JP7138082B2 - fast reactor fuel assembly and fast reactor core - Google Patents

Description

The present invention relates to a fast reactor fuel assembly and a fast reactor core.

A fast reactor has a core arranged in a reactor vessel, and the reactor vessel is filled with a liquid metal such as liquid sodium as a coolant. This core is loaded with a plurality of fuel assemblies having a plurality of fuel elements in which fissile materials such as fissile plutonium (for example, ²³⁹ Pu) and fuel fertile materials such as depleted uranium (U) are enclosed in cladding tubes. be done. Specifically, the fuel assembly has a plurality of fuel elements, a bugle tube and an entrance nozzle. An entrance nozzle supports the lower ends of the plurality of fuel elements. A wire spacer attached to the fuel element is wound around the outer surface of each fuel element, and the wire spacer maintains a predetermined distance between the fuel elements. A wrapper tube surrounds a bundle of fuel elements supported by an entrance nozzle. The lower end of the wrapper tube is attached to the entrance nozzle. The upper end of the wrapper tube defines a coolant outlet located above the fuel element.

Solid oxide fuels and solid metal alloy fuels are generally used as the nuclear fuel material contained in the fuel element, and they have been used extensively. In JP 2018-71997 A, a mixed oxide fuel (MOX fuel) in which uranium and plutonium, which are solid oxide fuels, are mixed is used as a core fuel, and uranium and minor actinides, which are other solid oxide fuels, are mixed. describe a fast reactor core using another mixed oxide fuel (MAOx fuel) as the inner blanket fuel.

However, there are also examples of using liquid fuels such as liquid metal alloys and molten salts as nuclear fuel material. For example, Japanese Patent Application Laid-Open No. 64-32189 discloses that a liquid metal fuel such as a plutonium (Pu)-uranium (U)-bismuth (Bi) alloy is enclosed in a cladding tube, and a molten salt region containing no nuclear fuel material is formed in the cladding tube. A fuel element is disclosed that is positioned within the cladding above the liquid metal fuel region. Further, Japanese Patent Application Laid-Open No. 2014-10022 discloses a fuel element in which a fluoride molten salt containing thorium (Th) and fissile materials (uranium 235 ( ²³⁵ U), plutonium 239 ( ²³⁹ Pu), etc.) is enclosed in a cladding tube. is disclosed. However, these fuel elements described in JP-A-64-32189 and JP-A-2014-10022 do not have blanket fuel.

JP 2018-71997 A JP-A-64-32189 JP 2014-10022 A

Due to the operation of the fast reactor, fissile materials ( ²³⁹ Pu, etc.), which are the main components of the core fuel, are consumed by nuclear fission reaction (burning of nuclear fuel). On the other hand, in the core fuel existing in the core fuel region in the core of the fast reactor, in addition to fissile materials, there are also fuel parent substances ( ²³⁸ U, etc.). Fissile material ( ²³⁹ Pu, etc.) is newly generated. In addition, when a blanket region is arranged above and below the core fuel region or around the core fuel region, the core fuel region is affected by the fuel affinity material ( ²³⁸ U, etc.), which is the main component of the blanket fuel in the blanket region. Fissile materials ( ²³⁹ Pu, etc.) are produced by capturing neutrons leaking from the reactor.

Here, the blanket fuel is a nuclear fuel that contains almost no fissionable material such as ²³⁹ Pu and instead contains a large amount of fertile material such as ²³⁸ U. The ^fertile fuel material ( ²³⁸ U, etc.) does not contribute to nuclear fission in the core of the fast reactor as it is. Contributes to nuclear fission in the core.

The ratio of fissile material production to consumption in a fast reactor is often referred to as the conversion ratio or breeding ratio. Here, the conversion ratio for fissile Pu, which is a fissile substance, is defined in the following three ways.

(1) Core fuel conversion ratio
Amount of Pu produced in core fuel/Amount of Pu consumed in core fuel (2) Conversion ratio of blanket fuel
Pu production in blanket fuel / Pu consumption in core fuel (3) Core conversion ratio
Sum of core fuel conversion ratio and blanket fuel conversion ratio Generally, in a fast reactor, the core fuel conversion ratio < 1, but by adding the blanket fuel conversion ratio, the conversion ratio ≥ 1 is realized in the entire core. (ie, fuel breeding). In this case, as the burning of the nuclear fuel progresses, the amount of Pu in the entire core including the blanket fuel increases, but the amount of Pu contained in the core fuel gradually decreases. Therefore, the fuel assemblies loaded into the core of the fast reactor are replaced with new fuel assemblies having a burnup of 0 GWd/t after a predetermined combustion period (operation cycle length) has elapsed. In order to improve the fuel economy of the core, long-life fuel assemblies that require less frequent replacement of fuel assemblies are required.

In addition, in order to reuse Pu produced in blanket fuel, a fuel assembly containing core fuel and blanket fuel, which is taken out as a spent fuel assembly from the core of the fast reactor after the lapse of a predetermined combustion period, is recycled. A series of recycling processes are required, such as transportation to a processing facility, recovery of Pu from the nuclear fuel in the spent fuel assembly at the reprocessing facility, and processing of the recovered nuclear fuel into core fuel.

Each fuel element described in JP-A-64-32189 and JP-A-2014-10022 does not have blanket fuel. Therefore, in the core of the fast reactor loaded with each fuel assembly containing each fuel element, the fuel conversion ratio of the core fuel is less than 1, and therefore the fuel needs to be replaced frequently.

Therefore, the inventors worked to develop a long-life fuel assembly that can further reduce the frequency of fuel replacement.

An object of the present invention is to provide a fast reactor fuel assembly and a fast reactor core capable of further reducing the replacement frequency of fuel assemblies loaded in the core.

Features of the present invention that achieve the above objectives include:
having multiple fuel elements,
The fuel element includes a first fuel zone in which a liquid metal fuel containing fertile fuel material resides, a second fuel zone located above the first fuel zone and a second fuel zone in which molten salt fuel containing fissionable material and fertile fuel material resides. , and a third fuel zone disposed above the second fuel zone and in which nuclear fuel material including the fertile fuel material is present;
The upper end of the first fuel zone is in contact with the lower end of the second fuel zone, and the upper end of the second fuel zone is in contact with the lower end of the third fuel zone.

During operation of the fast reactor, the fertile fuel in the second fuel region moves from the second fuel region to the first fuel region in contact with the second fuel region below the second fuel region in the fuel element during operation of the fast reactor. In the first fuel zone, the fertile fuel supplied from the second fuel zone is converted into fissile material, and the fissionable material produced by conversion in the first fuel zone is transferred from the first fuel zone to the first fuel zone. The nuclear fuel material contained in the nuclear fuel material in the second fuel zone is supplied to the second fuel zone, and the fuel parent material contained in the nuclear fuel material in the third fuel zone is supplied to the first fuel zone through the second fuel zone and converted into fissile material in the first fuel zone, Since this fissile material is also supplied from the first fuel zone to the second fuel zone, the life of the fuel assembly having the fuel element is further lengthened, and the replacement of the fuel assembly loaded in the core of the fast reactor is correspondingly longer. The frequency is further reduced.

The above purpose is
having multiple fuel elements,
The fuel element is arranged above the nuclear fuel material filling region filled with a mixture of liquid metal fuel containing fuel parent material and molten salt fuel containing fissionable material and fuel parent material, and above the nuclear fuel material filling region, having a nuclear fuel material region in which nuclear fuel material including parent fuel material exists;
It can also be achieved by contacting the upper edge of the nuclear fuel material filled area with the lower edge of the nuclear fuel material area.

