axial length of about 1 m is arranged in each fuel rod. Generally, fuel pellets and blanket fuel pellets of about 0.3 m are placed on the top and bottom of the fuel pellets, and a gas plenum that accumulates the gas generated from the fuel that secures a space with a spring or a sleeve is placed on the top and bottom. It was target.

The core using the fuel assembly for the conventional fast breeder reactor always has a negative output coefficient, but by making it more negative, the safety of the core can be improved. This output coefficient is composed of fuel Doppler coefficient, coolant density coefficient, etc. 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, there is a demand for an invention capable of making the coolant density coefficient more negative.

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.
As the temperature of the coolant passing through the fuel assembly rises, the density of sodium, which is the 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 of fuel such as plutonium due to fast neutrons increases. Further, the decrease in sodium density due to the rise in the coolant temperature also has the effect of increasing the leakage of neutrons to the outside of the fuel, which brings about a negative reactivity effect, but has a large electrical output of 300,000 kW or more. In the case of the core, the nuclear fission-increasing effect by fast neutrons was more dominant than 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 that can bring 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.

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 it is possible to reduce the coolant density coefficient. However, if the core height is lowered while the core power and core diameter are kept constant, the volume of the core decreases and the power density and fuel line output increase, so it is necessary to preserve the core volume. For this reason, it was necessary to increase the core diameter when lowering the core height. The increase in core diameter has the drawback of increasing core construction costs. Further, when the core height is reduced, the neutron leakage during normal operation also increases, so there is a drawback that the core nuclear characteristics such as increase in plutonium enrichment of fuel, increase in combustion reactivity, and decrease in breeding ratio may deteriorate. there were.

[0007] Due to the above-mentioned inconvenience in devising the reduction of the coolant density coefficient in the conventional core, the core is required to be increased in the plutonium enrichment, the combustion reactivity and the breeding ratio without increasing the core size. There is a demand for an invention of a core and a fuel capable of reducing the coolant density coefficient while suppressing the variation of nuclear characteristics as much as possible.

[0008]

However, if the core height is lowered while the core power and core diameter are kept constant, the volume of the core is reduced and the power density and fuel line output are increased. The volume needs to be preserved. For this,
When decreasing the core height, it was necessary to increase the core diameter. The increase in core diameter has the drawback of increasing core construction costs. Further, when the core height is reduced, the neutron leakage during normal operation also increases, so there is a drawback that the core nuclear characteristics such as increase in plutonium enrichment of fuel, increase in combustion reactivity, and decrease in breeding ratio may deteriorate. there were.

Due to the above-mentioned inadequacies in devising the reduction of the coolant density coefficient in the conventional core, the core is required to have an increased plutonium enrichment, an increased combustion reactivity and a reduced breeding ratio without increasing the core size. There is a demand for an invention of a core and a fuel capable of reducing the coolant density coefficient while suppressing the variation of nuclear characteristics as much as possible.

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

[0011]

[Means for Solving the Problems] A first means for achieving the above object is to provide a fuel assembly for a nuclear reactor having a blanket region for fuel breeding in upper and lower parts of a core fuel.
Of the multiple fuel rods that make up the fuel assembly, some fuel rods are removed downstream of the coolant to make them shorter than other fuel rods. Similarly, in the second means, the internal blanket fuel is a part of the fuel rod in the axial direction of the core fuel in the first means, and the position of the internal blanket fuel is more A lower part of the fuel assembly for a nuclear reactor, and the third means is the same as the first means, in the first means, the axial upper blanket of the fuel rods around the region where the fuel rods on the downstream side of the coolant of the short length fuel rods are removed. Is a fuel assembly for a nuclear reactor containing a neutron absorber in its part, and the fourth means is the same as the first means, in which the neutron absorber held by a support rod supported above the fuel assembly is arranged. At the same time, the support rod is used for other fuel such as fuel cladding. A fuel assembly for a nuclear reactor characterized in that the material has a thermal expansion larger than that of the constituent materials. The fifth means is the fourth means, in which the fuel loaded in the fuel assembly is a metal fuel, The fuel is a fuel assembly for a nuclear reactor in which a metal blanket fuel is loaded. Similarly, the sixth means is the first means, which has blanket regions for fuel breeding in the upper and lower portions of the core fuel, and the upper blanket thereof. A fuel assembly for a nuclear reactor, characterized in that a communication hole for a coolant is formed in a trumpet pipe adjacent to a fuel.

