JPH05297170A - Fuel assembly for fast breeder - Google Patents

Abstract

PURPOSE:To provide a fuel aggregate for fast breeder capable of stably continuing the reducing effect of coolant density coefficient to the last stage of combustion in a fast breeder. CONSTITUTION:The core of a fast breeder is formed, from the center to the outside, in the order of a core inside fuel aggregate areal, a core outside fuel assembly region 2, and a radial blanket fuel region 5. The core inside fuel assembly region 1 is formed, from the upper part, of an upper blanket fuel area 3, a sodium layer 6, a high plutonium enriched fuel region 8, a low plutonium enriched fuel region 9, and a lower blanket fuel region 4. The core outside fuel assembly area 2 is almost the same as the core inside fuel assembly region 1, and differed in the point where the inside blanket fuel area 7 in the core inside fuel assembly region 1 is changed to the low plutonium enriched fuel region 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a fast breeder reactor.

[0002]

2. Description of the Related Art Conventionally, the core of a fast breeder reactor is composed of a large number of fuel assemblies each having a long and hexagonal cross section, and uses plutonium mixed oxide as a fuel.

A large number of fuel pellets loaded with fuel in a small diameter cylindrical fuel pellet are stacked in the axial direction and housed in a stainless cladding tube to form a small diameter rod-shaped fuel rod. Further, the fuel assembly has a structure in which a large number of fuel rods are arranged in a trumpet tube and a wire is wound around the fuel rods so that the coolant flows through the gaps between the fuel rods.

The fuel assembly is divided into a core fuel region and a radial blanket fuel region. Generally, the core fuel region is loaded with plutonium mixed oxide, and the radial blanket fuel region is grown with plutonium fuel. Loaded with depleted uranium oxide.

That is, generally, in a conventional fuel assembly for a fast breeder reactor, a large number of fuel rods of the same length are arranged in a trumpet tube having a hexagonal cross section, and a stack length of 1 m is contained in the fuel rod.
Of core fuel pellets and blanket fuel pellets having an overlapping length of about 0.3 m are arranged above and below the core fuel pellets, and the space above and below is secured by springs or sleeves. A gas plenum is provided for accumulating the generated gas.

In the fast breeder reactor, when the conventional fuel assembly as described above is used, the power coefficient of the core always shows a negative value. However, in order to further improve the self-controllability of the core, it is desired to shift the power coefficient of the core to the negative side.

The power coefficient of the core is composed of a fuel Doppler coefficient, a coolant density coefficient, and the like.
In conventional fast breeder reactors, the Doppler coefficient is negative and the coolant density coefficient is slightly positive.

Therefore, if the coolant density coefficient is made negative and further shifted to the negative side, the power coefficient of the core shifts to the larger negative side, and the power control and self-controllability of the core are further improved, so that the cooling is improved. There is a demand for an invention in which the material density coefficient is made negative and is further shifted to the negative side.

In the fast breeder reactor, when a conventional fuel assembly is used, the coolant density coefficient is slightly positive, but the sodium content of the coolant as the coolant passing through the fuel assembly rises. Since the density decreases, the mass decreases, and the moderating effect on fast neutrons decreases, the energy of fast neutrons increases and uranium 23
This is because nuclear fission due to fast neutrons in fuels such as 8 and plutonium increases.

Further, due to the decrease in the density of sodium, a fast reactivity neutron is largely leaked to the outside of the fuel, and a negative reactivity effect is also produced.

Further, in the case of a large core having an electric output of about 300,000 kW or more, since the effect of increasing fission by fast neutrons is more dominant than the effect of fast neutron leakage, the coolant density coefficient becomes positive. There is a need for an invention of a fuel assembly that brings a coolant density coefficient close to zero.

That is, in the conventional fast breeder reactor, since the coolant density coefficient is positive, the coolant density coefficient is reduced,
Further research has been conducted to make it negative.

For example, if the core height is made low and the leakage of fast neutrons can be increased when the density of sodium decreases when the temperature of sodium rises, the coolant density coefficient can be reduced.

