JPH05323076A - Fast breeder rector core and control rod assembly - Google Patents

Abstract

PURPOSE:To increase the effect of radial direction core expansion negative reactivity by making the positive control rod assembly contribution to radial direction expansion reactivity of a fast breeder reactor to be zero or negative. CONSTITUTION:A fuel assembly 6 is constituted of a core fuel region 12 charged with core fuel material, upper axial blanket region 13 and lower axial blanket region 14 arranged in upper and lower core with <238>U as the main component, a hexagonal wrapper tube 7 surrounding them, an upper spacer pad 8 and the middle spacer pad 9 arranged in the side of the wrapper tube side above the fuel rod region. A control rod assembly is constituted of hexagonal column shape control rod guide tubes 2 for inserting and withdrawing the control rods in the core region during reactor operation, the upper part of control rod assembly, upper spacer pad 3 arranged on the side of control rod guide tube just above the fuel rod region and a middle spacer pad 4 thinner than the middle spacer pad of the fuel assembly.

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 control rod assembly suitable for improving the reactivity controllability of the core and a core using the same.

[0002]

2. Description of the Related Art Due to the radial expansion of the core of a nuclear reactor,
A decrease in the reactivity of the core is described by, for example, AE Walter and AB Reynolds, "Fast Breeder Reactors" (Pergamon Press).
Company).

Conventionally, fuel assemblies for fast breeder reactors have been known as Fast Breeder Reactors.
as described in "ors", a plurality of fuel rods in which a cylindrical pellet made of a nuclear fuel material is filled in a fuel cladding tube, a hexagonal column-shaped trumpet tube surrounding the fuel rods, an upper end portion of the fuel assembly, and a fuel rod region It consisted of a spacer pad (referred to as an upper spacer pad and an intermediate spacer pad, respectively) arranged on the side of the trumpet tube immediately above the. As shown in FIG. 2, the control rod assembly has a control rod protection tube 22 in which a plurality of neutron absorbing elements 23 in which a cylindrical pellet made of a neutron absorbing substance such as boron is filled in a cladding tube. And a control rod guide tube 2 in the shape of a hexagonal column for moving the control rod into and out of the core area during operation of the reactor, and the upper end of the control rod assembly, and the side surface of the control rod guide tube immediately above the fuel rod area. It consists of the upper spacer pad 3 and the intermediate spacer pad 4 which are arranged in the above. In this case, the main cause of the radial expansion of the core is that the distance between the fuel assemblies increases due to the thermal expansion of the core support plate and the spacer pad accompanying the temperature rise of the coolant.

In another prior art, Japanese Patent Application No. 2-102823, a radial projecting mechanism for a fuel assembly spacer pad is installed to improve the response of the radial expansion of the core to the coolant temperature change. By doing so, the reactivity controllability of the core is improved.

[0005]

As shown in FIG. 3, generally, in a fast breeder reactor core, the reactivity value of neutron absorbing material such as boron becomes maximum at the core center and becomes smaller as it gets closer to the periphery of the core. Become. Therefore, when the control rod assembly moves to the outer side in the radial direction of the core, the neutron absorption reactivity value decreases, and the positive reactivity is injected into the core.

In the prior art, as described above, the spacer pad of the control rod assembly having a positive contribution to the radial expansion reactivity and that of the fuel assembly having a negative contribution have the same structure. As the temperature of the coolant rises, the control rod assembly expands in the radial direction together with the fuel assembly.Especially during control rod insertion operation, the control rod assembly has a large positive contribution and the negative radial expansion reactivity of the core. There was a problem of reducing the effect of.

An object of the present invention is to increase the effect of the negative radial expansion reactivity of the core by setting the positive contribution of the control rod assembly to the radial expansion reactivity to 0 or negative, thereby cooling the core. It is to improve the safety during heat transients such as loss of material flow.

