JPH023157B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a thermal shielding device for a liquid metal cooled fast breeder reactor that uses liquid metal sodium or the like as a coolant.

[Technical background of the invention]

First, a general tank-type fast breeder reactor will be explained with reference to FIG.

A reactor vessel 1 having an upper opening 2 is filled with liquid metal sodium 3 as a coolant, and a reactor core 5 in which a large number of fuel assemblies 4 are arranged in an array is filled with liquid metal sodium 3 as a coolant.
is placed immersed in the sodium 3 so as to be located in the center of the furnace vessel 1. The inside of the reactor vessel 1 is vertically partitioned by a partition wall 6 connected to the outer periphery of the reactor core 5, and a pump 7 provided on the partition wall 6 is driven to move sodium 3 from below to above the core 5. It is designed to circulate within the furnace vessel 1 in a flowing manner.

The upper opening 2 of the reactor vessel 1 is closed by a roof slab 8, and in the center of the roof slab 8 is a reactor upper mechanism 9 facing directly above the reactor core 5.
is installed vertically. Further, a secondary sodium supply mechanism 10 is attached to this roof slab 8, and an intermediate heat exchanger 11 is provided in a portion penetrating the partition wall 6 below this secondary sodium supply mechanism 10.
is installed.

According to the configuration described above, the temperature in the core 5 is approximately 500 to 600℃.
The heated sodium 3 is transferred from the high temperature sodium pool 12 above the partition wall 6 to the intermediate heat exchanger 11.
is introduced into the secondary sodium supply mechanism 1.
It is cooled by heat exchange with secondary sodium from 0 and about
The temperature will be 300-400℃. Thereafter, this sodium 3 flows down to the low-temperature sodium pool 13 below the partition wall 6, is pressurized by the pump 7, and then recirculated to the core 5.

By the way, when replacing the fuel assembly 4 in the core 5, the temperature of the sodium 3 near the liquid level L is about 500 -
The temperature drops from 600℃ to about 300-400℃, and when restarted after refueling, the temperature rises again. During this temperature fluctuation, large thermal stress occurs in the thick plates of the furnace vessel 1, furnace upper mechanism 9, pump 7, intermediate heat exchanger 11, etc. that are in direct contact with the liquid level L of sodium 3, and these members May cause fatigue.

Therefore, in order to reduce temperature fluctuations in the furnace vessel 1 and reduce thermal stress, a heat shielding device, only the main part of which is partially shown in FIG. 2, has conventionally been provided.

The one in Fig. 2 shows an annular wall 2 provided around the inside of the furnace vessel 1.
1 forms an annular space (gas dam) 22 separated from the sodium 3, and a heat shield plate 23 is disposed within this annular space 22.

However, with this heat shielding device, if a large amount of sodium condenses on the inner wall of the furnace vessel 1 in the upper gas space and drips into the annular space (gas dam) and accumulates, or if sodium flows into the annular space 22 for some reason and accumulates, The thermal shielding effect (the function of inhibiting heat transfer and relieving thermal stress in the furnace vessel) is significantly reduced. The space above the free liquid level L of the sodium 3 is filled with a cover gas 14, and the roof slab 8
A gap 15 of approximately 10 mm to 50 mm is provided between the furnace vessel 1 and the furnace vessel 1 .

This cover gas 14 receives heat from the high-temperature free liquid surface L of sodium, and since the inside of the roof slab 8 has an insulating structure and the amount of heat dissipated from this part is suppressed to a low level, it can reach a considerably high temperature state. Retained.

On the other hand, since the heat source of the gap 15 is located at the bottom, the temperature becomes lower as it goes to the top.In particular, the top of the gap 15 is cooled to near room temperature, and the average temperature of the wall surface of the gap 15 is lower than that of the cover. The temperature is kept significantly lower compared to the temperature of the gas 14. Therefore, the cover gas 14 near the free liquid level L and the gap 1
A difference in density occurs between the cover gas 14 and the cover gas 14 within the cover gas 5 . Natural convection occurs between the heavy gas with high density cooled in the gap 15 and the light gas with low density near the free liquid level L, resulting in heat exchange. This natural convection causes not only a two-dimensional flow that ascends through the center of the gap and descends along the side wall, but also a flow that rotates in the circumferential direction of the roof slab 8. This creates a large temperature difference between the parts, causing large thermal stress and thermal deformation in the structural materials.

Moreover, on the contrary, since the cover gas 14 itself is cooled in the gap 15, the temperature becomes lower than that in the case where it is not cooled.

Furthermore, the saturated vapor pressure of sodium vapor in the cover gas 14 becomes lower, and the free liquid level L of sodium
Sodium evaporation from the cover gas 1
4, a large amount of sodium mist is generated, and sodium adhesion and condensation on the wall surface of the gap 15 increases.