However, if the core height and the core diameter are reduced while keeping the core power and core diameter as they are, the volume of the core decreases and the power density and the fuel line output in the core both increase.

That is, in order to lower the core height, it is necessary to increase the core diameter, and there is a drawback that the core construction cost increases if the core diameter is increased.

As described above, since the method of changing the core size has drawbacks, the conventional example in which the coolant density coefficient is made negative without changing the core size is the PHYSICAL GROUNDS FOR FUR.
THERIMPROVEMENT OF FAST SODIUM POWER REACTOR SAFE
TY (1990 International Fast Reactor Safety Meeting
Volume II 25).

The feature of this example is that a sodium layer is provided between the upper blanket fuel region and the core fuel region in the fuel assembly for a fast breeder reactor, and when the sodium density decreases when the temperature of sodium rises. In, in order to increase the leakage of fast neutrons from the sodium layer, to reduce the coolant density coefficient.

This conventional example will be described with reference to FIGS. FIG. 4 is a schematic vertical cross-sectional view of the core region of the conventional example, FIG. 5 is an explanatory view of the core inner fuel assembly of FIG. 4, and FIG. 6 is an explanatory view of the core outer fuel assembly of FIG.

5, (a) of FIG. 5 is a schematic longitudinal sectional view of the fuel assembly inside the core, (b) of FIG. 5 is a schematic vertical sectional view of the upper fuel rod in the fuel assembly inside the core, and (b) of FIG. (C) is a schematic longitudinal sectional view of a lower fuel rod in a fuel assembly inside a core.

Further, in FIG. 6, (a) of FIG. 6 is a schematic vertical cross-sectional view of the core outer fuel assembly, and (b) of FIG. 6 is a schematic vertical cross-sectional view of the upper fuel rod in the core outer fuel assembly. 6
(C) is a schematic longitudinal sectional view of a lower fuel rod in the fuel assembly outside the core.

Further, in the above figures, 1 is a core inner fuel assembly region, 2 is a core outer fuel assembly region, 3 is an upper blanket fuel region, 4 is a lower blanket fuel region, 5 is a radial blanket fuel region, 6 is a sodium layer, 7 is an inner blanket fuel region, 10 is an upper fuel rod, 11 is a lower fuel rod, 12 is a trumpet tube, 13 is an upper shield, 14 is a lower shield, 15 is a handling head,
16 is an entrance nozzle, 17 is an upper blanket fuel pellet, 18 is an upper end plug, 19 is a lower end plug, 22 is an inner blanket fuel pellet, 23 is a lower blanket fuel pellet, 24 is a gas plenum, 25 is a core fuel region, 26 is 3 shows a core fuel pellet.

In FIG. 4, in the core, a core inner fuel assembly region 1 is formed in the central portion, a core outer fuel assembly region 2 is formed outside thereof, and a radial blanket fuel region 5 is formed further outside thereof. There is.

In the core inner fuel assembly region 1, an upper blanket fuel region 3, a sodium layer 6, a core fuel region 25, an inner blanket fuel region 7, a core fuel region 25, and a lower blanket fuel region 4 are formed from above. There is.

The outer core fuel assembly region 2 has an upper blanket fuel region 3, a sodium layer 6, a core fuel region 25, and a lower blanket fuel region 4 formed from above. The specific points of FIG. 4 are as shown in FIGS. 5 and 6, but detailed description thereof will be omitted.

That is, by providing the sodium layer 6, it is possible to increase the volume ratio of sodium on the downstream side of sodium in the fuel assembly, and to increase the amount of fast neutrons from the sodium layer 6 when the density of sodium decreases when the output increases. It leaks and reduces the coolant density coefficient.

[0026]

As described above, if the sodium layer is provided between the upper blanket fuel region and the core fuel region, the coolant density coefficient can be reduced. However, it cannot be said to be sufficient yet, and an invention that further reduces the coolant density coefficient and changes it to a negative value is desired.

Further, as a measure for improving the above-mentioned method, a method has been proposed in which a part of the core fuel region is used as an internal blanket fuel region. Although this method significantly reduces the coolant density coefficient, in this case, because of the homogeneous core, plutonium accumulates in the internal blanket fuel region as combustion progresses. Therefore, there is a risk that the neutron flux in the internal blanket fuel region increases, the core upper power decreases, and the effect of reducing the coolant density coefficient does not continue until the end of combustion.