[0012]

In the first 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 the coolant flow path, so that one side of the fuel downstream side of the coolant is The volume ratio of the coolant is increased, and at the time of the density decrease due to the temperature rise of the coolant, neutrons are leaked from the coolant volume increase unit,
The effect of reducing the coolant density coefficient is obtained.

In the second means, in addition to the effect of the first means, the neutron flux distribution is enhanced above the core by the effect of the internal blanket fuel arranged in a part of the core fuel of the fuel rod, and When the density decreases due to the temperature rise, neutrons are leaked from the coolant volume ratio increasing portion, and the effect of further reducing the coolant density coefficient as compared with the case of the first means can be obtained.

In addition to the function of the first means, the third means utilizes the fact that neutrons in the coolant volume fraction increasing portion increase due to leakage when the density of the coolant decreases due to temperature rise. By including a neutron absorber around the volumetric ratio increasing part, when the density decreases due to the temperature rise of the coolant, the neutron absorber is made to absorb the neutrons flowing into the coolant volumetric ratio increasing part. The effect of further reducing is obtained as compared with the case of the first method.

According to the fourth means, in addition to the operation of the first means, neutrons in the coolant volume fraction increasing portion increase due to leakage when the density of the coolant decreases due to temperature rise, as in the third means. By including a neutron absorber in the coolant volume ratio increasing part, the absorption of neutrons is increased due to the density decrease due to the temperature rise of the coolant, and the neutron absorber is supported on the fuel upper part when the temperature rises of the coolant. Due to the thermal expansion of the absorber rod containing the neutron absorber, the neutron absorber is moved closer to the center side in the axial direction of the core, so that the absorption of neutrons is increased and the coolant density coefficient is reduced.

In the fifth means, in addition to the function of the fourth means, the fuel loaded in the fuel assembly is a metallic fuel,
The gas plenum of the fuel is the upper part of the fuel, and the support part of the absorber rod containing the neutron absorber can be made long, and the absorber bar of the absorber rod containing the neutron absorber supported on the upper part of the fuel when the temperature of the coolant rises. Since the thermal expansion is increased and the neutron absorber is moved to the center side in the axial direction of the reactor core, the absorption of neutrons is increased and the coolant density coefficient is reduced.

In the sixth means, in addition to the operation of the first means, a communication hole for the coolant is formed in the trumpet tube adjacent to the upper blanket fuel, so that when the density of the coolant decreases due to the temperature rise of the coolant, the trumpet tube By flowing the temperature-increased coolant to the outside of the, the neutrons are leaked in the axial direction outside the trumpet tube, and the effect of reducing the coolant density coefficient is obtained.

[0018]

Embodiments of the present invention will be described in detail below with reference to FIGS. 1 to 8.

FIG. 1 is a vertical sectional view of a fuel assembly showing an embodiment of a core fuel assembly of the present invention. 2 is shown in FIG.
2 is a radial cross-sectional view of the core fuel assembly of FIG.

The core fuel assembly of FIGS. 1 and 2 has 271 fuel rods, of which 234 in the peripheral portion are long fuel rods 1 and 37 in the central portion are short fuel rods. It is 10. Around the fuel rod, there are a trumpet tube 2, an upper shield 3, a lower shield 4 and an entrance nozzle 5 which is a coolant inlet. 1 and 2
Although not shown in Fig. 1, the distance between the fuel rods is held by a wire spirally wound around the fuel rod, which is conventionally called a wire spacer in fuel. Further, inside the fuel rod, 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 deteriorated uranium oxide are housed above and below the core fuel pellet 6. Further, a gas plenum 8 for storing gas generated from the fuel is provided under the fuel rod, and upper and lower ends of the fuel rod are hermetically sealed by welding end plugs 9.

As shown in FIGS. 1 and 2, the 37 fuel rods in the central portion of the fuel are shorter than the other fuel rods in this embodiment,
In the upper part of the core, the volume fraction of sodium coolant is increasing. The dimensions of the embodiment of FIGS. 1 and 2 are described below.