[0008]

In order to solve this problem, a plurality of fuel elements in which a cylindrical pellet made of a nuclear fuel material is filled in a fuel cladding tube and a hexagonal column-shaped trumpet tube surrounding the fuel element are formed. A plurality of fuel assemblies, and a plurality of blanket fuel elements in which a cylindrical pellet made of a fuel parent substance is filled in a fuel cladding tube to surround a core region in which the plurality of fuel assemblies are bundled in a cylindrical shape, and the blanket fuel. Blanket region composed of multiple blanket fuel assemblies composed of hexagonal-shaped trumpet tubes surrounding elements, and multiple neutron absorptions filled with cylindrical pellets composed of neutron-absorbing substances such as boron in the cladding tube A control rod having an element surrounded by a control rod protection tube and a control rod for moving the control rod into and out of the core region during the operation of the reactor. In a core of a fast breeder reactor composed of a plurality of hexagonal prism-shaped control rod guide tubes, the control rod guide tubes are installed on the outer periphery of the control rod guide tubes in order to maintain the distance between the adjacent fuel assemblies. The thickness of the intermediate spacer pad is smaller than the thickness of the intermediate spacer pad installed on the outer periphery of the trumpet tube in order to maintain the distance between the fuel assemblies.

Further, in the core of the fast breeder reactor, the coefficient of thermal expansion of the intermediate spacer pad installed on the outer circumference of the control rod guide tube is made smaller than the coefficient of thermal expansion of the intermediate spacer pad installed on the outer circumference of the trumpet tube.

Further, in the core of the fast breeder reactor, the bending rigidity of the intermediate spacer pad installed on the outer circumference of the control rod guide tube is made larger than the bending rigidity of the intermediate spacer pad installed on the outer circumference of the trumpet tube.

Further, in the core of the fast breeder reactor,
The contact load point of the intermediate spacer pad of the control rod guide tube with the intermediate spacer pad of the fuel assembly on the center side of the core is higher in the core height direction than the contact load point with the intermediate spacer pad of the fuel assembly on the core side. Located in.

[0012]

In the core of the fast breeder reactor of the present invention, even if the temperature of the coolant rises due to loss of the flow rate of the coolant and the intermediate spacer pad thermally expands, the thickness of the intermediate spacer pad of the control rod guide tube is reduced. It is smaller than that of the fuel assembly wrapper tube, the contact between the control rod guide tube and the intermediate spacer pad of the fuel assembly wrapper tube is prevented, and the radial movement amount of the control rod guide tube in the height direction core region is almost It can be suppressed to 0. Therefore, the positive contribution of the control rod assembly to the radial expansion reactivity can be made almost zero, and the effect of the negative core radial expansion reactivity is increased.

The same applies when the thermal expansion coefficient of the intermediate spacer pad of the control rod guide tube is smaller than that of the fuel assembly trumpet tube.

When the bending rigidity of the intermediate spacer pad of the control rod guide tube is larger than that of the fuel assembly trumpet tube, the coolant temperature rises, the intermediate spacer pad thermally expands, and the control rod. Even if a contact load is applied between the guide tube and the intermediate spacer pad of the fuel assembly wrapper tube, the amount of deformation of the control rod guide tube can be reduced, so that the positive contribution of the control rod assembly to the radial expansion reactivity is: The effect of negative radial expansion reactivity of the core can be increased.

Further, the contact load point of the intermediate spacer pad of the control rod guide tube with the intermediate spacer pad of the core center side fuel assembly is set to be smaller than the contact load point of the control rod guide tube with the intermediate spacer pad of the core peripheral side fuel assembly. If it is located on the upper side in the height direction, the coolant temperature rises, the intermediate spacer pad thermally expands, and a contact load acts between the intermediate spacer pad of the control rod guide tube and the fuel assembly wrapper tube. In the core region of the control rod guide tube below the intermediate spacer pad in the height direction, a bending moment toward the center of the core is generated. Therefore, since the core region of the control rod guide tube is bent toward the center of the core, the contribution of the control rod assembly to the radial expansion reactivity can be made negative, and the effect of the negative radial expansion reactivity of the core is reduced. Will increase.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is an elevation view of two fuel assemblies adjacent to a control rod assembly in the core of the fast breeder reactor according to the present invention.

