JPH1123773A - Core cooling structure of fast breeder reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance decay heat removal function by providing a side core support plate with a flow hole and a wrapper tube with an opening and forming a flow path with clear regions of an upper plenum, an in-core heat exchanger, core barrel wrapper tube gap. SOLUTION: During natural circulation, the secondary side of incore heat exchanger 8 placed in a reactor vessel 1 has natural circulation and the coolant in the upper plenum 9 is cooled by the natural circulation. The coolant flows in the heat transfer part of the heat exchanger 8, be cooled by heat exchanging with the secondary side and then exhausted from the outlet window below the heat exchanger 8 into the plenum 9. The coolant in the plenum 9 after cooled in the heat exchanger 8 spreads over the core upper part and lower temperature fluid dives into the barrel by the relation of buoyancy since low temperature and high density fluid goes over the opening provided in the side core support plate 4 placed in the periphery of the core barrel and high temperature and low density fluid goes below. This flow enhances the natural convection of the coolant in the wrapper tube gap region and works effectively on the decay heat removal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention makes use of the use of a liquid metal having high thermal conductivity as a coolant, and reduces the decay heat of the core by natural circulation of the coolant even when all AC power is lost. In a fast breeder reactor designed to remove and safely converge events, the natural convection of fluid existing between the wrapper pipes of the core (when a core cooling system is directly used for the decay heat removal system) The present invention relates to a fast breeder reactor core structure that promotes interwrapper flow (hereinafter abbreviated as IWF) and removes decay heat generated from fuel inside and outside the wrapper tube.

[0002]

2. Description of the Related Art In a fast breeder reactor, even if the operation of the reactor is continued, the core continuously generates heat for a predetermined time. Such heat generation is called decay heat. Conventionally,
Various devices have been proposed to remove this decay heat. One of them is a direct core cooling system in which a heat exchanger is immersed in a coolant in a reactor vessel to directly remove decay heat from the core. There is. FIG. 1 shows an example of the structure of a fast breeder reactor that employs a direct core cooling system including the submerged in-furnace heat exchanger, and FIG. 2 shows an example of an arrangement of core assemblies.

In FIG. 1, there is a core tank 3 in which a plurality of fuel assemblies 2 are loaded at a central portion of a reactor vessel 1, and a coolant supplied from a reactor vessel inlet pipe 18 passes through a core inlet plenum. After being distributed to each fuel assembly, and coming out of the reactor core upper plenum 9 and merging, it exits from the reactor vessel outlet pipe 17 and returns to the inlet pipe again.

When an accident such as loss of all AC power occurs, the forced circulation force of the primary coolant is lost, and decay heat is removed directly by natural circulation heat removal of the core cooling system. At this time, the in-furnace heat exchanger 8 of the direct core cooling system takes in the coolant of the upper plenum 9 heated by the decay heat from the upper window, exchanges heat, and then flows out from the lower window. In this scheme,
The major drawback is that the heat removal in the heat exchanger does not directly affect the natural circulation force of the primary system, so the coolant flow rate through the reactor core cannot be increased significantly and the cooling capacity of the fuel assembly is small. Has become.

Conventionally, as a measure for solving this drawback, a coolant existing between the fuel assembly wrapper tube 13 and the wrapper tube is used to promote natural convection in this portion, so that the fuel assembly is collected from outside the wrapper tube. Methods for cooling the inside of the body have been considered.

In the core arrangement shown in FIG. 2, decay heat is generated in the inner core 35 and the outer core 36, and a radial blanket 37, a gas expansion mechanism (abbreviated as GEM).
The wrapper 40 and the neutron shield 38 are in a region where the calorific value is small or there is no heat at all, so that the temperature distribution in the radial direction is high in the central part and low in the outer peripheral part in the core tank, and the wrapper which is originally in a stationary state is provided. The coolant in the area between the tube and the flaring tube will circulate by convection.

[0007] This natural convection of the coolant between the wrapper tubes is called an interwrapper flow (IWF). As a method of accelerating the IWF, it is conceivable to increase the gap between the pads attached to the upper part and the intermediate part as spacers of the flapper tube, or to provide a flow path in the pads.

