JPS6367160B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a nuclear reactor vessel, and particularly to a nuclear reactor vessel suitable for use in a tank-type fast breeder reactor.

[Background of the invention]

Liquid metallic sodium (hereinafter referred to as sodium)
In tank-type fast breeder reactors that use sodium as a coolant, the reactor vessel is filled with sodium, and the reactor core, core superstructure, multiple intermediate heat exchangers, and multiple primary side main circulation pumps are installed inside the reactor vessel. has been done. Furthermore, a partition wall is installed inside the reactor vessel to separate the high temperature primary sodium flowing out from the reactor core from the low temperature primary sodium flowing out from the intermediate heat exchanger. It is separated into a lower cold plenum.

The top of the reactor vessel is closed off by a thick lid plate (roof slab), and the roof slab itself is supported by a concrete reactor vessel expansion chamber. The reactor vessel is generally fixed to a roof slab, and the reactor vessel, its internal structures, and sodium are suspended from the roof slab through the top of the reactor vessel. This will cause stress.

Therefore, it is necessary to prevent the reactor vessel, which is in direct contact with sodium, from undergoing sudden temperature changes or from becoming excessively high during nuclear reactor operation.

In conventional technology, in order to prevent the reactor vessel wall in the hot plenum from becoming hot due to high-temperature sodium, a heat shield plate is installed circumferentially inside the reactor main vessel, and a heat shield plate is installed between the heat shield plate and the reactor vessel inner wall. A reactor vessel cooling mechanism is installed to forcefully cool the reactor vessel. However, in this method, due to the temperature difference between the cover gas region and the high temperature sodium, the temperature difference between the cover gas region and the high temperature sodium is caused by the temperature difference between the cover gas region and the high temperature sodium at the part where the cover gas region and the high temperature sodium come into contact between the roof slab and the high temperature sodium liquid surface, that is, the reactor vessel wall near the sodium liquid surface. Thermal stress and thermal shock are applied to the reactor vessel due to rapid temperature gradients and sudden changes in the temperature of the reactor vessel wall due to fluctuations in the sodium liquid level due to excessive operations such as reactor scrams. The disadvantage is that there are cases.

[Purpose of the invention]

An object of the present invention is to provide a nuclear reactor vessel equipped with a mechanism for alleviating thermal stress and thermal shock applied to the reactor vessel due to factors such as the temperature difference between the cover gas region and high-temperature sodium and fluctuations in the sodium liquid level. It is about providing.

[Summary of the invention]

Thermal stress and thermal shock applied to the reactor vessel, which are the problem of the present invention, are mainly caused by the following factors.

A cover gas region exists between the roof slab and the hot sodium liquid level. This temperature difference between the cover gas region and the hot sodium creates a sharp temperature gradient on the reactor vessel wall near the sodium liquid surface. In addition, when transient operations such as reactor scrams cause vertical fluctuations in the high-temperature sodium liquid level, the parts of the reactor vessel wall that were in contact with the cover gas may come into contact with the high-temperature sodium, or The change in parts coming into contact with the cover gas occurs quickly and rapidly, causing a rapid temperature change in the reactor vessel wall. In order to alleviate the thermal stress and thermal shock caused by such temperature gradients and temperature changes, it is necessary to provide the reactor vessel with a mechanism that softens sudden temperature gradients and temperature changes.

As a mechanism that satisfies the required functions as described above, the present invention uses a material that has a large number of holes in the wall of the reactor vessel near the free sodium liquid level and allows sodium to permeate into the holes. It uses a simple structure of attachment.

[Embodiments of the invention]

FIG. 1 shows the reactor vessel and its internal structure of a known tank-type fast breeder reactor to which the present invention is applied. A roof slab 2 is provided at the top of the reactor vessel 1 to isolate the inside of the reactor from the outside world and to mount reactor internals. Additionally, a plurality of main intermediate heat exchangers 3, a plurality of primary main circulation pumps 4, and a reactor core 5 are housed inside the reactor vessel 1, and above the reactor core 5, a device for controlling the output of the reactor is installed. An upper core mechanism 6 is installed, which consists of control rods and detectors that measure the temperature and flow rate at the core outlet. A hot plenum 7 that accommodates high-temperature sodium flowing out from the reactor core 5 and a cold plenum 8 that accommodates low-temperature sodium flowing out from the main intermediate heat exchanger 3 are separated by a partition wall 9. 1 leaked from core 5
The sodium on the secondary side flows into the main intermediate heat exchanger 3, descends while giving heat to the sodium on the secondary side flowing inside the heat transfer tube, and flows out into the cold plenum 8. This primary side sodium is sent to the reactor core 5 by the primary main circulation pump 4, heated within the reactor core, and then flows out to the hot plenum 7.