An object of the present invention is to provide a fuel assembly in which a stable reduction effect of the coolant density coefficient until the end of combustion is sustainable without changing the core size of the fast breeder reactor.

[0029]

The above object can be achieved as follows.

(1) In a fuel assembly for a fast breeder reactor, which has a sodium layer between the upper blanket fuel region and the core fuel region, and a part of the core fuel region is the inner blanket fuel region, Fuel pellets, which are divided into an upper core fuel region and a lower core fuel region in two directions in the vertical direction and are loaded with fuel of high plutonium enrichment in fuel rods in the upper core fuel region, are divided into fuel rods in the lower core fuel region. Fuel pellets loaded with low-plutonium-rich fuel are stored in each.

(2) In the fuel assembly for a fast breeder reactor, which has a sodium layer between the upper blanket fuel region and the core fuel region, and a part of the core fuel region is the inner blanket fuel region, It is divided into an upper core fuel region and a lower core fuel region in two vertical directions, and the plutonium enrichment of fuel in each fuel rod in the upper core fuel region and the lower core fuel region is the same, and the fuel rods in the upper core fuel region are divided. Solid fuel pellets and hollow fuel pellets in the fuel rods in the lower core fuel region are stored in the fuel rods in the upper core fuel region and the lower core fuel region, respectively. Plutonium-rich fuels have different amounts of fuel.

(3) In the fuel assembly for a fast breeder reactor, which has a sodium layer between the upper blanket fuel region and the core fuel region, and a part of the core fuel region is the inner blanket fuel region, It is divided into an upper core fuel region and a lower core fuel region in two vertical directions, and the plutonium enrichment of fuel in each fuel rod in the upper core fuel region and the lower core fuel region is the same, and the fuel rods in the upper core fuel region are divided. Fuel pellets loaded with plutonium-enriched fuel at a high density and fuel pellets loaded with plutonium-enriched fuel at a low density on fuel rods in the lower core fuel region are stored in the fuel cell.

(4) In the fuel assembly for a fast breeder reactor, which has a sodium layer between the upper blanket fuel region and the core fuel region, and a part of the core fuel region is the inner blanket fuel region, the core fuel region is Solid fuel pellets, which are divided into an upper core fuel region and a lower core fuel region in two directions in the vertical direction and are loaded with a high plutonium-enriched fuel in fuel rods in the upper core fuel region, are separated from fuel rods in the lower core fuel region. Hollow fuel pellets loaded with low-plutonium-enriched fuel are stored in each.

(5) In the fuel assembly for a fast breeder reactor, which has a sodium layer between the upper blanket fuel region and the core fuel region, and a part of the core fuel region is the inner blanket fuel region, Fuel pellets, which are divided into an upper core fuel region and a lower core fuel region in two directions in the up and down direction, and in which the fuel rods in the upper core fuel region are densely loaded with a high plutonium-rich fuel, Fuel pellets loaded with low-density plutonium-rich fuel at low density are stored in each.

(6) In the fuel assembly for a fast breeder reactor, which has a sodium layer between the upper blanket fuel region and the core fuel region, and a part of the core fuel region is the inner blanket fuel region, The fuel is divided into the upper core fuel region and the lower core fuel region in two directions in the vertical direction, and the plutonium enrichment of each fuel in the upper core fuel region and the lower core fuel region is the same. The solid fuel pellets loaded with the plutonium-enriched fuel in 1) and the hollow fuel pellets loaded with the low-density plutonium-enriched fuel in the fuel rods in the lower core fuel region are stored.

(7) In the fuel assembly for a fast breeder reactor, which has a sodium layer between the upper blanket fuel region and the core fuel region, and a part of the core fuel region is the inner blanket fuel region, The upper core fuel region and the lower core fuel region are divided into two parts in the two upper and lower directions, and solid fuel pellets loaded with high density high plutonium-rich fuel on the fuel rods of the upper core fuel region are replaced with the lower core fuel region. Hollow fuel pellets loaded with low-density low-plutonium-enriched fuel are stored in each fuel rod.

