JPH1152088A - Heavy-water-cooled breeder reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reactor using water coolant, having fuel breeding capability and improving safety. SOLUTION: In a reactor using liquid phase heavy water 27 as coolant and assembly type fuel bundling a multitude of fuel pins 1, oxide fuel is used, the volume ratio of coolant to fuel in the axial middle part of core is made 0.5 to 1.0 and the core height meaning the vertical regional length of core fuel pellets 2 excluding blanket is made 1 m or less. In other option, metal fuel or nitride fuel is used, the volume ratio of coolant to fuel in the axial middle part of core is made 0.5 to 1.7 and the core height meaning the vertical regional length of core fuel pellets 2 excluding blanket is made 1 m or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear reactor in which the conversion ratio (the amount of fissile material at the time of fuel removal with respect to the amount of fissile material at the time of fuel loading) is increased in order to effectively utilize uranium resources.

[0002]

2. Description of the Related Art In a conventional light water reactor, a neutron generated by nuclear fission is designed to be decelerated by a light water coolant in order to effectively utilize a nuclear fission reaction by uranium-235.
For this reason, the coolant volume Vc is set larger than the fuel volume Vf, and Vc / Vf is set to 2 or more. The conversion ratio at this time is about 0.5. On the other hand, in the case of the high conversion light water reactor, in order to promote the conversion of the parent material (uranium-238) to fissile material (plutonium-239) by neutron absorption, the coolant volume is reduced and Vc / Vf is reduced to 0. .5
About. The conversion ratio at this time is about 0.9.

A known example of a reactor having a high conversion ratio is disclosed in Journal of the Atomic Energy Society of Japan, Vol. 26, No. 6 (198).
As shown in 4), there is an example of a high conversion light water reactor in which the volume ratio of light water coolant to fuel (Vc / Vf) is reduced, and the high conversion furnace performance in the known example is such that Vc / Vf is about 0.5.
With a dense fuel, the conversion ratio is about 0.9.

On the other hand, in Japanese Patent Application Laid-Open No. 64-88396, "High Conversion Heavy Water Reactor", a high conversion ratio of 0.8 or more is achieved by using heavy water as a coolant, with a fuel having a square lattice arrangement equivalent to that of a light water reactor. There are known examples of nuclear reactors. Vc / V of light water reactor
The value f is not less than 2.0, and this known example is a technique for overcoming the drawback of a high conversion furnace utilizing light water, which makes fuel production difficult, by using heavy water coolant.

[0005] There are many nuclear reactors in the world using heavy water as a moderator and a coolant, but all of them are thermal neutron reactors that focus on the fact that thermal neutron absorption of heavy water is extremely small. All of them have Vc / Vf of 2.0 or more and a conversion ratio of about 0.8.
It is as follows.

First, a pressure vessel type heavy water reactor has been constructed in Argentina, Sweden.

From the Institute of Electrical Engineering, Modern Electrical Engineering, Lecture 6 Modern Nuclear Engineering (published in 1969), p. 237, the Swedish Marviken furnace is a boiling heavy water-cooled furnace, and the fuel is 1.
35% to 1.75% of slightly enriched uranium is used.

[0008] Directory of NUCLEAR POWER PLANTS in t
he world 1994 Japan NuclearEnergy Information cent
er Co., Ltd. ISSN 0912-7003, Sweden A
GESTA reactor, ATUCHA-1 and -2 in Argentina
The furnace is a heavy water cooled reactor and uses natural uranium as fuel.

Next, a pressure tube type heavy water reactor is a new type conversion reactor.
(ATR) and Canadian heavy water reactor (CANDU), all of which have a moderator area larger than that of light water reactors, and have a Vc / Vf of 8
To the extent, it enhances the moderating effect and effectively utilizes fission reactions caused by thermal neutrons.

[0010]

In the above prior art, when using light water in order to obtain a high conversion furnace, it is necessary to use a dense fuel for reducing the moderator area, and the productivity of the fuel is reduced. There was a problem that it was difficult. Also, when heavy water was used, a high conversion reactor could be obtained by suppressing fuel densification like a conventional light water reactor, but a breeder reactor concept that ensures safety while maintaining a conversion ratio of 1.0 or more has been obtained. Did not.

A first object of the present invention is to provide a fuel breeder reactor using heavy water coolant, and to improve the safety of the reactor.

A second object of the present invention is to provide a nuclear reactor using heavy water coolant, which further improves fuel breedability.