When the operation of the fast reactor is started, the liquid metal fuel containing the fuel parent material and the molten salt fuel containing the fissile material and the fuel parent material are mixed and filled in the nuclear fuel material filling area in the fuel element. There is a first fuel region that melts and within the fuel element resides a molten liquid metal fuel and a molten salt fuel positioned above and in contact with the first fuel region. A second fuel zone is formed. As a result, the philophilic substance in the second fuel region is supplied from the second fuel region to the first fuel region arranged below the second fuel region, and is supplied from the second fuel region in the first fuel region. The supplied proto-fuel material is converted into fissile material, the fissile material produced by the conversion in the first fuel zone is supplied from the first fuel zone to the second fuel zone, and the nuclear fuel in the nuclear fuel material zone The fuel parent material contained in the material is supplied to the first fuel area through the second fuel area and converted into fissile material in the first fuel area, and this fissile material is also supplied from the first fuel area to the second fuel area. Therefore, the life of the fuel assembly having the fuel element is further extended, and the replacement frequency of the fuel assembly loaded in the core of the fast reactor is further reduced.

A preferred configuration of a method of replenishing nuclear fuel material in a fuel assembly is described below.

(A) having a plurality of fuel elements, the fuel elements being arranged above the first fuel zone in which a liquid metal fuel containing the fissionable material and the fertile fuel material is present; and a third fuel region disposed above the second fuel region and containing nuclear fuel material containing the fuel parent material, the upper end of the first fuel region being the second A method of replenishing nuclear fuel material in a fast reactor fuel assembly in which the upper end of a second fuel region is in contact with the lower end of a third fuel region, the method comprising:
when the nuclear fuel material in the third region is extinguished, removing the fuel assemblies loaded in the core of the fast reactor out of the reactor vessel;
Remove the upper end plug blocking the upper end of the cladding tube of the fuel element contained in the removed fuel assembly,
Thereafter, a nuclear fuel material containing a fuel parent material is replenished from the upper end of the cladding tube into the cladding tube to form a third fuel area located above the second fuel area,
A method of replenishing nuclear fuel material in a fast reactor fuel assembly in which the upper end of a cladding tube is closed by an upper end plug.

According to the present invention, it is possible to further reduce the replacement frequency of the fuel assemblies loaded in the core.

1 is a vertical cross-sectional view of a fuel element used in a fast reactor fuel assembly of Embodiment 1, which is a preferred embodiment of the present invention; FIG. 2 is a cross-sectional view of a fuel assembly having a plurality of fuel elements shown in FIG. 1; FIG. FIG. 3 is a perspective view of a core of a fast reactor loaded with a plurality of fuel assemblies shown in FIG. 2; FIG. 4 is a vertical cross-sectional view of the core of the fast reactor shown in FIG. 3 along the central axis; FIG. 3 is an explanatory diagram showing a method of replenishing nuclear fuel material in the fuel assembly shown in FIG. 2, where (a) shows the state of fuel elements included in the fuel assembly before startup of the fast reactor, and (b) shows the fuel elements. (c) is the state in which the upper end plug of the fuel element is removed and the spent fuel is replenished in the cladding tube from the top end of the cladding tube; and (d) is an explanatory diagram showing a state in which the upper end of the cladding tube is closed with an upper end plug after the spent fuel has been replenished into the cladding tube. FIG. 4 is a vertical cross-sectional view of a fuel element included in a fuel assembly of a fast reactor with a burnup of 0 GWd/t according to Embodiment 2, which is another preferred embodiment of the present invention; FIG. 2 is a vertical cross-sectional view of a fuel element included in a fuel assembly of a fast reactor according to Japanese Patent Application No. 2017-238372, which is a prior application of the present invention; FIG. 3 is a vertical cross-sectional view of a fuel element included in a fast reactor fuel assembly of Example 3, which is another preferred example of the present invention; FIG. 9 is a longitudinal cross-sectional view along the central axis of the core of a fast reactor loaded with fuel assemblies having a plurality of fuel elements shown in FIG. 8; FIG. 10 is a longitudinal cross-sectional view of blanket fuel elements included in blanket fuel assemblies loaded in a blanket region surrounding a core fuel region in the core of a fast reactor according to a fourth embodiment, which is another preferred embodiment of the present invention; FIG. 11 is a vertical cross-sectional view of the core of the fast reactor of Example 4 shown in FIG. 10 along the central axis; FIG. 10 is a vertical cross-sectional view of a fuel element included in a fuel assembly of a fast reactor of Example 5, which is another preferred example of the present invention; FIG. 2 is a longitudinal cross-sectional view along the central axis of the core of a fast reactor loaded with fuel assemblies having a plurality of fuel elements shown in FIG. 1;

A long-life fuel assembly that reduces the frequency of fuel replacement is proposed in Japanese Patent Application No. 2017-238372. An outline of this fuel assembly will be described below with reference to FIG.

The fuel assembly with a burnup of 0 GWd/t proposed by Japanese Patent Application No. 2017-238372 supports the lower ends of the plurality of fuel elements 2D shown in FIG. It is constructed by surrounding 2D with a trumpet tube, which is a cylindrical structure. The lower end of the wrapper tube is attached to the entrance nozzle. A plurality of such fuel assemblies are loaded into a core of a cylindrical fast reactor to form a core of a cylindrical fast reactor.

The fuel element 2D is placed in a cladding tube 3 whose upper end is closed by an upper end plug 4 and whose lower end is closed by a lower end plug 5, from bottom to top, a liquid metal fuel region 7 and a molten salt fuel region. 6 is placed. The molten salt fuel zone 6 is arranged above the liquid metal fuel zone 7 . A gas plenum 13 is formed in the cladding tube 3 above the molten salt fuel region 6 . Molten salt fuel 9, such as PuCl ₃ , UCl ₃ , NaCl and MgCl ₂ , contains fissionable material (e.g. ²³⁹ Pu etc.) and fuel fertile material (e.g. ²³⁸ U) at a burnup of 0 GWd/t, The molten salt fuel region 6 is filled. A liquid metal fuel 10 , for example a U—Bi alloy, is filled in the liquid metal fuel region 7 . The liquid metal fuel 10 is blanket fuel. The liquid metal fuel 10 does not contain fissile material and contains a fuel parent material (for example, ²³⁸ U) at a burnup of 0 GWd/t.

During the operation of the fast reactor, in the molten salt fuel region 6 of the fuel element 2D, the fissile material such as ²³⁹ Pu, which is the main component of the molten salt fuel 9, is consumed by its own nuclear fission reaction (that is, burning of the nuclear fuel). be. Furthermore, fissile materials ( ²³⁹ Pu, etc.) are newly generated in the molten salt fuel region 6 by the neutron capture reaction in which ²³⁸ U, which is the parent material of the fuel contained in the molten salt fuel 9, captures the neutrons generated by the nuclear fission reaction of ²³⁹ Pu. generated in

Furthermore, in the liquid metal fuel region 7 of the fuel element 2D, ²³⁸ U, which is the parent fuel material that is the main component of the liquid metal fuel 10, captures neutrons leaking from the molten salt fuel region 6, and the neutron capture reaction causes fissile properties. It is transmuted into substances ( ²³⁹ Pu etc.). That is, in the liquid metal fuel region 7 of the fuel element 2D, fissile material ( ²³⁹ Pu, etc.) is produced.

At the boundary between the molten salt fuel region 6 and the liquid metal fuel region 7, the molten salt fuel 9 and the liquid metal fuel 10 contact each other in a molten state. , ²³⁸ U chloride contained in the molten salt fuel 9 becomes ²³⁸ U metal by the chemical reaction shown in formula (2) below, and ²³⁹ Pu metal contained in the liquid metal fuel 10 converts ²³⁹ Pu chloride becomes.

The chloride ²³⁹ Pu produced as described above has a lower specific gravity than the liquid metal fuel 10 , while the metal ²³⁸ U has a higher specific gravity than the molten salt fuel 9 . Therefore, ²³⁹ Pu of the chloride diffuses into the molten salt fuel 9 which is the core fuel existing in the molten salt fuel region 6 , and ²³⁸ U of the metal diffuses into the liquid metal which is the blanket fuel existing in the liquid metal fuel region 7 . Diffusion into the fuel 10 .

