JP3031644B2 - Fuel assemblies and cores - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Generally, in a liquid sodium-cooled fast breeder reactor, the reactivity of the reactor core changes as the temperature of liquid sodium as a coolant rises. Industrial newspaper).
Conventionally, a fuel assembly of a fast breeder reactor is, for example, as described in the above-mentioned `` fast breeder reactor '', a fuel element bundle in which a number of nuclear fuel substances are sealed in a cladding tube and bundled, a wrapper tube surrounding the fuel element bundle, It comprises a coolant outlet above the fuel element bundle and a coolant inlet below the fuel element bundle. Also,
The fuel element contains cladding tubes with plugs at the upper and lower ends, a core fuel pellet enriched in fissile material or a blanket fuel pellet mainly composed of a fuel parent material, and a gas for containing gas generated by the fission reaction. It consists of gas plenum.

[0003] In the core, a core region in which a plurality of core fuel assemblies loaded with fuel elements in which core fuel pellets are sealed in a cladding tube is bundled and filled in the core, and blanket fuel pellets are sealed in the cladding tube. A plurality of blanket fuel assemblies loaded with fuel elements are bundled, and include a radial blanket region surrounding the core region. JP-A-1-12919 discloses a core in which a blanket fuel assembly in which blanket fuel pellets and neutron moderator pellets are mixed is provided around a core region.

[0004]

In the above prior art,
During a reactor transient, if the temperature of the liquid sodium rises, the density of the liquid sodium decreases due to thermal expansion, making neutrons less likely to collide with the liquid sodium atoms. As a result, the average energy of neutrons in the core region increases, so that the reactivity of the core increases.

[0005] It is an object of the present invention to provide a system for
Provided is a fuel assembly that can suppress an increase in the average energy of neutrons in the core region even when the temperature of liquid sodium rises, and can alleviate an increase in the reactivity of the core, and a core made of this fuel assembly. Is to do.

Another object of the present invention is to promote the growth of fuel (conversion from fuel parent material to fission material) in the central portion of the core region, thereby reducing the reactivity loss associated with reactor operation (according to fuel combustion). It is an object of the present invention to provide a fuel assembly capable of reducing the reactivity loss) and a core constituted by the fuel assembly.

[0007]

The above object can be achieved as follows.

(1) core fuel enriched in fissile material,
In a blanket fuel containing a fuel parent material as a main component, and a fuel assembly loaded with a neutron moderating material, a central portion in the longitudinal direction of the fuel assembly is a region where the blanket fuel and the neutron moderating material are mixed, and a non-center portion in the longitudinal direction. The part shall be the area where the core fuel exists.

[0009]

(3) A fuel element in which a cladding tube is filled with a core fuel pellet enriched in fissile material, a blanket fuel pellet mainly containing a fuel parent material, and a neutron moderator pellet mainly containing a neutron moderating material. In a fuel assembly configured by bundling a plurality of fuel elements, the central portion in the longitudinal direction of the fuel element is a region where blanket fuel pellets and neutron moderator pellets are mixed, and the non-central portion in the longitudinal direction is a region where core fuel pellets are present. Having

(4) Nuclear fuel pellets enriched in fissile material, blanket fuel pellets mainly composed of a fuel parent material, and fuel elements in which neutron moderator pellets mainly composed of a neutron moderating material are enclosed in a cladding tube. In a fuel assembly formed by bundling a plurality of fuel elements, the central portion in the longitudinal direction of the fuel element is a region where the neutron moderator pellets are present,
The upper and lower portions sandwiching the neutron moderator pellets are regions where blanket fuel pellets are present, and the non-central portions are regions where core fuel pellets are present.

(5) Nuclear fuel pellets enriched in fissile material, blanket fuel pellets mainly composed of a fuel parent material, and fuel elements in which neutron moderator pellets mainly composed of a neutron moderating material are enclosed in a cladding tube. In the fuel assembly composed of a plurality of bundles, the central portion in the longitudinal direction of the fuel element is a region where the blanket fuel pellet exists, and the upper and lower portions of the blanket fuel pellet sandwiching the blanket fuel pellet are the neutron moderator pellet. The existing area and the non-center area shall be areas where core fuel pellets exist.

(6) Nuclear fuel pellets enriched in fissile material, blanket fuel pellets mainly composed of a fuel parent material, and fuel elements in which neutron moderator pellets mainly composed of a neutron moderating material are enclosed in a cladding tube. In a fuel assembly composed of a plurality of bundles, a core fuel pellet and a blanket fuel pellet are sealed in a cladding tube, a central portion in the longitudinal direction of the cladding tube is a region where blanket fuel pellets are present, and a non-central portion is the core. A first fuel element, which is a region where fuel pellets are present, and a core fuel pellet and a neutron moderator pellet are sealed in a cladding tube. In the part, a plurality of second fuel elements, which are areas where core fuel pellets are present, are mixed.

