JPH0258599B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to improvements in tank-type FBRs (fast breeder reactors);
In particular, it relates to improvements that take into account the effects of seismic behavior of liquid sodium on thermal shields under roof slabs.

[Background of the invention]

Figure 2 shows a tank type with a conventional heat shield.
The structure of the FBR is shown, showing the reactor core 9, a coolant (liquid sodium) for extracting the heat generated in the reactor core.
11, an intermediate heat exchanger 10 for transmitting the heat of the coolant to the outside, a main circulation pump 13 for circulating the coolant, and a core upper structure 8 that constitutes the core control rod group.
is contained in the reactor vessel 3 and surrounded by a safety vessel 4. The upper part of the reactor vessel 3 is joined to a roof slab 1 supported by a skirt 2. The heat shield is indicated by 5 and is suspended from the roof slab 1. This serves to block heat radiation from the free liquid surface 6 of high-temperature sodium, and consists of several layers of heat shielding plates. Furthermore, in order to suppress heat transfer due to convection, gaps between the heat shielding plates are set so that gas convection is difficult to occur. or,
Below the thermal shield 5 there is a cover gas space 12 and a free liquid level 6 of hot sodium.
Regarding the above heat shielding plate 5, for example, JP-A-54-
This is shown in Publication No. 140086.

By the way, when there is an earthquake input with a long period component, a phenomenon occurs in which the liquid level of high-temperature sodium sloshes and the free liquid surface collides with the heat shield plate 5 due to the action of the earthquake input. A phenomenon in which liquid sodium that flows between the heat shield plates 5 due to liquid level fluctuation does not flow out immediately and remains there for a while, and the weight of the sodium is applied as a load to the heat shield plates 5 (so-called seat pressure), and In this case, a phenomenon may occur in which liquid sodium remaining between the heat shield plates 5 begins to solidify due to a decrease in temperature, and the gaps between the heat shield plates 5 are connected by sodium (a so-called sodium bridge).

However, the structure of the conventional heat shielding plate 5 is disclosed in Japanese Patent Application Laid-Open No.
- As shown in No. 140086, the cover gas on the high-temperature sodium liquid surface is prevented from coming into direct contact with the slab, and the thermal radiation from the high-temperature sodium is blocked by a stack of heat shielding plates so that it does not reach the slab. and reduce the temperature and radiation dose on the top surface of the slab,
The above-mentioned phenomena that may occur during an earthquake are not taken into consideration.

[Purpose of the invention]

The purpose of the present invention is to reduce the sloshing impact pressure that occurs in a heat shield suspended from a roof slab when a seismic wave with a long period component is input to a tank-type FBR containing high-temperature liquid sodium; The objective is to increase the vertical rigidity of the slab. In an embodiment of the present invention in which the heat shield is a heat shield plate having a laminated structure with gaps between them, the seat pressure of the heat shield plate and sodium bridging caused by the inflow of liquid sodium due to sloshing is prevented. This is also the purpose.

[Summary of the invention]

Now, when the reactor vessel 3 of a rigid cylindrical tank-type FBR with a diameter D is filled with liquid sodium to a height H, the influence of vibrations such as an earthquake on the vessel will be considered. The influence of vibrations such as earthquakes largely depends on the relationship between the liquid level sloshing period (especially the first natural period) in the container and the dominant period of the input seismic wave. The dependence characteristics can be roughly classified into three types, as shown in FIG. 3, 1, 2, and 3. (However, the illustration of the roof slab and heat shield is omitted in Figure 3.) (1) Response to seismic wave input where the predominant period of the input seismic wave is shorter than the sloshing period); As shown in Figure 2, there is almost no liquid level fluctuation, and the sides and bottom face are subjected to shocking pressure. That is, when receiving such a short-period seismic input, the wave height of the high-temperature liquid sodium does not occur, so no sloshing wave crest impact occurs on the heat shield plate under the roof slab.