When such a trumpet tube shape is adopted,
The coolant cooled by the in-furnace heat exchanger is stratified below the upper plenum in relation to the density and covers the upper part of the core, and enters the gap between the pads to allow the low-temperature coolant to enter the gap of the trumpet pipe. The heat removal inside the assembly can be expected. In JP-A-59-44694 and JP-A-8-240686, measures are proposed to promote the decay heat removal function on the premise that an opening is provided in the wrapper tube pad.

On the other hand, JP-A-8-62374 discloses a method of providing a flow path from the upper plenum to the outside of the core tank (intermediate plenum) or to the inside of the core tank via the core tank vessel.
A method in which a primary outlet of an in-furnace heat exchanger is penetrated through an intermediate plenum and a through-hole is further provided between the intermediate plenum and a core tank, and a neutron shield as a non-heating assembly installed on the outer periphery of the core There has been proposed a method of providing a backflow channel in the apparatus.

[0010]

[Problems to be Solved by the Invention] Japan is one of the world's leading earthquake nations.
Particularly, since the seismic design conditions of the pad portion become severe, a head-wound pad adapted to the hexagonal shape of the fuel assembly wrapper tube is currently used. In this pad shape, since there is almost no gap between the pads, the low-temperature fluid from the in-furnace heat exchanger hardly enters the core tank, and the IWF in the core tank is a closed space in the core tank as shown in FIG. Therefore, a large heat removal effect cannot be expected.

On the other hand, the method proposed in Japanese Patent Application Laid-Open No. 8-62374 is effective as a method for feeding a low-temperature fluid from a heat exchanger in a furnace into a core vessel, but the supplied coolant is cooled in the core. There is no proposal for a flow path to return to the upper plenum after it has been warmed, and considerations on the inlet side alone are not enough to make effective use of IWF.

The present invention relates to a core structure in a direct core cooling type fast breeder reactor which promotes a function of removing decay heat during natural circulation by actively and effectively utilizing IWF without impairing the seismic resistance of the core. The purpose is to provide.

[0013]

The first means for achieving the above object is that a fuel assembly is provided at a central portion in a core tank, a blanket assembly and a neutron shield are provided at a peripheral portion, and a pad for a spacer is provided. In a fast breeder reactor equipped with an in-furnace heat exchanger connected to a cooling device in the upper plenum above the core tank, supporting the neutron shield by a side support plate attached to the core tank, A flow hole is provided in a side core support plate provided between a core tank and a neutron shield, and an opening is provided in a wrapper tube of a plurality of control rod assemblies provided in the core tank, and an upper plenum is provided. ~
This is a core cooling structure of a fast breeder reactor characterized by forming a clear flow path from the in-reactor heat exchanger to the core tank trumpet tube gap region.

A second means for achieving the above object is that a fuel assembly is provided at a central portion in a core vessel, a blanket assembly, a gas expansion mechanism (GEM) and a neutron shield are provided at a peripheral portion, and a spacer for a spacer is provided. High-speed breeder with neutron shields supported by side support plates attached to the core tank with pads interposed, and an internal heat exchanger connected to a cooling device in the upper plenum above the core tank In the furnace, a flow hole is provided in a side core support plate provided between a core tank and a neutron shield, and a part of a GEM area provided in the core tank is defined as a wrapper tube gap area in the core tank. Core cooling of a fast breeder reactor characterized by forming a clear natural circulation path from the upper plenum to the heat exchanger in the furnace to the core tank wrapper tube gap area by changing the communication mechanism between the upper plenum and the upper plenum. It is a structure.

[0015] According to the first means, after the coolant in the upper plenum is cooled by the in-furnace heat exchanger, it is covered over the upper part of the core, and the side core supporting plate installed on the outer peripheral portion of the core tank. The upper part of the opening provided in the lower part becomes a fluid having a low temperature and a high density, and the lower part becomes a fluid having a high temperature and a low density. This infiltration of the low-temperature coolant promotes the natural convection of the coolant in the flared tube gap region in the core tank, and has an effect of contributing to the cooling of the fuel assembly in the center where the decay heat is generated.