The chemical reaction of formula (2) occurring at the boundary between the molten salt fuel 9 and the liquid metal fuel 10 continues until the concentration of each substance involved in this chemical reaction reaches a predetermined equilibrium condition. For this reason, as long as the chemical reaction of formula (2) continues, ²³⁸ U, which is the parent fuel material, is transferred from the molten salt fuel region 6 where the molten salt fuel (core fuel) 9 exists to the liquid metal fuel (blanket fuel) 10. ²³⁹ Pu, which is fissile material, is continuously supplied from the liquid metal fuel region 7 to the molten salt fuel region 6 .

As a result, in this embodiment, the effective conversion ratio of the core is increased by the conversion ratio of the blanket fuel 10, as shown in Equation (1) below.

(Effective conversion ratio of core)
= (core fuel conversion ratio) + (blanket fuel conversion ratio) (1)
A fuel assembly loaded into the core of a fast reactor comprising a plurality of fuel elements 2D having a liquid metal fuel region 7 and a molten salt fuel region 6 disposed above the region 7 and in contact with the liquid metal fuel region 7 is: , the fissile material ²³⁹ Pu is burned in the molten salt fuel region 6 in each fuel element 2D, but the supply of ²³⁸ U from the molten salt fuel region 6 to the liquid metal fuel region 7, Since ²³⁹ Pu, which is a fissile material, is continuously supplied from the molten salt fuel area 6 to the molten salt fuel area 6, the fissile material such as ²³⁹ Pu can be burnt in the molten salt fuel area 6 for a long period of time. Therefore, the replacement frequency of the fuel assembly having a plurality of fuel elements 2D proposed in Japanese Patent Application No. 2017-238372 can be reduced. As a result, it is possible to greatly improve fuel economy in the core of a fast reactor loaded with fuel assemblies having a plurality of fuel elements 2D.

The inventors further conducted various studies in order to realize a fuel assembly that can further reduce the replacement frequency compared to the fuel assembly having a plurality of fuel elements 2D proposed in Japanese Patent Application No. 2017-238372. . As a result of this study, the inventors arranged a molten salt fuel region in contact with the liquid metal fuel region above the liquid metal fuel region, and brought the nuclear fuel material containing the parent fuel material into contact with the molten salt fuel region. It has been found that by arranging it above the molten salt fuel region, it is possible to further reduce the replacement frequency compared to the fuel assembly having multiple fuel elements 2D proposed by Japanese Patent Application No. 2017-238372.

Any one of spent fuel, depleted uranium, and natural uranium is used as the nuclear fuel material containing the fuel parent material, which is brought into contact with the molten salt fuel region and arranged above the molten salt fuel region.

Examples of the present invention that reflect the above study results will be described below.

Embodiment 1 A fast reactor fuel assembly of Embodiment 1, which is a preferred embodiment of the present invention, will be described with reference to FIGS. 1 and 2. FIG.

In the fuel assembly 24 of this embodiment, a bundle of a plurality of fuel elements 2 each having a wire spacer (not shown) wound around its outer surface is placed in a trumpet tube 14 which is a tubular structure having a regular hexagonal cross section. are placed. A plurality of fuel elements 2 are arranged in an equilateral triangular lattice within the wrapper tube 14 . The lower end of each fuel element 2 is supported by an entrance nozzle (not shown). The lower end of the wrapper tube 14 is attached to the entrance nozzle. A gap of predetermined width is formed between adjacent fuel elements 2 by the wound wire spacers. This gap is a coolant passage through which the liquid metal coolant flows.

The fuel element 2 includes a molten salt fuel zone (second fuel zone) 6 and a liquid metal fuel zone (second 1 fuel zone 7 is located within a cladding tube 3 closed by upper and lower end plugs 4 and 5 . The molten salt fuel region 6 is in contact with the upper edge of the liquid metal fuel region 7 and is positioned above the liquid metal fuel region 7 . The molten salt fuel 9 filled in the molten salt fuel region 6 is, for example, PuCl ₃ , UCl ₃ , NaCl and MgCl ₂ and is core fuel. Molten salt fuel 9 contains fissionable material (for example, ²³⁹ Pu, ²⁴¹ Pu, etc.) and fuel host material (for example, ²³⁸ U) in fuel assembly 24 with a burnup of 0 GWd/t. The liquid metal fuel 10 filled in the liquid metal fuel region 7 is, for example, a U--Bi alloy, which is blanket fuel. This liquid metal fuel 10 does not contain fissionable material in the fuel assembly 24 with a burnup of 0 GWd/t, but contains a fuel host material (for example, ²³⁸ U).

Molten salt fuel 9 and liquid metal fuel 10 are in contact with each other at the boundary between molten salt fuel region 6 and liquid metal fuel region 7 .

The melting point of PuCl ₃ , UCl ₃ , NaCl and MgCl ₂ as the molten salt fuel 9 is 837°C. For example, the melting point of PuCl ₃ , UCl ₃ , NaCl and MgCl ₂ was reduced to 500° C. by decreasing the respective concentrations of PuCl ₃ and UCl ₃ with the molar ratio of NaCl and MgCl ₂ ranging from 5:5 to 2:1. You can also: By adjusting the respective amounts of PuCl ₃ , UCl ₃ , NaCl and MgCl ₂ , the melting points of PuCl ₃ , UCl ₃ , NaCl and MgCl ₂ can be made to the desired melting point. In addition, the melting point of the U-Bi alloy that is the liquid metal fuel 10 is in the range of 880°C to 1200°C, and by changing the ratio of Bi, the melting point of the U-Bi alloy can be adjusted to the desired melting point within the range of 880°C to 1200°C. can do.

Unlike the fuel element 2D, the fuel element 2 is attached to the inner surface of the cladding tube 3 with a support member (for example, wire mesh) 12 having a large number of through-holes arranged at the upper end of the molten salt fuel region 6. Furthermore, in the cladding tube 3, a spent fuel zone (nuclear fuel material zone) (third fuel zone) 11D containing the spent fuel 11, which is a nuclear fuel material containing a fuel affinity substance (for example, ²³⁸ U), is attached to its support member. Forming on 12. Since the specific gravity of the spent fuel 11 is greater than that of the molten salt fuel 9 and the liquid metal fuel 10 , the support member 12 is positioned within the cladding tube 3 from above the molten salt fuel region 6 toward below the spent fuel 11 . supports the spent fuel 11 so that it does not fall. A gas plenum 13 is formed within the cladding tube 3 above the spent fuel region 11D. The gas plenum 13 is filled with inert gas.

Instead of the spent fuel 11, natural uranium or depleted uranium, which is a nuclear fuel material containing a parent fuel material, may be used. When natural uranium or depleted uranium is used, the natural uranium or depleted uranium is also filled on the support member 12 provided inside the cladding tube 3 .

In the fuel assembly 24 with a burnup of 0 GWd/t, the liquid metal fuel region 7 in the cladding tube 3 of the fuel element 2 is filled with a large number of solid liquid metal fuels 10 in the form of pellets. The salt fuel region 6 is filled with a large number of pellet-shaped solid molten salt fuel 9, and the spent fuel region 11D in the cladding tube 3 is filled with a large number of pellet-shaped solid spent fuel 11. there is The spent fuel 11 is oxide fuel in spent fuel assemblies removed from the cores of light water reactors, such as boiling water reactors and pressurized reactors, and other fast reactors.