(7) A plurality of core fuel assemblies loaded with core fuel are bundled and filled in the core region, and the periphery of the core region is formed by bundling blanket fuel assemblies loaded with blanket fuel. In a core including a blanket region, the core region is configured by bundling a plurality of fuel assemblies according to any one of (1) to (6).

(8) A plurality of core fuel assemblies loaded with core fuel are bundled and filled in the core region, and the periphery of the core region is formed by bundling blanket fuel assemblies loaded with blanket fuel. In the core comprising the blanket region, the first fuel assembly comprising the fuel assembly according to any one of (1) to (6), and the core fuel in both the central portion and the non-central portion in the longitudinal direction. A plurality of existing second fuel assemblies are bundled and filled in the core region, and the first fuel assembly is provided inside the core region in the radial direction of the core, and the second fuel assembly is provided outside the radial direction in the core region. The core region is configured by arranging the bodies.

[0016]

In the core using the fuel assembly according to the present invention, if the temperature of liquid sodium rises during a nuclear reactor transient, even if the density of sodium decreases due to thermal expansion, neutrons will be placed at the center of the fuel element. Due to the provision of the moderator, the neutrons in this region are still decelerated, and the average neutron energy in the core region does not increase significantly, and the increase in the reactivity of the core is mitigated.

Further, since the blanket fuel and the neutron moderating substance are mixed in the center of the fuel element and the average energy of neutrons in this region is lower than that in the non-center region, the fuel core is located in the center of the core region. The conversion from material to fissile material is promoted, and reactivity loss associated with fuel combustion can be reduced.

[0018]

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

FIG. 1 (a) is a schematic semi-longitudinal sectional view of the fuel assembly of the first embodiment, and FIG. 1 (b) is a schematic semi-longitudinal sectional view of a fuel element loaded with the fuel assembly of FIG. 1 (a). FIG. 1 (c)
FIG. 2 (b) is an enlarged schematic vertical sectional view of a portion A. FIG. 2 is an explanatory view of a core constructed using the fuel assembly of FIG. 1 (a) loaded with the fuel element of FIG. 1 (b), FIGS. FIG. 5 shows the second to fourth embodiments.
FIG. 6 (a) is a schematic semi-longitudinal sectional view of a fuel assembly of a fifth embodiment, and FIG. 6 (b) is a diagram of a fuel element loaded on the fuel assembly of FIG. 6 (a). Schematic semi-longitudinal sectional view, FIG.
(A) is a schematic semi-longitudinal sectional view of the fuel assembly of the sixth embodiment, FIG. 7 (b) is a schematic semi-longitudinal sectional view of a fuel element loaded on the fuel assembly of FIG. 7 (a), and FIG. 8) is a schematic semi-longitudinal sectional view of the fuel assembly of the seventh embodiment, and FIGS. 8B and 8C are FIGS.
2A is a schematic semi-longitudinal sectional view of a fuel element loaded in the fuel assembly of FIG. 1A, wherein 1 is a fuel assembly, 2 is a fuel element, 3 is a trumpet pipe, 4 is a coolant outlet at the upper end of the trumpet pipe, and 5 is A coolant inlet at the lower end of the wrapper tube, 6 a cladding tube, 7 an upper end plug, 8 a lower end plug, 9 a gas plenum, 10 a core fuel pellet,
Numeral 11 denotes a blanket fuel pellet, 12 denotes a neutron moderator pellet, 13 denotes a core fuel assembly, 14 denotes a radial blanket fuel assembly, and 15 denotes a fuel assembly composed of core fuel in both the central part and the non-central part.

FIG. 1A shows a fuel assembly 1 according to the first embodiment.
The fuel assembly 1 includes a fuel element 2 filled with nuclear fuel material, a wrapper pipe 3 surrounding the fuel element, a coolant outlet 4 at the upper end of the wrapper pipe, and a coolant inlet 5 at the lower end of the wrapper pipe.
And so on. FIG. 1 (b) shows a fuel element 2 loaded in the fuel assembly 1 of FIG. 1 (a). The fuel element 2 includes a cladding tube 6, an upper end plug 7, a lower end plug 8, and a core fuel pellet. 10, a blanket fuel pellet 11, a neutron moderator pellet 12, a gas plenum 9, and the like. The liquid sodium as the coolant flows from the bottom of the cladding tube 6 upward.