The communication path between the inside of the core tank provided in the control rod assembly wrapper tube and the upper plenum allows the IWF heated at the center of the core to pass through the control rod assembly to the upper plenum. A clear natural circulation path is formed between the in-furnace heat exchanger and the core tank wrapper tube gap region, and the core can be efficiently cooled by the IWF through the in-furnace heat exchanger.

According to the second means, in the core of the fast breeder reactor employing the GEM, the low temperature fluid in the upper portion of the side support plate is supplied to the core tank by the opening provided in the side support plate, as in the first means. By changing part of the GEM region into a communication mechanism between the wrapper tube gap region in the core tank and the upper plenum, the IWF heated at the center of the core through the control rod assembly Since the upper plenum exits, the same operation as the first means is obtained.

[0018]

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

Embodiment 1 FIG. 4 is a longitudinal sectional view showing an embodiment of a fast breeder reactor structure according to the first means of the present invention. In the figure, a core tank 3 containing a fuel assembly 2 is provided at a central portion of a reactor vessel 1, and a coolant that has exited the core during natural circulation is a core upper mechanism 7, an in-core heat exchanger 8, a coolant atomizer. After flowing into the upper plenum 9 including the reactor outlet pipe 17 and the like, and moving from the coolant reactor outlet pipe 17 to cooling system equipment (not shown), the flow returns to the coolant reactor inlet pipe 18 and the core inlet plenum 5 is removed. Through the reactor core.

At the time of natural circulation, the secondary side of the in-furnace heat exchanger 8 installed in the reactor vessel also flows by natural circulation, whereby the coolant in the upper plenum 9 is cooled by natural circulation. You. In particular, the coolant flows from the entrance window above the in-furnace heat exchanger to the heat transfer section of the in-furnace heat exchanger,
After being cooled by heat exchange with the next side, it is discharged into the upper plenum from an outlet window below the in-furnace heat exchanger.

FIG. 4 is a partial longitudinal sectional view of the fuel assembly. In the core tank, a large number of fuel assemblies housed in a hexagonally-shaped cross-section wrapper tube 13 are installed, and the wrapper tube is provided near the upper end and near the upper end of the fuel-assembly heat generating portion 14 for seismic support. Upper pads 15 and intermediate pads 16 are provided as spacers.

FIG. 6 shows a horizontal cross section of a portion of the fuel assembly where a pad is provided, and FIG. 7 shows a horizontal cross section of a portion where no pad is provided.
The wrapper tube 13 around the fuel assembly has a hexagonal shape. As shown in FIG. 7, a gap region exists between the wrapper tubes, and this region is also filled with the coolant.

In the area where the pad is located, the gap is very narrow as shown in FIG. Further, as shown in FIG. 2, the arrangement of the fuel assemblies is hexagonal, and the container of the core tank is cylindrical. Therefore, between the outermost layer portion of the fuel assembly and the core tank, A core side support plate 4 is installed at the same level as the upper pad and the intermediate pad, and secures horizontal support of the fuel assembly.

FIG. 5 shows a horizontal cross section of the core side support plate, which is a zigzag support plate along the surface of the wrapper tube of the outer peripheral assembly. The gap between the spacer pad and the spacer pad is extremely narrow, less than 1 mm in design dimension, and the temperature is high when the coolant is filled. The aggregates in the peripheral part are displaced in the radial direction due to thermal expansion compared to the aggregate in the center. It is considered that the gap between the pads on the outer peripheral portion is almost completely closed.

Since the in-furnace heat exchanger is usually installed on the outer periphery of the upper plenum as shown in FIG. 3, the coolant cooled by the in-furnace heat exchanger spreads from the outer periphery toward the center.
In order to allow the cooled coolant to enter the core tank, it is necessary to sink from the outer peripheral portion, which is a relatively low-temperature region in the core tank. It is difficult to dive as it is because it is almost blocked.