In this figure, a fuel assembly 6 includes a core fuel region 12 filled with a core fuel substance, and upper and lower parts in the core height direction, and an upper shaft blanket region 13, whose main component is ²³⁸ U, and a lower shaft. A blanket region 14, a hexagonal column-shaped wrapper pipe 7 surrounding the blanket region 14, a fuel assembly upper end portion, an upper spacer pad 8 and an intermediate spacer pad 9 arranged on the side face of the wrapper pipe immediately above the fuel rod region. To be done. Although not shown in the figure, the control rod assembly 1 is a control rod protection tube in which a plurality of neutron absorbing elements each having a cylindrical pellet made of a neutron absorbing substance such as boron filled in a cladding tube are surrounded by a control rod protection tube. Rods, a control rod guide tube 2 in the shape of a hexagonal column for moving the control rods into and out of the reactor core region during operation of the reactor, and an upper end portion of the control rod assembly, and a control rod guide pipe immediately above the fuel rod region. It is composed of an upper spacer pad 3 and an intermediate spacer pad 4 arranged on the side surface. The fuel assembly 6 and the control rod assembly 1 are attached to the core support plate 11 in a cantilever manner via the entrance nozzles 10 and 5 which are the coolant inflow portions, and the fuel assembly is shown in FIG. It is arranged so as to make light contact with other adjacent fuel assemblies not shown in the figure through the upper spacer pad 8 and the intermediate spacer pad 9. The feature of this embodiment is that the thickness of the intermediate spacer pad 4 of the control rod assembly 1 is smaller than the thickness of the intermediate spacer pad 9 of the fuel assembly 6.

During the rated operation of the nuclear reactor, the temperature of the coolant is within a certain range, and the thermal expansion amount of the upper and intermediate spacer pads is small, but the flow rate of the coolant decreases and the temperature rises. If such an event is assumed, the core fuel region 12 of the fuel assembly 6 moves outward in the core radial direction due to the contact between the adjacent assemblies due to the thermal expansion of the upper and intermediate spacer pads. On the other hand, since the intermediate spacer pad 4 of the control rod assembly 1 is thinner than the intermediate spacer pad 9 of the fuel assembly 6, the contact between the adjacent assemblies due to the thermal expansion of the intermediate spacer pad is prevented even if the coolant temperature rises. You can
The amount of movement of the region below the intermediate spacer pad outward in the radial direction of the core can be made almost zero.

As shown in FIG. 3, when the control rod assembly moves outward in the radial direction of the core, the neutron absorption reactivity value decreases and a positive reactivity is injected into the core. Therefore, according to the present embodiment, the positive contribution of the control rod assembly to the radial expansion reactivity due to factors such as loss of coolant flow rate and loss of heat removal source can be made almost zero, and negative radial expansion of the core can be achieved. The effect of reactivity can be emphasized, and the possibility of passively providing a safe reactor is increased.

Another embodiment of the present invention will be described with reference to FIG. A feature of this embodiment is that the thermal expansion coefficient of the intermediate spacer pad 4 of the control rod assembly 1 is smaller than the thermal expansion coefficient of the intermediate spacer pad 9 of the fuel assembly 6. In this embodiment, the thickness of the intermediate spacer pad of the control rod assembly 1 is
Even if the thickness of the intermediate spacer pad 9 of the fuel assembly 6 is the same, the thermal expansion amount of the control rod assembly intermediate spacer pad due to the temperature rise of the coolant can be suppressed to be small, and therefore the same effect as the embodiment of FIG. 1 can be obtained. can get. Particularly, in this embodiment, since the thickness of the intermediate spacer pad of the control rod assembly is the same as the thickness of the intermediate spacer pad of the fuel assembly, it is possible to reduce the vibration force due to the occurrence of an earthquake during the rated operation of the reactor. While avoiding the possibility of contact between the control rod assembly guide tube 2 and the fuel assembly trumpet tube 7, the above-described effects can be obtained.

Further, in the embodiment of FIG. 4, when the bending rigidity of the intermediate spacer pad 4 of the control rod assembly is made larger than the bending rigidity of the intermediate spacer pad 9 of the fuel assembly,
Even if the coolant temperature rises and the intermediate spacer pad thermally expands and a contact load acts between the control rod guide pipe and the intermediate spacer pad of the fuel assembly wrapper pipe, the amount of deformation of the control rod guide pipe can be reduced. Therefore, the same effect can be obtained.