The core fuel is composed of a mixture of plutonium and uranium oxide, the blanket fuel is composed of depleted uranium oxide, the neutron moderator is composed of beryllium oxide, and the cladding tube is composed of stainless steel. Blanket fuel pellet 1
Numerals 1 and neutron moderator pellets 12 are alternately arranged at the center in the longitudinal direction of the fuel element 2, and the core fuel pellets 10 are arranged at the upper and lower non-centers. The dimensions of each part are as follows.
mm, and the length of the pellet is 10.0 mm. The total length of the core fuel pellets 10 is 80 cm, the total length of the blanket fuel pellets 11 and the neutron moderator pellets 1
The sum of the two lengths is 20 cm.

FIG. 2 shows the fuel assembly 1 of FIG. 1 as a core fuel assembly 13, which is bundled into a plurality of cylindrical shapes and filled in a core region, and a plurality of radial blanket fuel assemblies 14 around the core region. FIG. 3 is a view illustrating a state in which a core fuel, a blanket fuel, and a neutron moderating substance are arranged in a longitudinal cross section of a state surrounded by.

Of the core fuel assemblies 13, the core fuel assemblies 13a are loaded with low plutonium-enriched core fuel, and the core fuel assemblies 13b are loaded with high plutonium-enriched core fuel. The core fuel assembly 1
3a and each central part 13ac and 13 of the core fuel assembly 13b
bc is filled with a mixed substance of a blanket fuel and a neutron moderating substance. Further, the radial blanket fuel assemblies 14 are filled with only blanket fuel.

In a core using a fuel assembly of the prior art, when a reactor transient such as a decrease in coolant flow rate occurs and the temperature of liquid sodium as a coolant rises, the entire core region is reduced. The reactivity increased because the average energy of the neutrons increased. On the other hand, in the core using the fuel assembly of this embodiment, since the blanket fuel and the neutron moderator are mixed in the center of the core, which most contributes to the reactivity of the core, the average of neutrons in this mixed region is The energy is still kept low and the increase in reactor reactivity is small.

Further, even during the steady operation, the neutron capture cross section of uranium-238 increases in this region because the average energy of neutrons in the center of the core region is lower than that of the conventional core. Production is promoted. Plutonium produced in the center of the core region
239 compensates for the reactivity loss associated with the combustion of the fuel in the non-center portion of the core region, so that the net reactivity loss associated with the combustion is small.

On the other hand, in the prior art disclosed in JP-A-1-12919, a fuel assembly in which a blanket fuel and a neutron moderating substance are mixed has a contribution to reactivity as a radial blanket fuel assembly. Since it is arranged around the small core region, the effect of this embodiment cannot be obtained.

Therefore, according to this embodiment, it is possible to provide a reactor core having inherent safety against a plant transient event such as a decrease in coolant flow rate, and having a small reactivity loss due to combustion during steady operation.

Next, the first embodiment shown in FIG.
Different second to fourth embodiments will be described with reference to FIGS. FIGS. 3 to 5 are similar to FIGS.
The fuel assembly 1 loaded with the fuel element 2 shown in (b) is bundled into a plurality of cylindrical shapes and charged into the core region as a core fuel assembly 13, and a plurality of radial blanket fuel assemblies are provided around the core region. FIG. 2 is a view showing a state in which a core fuel, a blanket fuel, and a neutron-reducing substance are arranged in a longitudinal section of a state surrounded by a body 14.

However, the arrangement states in the core fuel region in the core region, the mixed region of the blanket fuel and the neutron moderating substance, and the regions where the plutonium enrichment is different even in the same core fuel region are different from those in FIG. Each is different. Further, in the case of FIGS. 4 and 5, a plurality of core fuel assemblies 13 are bundled in a cylindrical shape inside the core region in the core region in the radial direction and filled, and the core fuel assemblies 13 are provided on both the center and the non-center portions on the outside. A different point is that the core fuel assembly 15 is filled.

FIG. 3 shows a second embodiment, in which a core fuel assembly 1 is provided.
3, the core fuel assemblies 13 c and 13 b
This is the case where the plutonium enrichment of each core fuel of d is the same. 13cc and 13dc are mixed areas of the blanket fuel pellet 11 and the neutron moderator pellet 12 in the core fuel assembly 13c and the core fuel assembly 13d, respectively. The feature of this embodiment is that the length of the mixed region of the blanket fuel pellets 11 and the neutron moderator pellets 12 is large in the core fuel assembly 13c and small in the core fuel assembly 13d. Similar effects can be obtained. Further, there is an effect that the power distribution in the core radial direction can be flattened while the plutonium enrichment of the core fuel remains one type.