In this embodiment, a flow hole 4a is provided in the core side support plate so that the upper plenum and the coolant in the core tank can easily communicate with each other.

In the core tank, several control rod assemblies 32 for inserting control rods for controlling the heat output during operation are installed in the inner core and the outer core which are the heat generating assemblies. In the present embodiment, as shown in FIG. 4, an opening 20 is provided in the wrapper tube of the control rod assembly so that the IW
The coolant heated by F facilitates the passage through the control rod assembly to the upper plenum.

FIG. 8 is a longitudinal sectional view showing the structural concept of a control rod assembly provided with an IWF outflow mechanism. As shown in the figure, the control rod assembly is composed of a handling head 23, a wrapper tube 13, an upper pad 15, an intermediate pad 16, and an entrance nozzle 24 at the outer peripheral portion, and the control rod guide tube 21, the control rod element 22, It is composed of various flow path components. In order to remove the heat generated by the control rod elements in response to neutrons from the fuel, a small amount of coolant is usually flowed inside the control rod elements.

In this embodiment, an opening 20 is provided in the wrapper tube below the upper pad 15 and below the intermediate pad. At the time of natural circulation, the fluid heated by the heat generating assembly by the IWF moves upward, and the position under each pad is expected to be the hottest.
The control rod assembly easily flows into the wrapper tube and flows out to the upper plenum through the flow paths 25 and 26 shown in FIG.

If there is an opening at this position, a flow will flow out of this hole to the outside of the trumpet tube during normal operation, but there is no problem if the flow goes out of the opening at the lower part of the upper pad above the control rod element. The opening below the intermediate pad is tapered as a flow path shape that has a different resistance ratio between the outflow and the inflow since the heat removal function of the control rod element is reduced if the flow rate flowing out of the wrapper tube is large.

In this embodiment, at the time of natural circulation, the low-temperature coolant flowing out of the in-furnace heat exchanger flows into the core tank from the flow hole attached to the core side support plate by buoyancy. There is also a flow that can sink below from the lower side core support plate at the level of the pad. The coolant that has sunk into the core tank moves to the center of the core along the intermediate pad or the lower surface of the core tank, and reaches the fuel assembly that emits decay heat in the center of the core, where it is assembled by heat conduction and heat transfer. The temperature rises due to heat exchange with the coolant inside, rises as an upflow, reaches the middle and lower end regions of the upper pad, passes through the opening of the control rod assembly, enters the control rod assembly, and then flows out to the upper plenum I do. Thereby, a natural circulation channel in the furnace as shown by 10 and 11 in FIG. 3 is formed, and the in-furnace heat exchanger can effectively act on the decay heat removal of the fuel assembly.

Embodiment 2 FIG. 9 is a longitudinal sectional view showing an embodiment of the fast breeder reactor according to the second means of the present invention. In the same drawing, a flow hole 4a in the side core support plate for allowing the low-temperature coolant from the in-furnace heat exchanger 8 to flow into the core tank as in the first means of the invention is provided.

Further, according to the present invention, in a reactor core structure provided with a gas expansion mechanism (GEM) 33 which is a means for ensuring the safety of the reactor core when a scram fails when the reactor is stopped, a part of the GEM region is provided. A core cooling structure is provided with an IWF outflow channel promoting structure.

When the control rod insertion fails during the reactor trip, the gas layer in the GEM increases the neutron leakage in the radial direction and reduces the reactivity of the core,
It is a safety mechanism to maintain fuel integrity.

FIG. 10 is a longitudinal sectional view showing the structure concept of the GEM. The GEM includes a handling head 23, an entrance nozzle 24, a wrapper tube, and upper / middle pads as shown in FIG.

During operation, the liquid level of the internal coolant is higher than the level of the fuel assembly heating section of the core.
Coolant is in communication with the core inlet plenum. When the reactor is shut down, the pump discharge pressure drops, so the internal liquid level drops below the fuel assembly heating part in the core, and the area around the heating part becomes a gas region, increasing leakage of neutrons to the outer periphery and increasing reactivity. It lowers. According to recent design considerations, GE
M need not necessarily be arranged on the entire circumference around the heat generating assembly, and a part may be designed as a dummy having no gas layer.