A third object of the present invention is to provide a nuclear reactor in which, in addition to the first or second object, safety against boiling of a coolant is improved.

A fourth object of the present invention, in addition to the third object, is to provide a nuclear reactor in which the void reactivity during boiling of the coolant is reduced.

A fifth object of the present invention, in addition to the first or second object, is to provide a nuclear reactor in which the reactivity is suppressed when the density of the coolant changes and the safety is improved. is there.

A sixth object of the present invention is to achieve the fifth object by using a short fuel rod.

A seventh object of the present invention is to provide a nuclear reactor in which, in addition to the first or second object, the safety of a primary or secondary cooling system in an accident is improved. .

An eighth object of the present invention, in addition to the first or second object, is to provide a reactor in which the diameter of a core pressure vessel of a breeder reactor using heavy water coolant is reduced. .

[0019]

Means for Solving the Problems The first aspect of the present invention.
Means of achieving the object is to use liquid-phase heavy water as a coolant, use an oxide fuel, set the volume ratio of the coolant to the fuel in the central portion in the axial direction of the core to 0.5 or more and 1.0 or less, and set the 1 m or less, or using a metal fuel or a nitride fuel, the volume ratio Vc / Vf of the coolant in the central portion in the axial direction of the core to the fuel is set to 0.5 or more and 1.7 or less, and the core height is set to 1 or less.
m or less.

The means for achieving the second object of the present invention, together with the means for achieving the first object, is achieved by using boiling heavy water as a coolant.

The means for achieving the third object of the present invention, together with the means for achieving the first or second object, is achieved by installing the sealed moderator rod in the range of 10 to 20% of the fuel rod volume. Is done.

According to a fourth aspect of the present invention, there is provided a moderator according to the third aspect, wherein the sealed moderator rod is a rod sealed with heavy water or light water, or a rod sealed with metal hydride. Is achieved by:

According to a fifth aspect of the present invention, together with the first or the second aspect, the volume ratio of the coolant to the fuel in the upper part in the core axial direction is larger than that in the central part in the core axial direction. Is achieved by doing

According to a sixth aspect of the present invention, together with the fifth aspect, a part of the fuel rod is formed into a short fuel rod, and a coolant penetrates above the short fuel rod through an end plug. Achieved by combining a heavy water rod in a mold.

The means for achieving the seventh object of the present invention is that, together with the means for achieving the first or second object, the reactor has an independent cooling system of a primary system and a secondary system. This is achieved by using boric acid-containing light water as the coolant of the system.

The means for achieving the eighth object of the present invention, together with the means for achieving the first or second object, is achieved by separating the reflector portion around the reactor core from heavy water coolant and using light water containing boric acid. Is done.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Heavy water has a very small thermal neutron absorption cross section of 1/600 of light water. The neutron moderating capacity of heavy water is about one-eighth of light water. Therefore, when heavy water is used as the moderator and the coolant, a neutron effective utilization type thermal neutron reactor having a small neutron absorption other than fuel can be obtained. But,
As long as it is a thermal neutron reactor, there is little reaction to absorb neutrons into non-fissionable uranium element, and it is difficult to have breeding performance to convert to plutonium. Therefore, in the present invention, deuterium is not used as a moderator, but is used as a coolant, and the volume of heavy water as a coolant in the core is used as a fuel, compared to the conventional heavy water thermal neutron reactor and heavy water high conversion reactor. By reducing the size, a nuclear reactor having breeding performance can be realized. Further, in the present invention, the volume ratio of the coolant to the fuel is defined according to the fuel substance.

However, the volume ratio of the coolant to the fuel Vc /
As Vf becomes smaller, it becomes necessary to make the gap between fuel rods narrower. Therefore, by setting this to 0.5 or more, a fuel and a core which are relatively easy to manufacture are obtained. Also, when heavy water is used as a coolant, if the volume ratio of coolant to fuel is reduced, a positive reactivity tends to be injected at the time of heavy water boiling, so that the core height is set to 1 m or less. ,
Neutron leakage in the axial direction of the core at the time of heavy water boiling can be increased, a negative reactivity can be injected, and the safety of the core can be improved.

Further, it was confirmed that when heavy water coolant was used, the coolant void reactivity did not increase linearly with the void ratio, but tended to suddenly increase when the void ratio was 80% or more. Therefore, by installing the sealed moderator rod within the range of 10 to 20% of the fuel rod volume, the coolant void reactivity can be reduced, and the reactivity controllability during normal operation and the safety in the event of an accident are improved. can do. A sealed moderator rod can be formed by using a rod sealed with heavy water, a rod sealed with light water, or a rod sealed with metal hydride.