In addition, in order to prevent the reactor vessel wall in the hot plenum from becoming high temperature, a heat shield plate 10 is provided in the inner circumferential direction of the reactor vessel, and the heat shield plate 10 and the reactor vessel 1
A reactor vessel cooling mechanism is installed between the inner walls of the reactor vessel 11 to cool the reactor vessel walls by flowing low-temperature sodium in the cold plenum.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 is a cross-sectional view of a reactor vessel in which the material having a large number of pores according to the present invention is attached to the inner wall near the sodium liquid level. That is, a material 12 having a large number of holes is attached to the inner wall of the reactor vessel 1 from the part on the hot plenum 7 side near the sodium liquid level to the part on the cover gas region 13 side (in the figure, the material 12 is surrounded by an ellipse). part A). Figures 3 and 4 are enlarged views of this part, and Figure 3 is a perspective view.
FIG. 4 is a sectional view. These figures show part of the reactor vessel 1 and the material 12 with many holes attached to its inner wall, and the material 12 with many holes also extends in the circumferential direction of the reactor vessel. installed.

Materials that can be used as the material with many pores mentioned here include, for example, combinations of various porous metals, metal fibers, ceramic materials, wire mesh, and molded foils or plates to create pores. In the following description, these will be collectively referred to as porous materials.

Porous materials include stainless steel, Inconel, and Hastelloy, which have heat resistance and corrosion resistance at high temperatures, and porous materials are already being used to purify molten sodium. . From these facts, when a porous material is attached to the inner wall of the reactor vessel of a fast breeder reactor that uses sodium as a coolant, the coexistence of the porous material and sodium is sufficient, and the present invention solves the practical problems. Less is.

In FIG. 4, the reactor vessel 1 in which the porous material 12 of the present invention is attached to the inner wall is not only below the sodium liquid level but also at the sodium liquid level due to vertical fluctuations 14, 15 of the sodium liquid level during transient operation or capillary action. Sodium also permeates into the porous material in contact with the upper cover gas region and retains sodium in its pores, creating a state in which the reactor vessel wall is wet with sodium even in the cover gas region.

The effective thermal conductivity λ _e of a mixed medium consisting of such a porous material and sodium permeated into its pores is:
It is given by equation (1).

λ _e /λ _f =ε+1−ε/φ+(2/3)(λ _f /λ _n )
(1) Where, λ _e : Effective thermal conductivity of mixed medium λ _f : Thermal conductivity of fluid (sodium) λ _n : Thermal conductivity of solid (porous material) ε : Porosity of porous material φ : (Effective thickness of the fluid film near the particle contact point of the porous material)/(Particle size of the porous material).

When stainless steel porous metal with a porosity of 40% is used as the porous material, the above values are: λ _f = 60 (Kcal/mh°C) λ _n = 13 (Kcal/mh°C) ε = 0.4 φ = 0.165, and the effective thermal conductivity of the mixed medium is λ _e = 35.1 (Kcal/mh°C).

In this way, the effective thermal conductivity of the mixed medium consisting of the porous material and the sodium that has penetrated into its pores is close to the thermal conductivity of sodium, 60 (Kcal/mh℃). is transported upward by heat conduction in the porous material located in the cover gas region above the sodium liquid level. The heat transported through the porous material is transferred to the reactor vessel wall, reducing the temperature gradient in the reactor vessel between the sodium liquid surface and the cover gas region, and
Alleviates thermal stress applied to the reactor vessel. FIG. 5 shows the effect of reducing this temperature gradient.

Next, the effect of sodium rising in the porous material to the cover gas region due to capillary action in this example will be described. The rising height h of the liquid when a capillary tube is erected in the liquid is expressed by the following formula.

h=2σcosθ/rρg (2) here, h: Difference in height of liquid inside and outside the tube σ: Surface tension of liquid θ: contact angle r: radius of tube ρ: density of liquid g: gravitational acceleration It is.