The fuel rod 1 has a diameter of 8.0 mm, and the trumpet tube 2 has inner and outer diameters of 150 mm and 158.1 mm, respectively. The core height is 1 m, and there are 350 mm axial blankets above and below it. The total length of 234 fuel rods 1 excluding 37 in the center of the fuel is 3 m,
The book is a short fuel rod 10 in which the axial upper part is removed and an end plug is welded in the middle of the core. The length of the short fuel rod 10 is 2.4 m 2 in this embodiment. Also, the fuel rod 1 and the short fuel rod 1
Reference numeral 0 denotes the lower shield 4 which is fixed in the axial direction, and has a structure in which the thermal expansion is performed freely upward in the axial direction.
Since the displacement of the fuel rods outside the 37 short fuel rods 10 in the center of the fuel to the center of the fuel is a problem, as shown in FIG. 1, the outer 234 fuel rods of the center 37 fuel rods are axial. The structure is such that the thermal expansion is allowed to proceed upward in the direction, but its radial position is fixed by a hexagonal fuel radial fixed tube 11 extending downward from the upper shield.

Although 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 present embodiment, 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 of enabling the reduction of the coolant density coefficient in the embodiment of the present invention 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.
As the temperature of the coolant passing through the fuel assembly rises, the density of sodium, which is the 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 of fuel such as plutonium due to fast neutrons increases. For uranium 238, fission increases even in the core when the coolant temperature rises, but fast neutrons in the blanket part outside the core increase relatively,
The fission increases significantly in the axial and radial blanket parts. In addition, the increase in neutron energy due to the decrease in sodium density due to the increase in coolant temperature also has the effect of increasing the leakage to the outside of the core, which brings about a negative reactivity effect. In the case of a large core having an electric power of about 300,000 KW or more, the effect of increasing fission due to fast neutrons was more dominant than the effect of leaking neutrons, 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 leakage effect of neutrons can be relatively increased when the coolant temperature rises, the coolant density coefficient can be shifted to the negative side. Further, if the leakage effect can be reduced at the coolant temperature at the rated output and the leakage effect can be increased when the output is further increased, it is possible to shift the coolant density coefficient to the negative side and suppress unnecessary leakage of neutrons. .

Therefore, in the present invention, paying attention to the fact that the coolant at the rated output also has the effect of shielding neutrons, and utilizing the fact that the density of the coolant decreases when the temperature rises, the coolant flow downstream of the core is used. A region containing only the coolant is provided on the side from the middle of the core so that the leakage in the axial upward direction at the rated output is small and the leakage in the axial upward direction is increased when the output from the rated is increased due to the decrease in the density of the coolant. As a result, the coolant density coefficient can be reduced.

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

Table 1 shows the coolant density coefficient of the core portion in the case of constructing a 1,000,000 kW electric power core using the fuel of the embodiment of the present invention in comparison with the conventional core. In the core, 421 core fuel assemblies and 78 radial blanket fuels are arranged.

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. Further, since the shaft blanket was slightly removed, the influence on the breeding ratio was confirmed, but as shown in Table 1, the influence was slight, and it was confirmed that the breeder reactor was sufficiently established.

Further, in the embodiment of the present invention, although the inner diameter of the trumpet tube and the number of fuel rods are the same as those of the conventional core example, the outer diameter of the fuel rod is increased from 7.6 mm to 8.0 mm. In addition, the core pressure loss can be set to be equal to or lower than that of the conventional core. This is because the flow path of the coolant is increased on the downstream side of the short fuel, so the pressure loss at that portion is reduced, and as a result, under the condition of equivalent pressure loss, the diameter of the fuel rod can be increased. . As a result, since the fuel volume was increased in the core, the breeding ratio could be hardly changed even though the upper blanket was slightly reduced.

In this embodiment, the number of short fuel rods in all the fuel assemblies of the core is 37. However, as a modification of this embodiment of the homogeneous core in two radial regions, the output of the outer core toward the center of the core is By increasing the number of short fuel rods in a large fuel assembly, 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, the coolant density coefficient in Table 1 can be reduced to about 0.4% Δk / k / Δρ / ρ.

Further, as a modification of this embodiment, instead of using a part of the fuel assembly as a short fuel rod, all the fuel rods in some of the fuel assemblies are short fuel rods, and a sodium region is provided above the fuel rods. May be provided, and other fuel assemblies may be ordinary long fuel rods. In this case, it is difficult to increase the diameter of a normal long fuel rod, but it is possible to reduce the coolant density coefficient with a simple structure.