Further, in the upper part in the core axial direction, the coolant density coefficient is negative, and in the case of heavy water cooling, when the output is increased during operation, the coolant density decreases first, so that By increasing the coolant-to-fuel volume ratio from the center in the axial direction, the effective coolant density coefficient can be reduced, improving reactivity controllability during operation and improving safety in the event of an accident. it can. The reason why the volume ratio of the coolant to the fuel is not increased in all the axial directions is that the growth rate is adversely affected. A part of the fuel rod is a short fuel rod, and a heavy water rod that penetrates the coolant is connected to the upper part of the short fuel rod via an end plug. Can be.

The reactor has independent cooling systems of a primary system and a secondary system, and as a coolant for the secondary cooling system,
By using light water or light water containing boric acid,
Tritium generated in the secondary system can be cut off from the turbine system. Also, in the unlikely event that the primary and secondary cooling waters are mixed and light water is mixed into the reactor core and thermal neutrons are generated due to the light water deceleration effect, the neutron absorption effect of boric acid Thereby, the subcriticality can be maintained.

Further, by separating the reflector portion around the reactor core from heavy water coolant and using light water containing boric acid, high-speed neutrons can be shielded more efficiently than heavy water by light water having a large deceleration effect. The neutron shielding thickness in the direction can be reduced, and the diameter and height of the core vessel can be reduced.

An embodiment of a heavy water cooled reactor for specifically realizing the above-mentioned contents of the present invention is as follows.

FIG. 8 is an overall view of a nuclear reactor system showing one embodiment of the present invention. In the reactor system, a reactor pressure vessel 15 having a built-in core, a primary cooling system 14 using heavy water as a coolant, a steam generator 16, and a primary coolant pump 17 are accommodated in a reactor containment vessel 23. Steam generator 1
The boiling light water of the secondary cooling system 24 generated in
1 to drive the generator 22. The secondary coolant that has passed through the turbine is returned to the liquid phase by the condenser 18 and is returned to the condensate pump 1.
9. A loop configuration in which the feed water pump 20 returns the steam to the steam generator 16.

Next, FIG. 9 shows a core arrangement according to an embodiment of the present invention. In the core, a plurality of core fuel assemblies 25 are arranged at the center, and a radial shield 2 for neutron shielding is arranged around the core fuel assemblies 25.
6 are arranged.

FIG. 1 is a sectional view of a core fuel assembly showing one embodiment of the present invention. The core fuel assembly has a large number of fuel pins 1 housed in a channel box 3.
Contains a core fuel pellet 2 and blanket fuel pellets 4 above and below it. A gas plenum 5 is disposed below the fuel pin, and the upper and lower sides of the fuel pin are sealed by an upper end plug 6 and a lower end plug 7. The height of the core fuel pellets is 1 m, and the height of the blanket fuel pellets is 0.3 m above and below the core. An upper shield 8 and a lower shield 9 for shielding neutrons are provided above and below the fuel assembly. The core fuel assembly has a structure in which heat due to nuclear fission is generated from each fuel pin 1, and the heat is cooled by heavy water 27 flowing from the lower part of the assembly and discharged to the upper part of the fuel.

FIG. 2 is a radial sectional view (Vc / Vf = about 1) of a part of the core fuel assembly of FIG.

A square lattice arrangement similar to that of a conventional light water reactor is used, except that the diameter of the fuel pin 1 is about 11 mm and the pin pitch is about 1 mm.
2.5 mm and the gap between pins is about 1.5 mm. A fuel pellet 2 housed in a cladding tube is disposed inside the fuel pin 1. In addition, heavy water is used as a coolant, and an oxide is used as a material of the core fuel pellet 2. FIG. 2 shows a part of the cross section of the fuel assembly. The assembly is composed of 12 × 12 rows of fuel pins, and the fuel pins are held by grid type spacers. In addition, Vc / Vf of the fuel of the conventional light water reactor
Is such that the coolant between the fuel assemblies is also reduced to Vc on average, and the boiling water reactor and the pressurized water reactor are both larger than about 2.