The surface tension σ of sodium is 157 dyn/cm, the specific weight ρg is 832 Kg/m ³ , and the contact angle θ is approximately 10° according to experimental observation. Substituting these values into equation (2) yields h=0.379/r(cm) (3). FIG. 6 illustrates the third equation.
As can be seen from the figure, a capillary tube with a radius of approximately 3 × 10 ^-3 mm is required to raise sodium by capillary action to the distance between the sodium liquid level and the roof slab, that is, the height of the cover gas area, which is approximately 1 m. It becomes necessary. Porous materials can be manufactured in a wide range of materials with porosity of 5 to 95% and pore diameters of 2 x 10 ^-6 to 2 (mm). It is possible to create a state in which the reactor vessel wall in the cover gas region is wetted with sodium by using this method. The porous material in which sodium permeates to the cover gas region above the liquid level by this capillary phenomenon transports the heat of the high temperature sodium in the hot plenum upwards by thermal conduction, and It reduces the temperature gradient in the reactor vessel and relieves the thermal stress applied to the reactor vessel.

Next, the thermal shock mitigation effect of this example will be described. In Figure 4, the liquid level changes from 14 to 15 or 15 due to fluctuations in the sodium liquid level during transient operation.
Suppose that it changes from to 14. Assuming that such liquid level fluctuations occur instantaneously, we will compare the temporal changes in the temperature of the reactor vessel when the porous material is not attached to the inner wall of the reactor vessel and when it is attached. First, let's consider a case where the liquid level rises and the area that used to be the cover gas area is submerged below the sodium liquid level.
If no porous material is installed, consider the calculation system as shown in Figure 7, and take the heat balance: ρCV∂T/∂t=q (4) q=h(T _S −T) (5) 1/h=δ/λ+δ _S /λ _S (6).

Here, ρ: Density of the reactor vessel C: Specific heat of the reactor vessel V: Volume per unit area of the reactor vessel T: Temperature of the reactor vessel t: Time q: Amount of heat flowing into the reactor vessel h: Sodium and the heat transfer coefficient of the reactor vessel T _S : Temperature of sodium λ : Thermal conductivity of the reactor vessel λ _S : Thermal conductivity of sodium δ : Distance from the inner wall surface of the reactor vessel to the temperature calculation point inside the reactor vessel _δS : Distance from the inner wall of the reactor vessel to the point inside the sodium.

The initial conditions are t=0 and T=T ₀ (7). When equations (4) to (6) are solved under the condition (7), the temporal change in the reactor vessel temperature becomes as follows.

T=T _S −(T _S −T ₀ )e ^−at (8) where a=h/ρCV (9).

Next, we calculate the temporal change in reactor vessel temperature when a porous material is attached to the inner wall of the reactor vessel. Considering the calculation system as shown in Figure 8 and taking the heat balance, ρCV∂T/∂t=q (10) ρ _S C _S V _P ∂T _P /∂t=q _P −q (11) q =h ^* (T _P −T) (12) q _P =h _P (T _S −T _P ) (13) 1/h ^* =δ/λ+δ _P /λ _e (14) 1/h _P =δ _P / λ _e + δ _S /λ _S (15). Here, ρ: Density of the reactor vessel C: Specific heat of the reactor vessel V: Volume per unit area of the reactor vessel T: Temperature of the reactor vessel t: Time q: Amount of heat flowing into the reactor vessel h ^* : Heat transfer coefficient between porous material and reactor vessel ρ _S : Density of sodium C _S : Specific heat of sodium V _P : Volume per unit area of porous material T _P : Temperature of porous material q _P : To the porous material Amount of heat flowing in T _S : Temperature of sodium h _P : Heat transfer coefficient between the porous material and sodium δ : Distance from the inner wall surface of the reactor vessel to the temperature calculation inside the reactor vessel δ _P : Thickness of the porous material δ _S : Distance from the surface of the porous material to a point inside the sodium λ : Thermal conductivity of the reactor vessel λ _S : Thermal conductivity of sodium λ _e : Effective thermal conductivity of the mixed medium consisting of the porous material and sodium. The initial conditions are t=0, T=T ₀ , T _P =T ₀ (16). Solving equations (10) to (15) under the condition of (16), we get
The temporal change in reactor vessel temperature is as follows.

T=a ₁ a ₂ T _S /m ² +n ² {1−e ^-mt secφcos(nt+φ)}
+T ₀ (cos nt+m/nsin nt)e ^-mt (17) Here, a ₁ = h ^* / ρCV, a ₂ = h _P / ρ _S C _S V _P , a ₃ = h ^* / ρ _S C _S V _P m=a ₁ +a ₂ +a ₃ /2, n ² =a ₁ +a ₂ −m ² φ=tan ⁻¹ (−m/n) (18).