[0035]

[Table 1]

FIG. 3 is a vertical 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. 3 is the same as the core fuel assembly shown in FIG. 1, the fuel rods, the wire spacers, the trumpet tubes that hold the periphery of the fuel rods, the upper and lower shield portions, and the entrance nozzle for the coolant. It is composed of parts. The fuel arrangement in the fuel rod is changed from that in FIG.

Also in this embodiment, the 37 fuel rods in the central portion of the fuel are shorter than the other fuel rods, and the volume ratio of the sodium coolant increases 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 portion of the other long fuel rods is plutonium mixed oxide fuel, but a part of the axial direction is depleted uranium oxide. It has an internal blanket fuel.

The dimensions of the embodiment of FIG. 3 are described below.

The diameter of the fuel rod, the inner and outer diameters of the trumpet 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 central 37 short fuel rods are the same as the fuel of FIG.

The method of radially fixing the outer long fuel rods of the 37 short fuel rods in the center of the fuel is also the same as that of the fuel of FIG.

The principle of enabling the reduction of the coolant density coefficient in this embodiment is the same as in the case of FIG.

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

Table 2 shows the coolant density coefficient in the case of constructing a 1,000,000 kW electric power core using the example fuel of the present invention in comparison with the conventional core. In the core, 421 core fuel assemblies and 78 radial blanket fuels are arranged.

As a result, as shown in Table 2, in the core using the fuel assembly shown in FIG. 3, the effect is greater than in the case of the conventional fast reactor core, and the coolant density coefficient is about 82. It turned out that it can be reduced by%. In the present embodiment, the effect of reducing the coolant density coefficient is larger than that in the embodiment of Table 1 because the core is an axially inhomogeneous core and the inner blanket of the core is below the center of the core in the axial direction. This is due to the fact that the neutron leakage in the axial upper direction when the density of the coolant decreases can be increased.

[0046]

[Table 2]

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

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

The core fuel assembly shown in FIG. 4 is the same fuel rod as the core fuel assembly shown in FIG. 1, a wire spacer and a trumpet tube for holding the wire spacer, the upper and lower shield portions, and an entrance nozzle which is a coolant inlet portion. It is composed of parts. The fuel arrangement in the fuel rod is changed from that in FIG. Further, in this embodiment, the neutron absorber is arranged on the upper shield part of the short fuel and on the uppermost part of the fuel rod adjacent to the fuel radial fixed pipe on the upper part of the short fuel. In the upper shield part, the neutron absorber contains the neutron absorber in a circular tube,
The neutron absorber is a boron carbide pellet. Further, the neutron absorber at the top of the fuel rod is also a pellet of boron carbide and is arranged above the fuel pellet.

In this embodiment, the 61 fuel rods in the central portion of the fuel are shorter than the other fuel rods, and the volume ratio of sodium coolant increases 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 are described below.

The diameter of the fuel rods, the inner and outer diameters of the trumpet tubes, the core height, and the dimensions of the axial blanket are the same as those of the fuel shown in FIG. Further, the lengths of 210 long fuel rods and 61 central short fuel rods are the same as the fuel of FIG.

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

The principle of enabling the reduction of the coolant density coefficient in this embodiment is the same as that in the case of FIG. 1, but in this embodiment,
Along with increasing neutron leakage to the upper axial direction when reducing the density of the coolant, neutrons flowing in the upper axial direction are positively absorbed by the neutron absorber placed in that portion, thereby making the coolant more effective. Reduction of the density coefficient can be performed.

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

Table 3 shows the coolant density coefficient in the case of constructing a 1,000,000 kW electric power core using the example fuel of the present invention in comparison with the conventional core. In the core, 421 core fuel assemblies and 78 radial blanket fuels are arranged.

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. .

