JP5896711B2 - Fuel assembly structure for fast reactor and fuel assembly arrangement structure in fast reactor - Google Patents

Description

  The present invention relates to a fuel assembly structure for a fast reactor used in a nuclear reactor such as a fast breeder reactor using fast neutrons and a fuel assembly arrangement structure in the fast reactor.

Examples of fast neutron reactors include fast breeder reactors and fast neutron reactors not intended for breeding. These are collectively referred to as “fast reactors”.
4 to 6 show the structure of a general fast reactor fuel assembly (hereinafter referred to as “fuel assembly”) 1 used in a fast reactor. The fuel assembly 1 used in the fast reactor has a regular hexagonal cross section different from the square cross section used in the light water reactor.

  The fuel rod 10 is a rod-shaped structure having a circular cross-sectional shape in which fuel elements called pellets are vertically connected. In order to increase the utilization efficiency of fast neutrons, the fuel rod 10 is arranged in a regular hexagonal tubular outer tube 11 (also called a “trumpet tube”) in a dense equilateral triangular lattice arrangement as shown in FIG. Is housed in. In other words, the fuel assembly 1 is an assembly of fuel rods 10 having a lattice arrangement in which the centers of adjacent fuel rods 10 form an equilateral triangle 12, and the assembly of fuel rods 10 is closely attached so as to surround the periphery thereof. The outer tube 11 is provided. The outer tube 11 is a cylinder having a regular hexagonal cross section.

Also, liquid sodium is generally used as a coolant that extracts heat from the fuel element as a liquid metal that is difficult to slow down fast neutrons. This coolant extracts heat generated from the fuel element in the process of passing from the lower part to the upper part of the outer tube 11 constituting the fuel assembly 1. In addition, the code | symbol 13 in a figure is an entrance nozzle into which a cooling material flows in into the outer tube 11 inside.
In such a fast reactor, in the unlikely event of core damage, if the molten fuel concentrates in part, it may lead to a phenomenon called “recriticality” that exceeds the immediate criticality and leads to nuclear runaway. Therefore, in order to avoid this re-criticality, it is necessary to take measures to permanently create a subcritical state by quickly discharging the molten fuel at the time of core damage to the outside and dispersing it.

In order to eliminate recriticality in the event of core damage, the following patent document 1 discloses that a control rod guide tube with an open top is installed in the fuel assembly to function as a molten fuel discharge duct. Have been described.
Patent Document 2 below describes that the shaft blanket fuel part is deleted from the fuel rods to function as a molten fuel discharge duct.
Patent Document 3 below describes that a thermal opening mechanism is installed in the fuel assembly to function as a molten fuel discharge duct.

JP 2009-85650 A JP 2002-55187 A Japanese Patent No. 2839222

As mentioned above, in a fast reactor, there is a possibility that a nuclear runaway phenomenon called recriticality will occur at the time of core damage, so in order to avoid this recriticality, the molten fuel at the time of core damage must be removed early. It is necessary to take measures to create a subcritical state permanently by discharging it out of the furnace and dispersing it.
The present invention has been made to solve the above-mentioned problems, and its object is to have a molten fuel discharge / dispersion capability at the time of core damage and for a fast reactor capable of avoiding recriticality. It is an object of the present invention to provide a fuel assembly structure and a fuel assembly arrangement structure in a fast reactor.

In order to solve the above problems, the present invention employs the following means.
A fuel assembly structure for a fast reactor according to claim 1 of the present invention is a fuel assembly structure for a fast reactor that is installed in the fast reactor in a state where adjacent outer peripheral surfaces are in close contact with each other. And a plurality of fuel rods accommodated in the outer tube, and the hollow cross-sectional shape of the outer tube is partially continuous with the outer peripheral surface of the adjacent outer tube in the axial direction. It is characterized by having a gap forming portion that does not adhere to the surface.

  According to such a fuel assembly structure for a fast reactor of the present invention, it is provided with a cylindrical outer tube having a hollow cross section and a number of fuel rods housed inside the outer tube, and the hollow cross section of the outer tube. Since it has a gap forming portion that is not partially in close contact with the adjacent outer tube outer peripheral surface in the axial direction, the shape is formed along the outer tube outer peripheral surface in the axial direction by the gap forming portion of the adjacent outer tube. A continuous molten fuel discharge duct is formed.