Figure 9 shows a comparison of the temporal changes in the temperature of the reactor vessel when a porous material is attached to the inner surface of the reactor vessel wall and when it is not attached using equations (8) and (17). As shown in the figure, when a porous material is attached, the change in the reactor vessel over time becomes more gradual than when it is not attached, and thermal shock is alleviated. Comparing the time constants in this case, l=t ₁ /t ₂ a/m (19), and actually calculating by substituting the following values, ρ=7820Kg/m ² , C=0.118Kcal/ Kg℃，V=
0.08m, δ=0.04m, δ _S =0.1m, λ=14Kcal/
mh℃, λ _S =60Kcal/mh℃, V _P =0.1m, δ _P =
0.1m, λ _e = 35.1Kcal/mh℃, ρ _S = 844Kg/m ³ ,
C _S =0.314Kcal/Kg℃, l0.35 (20). That is, by installing the porous material, the rate of temperature change in the reactor vessel is reduced to about 35% of the rate when no porous material is installed.

Similar calculations can be made in the case where the sodium liquid level falls and the part that was previously submerged below the sodium liquid level is exposed to the cover gas area. Here, only the results are shown. Figure 10 shows the temporal change in reactor vessel temperature when the sodium liquid level falls.
A comparison is made between a case where a porous material is attached and a case where a porous material is not attached. A comparison of the time constants in this case is as follows.

l=t ₁ /t ₂ 0.01 (21) In other words, this shows that when a porous material is attached, the rate of temperature drop in the reactor vessel is reduced to about 1% of the rate when it is not attached. This is because the presence of hot sodium held within the pores of the porous material prevents heat from escaping from the reactor vessel to the cover gas region.

As shown above, when a porous material is attached to the inner wall of the reactor vessel, it is possible to prevent rapid temperature changes associated with fluctuations in the sodium liquid level from directly applying to the reactor vessel. It has the effect of alleviating thermal shock.

The above is an embodiment of the present invention, and the porous material includes not only those with the structure shown in FIGS. 3 and 4, but also materials that combine wire mesh,
A similar effect can be obtained even if the material is made of fibrous material, a foil or plate formed to have holes or capillaries, as long as the structure allows the coolant to penetrate into the holes.

In addition, although the example describes the case where a porous material is attached to the inner wall of the reactor vessel, it is not limited to reactor internal structures such as the main intermediate heat exchanger and main circulation pump, but is also applicable to tanks and reactors that cause similar problems. It can also be used for containers, etc.

〔Effect of the invention〕

According to the present invention, the heat of high-temperature sodium in the hot plenum is
Heat is transported above the sodium surface by conduction in the porous material located in the cover gas region above the sodium surface. This heat is transferred to the reactor vessel wall, reducing the temperature gradient in the reactor vessel between the sodium liquid level and the cover gas region, and can relieve thermal stress on the reactor vessel. Furthermore, it is possible to prevent rapid temperature changes associated with up and down fluctuations in the sodium liquid level that occur during transient operations from being directly applied to the reactor vessel, thereby alleviating thermal shock to the reactor vessel.

[Brief explanation of drawings]

Fig. 1 is a structural diagram inside a tank type fast breeder reactor vessel according to a conventional example, Fig. 2 is a structural diagram inside a tank type fast breeder reactor vessel according to an embodiment of the present invention, and Fig. 3 is a structural diagram inside a tank type fast breeder reactor vessel according to an embodiment of the present invention. 4 is a cross-sectional view of an embodiment of the present invention, FIG. 5 is a temperature distribution diagram showing the temperature gradient relaxation effect near the sodium liquid surface of the present invention, and FIG. 6 is a diagram showing the radius of the capillary tube and the capillary phenomenon. Figure 7 is an explanatory diagram of the temperature change calculation system in the reactor vessel when the sodium liquid level rises when no porous material is installed. Figure 8 is a diagram showing the relationship between the height of rise in sodium. Figure 8 is when a porous material is installed. An explanatory diagram of the temperature change calculation system in the reactor vessel when the sodium liquid level rises in
FIG. 9 is a diagram showing the temporal change in the reactor vessel temperature when the sodium liquid level rises, and FIG. 10 is a diagram showing the temporal change in the reactor vessel temperature when the sodium liquid level falls. 1... Reactor vessel, 2... Roof slab, 3... Main intermediate heat exchanger, 4... Primary main circulation pump, 5... Reactor core,
6... Core upper mechanism, 7... Hot plenum, 8... Cold plenum, 9... Partition wall, 10... Heat shield plate, 1
1...Reactor vessel cooling channel.

Claims (1)

[Claims] 1. In a nuclear reactor that has liquid metal, which is the core coolant, in the reactor vessel, there are many holes in the reactor vessel wall near the free liquid surface of the coolant, and the coolant penetrates into the holes. A nuclear reactor vessel characterized by having a material attached thereto.