A fourth embodiment of the present invention will be described with reference to FIG. The feature of this embodiment is that the contact load point of the intermediate spacer pad 51 of the control rod guide tube with the intermediate spacer pad of the core center side fuel assembly 52 is defined as the core peripheral side fuel assembly 5
3 is located above the contact load point with the intermediate spacer pad in the core height direction. In this embodiment, when the coolant temperature rises, the intermediate spacer pad thermally expands, and the contact load acts between the intermediate spacer pad of the control rod guide tube and the fuel assembly trumpet tube, the intermediate rod of the control rod guide tube A bending moment toward the center of the core is generated in the core region below the spacer pad in the height direction. Therefore, since the core region of the control rod guide tube is bent toward the center of the core, the contribution of the control rod assembly to the radial expansion reactivity can be made negative, and the effect of the negative radial expansion reactivity of the core is reduced. Can be further increased. Further, in the case of using the prior art, during the control rod insertion operation at the initial stage of combustion where the positive contribution of the control rod assembly to the radial expansion reactivity becomes the largest, the present embodiment uses the negative radial direction of the core. The absolute value of the expansion reactivity can be increased by about 50%. Further, in the present embodiment, the thickness, the coefficient of thermal expansion, and the bending rigidity of the intermediate spacer pad 4 are changed in the opposite direction to the respective embodiments shown in FIGS. The effect of directional expansion reactivity can be further emphasized.

In each of the embodiments shown in FIGS. 1 and 4, the thickness of the intermediate spacer pad 4 of the control rod assembly 1, the coefficient of thermal expansion,
Although the above-described effect is obtained by changing the bending rigidity and the bending rigidity relative to the intermediate spacer pad 9 of the fuel assembly 6, the bending rigidity is changed relative to the upper spacer pad 3 of the control rod assembly itself. By doing so, the same effect can be obtained.

[0025]

According to the present invention, in a fast breeder reactor,
By setting the positive contribution of the control rod assembly to the radial expansion reactivity to 0 or negative, the effect of the negative radial expansion reactivity of the core is increased, and the thermal transients such as loss of coolant flow rate are increased. The safety can be increased.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a control rod assembly and two adjacent fuel assemblies in a core according to a first embodiment of the present invention.

FIG. 2 is an elevation view (a) and a cross-sectional view (b) of a control rod assembly used in the prior art.

FIG. 3 is a core radial distribution map of neutron absorbing material reactivity value.

FIG. 4 is a cross-sectional view of a control rod assembly and two adjacent fuel assemblies in the cores according to the second and third embodiments of the present invention.

FIG. 5 is a cross-sectional view of a control rod assembly and two adjacent fuel assemblies in a core according to a fourth embodiment of the present invention.

[Explanation of symbols]

1 ... Control rod assembly, 2 ... Control rod guide tube, 3, 8 ... Upper spacer pad, 4, 9 ... Intermediate spacer pad, 5, 10
... Entrance nozzle, 6 ... Fuel assembly, 7 ... Wrapper tube, 11 ... Core support plate, 12 ... Core fuel region, 13 ... Upper shaft bracket region, 14 ... Lower shaft bracket region, 2
1 ... Handling head, 22 ... Control rod protection tube, 23 ...
Control rod element, 24 ... Metal fittings for preventing fluid vibration.

Claims (14)