On the other hand, a similar phenomenon occurs in the annular space 22 between the annular wall 21 and the furnace vessel 1. The temperature of the inner wall of the furnace vessel 1 is the solidification temperature of sodium (approximately 98℃)
If the temperature is maintained above, the sodium adhering to and condensing on the inner wall surface of the furnace vessel 1, the gap 15, and the wall surface near the annular space 22 is always kept in a liquid state, so that it flows down as droplets and reaches the bottom of the annular space 22. A large amount of sodium accumulates in Since the thermal conductivity of the sodium accumulated in this annular space 22 is extremely high, the heat held by the sodium 3 in the annular wall 21 is transferred to the furnace vessel 1, and the heat shielding effect created by providing the annular space 22 is impaired. There is a problem that

That is, in the conventional heat shielding device, as described above, the heat shielding effect of not only the reactor vessel 1 but also the structural members such as the roof slab 8 was not sufficient, and there was a problem that the integrity of the reactor vessel etc. was impaired.

[Purpose of the invention]

The present invention was made based on the above circumstances, and its purpose is to constantly reduce the heat flux from the coolant to the furnace vessel and the roof slab that closes the furnace vessel, and furthermore, to prevent the generation of natural convection of the cover gas. To effectively prevent thermal stress and thermal deformation of the reactor vessel, etc. caused by them, to maintain each function, to ensure the integrity of the reactor vessel, and to improve the reliability of fast breeder reactor heat. The object of the present invention is to provide a shielding device.

[Summary of the invention]

The present invention includes a furnace vessel in which a coolant is stored and has a free liquid level, a cylindrical suppression wall attached to the lower end or peripheral edge of a roof slab that closes the furnace vessel, and an inner wall of the furnace vessel. , or an annular space (gas dam) partitioned from the coolant around the entire inner circumference of the inner wall of the reactor vessel and between the portion from the upper free liquid level of the coolant to the lower free liquid level and the inner wall of the reactor vessel. and an annular wall provided to form an annular wall, and the suppression wall is suspended from the roof slab to below the free liquid level (within 1.0 m below the liquid level) with a predetermined gap between it and the annular wall. This is a heat shielding device for a fast breeder reactor.

Therefore, by providing a cylindrical restraining wall that hangs below the liquid level (within 1 m) from the lower part of the roof slab at a predetermined distance from the inner wall of the furnace vessel or the inner surface of the annular wall, the wall surface of the furnace vessel can be reduced. This prevents sodium vapor generated from the liquid surface from adhering to the annular space and the gap between the furnace vessel and the roof slab due to natural convection and diffusion, condensation, and dripping to the bottom of the annular space, thereby preventing sodium accumulation. Thermal stress, thermal deformation, etc. caused by these factors can be constantly and reliably reduced, each function maintained, and a highly reliable fast breeder reactor obtained.

The present invention is applicable to any type of furnace vessel filled with coolant with a free liquid level, and is particularly applicable to loop type and tank type furnace vessels that are often operated at coolant temperatures of 500°C or higher. Suitable for fast breeder reactor vessels.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to FIGS. 3, 4, and 5.

In FIG. 3, reference numeral 1 denotes a furnace vessel, the upper end opening of which is closed by a roof slab 8 to form a closed vessel, and the interior thereof is filled with liquid metal sodium 3 as a coolant. The liquid level L of sodium 3
The cover gas space between the roof slab 8 and the lower surface of the roof slab 8 is filled with a cover gas 14 made of an inert gas such as argon gas or helium gas.
Further, a reactor core 5 is provided at the center bottom of the reactor vessel 1, and a reactor upper mechanism 9 is provided on a roof slab 8 above the reactor core 5. A pump 7 for forcibly circulating sodium 3 and an intermediate heat exchanger 11 for exchanging heat between primary and secondary sodium are suspended and supported on the roof slab 8. The inside of the furnace vessel 1 is separated by a partition wall 6 into an upper high temperature sodium pool 12 and a lower low temperature sodium pool 13. During reactor operation, the cover gas 14 in the cover gas space absorbs and alleviates the internal pressure change in the reactor vessel 1 when the sodium 3 expands its volume as its temperature changes, and the reactor Prevents adverse effects on component equipment.

Next, the heat shielding device of the present invention shown in FIGS. 3 and 4 will be explained.