FIG. 4 shows a third embodiment in which a core fuel assembly 1 is provided.
The core region is filled with a core fuel assembly 15 loaded with core fuel in both the central portion and the non-central portion, and the core fuel assembly 13 and the core fuel assembly 15 including the core fuel in both the central portion and the non-central portion are provided. In this case, the plutonium enrichment of each core fuel is the same. In this embodiment, the mixed region 13ec of the blanket fuel pellets 11 and the neutron moderator pellets 12 is provided only in the core fuel assembly 13 loaded radially inside the core region. In the area around the core,
Since the contribution to the reactivity is reduced, the same effect as in the first embodiment can be obtained without providing the mixed region of the blanket fuel pellets 11 and the neutron moderator pellets 12 in the peripheral region of the core.

FIG. 5 shows a fourth embodiment in which a core fuel assembly 1 is provided.
3, core fuel assemblies 15 loaded with core fuel in both the center and non-center portions are filled in the core region, and the core fuel assemblies 13 f and the core fuel assemblies 1 of the core fuel assemblies 13 are provided.
3 g of the core fuel has the same plutonium enrichment. 13fc and 13gc are the blanket fuel pellets 1 in the core fuel assemblies 13f and 13g, respectively.
1 and a neutron moderator pellet 12 mixed region. In this embodiment, the length of the mixed region is determined by the core fuel assembly 1
f is large, and 1 g of the core fuel assembly is small, so that not only the same effect as in the other embodiments can be obtained, but also the plutonium enrichment of the core fuel remains the same, and the core diameter remains unchanged. It is possible to further flatten the output distribution in the direction.

Further, the fifth to seventh embodiments are described with reference to FIGS.
It is shown in FIG. 6 to 8 differ from FIG. 1 showing the first embodiment in the types of pellets sealed in the fuel element 2 and the arrangement of the various pellets sealed.

FIG. 6A shows the fuel assembly 1 of the fifth embodiment.
FIG. 6B shows the fuel element 2 loaded on the fuel assembly 1 of FIG. 6A. In the case where the neutron moderator pellet 12 is disposed at the longitudinal center of the fuel element 2, the blanket fuel pellet 11 is disposed above and below the neutron moderator pellet 12, and the core fuel pellet 10 is disposed at the upper and lower non-central portions. It is.

FIG. 7A shows the fuel assembly 1 of the sixth embodiment.
FIG. 7B shows a fuel element 2 loaded on the fuel assembly 1 of FIG. 7A. A blanket fuel pellet 11 is placed at the center in the longitudinal direction of the fuel element 2, a neutron moderator pellet 12 is placed above and below the blanket fuel pellet 11, and a core fuel pellet 1 is placed at the upper and lower non-central portions.
This is the case where 0 is arranged.

FIG. 8A shows the fuel assembly 1 of the seventh embodiment.
8 (b) and 8 (c) show the fuel element 2 loaded on the fuel assembly 1 of FIG. 8 (a). FIG. 8B shows the first fuel element 2 in which the neutron moderator pellets 12 are arranged at the center in the longitudinal direction of the fuel element 2, and the core fuel pellets 10 are arranged at the upper and lower non-central parts, and FIG. This is a second fuel element 2 in which a blanket fuel pellet 11 is arranged in the central part in the longitudinal direction of the element 2 and a core fuel pellet 10 is arranged in the upper and lower non-central parts. The fuel assembly 1 includes a plurality of first fuel elements 2 and a plurality of second fuel elements 2.

That is, the fuel core described in any of the above embodiments, including the fifth to seventh embodiments, may be used alone or in combination of a plurality of types to constitute a core. The effect of can be obtained.

[0038]

According to the present invention, in a fast breeder reactor,
It is possible to mitigate an increase in the reactivity of the reactor core due to an increase in the temperature of the liquid sodium, and to realize inherent safety against a transient event of the reactor such as a decrease in coolant flow rate.

Further, the reactivity loss associated with the operation of the reactor can be reduced, and the economic effect of prolonging the operation period is great.

[Brief description of the drawings]

FIG. 1 is a schematic semi-longitudinal sectional view of a fuel assembly and a fuel element of a first embodiment, and an enlarged schematic longitudinal sectional view of a main part of a fuel element.

FIG. 2 is an explanatory view of an arrangement state of core fuel and the like in a core region in the first embodiment.

FIG. 3 is an explanatory diagram of an arrangement state of core fuel and the like in a core region in a second embodiment.