In FIG. 2, although the coolant area is reduced as compared with the conventional light water reactor, the gap between the fuel pins is about 1.5 mm, and the grid spacer can be manufactured.
Further, both the case where a channel box is not provided in the fuel assembly as in the pressurized water reactor and the case where a channel box is provided in the same manner as the boiling water reactor can be considered. The control rod channel always has a channel box installed. A B4C neutron absorber is used as the material of the control rod for output control during normal operation. Depleted uranium and depleted uranium are used as the material of the control rod for combustion reactivity compensation.

In FIGS. 1 and 2, the fuel pins are arranged in a square lattice, but as another example, Vc / Vf is set to 0.5.
In the case where the fuel pins are made as small as possible, the fuel pins may be arranged in a triangular arrangement and the assembly may be formed in a hexagonal cross section.

FIG. 3 shows the relationship between the breeding ratio and Vc / Vf indicating the range of the breeding performance of the heavy water coolant core of the present invention. FIG. 3 is a diagram showing a system in which a core of 1 m height and a blanket portion of 0.4 m in the axial direction are installed, and shows a total breeding ratio of the core and the blanket. The solid line in the figure indicates the case of the oxide fuel, and the dotted line indicates the characteristics of the case of the metal fuel or the nitride fuel. The thick line shows the relationship when the coolant is liquid-phase heavy water, and the thin line shows the relationship when boiling heavy water is used as the coolant when the average core void ratio is 40%.

As can be seen from the figure, when heavy water is used as the coolant, the breeding ratio can be made 1 or more. In the case of liquid-phase heavy water, when Vc / Vf is 1 or less in the case of an oxide fuel, the metal fuel or the nitride fuel In this case, Vc / Vf is set to 1.7 or less. In the case of 40% boiling heavy water, Vc / Vf is 1.5 or less for oxide fuel, and Vc / Vf is 2.0 or less for metal fuel or nitride fuel. In the present invention, the reason why the claim is set to 0.5 or more in terms of Vc / Vf is that the fuel including the coolant between the fuel bundle and the fuel assembly can be formed relatively easily only when Vc / Vf is about 0.5. This is due to the above case.

FIG. 4 shows the core of the embodiment of the present invention shown in FIGS. 1 and 2 in which oxide is used as fuel and Vc / Vf is about 1.0.
It is a result of organizing the relationship between the core height of the heavy water coolant core and the void reactivity when the core removal burnup is 50,000 MWd / t. From the viewpoint of easy control and operation of the reactor, it is desirable that the coolant void reactivity be negative or close to zero. FIG. 4 shows that Vc / Vf is about 1.0 using oxide fuel.
When the core height is set to 1 m or less, the void reactivity can be made negative and the safety can be improved.

The representative embodiment of the present invention has been described.

In the following, as an embodiment for improving safety, an embodiment in which the coolant void reactivity is reduced and the coolant density coefficient is reduced will be described.

When the burn-out of the core is increased from 50,000 MWd / t in the above embodiment or when the metal fuel or the nitride fuel is used, the neutron spectrum becomes hard, and the coolant void reaction occurs. It is known that the coolant density coefficient shifts to the more positive side. That is, when the burnup is further increased with an oxide fuel, or when a metal fuel or a nitride fuel is used, the reactivity caused by the change in the coolant density is set to be close to zero or negative, as shown in FIGS.
Some further ingenuity is desirable for the core. Although it is conceivable to further reduce the core height from 1 m, in order to keep the fuel linear output without increasing the fuel temperature, reducing the core height is accompanied by the disadvantage that the core diameter is significantly increased. Therefore, in the following, an embodiment will be described in which the coolant void reactivity and the coolant density coefficient are reduced by other measures without changing the core height.

FIG. 5 is a longitudinal sectional view of a fuel assembly having a reduced coolant void reactivity according to the present invention. The fuel of FIG. 5 features a sealed moderator rod 12 as a feature. The sealed moderator rods 12 are arranged about 10 to 20% of the fuel rods.
A sealing moderator 13 is housed inside. The sealing moderator 13 includes light water or heavy water. It is also possible to use a hydrogen-containing metal such as zirconium hydride as the sealing moderator 13. The coolant void reactivity tends to increase toward the positive side when the coolant-to-fuel volume ratio is reduced or the burnup is increased, and a device for reducing the reactivity is important for improving safety.