The liquid metal fuel 10 , the molten salt fuel region 6 and the spent fuel region 11 D within the fuel element 2 are the nuclear fuel material filling region 8 . The axial length of the nuclear fuel material filling region 8 is called the effective fuel length. The active fuel length of the fuel assembly 24 is the same as the active fuel length of the fuel element 2 . In all the fuel assemblies 24 loaded in the core 1, the position of the boundary between the liquid metal fuel region 7 and the molten salt fuel region 6 in each fuel element 2 from the lower end of the fuel effective length is the same. there is

A core 1 disposed in a reactor vessel (not shown) of a fast reactor has a plurality of fuel assemblies 24 containing a plurality of fuel elements 2, as shown in FIG. A dashed-dotted line shown in FIG. 4 indicates the central axis of the core 1 . FIG. 4 shows a longitudinal section of the core 1 along the central axis. A core 1 loaded with a plurality of fuel assemblies 24 containing a plurality of fuel elements 2 forms a liquid metal fuel layer 7B, a molten salt fuel layer 6B and a nuclear fuel material layer 11A from the lower end to the upper end in the axial direction. ing. The liquid metal fuel region 7 in each fuel element 2 contained in each fuel assembly 24 loaded in the core 1 is arranged in the liquid metal fuel layer 7B. 2 is formed by a liquid metal fuel region 7 . The molten salt fuel region 6 in each fuel element 2 included in each fuel assembly 24 loaded in the core 1 is arranged in the molten salt fuel layer 6B. 2 formed by the molten salt fuel region 6 . In the nuclear fuel material layer 11A, a spent fuel region ( A nuclear fuel material zone 11D is arranged and the nuclear fuel material layer 11A is formed by the spent fuel zone 11D in these fuel elements 2. FIG.

Some fuel assemblies 24 in the core 1 are fuel assemblies with a burnup of 0 GWd/t. Fast reactor operation begins. Liquid metal (e.g., liquid sodium), which is a coolant present in the reactor vessel, is fed from the entrance nozzle into the wrapper tube in the fuel assembly 24 and into the coolant passages formed between the fuel elements 2. rises and is released to the outside of the fuel assembly 24 .

During operation of the fast reactor, in each fuel element 2 of the fuel assembly 24, in the molten salt fuel region 6, fissile substances such as ²³⁹ Pu, which are the main components of the molten salt fuel 9, undergo nuclear fission by capturing neutrons, generate heat. The heat generated is transferred to the liquid metal coolant that rises in the coolant passages formed between the fuel elements 2 . This liquid metal cools the fuel element 2 . The liquid metal (coolant) whose temperature is raised by the generated heat is discharged from the upper end of the fuel assembly 24 into the reactor vessel. As the nuclear fission progresses, fissile materials such as ²³⁹ Pu contained in the molten salt fuel 9 are consumed. Further, fissile substances ( ²³⁹ Pu, etc.) are newly generated in the molten salt fuel region 6 by a neutron capture reaction in which ²³⁸ U contained in the molten salt fuel 9 captures neutrons generated by the nuclear fission reaction of ²³⁹ Pu.

When the operation of the fast reactor is started, nuclear fission of fissionable material such as ²³⁹ Pu occurs in each fuel element 2, and the temperature inside the fuel element 2 becomes higher than the melting points of the molten salt fuel 9 and the liquid metal fuel 10. , the molten salt fuel 9 and the liquid metal fuel 10 become molten. Therefore, at the boundary between the molten salt fuel region 6 and the liquid metal fuel region 7, the molten salt fuel 9 and the molten liquid metal fuel 10 come into contact with each other. Since the specific gravity of the molten salt fuel 9 is smaller than that of the molten liquid metal fuel 10, the molten salt fuel 9 is located above the molten liquid metal fuel 10 in the cladding tube 3.

Furthermore, in the liquid metal fuel region 7 of the fuel element 2, ²³⁸ U, which is the parent material of the liquid metal fuel 10 and is the main component of the liquid metal fuel 10, captures neutrons leaking from the molten salt fuel region 6, and the neutron capture reaction causes fissile properties. It is transmuted into substances ( ²³⁹ Pu etc.). That is, even in the liquid metal fuel region 7 of the fuel element 2, fissile material ( ²³⁹ Pu, etc.) is newly generated.

In the vicinity of the boundary between the molten salt fuel region 6 and the liquid metal fuel region 7 where the molten salt fuel 9 and the liquid metal fuel 10 are in contact with each other in a molten state, the chemical reaction shown in the following formula (2) causes the molten salt fuel 9 to ^{238 U of chloride contained becomes 238 U} of metal, and ²³⁹ Pu of metal contained in the liquid metal fuel 10 becomes ²³⁹ Pu of chloride.

Pu (metal) + UCl ₃ (chloride) → PuCl ₃ (chloride) + U (metal) (2)
The reason why the chemical reaction represented by formula (2) occurs is that between U and Pu, Pu is more likely to bond to chlorine (Cl).

The chloride ²³⁹ Pu has a lower specific gravity than the liquid metal fuel 10 and the metal ²³⁸ U has a higher specific gravity than the molten salt fuel 9 ^{. 238} U diffuses into the liquid metal fuel region 7 . As long as the chemical reaction of equation (2) continues, the metal ²³⁸ U will continue to be supplied to the liquid metal fuel region 7 and the chloride ²³⁹ Pu will continue to be supplied to the molten salt fuel region 6 .

By the way, in the liquid metal fuel region 7, as a result of the chemical reaction of equation (2), an amount of ²³⁹ Pu remains in equilibrium with the molten salt fuel 9. A nuclear fission reaction of ²³⁹ Pu remaining in the liquid metal fuel region 7 and a slight nuclear fission reaction of ²³⁸ U contained in the liquid metal fuel 10 in the liquid metal fuel region 7 also occur.

Of the fission product nuclides (hereinafter referred to as FP (Fission Products) nuclides) generated by the fission reaction in the liquid metal fuel region 7 of each fuel element 2, most of the FP nuclides excluding platinum group FP nuclides are liquid It has a lower specific gravity than the liquid metal fuel 10 in the metal fuel region 7 . Therefore, the generated FP nuclide rises to the boundary between the molten salt fuel region 6 and the liquid metal fuel region 7, and furthermore, the molten salt of the molten salt fuel 9 in contact with the liquid metal fuel 10 in the molten salt fuel region 6 A chemical reaction represented by the following formula (3) occurs with MgCl ₃ (chloride), which is a component, and diffuses into the molten salt fuel region 6 in the form of FP salt.

FP (simple substance) + (3/2) MgCl ₃ (chloride)
→FPCl ₃ (chloride) + (3/2) Mg (metal) (3)
In other words, almost no FP nuclides remain in the spent liquid metal fuel 10 except platinum group FP nuclides. Therefore, the process of recovering the FP nuclides from the spent liquid metal fuel 10 becomes almost unnecessary, that is, the process of separating the FP nuclides in the reprocessing of the spent liquid metal fuel 10 becomes almost unnecessary. Therefore, the reprocessing process of the spent fuel assembly 24 is greatly simplified.

In the fuel assembly 24 of this embodiment, a spent fuel region 11</b>D containing spent fuel 11 , which is a nuclear fuel material region, exists above the molten salt fuel region 6 in each fuel element 2 . The action of the spent fuel 11 within this spent fuel region 11D will be described below.

During operation of the fast reactor, molten salt fuel 9 is in contact with spent fuel 11 on support member 12 through numerous through-holes of support member 12 . When the uranium concentration in the molten salt fuel region 6 of the fuel element 2 decreases due to nuclear reactions during operation of the fast reactor, the spent fuel 11 dissolves into the contacting molten salt fuel 9 . Therefore, the uranium concentration in the molten salt fuel region 6 is kept uniform.

²³⁸ U (metal), which is a fuel parent substance contained in the spent fuel 11 dissolved in the molten salt fuel 9, diffuses into the molten salt fuel region 6 where the molten salt fuel 9 exists. Part of the ²³⁸ U (metal) diffused in the molten salt fuel region 6 captures neutrons in the molten salt fuel region 6 and is converted into ²³⁹ Pu, which is a fissile material. This ²³⁹ Pu becomes chloride ²³⁹ Pu in the molten salt fuel region 6 .