(2) When the dominant period of the input seismic wave is almost equal to the sloshing period (response to primary resonance seismic wave input); in this case, as shown in Figure 3,
A sloshing phenomenon occurs and a first-order liquid level fluctuation occurs. This shows a mode in which the surface 6 of the liquid vibrates strongly in a substantially planar shape, and is a direct cause of applying a large dynamic pressure to the side walls and the heat shield under the roof slab, causing their destruction. That is,
When receiving an earthquake input equal to the above-mentioned sloshing natural period, the wave height of the high-temperature liquid sodium increases, and the sodium free liquid level 6 exceeds the cover gas space, causing a sloshing wave crest impact on the heat shield. In addition, if the heat shield is of a laminated heat shield structure with mutual spacing, the high temperature liquid sodium will
The liquid sodium that has flowed into the gap between the heat shielding plates is difficult to flow down quickly, so it is easy to generate seat pressure. Further, the flowing liquid sodium remains on the heat shield plate, and as the temperature of the liquid sodium decreases, it begins to solidify, and the gaps between the heat shield plates are connected by the sodium. Create a factor that creates a so-called sodium bridge.
This sodium bridge significantly reduces the function of the heat shield plate because it facilitates heat transfer.

(3) When the dominant period of the input seismic wave is longer than the sloshing period (response to long-period seismic wave input); in this case, as shown in Figure 3,
The value of each response is very small. That is, when such an earthquake input with a period longer than the sloshing natural period is received, the wave height of the high temperature liquid sodium does not occur, and therefore no sloshing wave front impact occurs on the heat shield.

An input seismic wave usually has a component that is predominant in the acceleration wave region and a component that is predominant in the displacement wave region. The so-called sloshing phenomenon occurs when the free surface of the liquid in the container resonates with the latter, resulting in a large amplitude. The region where this displacement wave is dominant means a rather long period region with a period of about 1 to 10 seconds, and the first natural period of sloshing in a tank-type FBR vessel of normal dimensions is almost within this period region. exist. That is, when a tank-type FBR receives a rather long-period seismic input, the wave height of the high-temperature liquid sodium grows as described above, and a sloshing wave front impact occurs on the heat shield.

The present invention provides a tank-type FBR equipped with a heat shield structure whose primary purpose is to reduce these sloshing wave front impact forces.

The present invention is characterized in that a heat shield that is suspended from the lower surface of a roof slab and covers the lower surface of the roof slab is tilted downwardly toward the outside, and that the roof slab is also tilted downwardly toward the outside accordingly. It is characterized by the following. The former is effective in reducing the sloshing wave crest impact force acting on the heat shield, and the latter is effective in improving the vertical rigidity of the roof slab. Furthermore, in an embodiment of the present invention in which the heat shield is composed of laminated heat shield plates with gaps between them, the fact that the heat shields are inclined outwardly and downwardly allows the liquid sodium to be quickly drained from between the heat shield plates. It has the effect of preventing sitting pressure and sodium bridge by draining water.

The principle of the above effect will be explained below.

FIG. 4 is a model showing how the free liquid surface 6 of high-temperature liquid sodium collides with the lower layer 5 of the heat shield plate due to sloshing. In the figure,
3 is a side wall, which corresponds to the side wall of the reactor main vessel of a tank-type FBR. The angle indicates the angle between the heat shield plate 5 and the free liquid surface 6 of the high-temperature liquid sodium, and is =0° for a flat plate with a conventional structure. Here, the impact pressure P generated on the heat shield plate is the sloshing impact velocity ζ〓r
If it is constant, it depends on the angle between the heat shield plate 5 and the free liquid surface 6 of high temperature liquid sodium shown above,
The relational expression is as shown below.