FIG. 6 shows a typical relationship between the coolant void ratio and the coolant void reactivity. FIG. 6 shows an analysis result of a core using oxide fuel, using heavy water as a coolant, setting the coolant to fuel volume ratio to about 1.0, and increasing the burnup as compared with the case of FIG. As shown in FIG. 6, the relationship between the void fraction and the void reactivity is not proportional, and the rate of increase increases as the void fraction increases. In particular, when the void ratio is 80% or more, the rate of increase in the void reactivity becomes particularly large. Therefore, if 20% of the sealed moderator rod 12 containing heavy water is disposed with respect to the heavy water coolant as in the fuel of FIG. 5, the void reactivity is not effectively increased to 80% or more. It can be greatly reduced.

The sealed moderator rod 12 containing light water
10%, the deceleration effect of light water is greater than heavy water, so the void reactivity is effectively 80% of heavy water voids.
The following void reactivity can be suppressed.

FIG. 7 is a longitudinal sectional view of a fuel assembly having a reduced coolant density coefficient according to the present invention. In the fuel assembly, a large number of fuel pins 1 are stored in a channel box 3, and a core fuel pellet 2 and blanket fuel pellets 4 above and below the core fuel pellet 2 are stored in the fuel pin 1. A gas plenum 5 is disposed below the fuel pin, and the upper and lower sides of the fuel pin are sealed by an upper end plug 6 and a lower end plug 7. As a characteristic of the fuel according to the embodiment of the present invention, some fuel pins have an intermediate end plug 11, and a pipe for communicating heavy water coolant is arranged above the intermediate end plug. The upper pipe of the intermediate end plug is provided with a communication hole so that heavy water coolant flowing outside can pass through the inside. It is also possible to put heavy water or light water in the inside and seal it. The effect of the present embodiment is that even when the core average coolant density coefficient is on the positive side, the coolant density is reduced first in the negative coolant density coefficient portion at the upper part of the core, so that the coolant is reduced during normal operation. Since the density coefficient can be made negative, operation control becomes easy.

FIG. 8 is a system configuration diagram showing an embodiment of the present invention in which boric acid-containing light water is used as a coolant for a secondary cooling system. By using boric acid-containing light water as the coolant for the secondary cooling system, a negative reactivity is injected into the core even if the coolant for the secondary cooling system is mixed with the primary coolant in the event of an accident. Can improve safety. FIG. 9 is a core layout diagram having a radial shield showing one embodiment of the present invention.

As the shield around the core, a radial shield assembly 26 is arranged on the outer periphery of the core fuel assembly 25. This is an example in which a number of boric acid-containing light water moderator rods are housed and arranged in a metal tube in the radial shielding assembly 26. As a result, the shielding thickness in the core radial direction can be reduced, so that the diameter of the pressure vessel can be reduced. In addition, by adding boric acid to the light water moderator in advance, the shielding effect is improved, and even if the light water moderator is mixed into the heavy water coolant, the reactor will have a negative reactivity and the safety will be improved. it can.

[0053]

According to the first aspect of the present invention, it is possible to provide a fuel breeder type nuclear reactor using heavy water coolant and to improve the safety of the reactor.

According to the second aspect of the present invention, it is possible to provide a nuclear reactor in which fuel breedability is further improved as compared with the nuclear reactor according to the first aspect of the present invention.

According to the third aspect of the invention, in addition to the effects of the first or second aspect of the invention, it is possible to provide a nuclear reactor with improved safety against boiling of the coolant.

According to the invention of claim 4, in addition to the effect of the invention of claim 3, it is possible to provide a nuclear reactor in which the void reactivity at the time of boiling of the coolant is reduced.

According to the fifth aspect of the present invention, in addition to the effects of the first or second aspect of the present invention, there is provided a nuclear reactor having improved safety by suppressing a change in reactivity when the density of the coolant changes. Can be provided.

According to the sixth aspect of the invention, the effect of the fifth aspect of the invention can be achieved by using a short fuel rod.

According to the seventh aspect of the present invention, in addition to the effects of the first or second aspect, the primary system of the nuclear reactor,
It is possible to provide a nuclear reactor with improved safety in a secondary cooling system accident.

According to the eighth aspect of the present invention, in addition to the effect of the first or second aspect of the present invention, the shield around the core is separated from the heavy water coolant, and light water containing boric acid is used. The shielding effect can be improved compared to the case of using heavy water, and the shielding thickness can be reduced, so that the size of the pressure vessel can be reduced and the diameter of the core pressure vessel of the breeder reactor using heavy water coolant can be reduced. I can do it.

[Brief description of the drawings]

FIG. 1 is a sectional view of a fuel assembly showing one embodiment of the present invention.

FIG. 2 is a fuel grid diagram showing one embodiment of the present invention.