In addition, the remainder of ²³⁸ U (metal) that occupies most of the ²³⁸ U (metal) diffused in the molten salt fuel region 6 is the liquid metal fuel 10 in which ²³⁸ U (metal) is melted in the molten salt fuel region 6 as it is. descends to the liquid metal fuel region 7 where is present. ²³⁸ U (metal) descending to the liquid metal fuel region 7 captures neutrons leaking from the molten salt fuel region 6 in the liquid metal fuel region 7 and is converted to ²³⁹ Pu. In the vicinity of the boundary between the molten salt fuel region 6 and the liquid metal fuel region 7, the spent fuel 11 on the support member 12 diffuses into the molten salt fuel region 6 due to the chemical reaction shown in the above formula (2), and the liquid ²³⁹ Pu converted from the aforementioned ²³⁸ U (metal) descending to the metal fuel region 7 also becomes chloride ²³⁹ Pu in the liquid metal fuel region 7 . The chloride ²³⁹ Pu generated in the liquid metal fuel region 7 diffuses into the molten salt fuel region 6 because it has a lower specific gravity than the liquid metal fuel 10 . Further, due to the chemical reaction, the chloride ²³⁸ U contained in the molten salt fuel 9 becomes metal ²³⁸ U, and this ²³⁸ U diffuses into the liquid metal fuel region 7 . As described above, ²³⁹ Pu converted from ²³⁸ U (metal), which is a fuel parent substance contained in the spent fuel 11 , in the liquid metal fuel region 7 is supplied to the molten salt fuel region 6 .

In the fuel assembly 24 with a burnup of 0 GWd/t, the spent fuel 11 on the support member 12 contains fissionable Pu ( ²³⁹ Pu, etc.) and minor actinides (hereinafter referred to as MA) in addition to the fuel parent material. . MA includes ²³⁷ Np, ²⁴¹ Am, ^242M Am, ²⁴³ Am, ²⁴³ Cm, ²⁴⁴ Cm, ²⁴⁵ Cm and ²⁴⁶ Cm. The fissile Pu and MA contained in the spent fuel 11 dissolved in the molten salt fuel 9 diffuse into the molten salt fuel region 6 . The fissionable Pu diffused in the molten salt fuel region 6 captures neutrons in the molten salt fuel region 6 and undergoes nuclear fission. This nuclear fission also consumes fissile Pu contained in the spent fuel 11 . The MA that has diffused into the molten salt fuel region 6 also captures neutrons and undergoes nuclear fission. Due to this nuclear fission, the MA contained in the spent fuel 11 is also consumed in the molten salt fuel region 6 and eventually disappears. MA generated by fission of fissile substances ( ²³⁹ Pu, etc.) in the molten salt fuel region 6 also captures neutrons, undergoes nuclear fission, and annihilates.

A portion of ²³⁸ U existing in the spent fuel region 11D filled with the spent fuel 11 in the fuel element 2 captures neutrons leaking from the molten salt fuel region 6 and is converted to ²³⁹ Pu. ²³⁹ Pu produced by this conversion is also contained in the spent fuel 11 dissolved in the melted molten salt fuel 9 . The ²³⁹ Pu produced in the spent fuel region 11D also captures neutrons in the molten salt fuel region 6 and undergoes nuclear fission.

This embodiment has a liquid metal fuel region 7 in which liquid metal fuel 10 is present and a molten salt fuel region 6 in which molten salt fuel 9 is present and which is positioned above and in contact with the liquid metal fuel region 7 . A fuel assembly that includes a plurality of fuel elements 2 and is loaded into a core 1 of a fast reactor burns ²³⁹ Pu, which is a fissile material, in a molten salt fuel region 6 within each fuel element 2. The supply of fertile fuel material (for example, ²³⁸ U) from the region 6 to the liquid metal fuel region 7 and the supply of ²³⁹ Pu, which is a fissile material, from the liquid metal fuel region 7 to the molten salt fuel region 6 continue. Therefore, the fissile material such as ²³⁹ Pu in the molten salt fuel region 6 can be burnt for a long period of time. Therefore, the fuel assembly 24 of the present embodiment loaded in the core 1 of the fast reactor has a replacement frequency similar to that of the fuel assembly having a plurality of fuel elements 2D proposed in Japanese Patent Application No. 2017-238372. can be reduced.

In the fuel assembly 24 of this embodiment, the liquid metal fuel 10 in the liquid metal fuel region 7, i.e., the blanket fuel, is not reprocessed, and during the operation of the fast reactor, the fertile fuel is removed in the liquid metal fuel region 7. The fissile material (e.g., ²³⁹ Pu) converted from (e.g., ²³⁸ U) can be continuously supplied to the molten salt fuel area 6, and fission of the fissile material in the molten salt fuel area 6 can be continued. persists over the long term.

In this embodiment, the spent fuel region 11D in which the spent fuel 11 exists is formed above the molten salt fuel region 6 within the fuel element 2. As described above, the spent fuel region 11D and the molten salt In each of the fuel region 6 and the liquid metal fuel region 7, the fuel parent material (eg, ²³⁸ U) contained in the spent fuel 11 captures neutrons and is converted into fissile material (eg, ²³⁹ Pu). Each fissile material (for example, ²³⁹ Pu) converted from the fuel parent material (for example, ²³⁸ U) contained in the spent fuel 11 in each of the spent fuel region 11D and the liquid metal fuel region 7 is molten salt fuel. Fissile material (e.g., ²³⁹ Pu) supplied to region 6 and converted from fuel parent material (e.g., ²³⁸ U) contained in spent fuel 11 within molten salt fuel region 6 contribute to nuclear fission in

Thus, in the fuel assembly 24 of this embodiment, the spent fuel region 11D in which the spent fuel 11 exists is formed within the fuel element 2. Therefore, the fuel proposed in Japanese Patent Application No. 2017-238372 Compared to the fuel assembly with element 2D, the fuel assembly 24 has substantially as ^{much fissile material (e.g. 239 Pu} ) can be increased. Therefore, the fuel assembly 24 of this embodiment has a longer life than the fuel assembly having the fuel elements 2D proposed in Japanese Patent Application No. 2017-238372, Also the replacement frequency is reduced. This embodiment can extend the effective operating cycle length of the fast reactor. Fuel economy in the core 1 of the fast reactor loaded with the fuel assemblies 24 can be further improved.

According to this embodiment, since each fuel element 2 included in the fuel assembly 24 is filled with the spent fuel 11, the spent fuel 11 can be reprocessed while the fuel assembly 24 is in operation. can be carried out within

In the fuel assembly 24 of this embodiment, the life of the fuel assembly 24 can be further extended by replenishing each fuel element 2 with nuclear fuel material, that is, spent fuel. A method of replenishing the nuclear fuel material in the fuel assembly 24 will be specifically described with reference to FIG. In FIG. 5, (a) shows the state of the fuel element 2 included in the fuel assembly 24 with a burnup of 0 GWd/t before the startup of the fast reactor, and (b) shows the molten salt fuel region in the fuel element 2. 6 shows a state in which the spent fuel 11 existing above 6 has disappeared, and (c) removes the upper end plug 4 of the fuel element 2 and replenishes the spent fuel 11 in the cladding tube 3 from the upper end of the cladding tube 3. and (d) shows the state in which the upper end of the cladding tube 3 is closed with the upper end plug 4 after the spent fuel 11 has been replenished into the cladding tube 3 .

In the fuel element 2 included in the fuel assembly 24 with a burnup of 0 GWd/t before startup of the fast reactor shown in FIG. A large number of filled solid molten salt fuels 9 in the form of pellets are filled in the molten salt fuel region 6, and a large number of solid spent fuels 11 in the form of pellets are filled in the spent fuel region 11D. Molten salt fuel 9 and liquid metal fuel 10 are not included.

When the fast reactor starts operating and the temperature of the nuclear fuel material in the fuel element 2 becomes higher than the melting points of the molten salt fuel 9 and the liquid metal fuel 10, the molten salt fuel 9 and the liquid metal fuel 10 melt in the fuel element 2. melt. Even if the molten salt fuel 9 and the liquid metal fuel 10 are melted, the arrangement of the molten salt fuel region 6 and the liquid metal fuel region 7 within the fuel element 2 is maintained as shown in FIG. 5(a). When the operating time of the fast reactor elapses, the spent fuel 11 filled in the fuel element 2 of the fuel assembly 24 eventually dissolves into the molten salt fuel 9 in the molten salt fuel region 6, and the spent fuel 11 disappears (see FIG. 5(b)).