  In the above invention, it is preferable that the fuel rod has a circular cross section and is accommodated in a large number as an equilateral triangular lattice array inside the mantle tube, thereby making it possible to increase the utilization efficiency of fast neutrons. Become.

  In the above-mentioned invention, it is preferable that the hollow cross-sectional shape of the outer tube is a regular dodecagon, which is easy to change because it lies on an extension line of a regular hexagonal cross section that has been generally adopted conventionally, There is also an advantage that there is no limit to the direction of assembly.

  The fuel assembly arrangement structure in the fast reactor according to the present invention is such that the fast reactor fuel assembly having the fast reactor fuel assembly structure according to any one of claims 1 to 3 surrounds at least the blanket fuel. It is arrange | positioned at the core center part enclosed.

  According to such a fuel assembly arrangement structure in a fast reactor, the fast reactor fuel assembly having the fast reactor fuel assembly structure according to any one of claims 1 to 3 is at least a blanket fuel. Since it is disposed in the central part of the core surrounded by the periphery, the molten fuel discharge duct can be formed at least in the central part of the core. In the blanket fuel installation area where no molten fuel discharge duct is required, a fast reactor fuel assembly having a conventional structure can be arranged to minimize the reduction in the total number of fuel rods housed and installed in the fast reactor. .

  According to the fuel assembly structure for a fast reactor and the fuel assembly arrangement structure in the fast reactor according to the present invention, in the unlikely event of core damage, the molten fuel that has become hot melts the outer tube and forms a melt in the core portion. Since it is discharged and dispersed out of the reactor early through the fuel discharge duct, it has the ability to discharge and disperse molten fuel at the time of core damage that permanently creates a subcritical state and can avoid recriticality. That is, even if the core is damaged, it is possible to avoid a nuclear runaway phenomenon called recriticality.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows one Embodiment of the fuel assembly structure for fast reactors which concerns on this invention, (a) is sectional drawing which shows the example of several adjacent fuel assembly arrangement | positioning, (b) is the fuel assembly structure for fast reactors It is an expanded sectional view showing an example. It is sectional drawing which shows the example of arrangement | positioning in the reactor of the fuel assembly for fast reactors in a fast reactor. FIG. 6 is a cross-sectional view showing a modification of the fast reactor fuel assembly structure and fuel assembly arrangement example shown in FIG. 1. It is a perspective view which shows the conventional general fuel assembly structure for fast reactors. (A) is AA sectional drawing of FIG. 4, (b) is explanatory drawing which shows the equilateral triangular lattice arrangement | sequence of an adjacent fuel element. It is sectional drawing which shows the example of arrangement | positioning in the reactor of the fuel assembly for fast reactors in a fast reactor as a prior art example.

Hereinafter, an embodiment of a fuel assembly structure for a fast reactor and a fuel assembly arrangement structure in the fast reactor according to the present invention will be described with reference to the drawings.
The fuel assembly structure for a fast reactor according to the present embodiment is a fuel used in a nuclear reactor called a “fast reactor” using fast neutrons, and is compared with the fuel assembly 1 having a conventional structure shown in FIG. The outer tube 11 is basically the same except for the cross-sectional shape.

A large number of fast reactor fuel assemblies (hereinafter referred to as “fuel assemblies”) 1A shown in FIG. 1 are installed in the reactor of the fast reactor with their adjacent outer peripheral surfaces being in close contact with each other. The five fuel assemblies 1A shown in FIG. 1 are a part of many fuel assemblies installed in the furnace, and each fuel assembly 1A is arranged so that the axis of the cylinder is in the vertical direction. Adjacent cylinder side wall surfaces (outer peripheral surfaces) are in close contact with each other.
The illustrated fuel assembly 1 </ b> A includes a cylindrical outer tube 11 </ b> A having a hollow cross section, and a large number of fuel rods 10 having a circular cross section housed as an equilateral triangular lattice array inside the outer tube 11 </ b> A. The outer tube 11A of the present embodiment is provided with a gap forming portion 20 that is not in close contact with the outer peripheral surface of the adjacent outer tube 11A in a part of the cross-sectional shape by making the hollow cross-sectional shape a regular dodecagon. The illustrated fuel rod 10 has a circular cross section that is accommodated in the outer tube 11A as an equilateral triangular lattice arrangement, but is not limited thereto.