FIG. 3 is a diagram showing a relationship between a heavy water coolant / fuel volume ratio and a breeding ratio.

FIG. 4 is a diagram showing a relationship between a core height and a coolant void reactivity according to one embodiment of the present invention.

FIG. 5 is a sectional view of a fuel assembly showing one embodiment of the present invention.

FIG. 6 is a diagram showing a relationship between a coolant void ratio and a coolant void reactivity according to one embodiment of the present invention.

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

FIG. 8 is a system configuration diagram showing an embodiment of the present invention using light water containing boric acid for a secondary cooling system.

FIG. 9 is a core layout diagram having a shield showing one embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel pin, 2 ... Core fuel pellet, 3 ... Channel box, 4 ... Blanket fuel pellet, 5 ... Gas plenum, 6 ... Upper end plug, 7 ... Lower end plug, 8 ... Upper shield, 9 ... Lower shield, 10 ... heavy water coolant communication pipe, 11 ...
Middle end plug, 12: sealed moderator rod, 13: sealed moderator,
14: Primary cooling system, 15: Reactor pressure vessel, 16: Steam generator, 17: Secondary coolant pump, 18: Condenser, 19 ...
Condenser pump, 20: feed water pump, 21: turbine, 22
... generator, 23 ... reactor containment vessel, 24 ... secondary cooling system,
25: core fuel assembly, 26: radial shielding assembly, 27
…heavy water.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor, Yukinori Goto 3-1-1, Sachimachi, Hitachi, Ibaraki Prefecture Inside the Hitachi Works, Hitachi, Ltd. (72) Megumi Yamadate 3-chome, Sachimachi, Hitachi, Ibaraki No. 2 Hitachi Engineering Co., Ltd. (72) Inventor Michio Yokomi 3-1-1 Sachimachi, Hitachi City, Ibaraki Pref. Hitachi, Ltd. Hitachi Plant

Claims (8)

[Claims] In a nuclear reactor using a fuel assembly type fuel in which a number of fuel pins are bundled using liquid phase heavy water as a coolant, an oxide fuel is used, and a fuel of a coolant in a central portion in a core axial direction is used. A core ratio of 0.5 to 1.0, and a core height meaning a vertical region length of a core fuel pellet not containing a blanket of 1 m or less, or
Using a metal fuel or a nitride fuel, the volume ratio of the coolant to the fuel in the central part in the axial direction of the core is set to 0.5 or more and 1.7 or less, and means the vertical region length of the core fuel pellet not including the blanket. A heavy water-cooled breeder reactor having a core height of 1 m or less. 2. A reactor using a fuel assembly type fuel in which a plurality of fuel pins are bundled using boiling heavy water as a coolant. A volume ratio of 0.5 or more and 1.5 or less, and a core height of 1 m or less, which means a vertical region length of a core fuel pellet not including a blanket, or
Using a metal fuel or a nitride fuel, the volume ratio of the coolant to the fuel in the central part in the axial direction of the core is set to 0.5 or more and 2.0 or less, and means the vertical region length of the core fuel pellet not including the blanket. A heavy water-cooled breeder reactor having a core height of 1 m or less. 3. The heavy water cooled breeder reactor according to claim 1, wherein the sealed moderator rod is provided in a range of 10 to 20% of the volume of the fuel rod. 4. The heavy water cooled breeder reactor according to claim 3, wherein the sealed moderator rod is a rod sealed with heavy water, a rod sealed with light water, or a rod sealed with metal hydride. 5. The heavy water cooled breeder reactor according to claim 1, wherein a volume ratio of the coolant to the fuel in the upper part in the axial direction of the core is larger than that in a central part in the axial direction of the core. 6. A fuel injection system according to claim 5, wherein a portion of the fuel rod is a short fuel rod at a top portion in the axial direction of the core to increase a volume ratio of the coolant to the fuel compared to a central portion in the axial direction of the core. A heavy water-cooled breeder reactor characterized in that a coolant penetrating heavy water rod is connected to an upper part of the rod through an end plug. 7. A nuclear reactor according to claim 1 or 2, wherein the reactor has independent cooling systems of a primary system and a secondary system.
A heavy water-cooled breeder reactor, wherein light water containing boric acid is used as a coolant for the secondary cooling system. 8. The heavy water-cooled breeder reactor according to claim 1, wherein the reflector portion around the reactor core is separated from heavy water coolant and light water containing boric acid is used.