[0058]

[Table 3]

FIG. 6 is a vertical 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 is the same fuel rod as the core fuel assembly shown in FIG. 1, a wire spacer, a trumpet tube for holding the wire spacer, the upper and lower shield portions, and an entrance nozzle for the coolant inlet. It is composed of parts. The structure of the upper part of the fuel assembly is changed from that of FIG. 1, and in this embodiment, the neutron absorber fixed to the upper shield by the support rod is arranged above the short fuel. The neutron absorber fixed by the support rod is a pellet of boron carbide held in a circular tube. The supporting rod is made of metal having a large coefficient of thermal expansion as compared with other trumpet pipes and fuel cladding pipes. Regarding the material of each part of the fuel assembly of this embodiment, the core fuel pellets 6 are uranium-plutonium mixed oxide fuels, and the blanket fuel pellets 7 are depleted uranium oxides. Further, the fuel rod 1, the trumpet tube 2, and the short length fuel rod 10 are ferritic stainless steel, and the support rod 13 is austenitic stainless steel. The core outlet sodium temperature of the fast breeder reactor is about 500 ° C., but 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 /
℃.

In this embodiment, the 61 fuel rods in the central portion of the fuel are shorter than the other fuel rods, and the volume ratio of the sodium coolant increases in the upper part of the core.

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

The diameter of the fuel rods, the inner and outer diameters of the trumpet tubes, the core height, and the dimensions of the axial blanket are the same as those of the fuel shown in FIG. Further, the lengths of 210 long fuel rods and 61 central short fuel rods are the same as the fuel of FIG.

The method of radially fixing the outer long fuel rods outside the 61 short fuel rods in the center of the fuel is also the same as that of the fuel of FIG.

The principle of enabling the reduction of the coolant density coefficient in this embodiment is the same as that in the case of FIG. 1, but in this embodiment, the neutron leakage to the axial upper part at the time of the coolant density reduction is increased. In addition, by positively absorbing the neutrons flowing in the axially upper portion by the neutron absorber arranged in that portion, the coolant density coefficient can be reduced more effectively.

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, so that the control rod is inserted. It is equivalent to that, in addition to the above effects,
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 embodiments of the present invention will be summarized below.

Table 4 shows the temperature coefficient of the coolant in the core portion in the case of constructing a 1,000,000 kW electric power core using the fuel of the example of the present invention, in comparison with the conventional core. In the core, 421 core fuel assemblies and 78 radial blanket fuels are arranged.

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.

[0070]

[Table 4]

FIG. 7 is a vertical 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 in the peripheral portion are long fuel rods 1 and 61 in the central portion are short fuel rods 10. . Also,
Around the fuel rod, there are a trumpet tube 2, an upper shield 3, a lower shield 4 and an entrance nozzle 5 which is a coolant inlet. Although not shown in FIG. 7, the distance between the fuel rods is held by the wire spirally wound around the fuel rods. Further, a metal core fuel 14, which is an alloy of uranium, plutonium and zirconium, is housed inside the fuel rod. In addition, the metal core fuel 14
A metal blanket fuel 15, which is an alloy of uranium and zirconium, is stored in the upper and lower parts of the upper part. Further, inside the fuel rod, sodium is filled between the metallic fuel and the cladding tube. A gas plenum 8 for storing gas generated from the fuel is provided on the upper portion of the fuel rod, and upper and lower ends of the fuel rod are hermetically sealed by welding end plugs 9.

In this embodiment, similarly to the embodiment shown in FIG. 6, the neutron absorber fixed to the upper shield by the support rod is arranged above the short fuel. The neutron absorber fixed by the support rod is a pellet of boron carbide held in a circular tube. The supporting rod is made of metal having a large coefficient of thermal expansion as compared with other trumpet pipes and fuel cladding pipes. The difference between the embodiment of FIG. 6 and this embodiment is that, since this embodiment uses a metal fuel as the fuel, the gas plenum of the fuel is in the upper part of the core and the length of the support rod of the neutron absorber fixed to the upper shield is long. It is possible to increase the length and to use a metal blanket fuel as the fuel in the short length fuel.

In this embodiment, the 61 fuel rods in the center of the fuel are shorter than the other fuel rods, and the volume ratio of the sodium coolant increases in the upper part of the core.

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

The fuel rod 1 has a diameter of 7.1 mm, and the trumpet tube 2 has inner and outer diameters of 150 mm and 158.1 mm, respectively. The core height is 0.6m. Also, no axial blanket is installed. 27 excluding 61 in the center of fuel
The total length of 0 fuel rods 1 is 2.0m, but the center 61
The book is a short fuel rod 10 in which the axial upper part is removed 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. Regarding the material of each part of the fuel assembly of this embodiment, the core fuel pellets 6 are metallic fuels of uranium, plutonium and zirconium alloys, and the fuel rods 1, trumpet tubes 2 and short fuel rods 10 are ferritic stainless steels. Yes, the support bar 13 is austenitic stainless steel.