[0037]

The operation of each of the above-mentioned first to seventh means is as follows.

In each of the above means, a fuel assembly for a fast breeder reactor having a sodium layer between the upper blanket fuel region and the core fuel region, and a part of the core fuel region comprising the inner blanket fuel region is provided. This is also the case when the core fuel region is divided into an upper core fuel region and a lower core fuel region in two vertical directions.

In the first means, fuel rods in the upper core fuel region are loaded with fuel pellets having a high plutonium enrichment, and fuel rods in the lower core fuel region are loaded with fuel having a low plutonium enrichment. Each of the fuel pellets is stored.

Therefore, during combustion, plutonium may accumulate in the inner blanket fuel region, increasing the neutron flux in the inner blanket fuel region. However, since the plutonium enrichment in the upper and lower two regions has the above difference, the effect of increasing the neutron flux in the upper core fuel region is much larger than the effect of increasing the neutron flux in the inner blanket fuel region. , The stable reduction effect of the coolant density coefficient continues until the end of combustion.

In the second means, the plutonium enrichment of the fuel in each fuel rod in the upper core fuel region and the lower core fuel region is the same, and solid fuel pellets are placed in the fuel rods in the upper core fuel region and the lower core fuel region. Hollow fuel pellets are housed in the fuel rods in the region, and there is a difference in the amount of plutonium-enriched fuel loaded on each fuel pellet between the fuel rods in the upper core fuel region and the fuel rods in the lower core fuel region. It is provided.

Therefore, the neutron flux distribution is increased in the upper part of the core, and when the sodium density decreases due to the temperature increase of sodium at the time of power increase, more fast neutrons leak from the sodium layer, so that the coolant density coefficient decreases.

Further, by the same action as the first means, the stable effect of reducing the coolant density coefficient is maintained until the end of combustion.

In the third means, the fuel rods in the upper core fuel region and the lower core fuel region have the same fuel plutonium enrichment, and the fuel rods in the upper core fuel region are densely loaded with the plutonium enriched fuel. The fuel pellets prepared by loading the fuel rods in the lower core fuel region with plutonium-enriched fuel at a low density are housed.

Therefore, the operation of the third means is similar to that of the second means, and the stable effect of reducing the coolant density coefficient is maintained until the end of combustion.

In the fourth means, solid fuel pellets in which the fuel rods in the upper core fuel region are loaded with the high plutonium-enriched fuel and solid hollow fuel rods in which the fuel rods in the lower core fuel region are loaded with the low plutonium-enriched fuel are used. Each of the fuel pellets is stored.

Therefore, since the effects of the first and second means are synergized, the effect is further enhanced.

In the fifth means, fuel rods in the upper core fuel region are loaded with high-density plutonium-rich fuel at high density, and fuel rods in the lower core fuel region are loaded with low-plutonium-rich fuel in low density. Each loaded fuel pellet is stored.

Therefore, the effects of the first and third means are synergized, and the effect is further enhanced.

In the sixth means, the fuel in each of the fuel rods in the upper core fuel region and the lower core fuel region has the same plutonium enrichment, and the fuel rods in the upper core fuel region are loaded with the high density plutonium enriched fuel. Each of the solid fuel pellets is a hollow fuel pellet in which a fuel rod in the lower core fuel region is loaded with a low-density plutonium-rich fuel.

Therefore, the effects of the second and third means are synergized, and the effect is further enhanced.

In the seventh means, solid fuel pellets in which the fuel rods in the upper core fuel region are loaded with the high density and high plutonium enrichment fuel, the fuel rods in the lower core fuel region are enriched in the low density and low plutonium enrichment. Hollow fuel pellets loaded with fuel are stored respectively.

Therefore, the effects of the first, second and third means are synergized, and the effect is further enhanced.

[0054]

Embodiments of the present invention will be described below with reference to FIGS.