An annular space 2 is provided inside the inner wall of the furnace vessel 1 between the upper part of the liquid level L of the sodium 3 and the lower part of the liquid level, and is separated from the inner wall of the furnace vessel 1.
2, an annular wall 21 having an L-shaped cross section is provided. A predetermined gap exists between the annular wall 21 and the lower part (lower end or peripheral part) of the roof slab 8.
A cylindrical suppression wall 31 is attached to the roof slab 8 and hangs down from the lower part of the roof slab 8 to below the lower liquid level (within 1.0 m of the liquid level). The suppression wall 31 has a cover gas 1 between the roof slab 8 and the sodium 3
4 and the gas pressure in the gap 33 partitioned by the suppression wall 31 are provided with a plurality of small diameter through holes 32.

Next, the operation of one embodiment of the present invention constructed as above will be explained.

The sodium in the low-temperature sodium pool 13 is heated to about 500° C. while passing through the reactor core 5, reaches the high-temperature sodium pool 12, and evaporates from the liquid level L into the cover gas 14 space.

This cover gas 14 receives heat from the high-temperature free liquid surface L of sodium, and since the inside of the roof slab 8 has an insulating structure and the amount of heat dissipated from this part is suppressed to a low level, it is in a considerably high temperature state. is maintained.

On the other hand, since the heat source of the gap 15 is located at the bottom, the temperature becomes lower as it goes to the top. In particular, the top of the gap 15 is cooled to near room temperature, and the average temperature of the gap wall surface is the same as that of the cover gas 14. temperature is kept considerably lower than that of Therefore, a density difference occurs between the cover gas 14 near the free liquid level and the cover gas in the gap 15, and the dense heavy gas cooled in the gap 15 and the free liquid level L
Natural convection occurs and heat exchange occurs between small and light gases nearby. This natural convection causes not only a two-dimensional flow that ascends through the center of the gap and descends along the side wall, but also a flow that rotates in the circumferential direction of the roof slab 8. etc., causing large temperature differences, causing large thermal stress and thermal deformation in structural materials.

The upper surface of the roof slab 8 is at a low temperature close to room temperature, and therefore the temperature of the lower surface facing the cover gas 14 space is also lower than the temperature of the sodium 3. Therefore,
The evaporated sodium 3 into the cover gas space condenses on the lower surface of the roof slab 8. Further, the wall surface of the reactor vessel 1 is also cooled by dissipating heat to the outside through the safety vessel. The temperature of the inner wall in the cover gas space of the reactor vessel 1 is sodium 3
is lower than the liquid temperature of , so that the sodium 3 vapor condenses. This condensation of sodium 3 always progresses during reactor operation or while sodium 3 is maintained at a higher temperature than the inner wall temperature of reactor vessel 1.

Therefore, the temperature of the inner wall of the reactor vessel 1 is equal to the solidification temperature (approximately 98°C) of the liquid metal sodium 3, which is the coolant.
If the temperature is maintained above, the sodium condensed on the inner wall surface of the reactor vessel 1 is always kept in a liquid state, so that it flows down as droplets and accumulates at the bottom of the annular space 22. Since the thermal conductivity of the liquid metal sodium accumulated in this annular space 22 is extremely high, the heat held by the sodium 3 in the annular wall 21 is transferred to the reactor vessel 1.

Therefore, in this embodiment, a cylindrical restraining wall 31 is provided which hangs down from the lower part of the roof slab 8 to below the liquid level (within 1.0 m below the liquid level) with a predetermined gap from the annular wall 21. This prevents natural convection and diffusion of the sodium 3 vapor toward the inner wall surface of the reactor vessel 1, the gap 15, and the annular space 22. Furthermore, it is possible to prevent a large amount of sodium from adhering to the walls of the gap 15 and the annular space 22, condensing the same, and turning the sodium into a liquid pool and flowing down, thereby preventing sodium from accumulating at the bottom of the annular space 22.

Thereby, the annular space 22 is always filled with inert gas, so that its heat shielding effect is completely fulfilled, and generation of large thermal stress in the reactor vessel 1 can be prevented. Further, there is no large temperature difference in the gap 15 between the roof slab 8 and the inner wall surface of the reactor vessel 1, and it is possible to prevent large thermal stress and thermal deformation from occurring in structural members such as the roof slab 8.

On the other hand, it is generally known that the amount of sodium evaporated from the sodium liquid level into the cover gas is expressed by the following equation.

Δm=D・((C-Co)/y)×A×Δt Δm: Amount of diffused Na (g) D: Diffusion coefficient (cm ² /sec) C: Na evaporation concentration at the top of the Na liquid level (g/cm ² ) Co: Na vapor concentration at the top opening (g/cm ² ) y: Distance from the opening to the Na liquid level (cm) A: Surface area (cm ² ) Δt: Time (sec) From the above formula, Δm( It can be seen that (amount of sodium) ∝A (surface area).