  The gap forming portion 20 is a portion that is provided continuously in the axial direction in the cross-sectional shape of the outer tube 11A and does not adhere to the outer peripheral surface of the adjacent outer tube 11A. Thus, by providing the gap forming portion 20 that is not in close contact with the outer peripheral surface of the adjacent mantle tube 11A in the axial direction, the mantle tube is provided in the furnace in which many fuel assemblies 1A are arranged. A molten fuel discharge duct 30 that is continuous (communicated) in the axial direction is formed along the outer peripheral surface of 11A. That is, the fast reactor fuel assembly structure of the present embodiment forms a gap generated between adjacent fuel assemblies 1 </ b> A, and this gap functions as the molten fuel discharge duct 30.

  In the case of the fuel assembly 1A shown in the present embodiment, since the mantle tube 11A has a regular dodecagonal cross section, the twelve sides constituting the regular dodecagon have six sides alternately adjacent to the mantle. The contact portion 21 is in close contact with the tube 11A, and the remaining six sides are the gap forming portion 20 that is not in close contact with the adjacent outer tube 11A. As a result, as shown in FIG. 1A, a single molten fuel discharge duct 30 having an equilateral triangular cross section is formed by the adjacent three outer tube 11A. In addition, six molten fuel discharge ducts 30 are formed around one fuel assembly 1A.

Like the outer tube 11 having a regular hexagonal cross section, a large number of fuel rods 10 are accommodated in the outer tube 11A as an equilateral triangular lattice arrangement.
As described above, when the fast reactor fuel assembly structure having the outer tube 11A having a regular dodecagonal cross section is adopted, the hollow cross-sectional shape of the outer tube 11A is partially continuous with the outer surface of the adjacent outer tube in the axial direction. Since the gap forming portion 20 that does not adhere to the outer tube 11A is provided, the molten fuel discharge duct 30 that continues in the axial direction along the outer peripheral surface of the outer tube 11A can be obtained by cooperating with the gap forming portion 20 of the adjacent outer tube 11A. It is formed.

That is, in the unlikely event of damage to the core, the molten fuel that has reached a high temperature melts the outer tube 11A and flows out of the fuel assembly 1A. It functions as a molten fuel discharge passage having a molten fuel discharge / dispersion capability for quick discharge to the outside. In this case, the main driving force for discharging and dispersing the molten fuel out of the reactor is the gas pressure generated by fission of the core fuel, that is, the pressure of the fission gas held in the fuel element.
As a result, even if the core is damaged, the molten fuel at the time of core damage that permanently creates a subcritical state is discharged and dispersed through the molten fuel discharge duct 30 to the outside of the reactor at an early stage. It is possible to avoid a nuclear runaway phenomenon called recriticality caused by the concentration of.

  By the way, the outer tube 11A having a regular dodecagonal cross section has a shape in which the corners of the regular hexagonal cross section are cut as shown by a broken line in FIG. For this reason, in the outer tube 11A having a regular dodecagonal cross section, the accommodation of the fuel rods 10 accommodated in an equilateral triangular lattice arrangement is reduced around the corner cut portion. As a result, the total number of fuel rods 10 that can be accommodated is reduced. The number decreases somewhat.

  Therefore, for example, in the example of the arrangement of the fast reactor fuel assembly shown in FIG. 2, the fuel assembly 1A capable of forming the molten fuel discharge duct 30 with the outer tube 11A having a regular dodecagonal cross-sectional shape is formed in the central portion of the core. As in the prior art, a fuel assembly structure for a fast reactor using a sheath tube 11 having a regular hexagonal cross section is disposed in the fuel assembly 1 disposed as a blanket fuel on the outer periphery thereof. That is, in the fast reactor, a number of fuel assemblies 1A having the fuel assembly structure for the fast reactor according to the above-described embodiment are disposed in the center of the core, and are blankets disposed around the fuel assemblies 1A. As a fuel, a fuel assembly 1 in which a large number of fuel rods 10 having a circular cross section are accommodated in a regular triangular lattice arrangement is disposed inside a sheath tube 11 having a hollow hexagonal cross section. In other words, the fuel assembly 1 </ b> A only needs to be arranged at the center of the core surrounded by at least the blanket fuel assembly 1.