FIG. 1 is a schematic longitudinal sectional view of a core region of one embodiment of the present invention, FIG. 2 is an explanatory view of a fuel assembly in a core inner region of FIG. 1, and FIG. 3 is a fuel assembly of a core outer region of FIG. It is explanatory drawing of a body.

FIG. 2 (a) is a schematic vertical sectional view of the core inner fuel assembly, FIG. 2 (b) is a schematic vertical sectional view of the upper fuel rod in the core inner fuel assembly, and FIG. 2 (c) is FIG. 3 is a schematic vertical sectional view of a lower fuel rod in a fuel assembly inside a core.

Further, FIG. 3 (a) is a schematic vertical sectional view of the core outer fuel assembly, FIG. 2 (b) is a schematic vertical sectional view of the upper fuel rod in the core outer fuel assembly, and FIG. 2 (c). 8] is a schematic longitudinal sectional view of a lower fuel rod in a fuel assembly outside the core.

Further, in the above figure, 8 is a high plutonium rich fuel region, 9 is a low plutonium rich fuel region, 20 is a high plutonium rich fuel pellet, and 21 is a low plutonium rich fuel pellet. The other symbols are the same as those described above.

In FIG. 1, the core forms a core inner fuel assembly region 1 in the central portion, a core outer fuel assembly region 2 in the outer portion, and a radial blanket fuel region 5 in the outer portion. ..

On the other hand, in the vertical direction, the core inner fuel assembly region 1 is, from above, the upper blanket fuel region 3, the sodium layer 6, the high plutonium enriched fuel region 8, the inner blanket fuel region 7, and the low plutonium enriched region. A fuel region 9 and a lower blanket fuel region 4 are formed.

Further, in the outer core fuel assembly area 2, the inner blanket fuel area 7 in the inner core fuel assembly area 1 is the low plutonium enriched fuel area 9, and the other parts are in the inner core area. It is the same as the fuel assembly region 1.

As shown in FIG. 2, the fuel assembly in the inner core fuel assembly region 1 of FIG. 1 is divided into an upper fuel rod 10 and a lower fuel rod 11, and a sodium layer 6 is formed between both fuel rods. The sodium layer 6 is filled with sodium as a coolant. The number of the upper fuel rods 10 and the number of the lower fuel rods 11 in this embodiment are both 271.

The upper fuel rod 10, the sodium layer 6 and the lower fuel rod 11 are surrounded by the same trumpet tube 12,
An upper shield 13 is installed above the upper fuel rod 10 located on the outer peripheral side, and a lower shield 14 is installed below the lower fuel rod 10 located on the outer peripheral side. is there.

A handling head 15 is provided at the upper end of the upper shield 13, and an entrance nozzle 16 into which a coolant flows is provided below the lower shield 14. An upper blanket fuel pellet 17 is housed in the upper fuel rod 10, an upper end plug 18 is attached to the upper part of the upper fuel rod 17, and a lower end plug 19 is attached to the lower part thereof. It has a sealed structure.

Further, in the lower fuel rod 11, a high plutonium enriched fuel pellet 20, an inner blanket fuel pellet 22, a low plutonium enriched fuel pellet 21 and a lower blanket fuel pellet 23 are housed from above,
A gas plenum 24 is internally provided, an upper end plug 18 is attached to the upper part, and a lower end plug 19 is attached to the lower part. These end plugs are welded to each other, and the lower fuel rod 11 has a sealed structure.

The upper and lower fuel rods 10 and 11 are fixed in the axial direction by the lower end plugs 19 and have a structure in which they are thermally expanded upward in the axial direction.

The high plutonium-rich fuel pellets 20 and the low plutonium-rich fuel pellets 21 are loaded with ceramics obtained by mixing and sintering plutonium oxide and uranium oxide. The high plutonium-rich fuel pellet 20 has a high plutonium enrichment, and the low plutonium-rich fuel pellet 21 has a low plutonium enrichment.

The upper, inner and lower blanket fuel pellets 17, 22 and 23 are all loaded with depleted uranium oxide, and the gas plenum 24 is a storage location for the gas produced by the fuel.