[Claims] 1. A plurality of fuel assemblies composed of a plurality of fuel elements in which a cylindrical pellet made of a nuclear fuel material is filled in a fuel cladding tube and a hexagonal-shaped trumpet tube surrounding the fuel element, and the plurality of fuel assemblies. From a plurality of blanket fuel assemblies surrounding a core region where the fuel assemblies are bundled in a cylindrical shape, and comprising a plurality of blanket fuel elements filled with the pellets in the fuel cladding tube and the trumpet tubes surrounding the blanket fuel elements. A blanket region constituted, and a control rod in which a plurality of neutron absorbing elements filled with a cylindrical pellet made of a neutron absorbing substance such as boron in the fuel cladding pipe is surrounded by a control rod protection pipe, and the control rod is an atomic atom. A fast breeder reactor composed of a plurality of hexagonal prismatic control rod guide tubes for moving in and out of the core region during operation of the reactor. In the heart, and the control rod guide tubes,
The thickness of the intermediate spacer pad installed on the outer circumference of the control rod guide tube to maintain the distance between the adjacent fuel assemblies is equal to the outer circumference of the trumpet tube to maintain the distance between the fuel assemblies. A core of a fast breeder reactor, which is smaller than the thickness of the installed intermediate spacer pad. 2. The core of a fast breeder reactor according to claim 1, wherein a thermal expansion coefficient of the intermediate spacer pad of the control rod guide tube is smaller than a thermal expansion coefficient of the intermediate spacer pad of the fuel assembly. 3. The core of a fast breeder reactor according to claim 1, wherein a bending rigidity of the intermediate spacer pad of the control rod guide tube is higher than a bending rigidity of the intermediate spacer pad of the fuel assembly. 4. The fast breeder reactor according to claim 1, wherein a load point on the center side of the intermediate spacer pad of the control rod guide tube on the core center side is located above a load point on the core periphery side in the core height direction. Core. 5. The core of a fast breeder reactor according to claim 4, wherein the thickness of the intermediate spacer pad of the control rod guide tube is larger than the thickness of the intermediate spacer pad of the fuel assembly. 6. The core of a fast breeder reactor according to claim 4, wherein the coefficient of thermal expansion of the intermediate spacer pad of the control rod guide tube is higher than the coefficient of thermal expansion of the intermediate spacer pad of the fuel assembly. 7. The core of a fast breeder reactor according to claim 4, wherein the bending rigidity of the intermediate spacer pad of the control rod guide tube is smaller than the bending rigidity of the intermediate spacer pad of the fuel assembly. 8. A control rod in which a plurality of neutron absorbing elements, each of which has a cylindrical pellet made of a neutron absorbing substance such as boron filled in a cladding tube, is surrounded by a control rod protection tube, and the control rod is provided during operation of a nuclear reactor. In a control rod assembly of a fast breeder reactor, which is composed of a hexagonal column-shaped control rod guide tube for moving in and out of a core, the control rod guide tube and the fuel rod adjacent to each other are provided for maintaining a space between the adjacent fuel assemblies. The control rod assembly, wherein the thickness of the intermediate spacer pad installed on the outer circumference of the control rod guide tube is smaller than the thickness of the upper spacer pad installed on the outer circumference of the control rod guide tube. 9. The control rod guide tube according to claim 8,
The coefficient of thermal expansion of the intermediate spacer pad installed on the outer periphery of the control rod guide tube to maintain the distance between the adjacent fuel assemblies is controlled by the control rod guide in order to maintain the distance between the fuel assemblies. A control rod assembly having a coefficient of thermal expansion smaller than that of the upper spacer pad installed on the outer circumference of the tube. 10. The bending rigidity of the intermediate spacer pad installed on the outer periphery of the control rod guide pipe for maintaining a space between the control rod guide pipe and the adjacent fuel assemblies according to claim 8. A control rod assembly having a bending rigidity greater than that of the upper spacer pad installed on the outer periphery of the control rod guide tube in order to maintain the distance between the fuel assemblies. 11. The control rod according to claim 8, wherein a load point on the core center side of the intermediate spacer pad of the control rod guide tube is positioned above a load point on the core periphery side in the height direction of the core. Aggregation. 12. The control rod assembly according to claim 11, wherein the thickness of the intermediate spacer pad of the control rod guide tube is larger than the thickness of the upper spacer pad. 13. The control rod assembly according to claim 11, wherein a thermal expansion coefficient of the intermediate spacer pad of the control rod guide tube is larger than a thermal expansion coefficient of the upper spacer pad. 14. The control rod assembly according to claim 11, wherein the bending rigidity of the intermediate spacer pad of the control rod guide tube is smaller than the bending rigidity of the upper spacer pad.