Further, the fuel rods 1 and the short fuel rods 10 are fixed in the axial direction at the lower shield 4, and the structure is such that they can be thermally expanded toward the axially upper part. 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 outer 270 fuel rods of the 61 fuel rods at the center are axial as shown in FIG. The structure is such that the thermal expansion is allowed to proceed upward in the direction, but its radial position is fixed by a hexagonal fuel radial fixed tube 11 extending downward from the upper shield.

The principle of enabling the reduction of the coolant density coefficient in this embodiment is the same as that in the case of FIG. 1, but in this embodiment,
Along with increasing neutron leakage to the upper axial direction when reducing the density of the coolant, neutrons flowing in the upper axial direction are positively absorbed by the neutron absorber placed in that portion, thereby making the coolant more effective. Reduction of the density coefficient can be performed.

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

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

Table 5 shows the coolant density coefficient in the case of constructing a 1 million kW electric power core using the example fuel of the present invention in comparison with the conventional metal fuel core. In the core, 421 core fuel assemblies and 78 radial blanket fuels are arranged.

As a result, as shown in Table 5, in the core using the fuel assembly shown in FIG. 7, the coolant density coefficient can be reduced by about 94% as compared with the conventional fast reactor metal fuel core. Further, in the present embodiment, since the pressure loss of the coolant in the gas plenum portion can be greatly reduced, the fuel pin diameter can be greatly increased with the same pressure loss. In contrast to the outer diameter of the conventional fuel of 6.4 mm, this embodiment has an outer diameter of 7.1 mm. As a result, a large amount of uranium 238, which is the fuel parent substance, can be loaded in 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.

[0083]

[Table 5]

FIG. 8 is a vertical cross-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 is the same fuel rod as that of the core fuel assembly shown in FIG. 1, a wire spacer, a trumpet tube for holding the wire spacer, the upper and lower shield portions, and an entrance nozzle serving as a coolant inlet portion. It is composed of parts. Further, the use of the short fuel is similar to that of FIG. 1, but this embodiment is different from that of FIG. 1 in that a communication hole for the coolant is further provided in the trumpet pipe adjacent to the upper blanket in the axial direction. Further, the thickness of the portion where the communication hole is opened is increased in order to secure the strength of the trumpet tube.

The dimensions of the embodiment of FIG. 8 are described below.

The diameter of the fuel rods, the inner and outer diameters of the trumpet tubes, 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.

The principle of enabling the reduction of the coolant density coefficient in this embodiment is the same as that in the case of FIG. 1, but in this embodiment, the increase in the axial leakage of neutrons is caused by the trumpet tube when the coolant density is further reduced. The effect of decreasing the sodium density between the trumpet tubes by the communication hole of the opened coolant can be enhanced.

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

Table 6 shows the coolant density coefficient in the case of constructing a 1,000,000 kW electric power core using the example fuel of the present invention in comparison with the conventional core. In the core, 421 core fuel assemblies and 78 radial blanket fuels are arranged.

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

In this embodiment, short fuel is used and
The coolant density coefficient can be greatly reduced by a simple method of forming a communication hole for the coolant in the trumpet pipe adjacent to the upper axial blanket.

[0093]

[Table 6]

[0094]

According to the first aspect of the present invention, for some or all of the multiple fuel assemblies arranged in the core, some of the fuel rods constituting the fuel assembly are made shorter than the other fuel rods, By using the downstream side as the coolant flow path, a part of the fuel downstream side of the coolant increases the volume ratio of the coolant, and when the density decreases due to the temperature increase of the coolant, the neutrons from the coolant volume ratio increasing part Is obtained, and the effect that the coolant density coefficient can be reduced can be obtained.

According to the invention of claim 2, in addition to the effect of the invention of claim 1, the neutron flux distribution can be enhanced in the upper part of the core by the effect of the internal blanket fuel arranged in a part of the core fuel of the fuel rod. When the density of the coolant decreases due to the temperature rise, neutrons are leaked from the coolant volume ratio increasing portion, and the coolant density coefficient can be further reduced as compared with the case of claim 1.