The plutonium enrichment in this example is
27 wt% for high plutonium enriched fuel pellets 20
%, 21w for low plutonium enriched fuel pellets 21
t%.

Further, the dimensions of the respective portions of FIG. 2 in the present embodiment are as follows.

The upper and lower fuel rods 10 and 11 each have a diameter of 8.0 mm, and the trumpet tube 12 has inner and outer diameters of 150 mm and 158.1 mm, respectively.

The length of the upper fuel rod 10 is 450 mm, and the length of the upper blanket fuel pellet 17 portion housed inside the upper fuel rod 10 is 300 mm.
The length of the sodium layer is about 400 mm.

On the other hand, the length of the lower fuel rod 11 is 2.6 m, and the partial lengths of the high plutonium-enriched fuel pellets 20, the inner blanket fuel pellets 22 and the low plutonium-enriched fuel pellets 21 are added together. The length is 1 m, of which the length of the inner blanket fuel pellet 22 portion is 300 mm.

Furthermore, the length of the lower blanket fuel pellet 23 accommodated thereunder is 400 mm, and the length of the gas plenum 24 is 1.2 mm.

Although not shown in FIG. 2, a wire conventionally called a wire spacer is wound around the fuel rod in a spiral shape in order to maintain a gap between the fuel rods. The present embodiment is characterized in that, in the lower fuel rod 11, in the conventional example, the same core fuel pellets 25 (see FIG. 5) are stored above and below the internal blanket fuel pellets 22, respectively. In the embodiment, a high plutonium enriched fuel pellet 20 and an internal blanket fuel pellet 22 are provided above the internal blanket fuel pellet 22.
The low plutonium-rich fuel pellets 21 are housed in the lower part of each.

Next, the fuel assemblies in the core outer fuel assembly region will be described.

The fuel assembly in the outer core fuel assembly area 2 shown in FIG. 1 is different from the fuel assembly in the inner core fuel assembly area 1 in the fuel assembly in the outer core fuel assembly area 2. The lower fuel rod 11 does not have the internal blanket fuel pellet 22 region, but this portion is the low plutonium-rich fuel pellet 21 region.

Therefore, as shown in FIG. 3, the fuel assembly in the core outer fuel assembly region 2 has a lower plutonium enrichment fuel pellet 21 region than that in the core inner fuel assembly region 1. The length is increased by adding the length of the inner blanket fuel pellet 22 region.

In the first embodiment, the fuel assembly is constructed as described above, the neutron flux in the upper part of the core is increased, the coolant density coefficient can be reduced, and the end of combustion can be improved. The stable reduction effect of the coolant density coefficient can be maintained.

The effects of the first embodiment will be described below in comparison with the conventional example. The comparison is made when the first embodiment and the conventional example have the same conditions as described below.

That is, the heat output of the core: 2600 MW,
Core electrical output: 1000 MW, core fuel number: 421
Body, blanket fuel number: 78 body, fuel rod outer diameter: 7.6
mm, trumpet tube inner diameter: 147 mm, trumpet tube outer diameter: 15
5 mm, fuel assembly pitch: 164 mm, in the core,
This is a case where 241 inner core fuel assemblies, 180 outer core fuel assemblies, and 78 radial blanket fuels are arranged.

Under the same conditions as above, core pressure loss: 3.3 kgf / c in both the first embodiment and the conventional example.
m ² and breeding ratio were 1.11, which were the same values, but the coolant density coefficient (% Δk / k / Δρ / ρ) was 1.7 in the conventional example, whereas The value is 0.79 in the first embodiment, and is -54% in the first embodiment when the conventional example is used as a reference.
Has decreased.

That is, the sodium layer 6 is provided between the upper blanket fuel region 3 and the core fuel region 25 (see FIG. 4).
In comparison with the core of the conventional fast breeder reactor, the fuel has a high plutonium-rich fuel region 8 in the upper region and a low plutonium-rich fuel 9 in the lower region in the conventional example. It has been found that the coolant density coefficient can be reduced by about 54% in the core of the present example (FIG. 1) using the aggregate.

The above-mentioned comparative values of the coolant density coefficient are those at the end of combustion. Therefore, according to this example, it is apparent that the effect of reducing the coolant density coefficient can be maintained until the end of combustion.