When the spent fuel 11 in the fuel element 2 is extinguished, the fast reactor is shut down. A fuel assembly 24 loaded in the core 1 of the fast reactor and in which the spent fuel 11 in the fuel element 2 has disappeared is taken out of the reactor vessel of the fast reactor. The fuel assembly 24 removed from the reactor vessel is transported to a replenishment area where replenishment of nuclear fuel material (eg, spent fuel 11) is performed.

In this replenishment area, the dismantling process of the fuel assembly 24 is performed. In this step the wrapper tube of the fuel assembly 24 is removed from the entrance nozzle and the lower end plug 5 of each fuel element 2 is removed from the entrance nozzle. A wire spacer wrapped around the outer surface of the cladding tube 3 of the fuel element 2 is removed. The upper end plug 4 is removed from the upper end of the cladding tube 3 . After removing the upper end plug 4, an opening is formed at the upper end of the cladding tube 3 (see FIG. 5(c)).

After the dismantling process of the fuel assembly 24 is completed, the process of supplying the spent fuel 11 into the cladding tube 3 is carried out. That is, the cladding tube 3 is filled with a large number of pellet-like solid spent fuels 11 through an opening formed at the upper end of the cladding tube 3 . A predetermined number of pellets of charged spent fuel 11 are placed on a support member (eg wire mesh) 12 located above the molten salt fuel region 6 . An upper end plug 4 is attached to the upper end of the cladding tube 3 to seal the cladding tube 3 (see FIG. 5(d)).

After that, the reassembly process of the fuel assembly 24 is carried out in the replenishment area. That is, one end of the wire spacer is attached to the lower end plug 5 , the wire spacer is wound around the outer surface of the cladding tube 3 in which the wire spacer is sealed, and the other end of the wire spacer is attached to the upper end plug 4 . The lower end plug 5 of each fuel element 2 is attached to the entrance nozzle and the bundle of fuel elements 2 is inserted into the wrapper tube. The lower end of the wrapper tube surrounding the bundle of fuel elements 2 is attached to the entrance nozzle.

The dismantling process of the fuel assembly 24, the replenishment process of the spent fuel 11, and the reassembly process of the fuel assembly 24 are performed by remote control in a sealed area within the replenishment area.

If natural uranium or depleted uranium, which is a nuclear fuel material containing a parent fuel material, is used instead of the spent fuel 11, the cladding tube A large number of solid natural uranium or depleted uranium pellets are filled into the cladding tube 3 through an opening formed at the upper end of the cladding tube 3 .

The reassembled fuel assemblies 24 are reloaded into the core 1 of the fast reactor. The fast reactor is then restarted. After restarting, the molten salt fuel 9 and liquid metal fuel 10 in each fuel element 2 contained in the reloaded fuel assembly 24 melts. The operation of the fast reactor with the fuel assemblies 24 loaded in the core 1 continues until the replenished spent fuel 11 in the fuel elements 2 disappears. The life of the fuel assembly 24 having the fuel element 2 replenished with the spent fuel 11 is further extended, and the replacement frequency of the fuel assembly 24 is further reduced.

Embodiment 2 A fast reactor fuel assembly of Embodiment 2, which is another preferred embodiment of the present invention, will be described with reference to FIG.

The fuel assembly of Example 2 has a plurality of fuel elements 2A shown in FIG. In the fuel element 2</b>A, the particulate (or powder) molten salt fuel 9 and the liquid metal fuel 10 are mixed and filled below the support member (for example, wire mesh) 12 . Spent fuel 11 is filled above the support member 12 within the cladding tube 3 of the fuel element 2A. In the fuel element 2A, the region below the support member 12 and filled with mixed particulate (or powder) molten salt fuel 9 and liquid metal fuel 10 is called a nuclear fuel material filling region. The nuclear fuel material zone is in contact with spent fuel 11 .
The mixing ratio of the particulate (or powdery) molten salt fuel 9 and the particulate (or powdery) liquid metal fuel 10 in the mixed cladding tube 3 is the fuel element 2 of the fuel assembly 24 of the first embodiment. is the same as the ratio of the molten salt fuel 9 in the molten salt fuel region 6 and the liquid metal fuel 10 in the liquid metal fuel region 7 in .

In a fuel assembly having a burnup of 0 GWd/t and having fuel elements 2A, the particulate (or powder) molten salt fuel 9 and the particulate (or powder) liquid metal fuel 10 in the fuel elements 2A are molten. do not have. After the fuel assemblies are loaded into the core of the fast reactor, the fast reactor starts operating. When the temperature of the fuel element 2A rises above the respective melting points of the molten salt fuel 9 and the liquid metal fuel 10 due to nuclear fission of the fissionable material (239Pu, etc.) contained in the molten salt fuel 9 in the fuel element 2A, the fuel element 2A melts. The salt fuel 9 and the liquid metal fuel 10 are melted, and the melted salt fuel 9 and the melted liquid metal fuel 10 are separated below the support member 12 . As a result, similarly to the fuel element 2 of the fuel assembly 24 of Example 1, the molten salt fuel region 6 in which the molten salt fuel 9 exists and the liquid metal fuel region 7 in which the liquid metal fuel 10 exists are formed in the fuel element 2A. is formed and a molten salt fuel region 6 is formed above the liquid metal fuel region 7 in contact with the liquid metal fuel region 7 .

The fuel assembly of this embodiment having the fuel element 2A can obtain the effects obtained in the first embodiment.

Embodiment 2 A fast reactor fuel assembly of Embodiment 2, which is another preferred embodiment of the present invention, will be described with reference to FIGS. 8 and 9. FIG.

The fuel assembly of this embodiment has a plurality of fuel elements 2B shown in FIG. The fuel element 2B differs from the fuel element 2 used in Example 1 only in the molten salt fuel region where the molten salt fuel exists and the liquid metal fuel region where the liquid metal fuel exists. Same as 2. That is, fuel element 2B has molten salt fuel region 6A in which molten salt fuel 9A resides and liquid metal fuel region 7A in which liquid metal fuel 10A resides. In a fuel assembly having a burnup of 0 GWd/t and having fuel elements 2B, molten salt fuel 9A containing highly enriched fissile Pu (for example, ²³⁹ Pu) exists in molten salt fuel region 6A, and liquid metal fuel In region 7A there is liquid metal fuel 10A containing low enrichment of fissile Pu (eg ²³⁹ Pu). The fissile Pu enrichment of the liquid metal fuel 10A is smaller than the fissile Pu enrichment of the molten salt fuel 9A. The molten salt fuel 9A containing highly enriched fissile Pu specifically contains PuCl ₃ , UCl ₃ , NaCl, and MgCl ₂ . The liquid metal fuel 10A containing low-enrichment fissile Pu specifically contains a Pu--U--Bi alloy. A major difference of the fuel element 2B of this embodiment from the fuel element 2 of Embodiment 1 is that the liquid metal fuel region of the fuel element 2B contains fissionable Pu (for example, ²³⁹ Pu).

The molten salt fuel region 6A of the fuel element 2B is filled with a large number of solid liquid metal fuels 10 in the form of pellets, and the molten salt fuel region 6 in the cladding tube 3 is filled with a large number of solid molten salt fuel in the form of pellets. 9 is filled, and the spent fuel region 11D in the cladding tube 3 is filled with a large number of solid spent fuels 11 in the form of pellets.

The fissile Pu enrichment in the molten salt fuel 9A may be the same as the fissile Pu enrichment in the molten salt fuel 9 in the fuel element 2 included in the fuel assembly 24 of the first embodiment.