According to the invention of claim 3, in addition to the effect of the invention of claim 1, the fact that neutrons in the coolant volume fraction increasing portion increase due to leakage when the density decreases due to the temperature rise of the coolant is utilized. , By containing a neutron absorber around the coolant volume ratio increasing portion, when the density decreases due to the temperature increase of the coolant, the neutron absorber to absorb neutrons flowing into the coolant volume ratio increasing portion, the coolant The density coefficient can be further reduced as compared with the case of claim 1.

According to the invention of claim 4, in addition to the effect of the invention of claim 1, the neutron absorber is produced by thermal expansion of the absorber rod containing the neutron absorber supported above the fuel when the temperature of the coolant rises. Is moved toward the center side in the core axis direction, absorption of neutrons is increased, and the coolant density coefficient can be further reduced as compared with the case of claim 1.

According to the invention of claim 5, in addition to the effect of the invention of claim 4, the fuel loaded into the fuel assembly is a metallic fuel, the gas plenum of the fuel is the upper part of the fuel, and the absorption containing the neutron absorber is performed. The support part of the body rod can be made long, and the thermal expansion of the absorber rod containing the neutron absorber supported above the fuel increases when the temperature of the coolant rises, and the neutron absorber is more centered in the core axial direction. Since it is moved to the side, the absorption of neutrons can be increased, and the coolant density coefficient can be further reduced as compared with the case of claim 4.

According to the sixth aspect of the present invention, in addition to the effect of the first aspect of the invention, the density of the density of the coolant decreases due to the temperature rise of the coolant by forming the communication hole of the coolant in the trumpet pipe adjacent to the upper blanket fuel. Sometimes, by flowing the coolant whose temperature has risen to the outside of the trumpet tube, neutrons are also leaked in the axial direction outside the trumpet tube, and the coolant density coefficient can be further reduced as compared with the case of claim 1.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a core fuel assembly according to an embodiment of the present invention.

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 another embodiment of the present invention.

FIG. 4 is a vertical cross-sectional view of a core fuel assembly according to still another embodiment of the present invention.

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

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

[Explanation of symbols]

1 ... Fuel rod, 2 ... Trumpet pipe, 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 rods, 11 ... Fuel radial fixed pipes, 12 ... Neutron absorbers, 13 ... Support rods, 14 ... Coolant communication holes.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kunikazu Kanato             3-1-1 Sachimachi, Hitachi City, Ibaraki Prefecture Stock Association             Hitachi, Ltd.Hitachi factory (72) Inventor, Watanabe             3-1-1 Sachimachi, Hitachi City, Ibaraki Prefecture Stock Association             Hitachi, Ltd.Hitachi factory

Claims (6)

[Claims] 1. A fuel assembly for a nuclear reactor having blanket regions for fuel breeding above and below a core fuel, wherein some of the fuel rods constituting the fuel assembly are replaced by other fuel rods. A fuel assembly for a nuclear reactor, characterized in that the length of the fuel rod is shorter than that of the fuel rod, and the upper portion of the short fuel rod serves as a coolant passage to the end portions of other long fuel rods. 2. The fuel assembly for a nuclear reactor according to claim 1, wherein
A core fuel of a fuel rod is a fuel assembly for a nuclear reactor, characterized in that a part of an axial direction thereof is an internal blanket fuel, and the position of the internal blanket fuel is below a center of the core axial direction. 3. The fuel assembly for a nuclear reactor according to claim 1,
A fuel assembly for a nuclear reactor, comprising a neutron absorber in an axially upper blanket part of the fuel rod around a region where the fuel rod on the downstream side of the coolant of the short fuel rod is removed. 4. The fuel assembly for a nuclear reactor according to claim 1, wherein
A neutron absorber held by a support rod supported above the fuel assembly is arranged, and the support rod is made of a material having a larger thermal expansion than other fuel constituent materials such as a fuel cladding tube. Fuel assembly. 5. The fuel assembly for a nuclear reactor according to claim 4,
A fuel assembly for a nuclear reactor, wherein the fuel loaded into the fuel assembly is a metal fuel, and the short length fuel is a metal blanket fuel. 6. The fuel assembly for a nuclear reactor according to claim 1, wherein
A fuel assembly for a nuclear reactor, comprising: a blanket region for fuel breeding above and below a core fuel; and a communication hole for a coolant formed in a trumpet tube adjacent to the upper blanket fuel.