In the second embodiment, the plutonium enrichment of each fuel of the high plutonium enriched fuel pellet 20 and the low plutonium enriched fuel pellet 21 in FIGS. Degree fuel pellet 2
Solid inside 0, low plutonium enriched fuel pellet 2
This is a case where the inside of 1 is made hollow and the amount of fuel of the former is made larger than that of the latter. In this case, the same effect as the first embodiment can be obtained.

The third embodiment is a high plutonium enriched fuel pellet 20 and a low plutonium enriched fuel pellet 2.
The fuel in the high plutonium enriched fuel pellets 20 has a high density, and the fuel in the low plutonium enriched fuel pellets 21 has a low density. Also in this case, the same effect as that of the first embodiment can be obtained.

The fourth embodiment is a case where the high plutonium-rich fuel pellet 20 is made solid and the low plutonium-rich fuel pellet 21 is made hollow. In this case, the effects obtained by synergizing the effects of the first and second embodiments can be obtained.

The fifth embodiment is a case where the fuel in the high plutonium-enriched fuel pellets 20 has a high density and the fuel in the low plutonium-enriched fuel pellets 21 has a low density. In this case, the effects obtained by synergizing the effects of the first and third embodiments can be obtained.

The sixth embodiment is a high plutonium enriched fuel pellet 20 and a low plutonium enriched fuel pellet 2.
The plutonium enrichment of 1 is the same, the inside of the high plutonium-rich fuel pellet 20 is solid, the inside of the low plutonium-rich fuel pellet 21 is hollow, and the fuel in the high plutonium-rich fuel pellet 20 is This is a case where the fuel in the low plutonium-rich fuel pellet 21 is made to have a high density and a low density. In this case, the effects obtained by synergizing the effects of the second and third embodiments can be obtained.

In the seventh embodiment, the inside of the high plutonium-rich fuel pellet 20 is made solid, the inside of the low plutonium-rich fuel pellet 21 is made hollow, and the fuel in the high plutonium-rich fuel pellet 20 is densely packed. In addition, the fuel in the low plutonium-rich fuel pellet 21 has a low density. In this case, the effects obtained by synergizing the effects of the first embodiment, the second embodiment, and the third embodiment can be obtained.

[0091]

According to the present invention, in a fast breeder reactor,
It is possible to provide a fuel assembly in which the effect of stable reduction of the coolant density coefficient until the end of combustion is sustainable without affecting the core size.

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view of a core region according to an embodiment of the present invention.

2 is an explanatory view of a fuel assembly in a core inner region of FIG. 1. FIG.

FIG. 3 is an explanatory diagram of a fuel assembly in a core outer region of FIG. 1.

FIG. 4 is a schematic vertical sectional view of a core region of a conventional example.

5 is an explanatory diagram of a fuel assembly in a core inner region of FIG. 4. FIG.

6 is an explanatory view of a fuel assembly in an area outside the core of FIG.

[Explanation of symbols]

3 ... Upper blanket fuel region, 6 ... Sodium layer, 7
... internal blanket fuel region, 8 ... high plutonium enriched fuel region, 9 ... low plutonium enriched fuel region, 20
... high plutonium enriched fuel pellets, 21 ... low plutonium enriched fuel pellets, 25 ... core fuel region, 26
… Core fuel pellets.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Satoshi Tokame 3-1-1, Saiwai-cho, Hitachi, Ibaraki Hitachi Ltd., Hitachi Works (72) Inventor, Kunikazu Kanade 3-chome, Saiwai-cho, Hitachi, Ibaraki No. 1 No. 1 Stock Company Hitachi Ltd. Hitachi factory

Claims (7)