FIG. 4 is an explanatory view of an arrangement state of core fuel and the like in a core region in a third embodiment.

FIG. 5 is an explanatory diagram of an arrangement state of core fuel and the like in a core region in a fourth embodiment.

FIG. 6 is a schematic semi-longitudinal sectional view of a fuel assembly and a fuel element of a fifth embodiment.

FIG. 7 is a schematic semi-longitudinal sectional view of each of a fuel assembly and a fuel element of a sixth embodiment.

FIG. 8 is a schematic semi-longitudinal sectional view of each of a fuel assembly and a fuel element of a seventh embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel assembly, 2 ... Fuel element, 3 ... Wrapper tube, 6 ... Cladding tube, 10 ... Core fuel pellet, 11 ... Blanket fuel pellet, 12 ... Neutron moderator pellet, 13 ... Core fuel assembly, 14 ... Diameter Directional blanket fuel assembly, 1
5. A core fuel assembly composed of core fuel in both the central part and the non-central part.

Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G21C 3/28 G21C 3/30 G21C 5/18 G21C 5/20

Claims (7)

(57) [Claims] 1. A fuel assembly loaded with a core fuel enriched in fissile material, a blanket fuel containing a fuel parent material as a main component, and a neutron moderating material, wherein the longitudinal center of the fuel assembly is A fuel assembly comprising: a region where a blanket fuel and the neutron moderating substance are mixed; and a non-central portion in the longitudinal direction is a region where the core fuel exists. 2. A core fuel pellet enriched in fissile material.
Blanket fuel pellets containing fuel parent
And a neutron moderator containing neutron moderator as a main component
Bundling multiple fuel elements with the
In the fuel assembly consisting of, in the longitudinal direction of the fuel element
The central part is the blanket fuel pellet and the neutron
The area where the fast material pellets are mixed, the non-central part in the longitudinal direction is the front
It is characterized by the area where the core fuel pellets exist.
Fuel assembly. 3. A core fuel pellet enriched in fissile material.
Blanket fuel pellets containing fuel parent
And a neutron moderator containing neutron moderator as a main component
Bundling multiple fuel elements with the
In the fuel assembly consisting of, in the longitudinal direction of the fuel element
The central part is the area where the neutron moderator pellets are located,
The upper and lower parts of the neutron moderator pellet
The area where the fuel pellets are present, and the
It is characterized by the area where the core fuel pellets exist
Fuel assembly. 4. A core fuel pellet enriched in fissile material.
Blanket fuel pellets containing fuel parent
And a neutron moderator containing neutron moderator as a main component
Bundling multiple fuel elements with the
In the fuel assembly consisting of, in the longitudinal direction of the fuel element
The center is where the blanket fuel pellets are located.
Area, the upper and lower parts sandwiching the blanket fuel pellet
The area where the neutron moderator pellets exist and the non-center area
Is the area where the core fuel pellets are located.
Fuel assembly to be featured. 5. A core fuel pellet enriched in fissile material.
Blanket fuel pellets containing fuel parent
And a neutron moderator containing neutron moderator as a main component
Bundling multiple fuel elements with the
In the fuel assembly consisting of Te, the reactor core fuel pellets and before
The blanket fuel pellet is sealed in a cladding tube,
The center of the cladding in the longitudinal direction is the blanket fuel pellet.
The area where the fuel exists, and the non-central part
A first fuel element that is an existing region, and the core fuel
Pellet and neutron moderator pellet are sealed in cladding tube
And the central part in the longitudinal direction of the cladding tube is the neutron moderator
The area where the pellets exist, and the non-central part, is the core fuel pellet.
A plurality of second fuel elements in the area where the fuel exists
A fuel assembly, comprising: 6. A core fuel assembly loaded with core fuel comprises a plurality of core fuel assemblies.
Several core regions exist around the core region.
There are multiple blanket fuel assemblies loaded with fuel
Core in a radial blanket zone
Bundling a plurality of fuel assemblies according to any one of claims 1 to 6
, Comprising the core region
heart. 7. A core fuel assembly loaded with core fuel comprises a plurality of core fuel assemblies.
Core area where several bundles exist, and blanket around the core area
Multiple blanket fuel assemblies loaded with fuel
Core in the existing radial blanket region,
7. The method according to claim 1, further comprising:
A first fuel assembly comprising a fuel assembly according to any of the preceding claims.
And the central and non-central portions in the longitudinal direction on the outside in the core radial direction
A second fuel assembly in which core fuel exists in each area of
A core characterized by being arranged.