  Thus, according to the fuel assembly arrangement structure in the fast reactor using the fuel assembly 1A installed in the center of the core and the fuel assembly 1 arranged around the core as a blanket fuel, the high-temperature molten fuel Forms a molten fuel discharge duct 30 at the center of the core where the outer tube 11A is melted and flows out of the fuel assembly 1A to provide molten fuel discharge / dispersion capability, and further, the outer tube 11 is melted at a high temperature. In the blanket fuel installation area where the molten fuel discharge duct 30 is not required, the conventional fuel assembly 1 is arranged to minimize the total number of fuel rods 10 housed and installed in the fast reactor. Yes.

In the above-described embodiment, the hollow cross-sectional shape of the mantle tube 11A is a regular dodecagon, but it is not limited to this. For example, as in the modification shown in FIG. 3, the mantle tube 11B of the fuel assembly 1B. May have a circular cross section, and the molten fuel discharge duct 30 'may be formed by the arc-shaped gap forming portion 20'.
That is, in this embodiment, the outer tube 11A, 11B has a cross-sectional shape as long as the molten fuel discharge ducts 30, 30 ′ can be formed by the adjacent gap forming portions 20, 20 ′. In addition to the circular shape, various polygonal cylindrical shapes that can form a molten fuel discharge duct, such as a regular octagon and a regular dodecagon, can be employed.

However, the outer tube 11A in which the hollow cross-sectional shape of the outer tube 11 is a regular dodecagon is on the extension line of the regular hexagonal cross section that is most efficient in arranging the fuel rods 10 in a regular triangular lattice shape. Easy to change from structure. In other words, the outer tube 11A having a regular dodecagonal tube shape can be easily considered as an extension of the existing hexagonal tube shape that is the existing mantle tube 11. Therefore, when the blanket fuel is left in a regular hexagonal shape, the dodecagonal tube shape is easy. Easy to combine with.
Further, the outer tube 11B having a circular cross section has an advantage that it is easy to manufacture as compared with a cylindrical body having a polygonal cross section.

As described above, according to the fast reactor fuel assembly structure of the present embodiment, in the unlikely event of core damage, molten fuel is discharged and dispersed out of the reactor early through the molten fuel discharge duct 30 formed in the core. Therefore, it is possible to avoid a nuclear runaway phenomenon called recriticality.
In addition, this invention is not limited to embodiment mentioned above, In the range which does not deviate from the summary, it can change suitably.

1,1A, 1B Fast Reactor Fuel Assembly (Fuel Assembly)
DESCRIPTION OF SYMBOLS 10 Fuel rod 11, 11A, 11B Mantle tube 20, 20 'Gap formation part 21 Contact | adherence part 30, 30' Molten fuel discharge duct

Claims (4)

A fuel assembly structure for a fast reactor that is installed in the fast reactor in a state where the adjacent outer peripheral surfaces are in close contact with each other,
A cylindrical outer tube having a hollow cross section, and a number of fuel rods housed in the outer tube,
A hollow cross-sectional shape of the outer tube is provided with a gap forming part that does not partially adhere to the outer peripheral surface of the adjacent outer tube continuously in the axial direction ,
A fuel assembly structure for a fast reactor, wherein a hollow cross-sectional shape of the outer tube is a regular dodecagon .   2. The fuel assembly structure for a fast reactor according to claim 1, wherein the fuel rod has a circular cross section and is accommodated in the outer tube as a regular triangular lattice array. A fast reactor fuel assembly having the fuel assembly structure for a fast reactor according to claim 1 or 2 , wherein the fast reactor fuel assembly is disposed at least in a central portion of the core surrounded by a blanket fuel. Fuel assembly arrangement structure. The outer tube having a hollow cross-sectional shape of a regular dodecagon is disposed at the core central portion, and the outer tube having a hollow cross-sectional shape of a regular hexagon is disposed at an outer peripheral portion thereof. The fuel assembly arrangement structure in the fast reactor.

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