In the present embodiment, from the viewpoint of effectively utilizing the dummy GEM region, a mechanism for discharging IWF to the upper plenum of the furnace as shown in a vertical sectional view of FIG. It aims to obtain the same effect as the means. The IWF outflow promotion mechanism of FIG. 11 has a handling head 23 serving as a communication portion between the upper plenum and the internal coolant, and openings 20 provided below the upper pad and the lower intermediate pad for flowing the IWF into the inside. The main feature is that the entrance nozzle has no communication port with the lower core inlet plenum.

Normally, it is not necessary to provide a forced flow in the wrapper tube, so that there are few restrictions on the opening 20 and it is possible to provide as many openings as the strength allows. During natural circulation, the temperature of I
The WF passes through this opening and into the upper plenum through channels such as 30 and 31 in the figure. Since there is almost no need to provide an internal structure like a control rod assembly in the wrapper tube, the flow resistance when the coolant flows through the flow passages 30 and 31 can be extremely small.

In this embodiment, at the time of natural circulation, the low-temperature coolant flowing out of the in-furnace heat exchanger flows into the core tank from the flow hole attached to the core side support plate by buoyancy. There is also a flow that sinks down from the lower side core support plate at the level of the pad. The coolant that has sunk into the core tank moves to the center of the core along the intermediate pad or the lower surface of the core tank, and reaches the fuel assembly that emits decay heat in the center of the core, where it is assembled by heat conduction and heat transfer. The temperature rises to exchange heat with the internal coolant, and the temperature rises to reach the middle part and the lower end area of the upper pad. The upper plenum passes through the opening of the IWF outflow channel promotion mechanism installed in a part of the GEM area. Leaked to

As a result, a natural circulation flow path in the furnace as shown by 10 and 11 in FIG. 9 is formed, and the heat exchanger in the furnace can effectively act to remove decay heat of the fuel assembly.

Since the flow resistance in the IWF outflow flow rate promoting mechanism can be made very small, the flow resistance of the entire natural circulation passage in the furnace also becomes small, and the core structure has a structure in which the IWF flows more easily.

[0042]

According to the present invention, by providing a flow hole in the core side support structure, the low temperature fluid from the in-furnace heat exchanger can easily flow into the core tank, and the control rod wrapper tube can be used. By providing a communication path between the core tank and the upper plenum, the coolant heated by the convection phenomenon in the core tank easily flows out to the upper plenum, and flows through the heat exchanger in the furnace to the core tank to the upper plenum. The IWF as a pass is promoted, and the effect of improving the core decay heat removal performance during the natural circulation of the fast breeder reactor is obtained.

By providing a flow hole in the core side support structure, the low temperature fluid from the in-furnace heat exchanger can easily flow into the core tank, and a part of the GEM region communicates with the core tank and the upper plenum. By providing an IWF outflow flow rate promotion mechanism provided with a path, the coolant heated by the IWF in the core tank easily flows out to the upper plenum, and the flow path passes through the heat exchanger in the furnace, the core tank, and the upper plenum. IWF
Is promoted, and the effect of improving the core decay heat removal performance during the natural circulation of the fast breeder reactor is obtained. Since the flow resistance in the IWF outflow flow rate promoting mechanism can be made extremely small, it is expected that the IWF effect will be further improved.

[Brief description of the drawings]

FIG. 1 shows an example of a conventional fast breeder reactor structure.

FIG. 2 shows an example of a conventional fast breeder reactor fuel assembly arrangement.

FIG. 3 is a schematic longitudinal sectional view of the first embodiment of the present invention.

FIG. 4 is a partial longitudinal sectional view of a fuel assembly according to the first embodiment of the present invention.

FIG. 5 is a horizontal partial cross-sectional view of a core vessel container according to the first embodiment of the present invention.

FIG. 6 is a horizontal partial sectional view of a fuel assembly (pad portion).