A core 1A of a fast reactor loaded with a plurality of fuel assemblies having fuel elements 2B of this embodiment will be described with reference to FIG. A core 1A loaded with a plurality of fuel assemblies including a plurality of fuel elements 2B forms a liquid metal fuel layer 7C, a molten salt fuel layer 6C and a nuclear fuel material layer 11A from the lower end to the upper end in the axial direction. there is The liquid metal fuel region 7A in each fuel element 2B contained in each fuel assembly loaded in the core 1A is arranged in the liquid metal fuel layer 7C. is formed by the liquid metal fuel region 7A within. A molten salt fuel region 6A in each fuel element 2B included in each fuel assembly loaded in the core 1A is arranged in the molten salt fuel layer 6C. formed by the molten salt fuel region 6A within. The nuclear fuel material layer 11A is the same as the nuclear fuel material layer 11A of the core 1 .

Also in the fuel assembly of this embodiment, in the fuel element 2B, like the fuel element 2 in the fuel assembly 24 of Embodiment 1, the nuclear fission reaction of ²³⁹ Pu occurs in the molten salt fuel region 6A, and the liquid metal fuel region 7A ²³⁹ Pu is produced by the neutron capture reaction of ²³⁸ U at .

Furthermore, in the vicinity of the boundary between the molten salt fuel region 6A and the liquid metal fuel region 7A, the chemical reaction represented by the formula (2) occurs as in the first embodiment. Therefore, due to this chemical reaction, ²³⁹ Pu, which has become a chloride, diffuses from the liquid metal fuel region 7A into the molten salt fuel region 6A in the fuel element 2B, and ²³⁸ U, which has become a metal, diffuses into the molten salt fuel region. It diffuses from 6A into the liquid metal fuel region 7A.

The spent fuel 11 filled on the support member 12 within the fuel element 2B also acts in the same manner as the spent fuel 11 within the fuel element 2 in the fuel assembly 24 of the first embodiment.

This embodiment can obtain each effect produced in the first embodiment. Furthermore, in this embodiment, in the fuel element 2B of the fuel assembly with a burnup of 0 GWd/t, since the liquid metal fuel 10A in the liquid metal fuel region 7A contains low-enrichment fissile Pu, the liquid metal The supply of ²³⁹ Pu from the fuel zone 7A to the molten salt fuel zone 6A is started at an early point after the start of operation of the fast reactor. Also, the amount of ²³⁹ Pu supplied is greater than in the first embodiment. Therefore, the effective conversion ratio of the core in this embodiment is higher than in the first embodiment.

Further, in the present embodiment, in the reprocessing of the liquid metal fuel 10A containing ^fission Pu of low enrichment, which is present in the liquid metal fuel region 7A, platinum group FP nuclide Only a small number of FP nuclides, such as , need to be removed. That is, the liquid metal fuel 10A can be refined rather than reprocessed, so that the recycling process is greatly simplified.

A core of a fast reactor of Example 4, which is another preferred example of the present invention, will be described with reference to FIGS. 10 and 11. FIG.

A core 1B, which is the core of the fast reactor of this embodiment, has a central region 21 and an annular peripheral region 22 surrounding the central region 21, as shown in FIG. This peripheral area 22 is a blanket area. The central region 21 is loaded with a plurality of fuel assemblies 24 used in Example 1, and the outer peripheral region 22 is loaded with a plurality of blanket fuel assemblies (Fig. not shown) is loaded.

A blanket fuel assembly comprises a bundle of a plurality of blanket fuel elements 16 with wire spacers (not shown) wrapped around their outer surfaces disposed within a wrapper tube 14, as shown in FIG. A plurality of blanket fuel elements 16 are arranged in an equilateral triangular lattice within the wrapper tube 14 . The lower end of each blanket fuel element 16 is supported by an entrance nozzle (not shown). The lower end of the wrapper tube 14 is attached to the entrance nozzle. Between the adjacent fuel elements 2, the wound wire spacers form a gap of a predetermined width, which is a coolant passage through which the liquid metal coolant flows.

The blanket fuel element 16 comprises a molten salt blanket fuel region 17 in which a molten salt blanket fuel 19 resides and a liquid metal blanket fuel 20 in a cladding tube 3 sealed at its upper and lower ends by upper and lower end plugs 4 and 5 . An existing liquid metal blanket fuel region 18 is formed. Molten salt blanket fuel region 17 is positioned above liquid metal blanket fuel region 18 and contacts the top edge of liquid metal blanket fuel region 18 . Molten salt blanket fuel 19 is a molten salt blanket fuel containing UCl ₃ , NaCl and MgCl ₂ . Liquid metal blanket fuel 20 is a liquid metal blanket fuel containing a U—Bi alloy.

Specifically, in the blanket fuel element 16, the molten salt blanket fuel region 17 within the cladding tube 3 is filled with a multiplicity of solid molten salt blanket fuel 19 in the form of pellets, and the liquid metal blanket fuel within the cladding tube 3 is filled. Region 18 is filled with a multiplicity of solid liquid metal blanket fuel 20 in the form of pellets.

In blanket fuel element 16, molten salt blanket fuel region 17 and liquid metal blanket fuel region 18 are nuclear fuel material filling region 8A. The axial length of the nuclear fuel material filling region 8A is called the effective fuel length. The effective fuel length of a blanket fuel assembly with blanket fuel elements 16 is the same as the effective fuel length of the blanket fuel elements 16 . In all the blanket fuel assemblies loaded in the core 1B, the position of the boundary between the molten salt blanket fuel region 17 and the liquid metal blanket fuel region 18 in each blanket fuel element 16 from the lower end of the fuel effective length is the same position. It has become.

In a blanket fuel assembly with a burnup of 0 GWd/t having blanket fuel elements 16, the molten salt blanket fuel 19 filled in the molten salt blanket fuel region 17 and the liquid metal blanket fuel 20 filled in the liquid metal blanket fuel region 18 are , are both solid. At a burnup of 0 GWd/t, the molten salt blanket fuel 19 and the liquid metal blanket fuel 20 contain fertile material (such as ²³⁸ U) but do not contain fissile material (such as ²³⁹ Pu).

When the operation of the fast reactor having the core 1B is started, fission reactions of fissile substances ( ²³⁹ Pu, etc.) first occur in the molten salt fuel regions 6 of the fuel elements 2 existing in the central region 21 . When the temperature inside the fuel element 2 of the fuel assembly 24 becomes higher than the melting points of the molten salt fuel 9 and the liquid metal fuel 10, each of the molten salt fuel 9 and the liquid metal fuel 10 becomes molten. Also, when the temperature within blanket fuel element 16 rises above the melting points of molten salt blanket fuel 19 and liquid metal blanket fuel 20, each of molten salt blanket fuel 19 and liquid metal blanket fuel 20 also becomes molten.

At the boundary between molten salt blanket fuel region 17 and liquid metal blanket fuel region 18, molten salt blanket fuel 19 and molten liquid metal blanket fuel 20 come into contact. Since the specific gravity of the molten salt blanket fuel 19 is smaller than the specific gravity of the molten liquid metal blanket fuel 20, the molten salt blanket fuel 19 is positioned above the molten liquid metal blanket fuel 20 in the cladding tube 3. It will be.

In each fuel assembly 24 loaded in the central region 21, as described in Example 1, metal ²³⁸ U of molten salt fuel region 6 feeds liquid metal fuel region 7, chloride ²³⁹ Pu liquid Supply from the metal fuel region 7 to the molten salt fuel region 6 is performed, and further, ²³⁸ U contained in the spent fuel 11 in the spent fuel region 11D, the spent fuel region 11D, the molten salt fuel region 6 and the liquid Conversion to ²³⁹ Pu in the metal fuel region 7 takes place.

In each blanket fuel element 16 of the blanket fuel assembly, the fuel affinity material ( ²³⁸ U, etc.) present in each of the molten salt blanket fuel region 17 and the liquid metal blanket fuel region 18 is also the molten salt fuel region of the fuel element 2 . Neutrons leaking from 6 are captured and transmuted into fissile materials ( ²³⁹ Pu, etc.). The fissionable material ( ²³⁹ Pu, etc.) thus produced burns by fission in the molten salt blanket fuel region 17 and the liquid metal blanket fuel region 18 .