[Claims] 1. A fast breeder reactor fuel assembly having a sodium layer between an upper blanket fuel region and a core fuel region, wherein a part of the core fuel region comprises an internal blanket fuel region, wherein the core fuel region Is divided into an upper core fuel region and a lower core fuel region in two vertical directions, and fuel pellets loaded with fuel of high plutonium enrichment on the fuel rods of the upper core fuel region are connected to the lower core fuel region. A fuel assembly for a fast breeder reactor, characterized in that fuel pellets in which the fuel of low plutonium enrichment is loaded in the fuel rods of 1. 2. A fast breeder reactor fuel assembly having a sodium layer between an upper blanket fuel region and a core fuel region, wherein a part of the core fuel region comprises an internal blanket fuel region, wherein the core fuel region Is divided into an upper core fuel region and a lower core fuel region in two vertical directions, and the plutonium enrichment of fuel in each fuel rod of the upper core fuel region and the lower core fuel region is the same, and the upper core fuel region is Solid fuel pellets are housed in the fuel rods of the region, hollow fuel pellets are housed in the fuel rods of the lower core fuel region, respectively, and the fuel rods and the lower core fuel region of the upper core fuel region are accommodated. 2. A fuel assembly for a fast breeder reactor, characterized in that there is a difference in the amount of the plutonium-enriched fuel loaded on each fuel pellet of the fuel rod. 3. A fast breeder reactor fuel assembly having a sodium layer between an upper blanket fuel region and a core fuel region, wherein a part of the core fuel region comprises an internal blanket fuel region. Is divided into an upper core fuel region and a lower core fuel region in two vertical directions, and the plutonium enrichment of fuel in each fuel rod of the upper core fuel region and the lower core fuel region is the same, and the upper core fuel region is A fuel pellet in which the plutonium-enriched fuel is loaded at high density on the fuel rods in the region, and a fuel pellet in which the plutonium-enriched fuel is loaded at low density in the fuel rods in the lower core fuel region are respectively stored. A fuel assembly for a fast breeder reactor characterized by being provided. 4. A fast breeder reactor fuel assembly having a sodium layer between an upper blanket fuel region and a core fuel region, wherein a part of said core fuel region comprises an internal blanket fuel region, wherein said core fuel region Is divided into an upper core fuel region and a lower core fuel region in two vertical directions, and solid fuel pellets loaded with high plutonium-enriched fuel on the fuel rods of the upper core fuel region are replaced by the lower core fuel region. A fuel assembly for a fast breeder reactor, characterized in that hollow fuel pellets loaded with a low plutonium enrichment fuel are stored in each of the fuel rods. 5. A fuel assembly for a fast breeder reactor having a sodium layer between an upper blanket fuel region and a core fuel region, wherein a part of the core fuel region comprises an internal blanket fuel region, wherein the core fuel region Is divided into an upper core fuel region and a lower core fuel region in two vertical directions, and fuel pellets in which the fuel rods in the upper core fuel region are densely loaded with a high plutonium-enriched fuel are added to the lower core fuel region. A fuel assembly for a fast breeder reactor, characterized in that fuel pellets in which a low plutonium-enriched fuel is loaded at a low density in each of the fuel rods are stored. 6. A fast breeder reactor fuel assembly having a sodium layer between an upper blanket fuel region and a core fuel region, wherein a part of said core fuel region comprises an internal blanket fuel region, wherein said core fuel region Is divided into an upper core fuel region and a lower core fuel region in two vertical directions, and the plutonium enrichment of each fuel in the upper core fuel region and the lower core fuel region is the same, and the fuel in the upper core fuel region is A solid fuel pellet loaded with the high density plutonium-enriched fuel in a rod and a hollow fuel pellet loaded with the low density plutonium-enriched fuel in the fuel rod in the lower core fuel region are respectively stored. A fuel assembly for a fast breeder reactor characterized by being provided. 7. A fast breeder reactor fuel assembly having a sodium layer between an upper blanket fuel region and a core fuel region, wherein a part of said core fuel region comprises an internal blanket fuel region, wherein said core fuel region Is divided into an upper core fuel region and a lower core fuel region in two vertical directions, and solid fuel pellets loaded with high density high plutonium-enriched fuel are loaded on the fuel rods of the upper core fuel region. A fuel assembly for a fast breeder reactor, characterized in that hollow fuel pellets loaded with low-density low-plutonium-enriched fuel are housed in the fuel rods in the core fuel region, respectively.