FIG. 7 is a horizontal partial sectional view of a fuel assembly (portion without a pad).

FIG. 8 is a longitudinal sectional view of a control rod assembly structure according to the first embodiment of the present invention.

FIG. 9 is a schematic longitudinal sectional view of a second embodiment of the present invention.

FIG. 10 is a longitudinal sectional view of a gas expansion mechanism (GEM) structure.

FIG. 11 is a longitudinal sectional view of an IWF-accelerated structure according to a second embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reactor vessel, 2 ... Fuel assembly, 3 ... Core tank wall, 4 ...
Side core support plate, 4a: flow hole in side core support plate, 5: core inlet plenum, 6: lower plenum, 7: upper core mechanism (UIS), 8: in-core heat exchanger (DHX),
Reference numeral 9: upper plenum, 10: DHX flow path, 11: core tank inter-wrapper flow, 12: flow path in the assembly, 13: fuel assembly wrapper tube, 14: fuel assembly heating section, 15: wrapper tube Upper pad, 16 ... middle pad of wrapper pipe, 17 ... coolant reactor outlet pipe, 18 ... coolant reactor inlet pipe, 19 ... horizontal bulkhead, 20 ... IWF outlet channel, 21 ... control rod guide pipe, 22 ... control Rod element, 23 ... Handling head, 24 ... Entrance nozzle, 25 ...
Upper IWF outflow channel in the control rod assembly, 26 ... Middle IWF outflow channel in the control rod assembly, 27 ... Liquid metal in GEM,
28: inert gas in GEM, 29: inlet of liquid metal in GEM, 30: outflow channel of upper IWF, 31: intermediate IWF
Outflow channel, 32 ... IWF-promoted control rod assembly, 33 ... G
EM part IWF outflow flow rate promotion mechanism, 34 ... shielding assembly,
35: inner core, 36: outer core, 37: radial blanket, 38: neutron shield, 39: control rod assembly,
40 ... GEM (gas expansion mechanism).

Claims (2)

[Claims] 1. A fuel assembly is provided at a central portion in a core tank, and a blanket assembly and a neutron shield are provided at peripheral portions thereof with spacer pads interposed therebetween, and neutrons are provided by side support plates attached to the core tank. A fast breeder reactor that supports a shield and has a submerged-type in-furnace heat exchanger connected to a cooling device in the upper plenum above the core tank, provided between the core tank and the neutron shield. By providing a flow hole in the side core support plate, the low-temperature coolant discharged from the in-furnace heat exchanger to the upper plenum can easily flow into the gap area between the assemblies in the core tank, and into the core tank. An opening is provided in the wrapper tube of the plurality of control rod assemblies, and after the coolant in the wrapper tube gap in the core tank is heated by the IWF, the coolant flows into the wrapper tube through the control rod wrapper tube opening, In the upper plenum By order issued, the upper plenum -
A core cooling structure for a fast breeder reactor, characterized in that a clear natural circulation path is formed between a heat exchanger in a reactor and a core tank wrapper tube gap region. 2. A fuel assembly is disposed at a central portion in a core tank, and a blanket assembly, a gas expansion mechanism (GEM) and a neutron shield are disposed at a peripheral portion with a spacer pad interposed therebetween.
In a fast breeder reactor equipped with a submerged-type in-furnace heat exchanger that supports a neutron shield by a side support plate attached to a core tank and is connected to a cooling device in an upper plenum above the core tank, By providing a flow hole in the side core support plate provided between the neutron shield and
The low-temperature coolant discharged from the in-furnace heat exchanger to the upper plenum easily flows into the gap area between the assemblies in the core tank, and a part of the GEM area provided in the core tank is removed from the core tank. By changing to a communication mechanism between the trumpet pipe gap region and the upper plenum, the coolant in the trumpet pipe gap in the core tank is heated by the IWF, and then flows out to the upper plenum via the communication mechanism, and the upper plenum to the furnace A core cooling structure for a fast breeder reactor, wherein a clear natural circulation path is formed between the internal heat exchanger and the core tank trumpet tube gap region.