This embodiment can obtain each effect produced in the first embodiment. Furthermore, in this embodiment, since the core 1B surrounds the central region 21 with the outer peripheral region 22, the conversion ratio of the entire core 1B is the conversion ratio of the core fuel (molten salt fuel 9) in the central region 21, the conversion ratio of the core fuel (molten salt fuel 9) in the central region It is the sum of the conversion ratio of blanket fuel (liquid metal fuel 10) at 21 and the conversion ratios of molten salt blanket fuel 19 and liquid metal blanket fuel 20 at peripheral region 22, respectively.

Therefore, in this embodiment, the effective conversion ratio of the core can be greatly increased compared to the first embodiment, and the life of the fuel assemblies 24 having the fuel elements 2 can be extended. , the fuel economy of fast reactors can be improved.

The blanket fuel element 16 also produces many fission product nuclides (FP nuclides) due to the fission reaction of fissile material ( ²³⁹ Pu, etc.) in the liquid metal blanket fuel 20 . These FP nuclides have a lower specific gravity than the liquid metal blanket fuel 20, except for the platinum group FP nuclides. Therefore, the FP nuclides other than the platinum group FP nuclides rise to the boundary between the molten salt blanket fuel region 17 and the liquid metal blanket fuel region 18 . The FP nuclides that have risen to the boundary between the molten salt blanket fuel region 17 and the liquid metal blanket fuel region 18 undergo a chemical reaction represented by the formula (3) with the molten salt blanket fuel 19 to become FP salts. The generated FP salt diffuses into the molten salt blanket fuel region 17 .

Therefore, almost no FP nuclides remain in the spent liquid metal blanket fuel 20 except platinum group FP nuclides. Therefore, the process of recovering the FP nuclides from the spent liquid metal blanket fuel 20 becomes almost unnecessary, so that the reprocessing process of the spent liquid metal blanket fuel 20 is greatly simplified.

In the central region 21 of the core 1B of this embodiment, fuel assemblies having the fuel elements 2A of the second embodiment and fuel assemblies having the fuel elements 2B of the third embodiment are provided instead of the fuel assemblies 24 of the first embodiment. , and a fuel assembly having fuel elements 2C of Example 5 described below.

A fast reactor fuel assembly of Example 5, which is another preferred example of the present invention, will be described with reference to FIGS. 12 and 13. FIG.

The fuel assembly of Example 5 has a plurality of fuel elements 2C shown in FIG. The fuel element 2C has a configuration in which an adsorbent 23 that adsorbs FP nuclides is added to the fuel element 2 used in the fuel assembly 24 of the first embodiment. This adsorbent 23 is installed on the inner surface of the cladding tube 3 of the fuel element 2C and arranged above the spent fuel region 11D. The adsorbent 23 is specifically arranged within the gas plenum 13 . Other configurations of the fuel element 2C are the same as those of the fuel element 2C.

A core 1C of a fast reactor loaded with fuel assemblies having a plurality of fuel elements 2C is, as shown in FIG. It has an additional configuration. The adsorbents 23 in the fuel elements 2C used in the fuel assemblies 24 are arranged in the adsorbent layer 23A of the core 1C.

When the fast reactor in which the fuel assembly having a plurality of fuel elements 2C is loaded into the core starts operating, in the fuel element 2C, metal ²³⁸ U is supplied from the molten salt fuel area 6 to the liquid metal fuel area 7, ²³⁹ Pu of chloride is supplied from the liquid metal fuel area 7 to the molten salt fuel area 6, and furthermore, in the spent fuel area 11D ²³⁸ U contained in the spent fuel 11 is converted into ²³⁹ Pu in the spent fuel region 11D, the molten salt fuel region 6 and the liquid metal fuel region 7.

FP nuclides such as Cs produced by the nuclear fission reaction of ²³⁹ Pu reach the gas plenum 13 from the molten salt fuel region 6 . FP nuclides such as Cs in the gas plenum 13 are adsorbed by the adsorbent 23 and removed.

The adsorbent 23 provided in the fuel element 2C in this embodiment is applied to each of the fuel element 2A used in the fuel assembly of the second embodiment and the fuel element 2B used in the fuel assembly of the third embodiment. may

This embodiment can obtain each effect produced in the first embodiment. Furthermore, in this embodiment, since the adsorbent 23 is arranged above the spent fuel region 11D in the fuel element 2C, the adsorbent 23 adsorbs FP nuclides such as Cs generated by the nuclear fission reaction of ²³⁹ Pu. Since it can be removed, the pressure rise in the cladding tube 3 can be suppressed.

1, 1A, 1B, 1C... core, 2, 2A, 2B, 2C... fuel element, 3... cladding tube, 6, 6A... molten salt fuel region, 6B, 6C... molten salt fuel layer, 7, 7A... liquid metal Fuel regions 7B, 7C Liquid metal fuel layer 8, 8A Nuclear fuel material filling region 9, 9A Molten salt fuel 10, 10A Liquid metal fuel 11 Spent fuel 11A Nuclear fuel material layer 11D Spent fuel region (nuclear fuel material region) 12 Support member 13 Gas plenum 16 Blanket fuel element 17 Molten salt blanket fuel region 18 Liquid metal blanket fuel region 19 Molten salt blanket fuel 20 ... liquid metal blanket fuel, 21 ... central region, 22 ... peripheral region, 23 ... adsorbent, 24 ... fuel assembly.

Claims (10)

having multiple fuel elements,
The fuel element comprises a first fuel region in which a liquid metal fuel containing a fuel fertile material is present, a second fuel region disposed above the first fuel region and a molten salt fuel containing fissile material and a fuel fertile material is present. and a third fuel region disposed above the second fuel region and in which nuclear fuel material including fertile fuel material is present;
A fuel for a fast reactor, wherein the upper end of the first fuel zone is in contact with the lower end of the second fuel zone, and the upper end of the second fuel zone is in contact with the lower end of the third fuel zone. Aggregation. 2. The fast reactor fuel assembly according to claim 1, wherein said nuclear fuel material present in said third fuel zone is present on a support member having a plurality of through-holes provided within said fuel element. 3. The fast reactor fuel assembly according to claim 1, wherein an adsorbent is arranged in said fuel element above said third fuel zone. The liquid metal fuel contains a fissile material in addition to the fuel parent material,
4. The fast reactor according to any one of claims 1 to 3, wherein the enrichment of the fissile material contained in the liquid metal fuel is lower than the enrichment of the fissile material contained in the molten salt fuel. fuel assembly. 5. The fast reactor fuel assembly according to any one of claims 1 to 4, wherein said nuclear fuel material present in said third fuel zone is any one of spent fuel, natural uranium, and depleted uranium. having multiple fuel elements,
The fuel element is arranged above a nuclear fuel material filling region filled with a mixture of a liquid metal fuel containing a fuel parent material and a molten salt fuel containing a fissile material and a fuel parent material, and above the nuclear fuel material filling region, having a nuclear fuel material region in which nuclear fuel material including parent fuel material exists;
A fuel assembly for a fast reactor, wherein an upper end of said nuclear fuel material filled region is in contact with a lower end of said nuclear fuel material region. 7. The fast reactor fuel assembly according to claim 6, wherein said nuclear fuel material existing in said nuclear fuel material zone exists on a support member having a plurality of through-holes provided in said fuel element. 8. The fast reactor fuel assembly according to claim 6, wherein an adsorbent is arranged in said fuel element above said nuclear fuel material zone. A core of a fast reactor, characterized in that a plurality of fuel assemblies according to any one of claims 1 to 8 are loaded. a central region in which the plurality of fuel assemblies are arranged; a fourth fuel region in which a liquid metal blanket fuel containing a philic substance is present; a plurality of blanket fuel assemblies having a plurality of blanket fuel elements arranged therein including a fifth fuel region in which salt blanket fuel resides and having an outer peripheral region surrounding said central region;
10. The fast reactor core according to claim 9, wherein the upper end of said fourth region is in contact with the lower end of said fifth fuel region.

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