P=ρ・π/2・cot(ζ〓r)〓 ………(1) where P; Impact pressure generated by sloshing ρ; Density of high-temperature liquid sodium; Angle ζ between the heat shield plate and high-temperature liquid sodium 〓r: sloshing collision speed α: constant The above equation (1) is an experimental formula, and if the sloshing collision speed ζ〓r is constant, the impact pressure P depends on the angle. Figure 5 shows the impact pressure P obtained from this experimental formula (1).
This is a graph of the relationship between angle and angle. As mentioned above, the impingement pressure P depends on the angle between the heat shield and the hot liquid sodium. From this figure, impact pressure P
It can be seen that the angle becomes smaller by making the angle several degrees larger than zero. Therefore, in order to reduce the sloshing impact pressure of the heat shield plate, it is very effective to tilt the heat shield plate outward and downward. That is, since the heat shield plate is tilted, the flow of liquid due to the sloshing impact can easily escape to the upper part of the tilted heat shield plate. On the other hand, in the horizontal flat heat shielding plate of the conventional structure, there is no place for the flow of liquid due to sloshing, so a very large sloshing impact pressure is generated.

Next, the effect of improving the vertical rigidity of the roof slab according to the present invention will be explained. FIG. 6 shows the relationship between the roof slab inclination angle θ and the roof slab rigidity _KV . From this figure, it can be seen that the vertical stiffness K _V is the smallest when the roof slab support angle θ = 0°, which has a conventional horizontal flat plate structure heat shield plate. However, by adding a slight slope to the roof slab, the roof slab structure becomes cone-shaped, and the vertical stiffness K _V suddenly increases.
FIG. 7 shows the relationship between the roof slab support angle θ and the vertical natural frequency _fV of the roof slab. Also in this figure, the natural frequency _fV can be increased by slightly tilting the roof slab support angle θ, reflecting the increase in rigidity shown above. In other words, by tilting the heat shield plate to reduce the sloshing impact pressure, it is also possible to tilt the roof slab at the same time, which further improves the effect of vertical earthquake resistance compared to tank-type FBR reactors with conventional structure. A certain reactor structure can be used.

[Embodiments of the invention]

Figure 1 shows a tank-type FBR that is an embodiment of the present invention.
1 is a schematic elevational view of the roof slab, with the roof slab sloping outwardly and downwardly, except for the penetrations of the core superstructure 8, the intermediate heat exchanger 10 and the main circulation pump 13. A heat shielding plate 5 having a laminated structure and slanting downward toward the outside is suspended so as to cover the lower surface of the housing. The configuration of other parts is the same as that in FIG. 2. FIG. 8 is an enlarged perspective view showing the vicinity of the heat shield plate of this embodiment. however,
The number of heat shielding plates shown in this figure is merely an example, and is not limited to this. As shown, the heat shield plates are suspended from the roof slab 1 with screw rods and nuts at intervals from each other.

According to the above configuration, since the heat shield plate 5 is inclined, the impact force acting on the heat shield plate due to the sloshing of the high temperature liquid sodium is greatly reduced as described above. If it collides with the shielding plate 5 and flows into the gap between the heat shielding plates, it quickly flows out because the heat shielding plates are inclined. For this reason, seat pressure due to sloshing is less likely to occur. Furthermore, as mentioned above, the liquid sodium that has flowed in does not remain in the gap between the heat shield plates, so there is no concern about the formation of sodium bridges due to solidification of the liquid sodium.

FIG. 9 shows a modification in which corrugated corrugated unevenness is applied to the inclined heat shield plate radially outward from the center of the heat shield plate. With this structure, the bending rigidity of the heat shield plate is improved compared to a flat heat shield plate. That is, deformation of the heat shield plate due to the sloshing wave crest impact can be prevented, and the effect of keeping the heat shield plate gap constant even after the sloshing wave crest impact can be expected. Furthermore, since the uneven wavy structure is applied radially outward from the center of the inclined heat shield plate, the liquid sodium flowing in through sloshing and the liquid sodium generated by the sodium mist can be quickly discharged. It is even more effective in preventing pressure build-up and sodium brittle formation.