That is, according to the present invention, by providing an extremely narrow gap 33 between the suppression wall 31 and the inner wall surface of the reactor vessel 1, the surface area on which sodium vapor is generated can be reduced, and The amount of sodium evaporated can be reduced. Taking a tank-type nuclear reactor as an example, the reactor inner diameter is approximately 22 m, and the sodium liquid surface area is approximately 380 m ² . On the other hand, if the suppression wall 31 according to the present invention is provided with a gap 33 of 0.1 m, the sodium liquid surface area in the gap 33 will be 3 m ² , and the sodium surface area ratio depending on the presence or absence of the suppression wall 31 will be 1/127. As can be seen from the relationship in the above equation, the amount of sodium evaporation is reduced to 1/127, and the amount of sodium evaporated is reduced to 1/127.
The amount of sodium penetrating into the water is greatly reduced.

Furthermore, by providing a plurality of small-diameter through holes 32 in the suppression wall 31, even if pressure fluctuations occur in the cover gas 14, the cover gas pressure is transmitted through the through holes 32, causing stress due to the pressure difference in each structural member. etc. can be prevented from occurring. Furthermore, by making the diameter of the conduction hole 32 minute, diffusion of sodium vapor can be prevented.

Furthermore, during reactor operation, the sodium liquid level fluctuates within a few percent, and in the tank type, where the fluctuation is the greatest, it is about 0.7 to 0.8 m. Suppression wall 31
is always below the sodium liquid level (within 1.0m below the liquid level)
The function is fully satisfied by maintaining the function.

FIG. 5 shows another embodiment of the present invention, which is installed in a loop-type reactor vessel.

Note that the present invention is not limited to the above embodiments.

For example, the restraining wall provided all around the inner circumference of the inner wall of the reactor vessel may be arranged by dividing the inner circumferential direction of the inner wall of the reactor vessel within a range that can maintain its function.

Further, the restraining walls may be arranged in multiple layers in the circumferential direction.

〔Effect of the invention〕

As described above, the fast breeder reactor heat shielding device of the present invention prevents natural convection of sodium vapor in the reactor to the reactor vessel wall surface, the gap between the roof slab and the reactor vessel, and the annular space, as well as adhesion and condensation of sodium. Reliably prevents stagnation due to dripping.

In addition, the high-temperature coolant can be reliably isolated from the inner surface of the furnace vessel near the liquid surface of the coolant by the gas space, reducing the amount of heat transferred from the coolant to the furnace vessel and reducing thermal stress in the furnace vessel. This makes it possible to ensure the integrity of the reactor vessel and provide a highly reliable fast breeder reactor.

[Brief explanation of drawings]

Fig. 1 is a vertical cross-sectional view schematically showing a general fast breeder reactor, Fig. 2 is a longitudinal cross-sectional view showing only the main parts of a conventional heat shielding device in an enlarged manner, and Fig. 3 is an embodiment of the present invention. FIGS. 4 and 5 are longitudinal cross-sectional views of a fast breeder reactor equipped with a fast breeder reactor equipped with a fast breeder reactor. DESCRIPTION OF SYMBOLS 1... Furnace vessel, 3... Sodium, 5... Core, 6... Partition wall, 7... Pump, 8... Roof slab, 9... Furnace upper mechanism, 11... Intermediate heat exchanger, 12... ...High temperature sodium pool, 13...Low temperature sodium pool, 14...Cover gas, L...
...liquid level, 21... annular wall, 22... annular space, 3
1... Suppression wall, 32... Conduction hole, 33... Gap between the suppression wall and the furnace vessel.

Claims (1)

[Claims] 1. Along the inner peripheral surface of the furnace vessel in which the coolant is stored,
and a cylindrical suppression wall attached to the lower end or peripheral edge of the roof slab that closes the upper opening of the furnace vessel, and a free liquid level of the coolant on the inner wall of the furnace vessel or the entire inner circumference of the inner wall of the furnace vessel. An annular wall is provided between a portion from the upper part to the lower part of the free liquid level and the inner wall of the reactor vessel to form an annular space separated from the coolant, and the suppression wall is provided between the annular wall and the inner wall of the reactor vessel. A heat shielding device for a fast breeder reactor, characterized in that the device is suspended from a lower part of the roof slab to below the free liquid level with a predetermined gap between the roof slab and the roof slab. 2. The heat shielding device for a fast breeder reactor according to claim 1, wherein the suppression wall is provided with a plurality of micro-diameter through holes in a portion located in the cover gas. 3 The lower part of the suppression wall is 1.0m from the free liquid level of the coolant.
2. A heat shielding device for a fast breeder reactor according to claim 1, wherein the heat shielding device is hung down to within a range of 100 to 100 cm.