Figure 10 shows how the heat shield plate, which has a laminated structure installed at an angle, is cut off from contact with the cover gas atmosphere above the high-temperature sodium liquid surface, and at the same time prevents sodium mist from entering from the cover gas. This is a modified embodiment of the invention in which a thin membrane 14 is provided on the outside. JP-A-54-140086 also discloses a heat shield plate coated with a thin film, but it is not arranged at an angle, and the purpose of reducing impact pressure by sloshing cannot be achieved. In contrast, in the embodiment of the present invention shown in FIG. 10, the heat shielding plate provided with the sodium contamination prevention film is tilted, which has the effect of reducing the impact pressure caused by sloshing.

FIG. 11 shows an example of this structure in which a heat insulating material 15 covered with a thin film 14 is used instead of the laminated heat shield plate, and the impact pressure due to sloshing is similarly reduced by tilting the insulating material. can.

〔Effect of the invention〕

According to the present invention, in a tank-type FBR, the heat shield suspended to cover the bottom surface of the roof slab is tilted downward toward the outside for the purpose of reducing the roof slab top surface temperature and radiation dose. Since the fluid caused by the wave crest impact caused by sloshing can easily escape, the sloshing wave crest impact force applied to the heat shield can be reduced compared to a conventional horizontal heat shield. Since it is slanted, it has the effect of improving its vertical rigidity.

In addition, in the embodiment of the present invention in which the inclined heat shield is constructed of laminated heat shield plates with mutual gaps as described above, the high temperature liquid sodium that flows into the space between the heat shield plates as the heat shield quickly flows out due to sloshing. Therefore, there is an effect of preventing the generation of seat pressure and an effect of preventing the generation of sodium bridge, which significantly reduces the heat shielding effect. ”

[Brief explanation of drawings]

Figure 1 is an elevational view of a tank-type FBR structure with an inclined heat shield structure, which is an embodiment of the present invention.
The figure shows a tank type with a conventional heat shield structure.
Elevation view of the FBR structure, Figure 3 1, 2, and 3 are illustrations of the principle of sloshing generation, Figure 4 is a model of how high-temperature liquid sodium collides with the heat shield due to sloshing, and Figure 5 is the Figure 6 shows the relationship between the sloshing impact pressure and the inclination angle of the heat shield, Figure 6 shows the relationship between the roof slab inclination angle θ and the roof slab vertical stiffness _KV , and Figure 7 shows the relationship between the inclination angle θ and the roof slab. Vertical natural frequency
Fig. ₈ is a diagram showing the vicinity of the structure of the heat shield plate according to the embodiment of the present invention, and Fig. 9 is a diagram showing the structure of the heat shield plate according to the modified embodiment of the present invention. FIG. 10 is a diagram showing a modified embodiment of the present invention in which the coated heat shield plate is tilted, and FIG. 11 is a diagram showing a modified embodiment of the present invention in which the heat shield made of a heat insulating material is tilted. It is a figure showing an example. 1... Roof slab, 2... Roof slab skirt, 3... Main container body, 4... Safety container, 5...
Heat shield plate, 6...High temperature sodium free liquid level, 7...
... Core support structure, 8 ... Core superstructure, 9 ... Core, 10 ... Intermediate heat exchanger, 11 ... Low temperature sodium region, 12 ... Cover gas space, 13 ... Main circulation pump, 14 ... Heat insulation Material, 15... Slothing free liquid level.

Claims (1)

[Scope of Claims] 1. A tank characterized in that a heat shield sloped downwardly toward the outside is suspended from the lower surface of the roof slab sloped downwardly toward the outside so as to cover the lower surface of the roof slab. type fast breeder reactor. 2. The tank-type fast breeder reactor according to claim 1, wherein the heat shield is comprised of a plurality of heat shield plates stacked with space between them.