JPS6047989A - Cooling device for melted core - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an apparatus for holding and cooling a molten core that occurs in an nuclear couplant during an accident.

[Background of the invention]

If an accident occurs in a nuclear couplant, the emergency core cooling system is activated and can sufficiently remove all the heat from the reactor and cool it down. However, if the emergency core cooling system were to fail, the core could melt, penetrate the pressure vessel, and fall into the cavity. At this time, the molten core contacts the water in the capacity and generates water vapor, and reacts with the concrete to generate gases such as water vapor and carbon dioxide.If the amount is large, emergency There is a risk that the capacity of the gas treatment system will be exceeded and the containment vessel will be damaged due to overpressure. In the case of such a core meltdown accident, 4.
Among the types of barriers, 1) fuel pellets, 2) fuel cladding tubes, and 3) secondary systems have already been lost, so the containment vessel is the last barrier.

According to an assessment by the Sampuia National Laboratory in the US, the risk of overpressure failure of the containment vessel accounts for approximately 30% of the total risk of nuclear couplants. Therefore, it is an important safety issue to prevent the molten core from coming into contact with water, concrete, etc., generating steam and gas, and increasing the pressure in the containment vessel.

Conventionally, some fast breeder reactors are provided with a molten core holding device as shown in FIG. The installation of similar holding devices in light water reactors has also begun to be considered as a measure against core meltdown accidents. A core holding device 3 is provided at the lower part of the containment vessel 2 to receive the core that has melted and penetrated the pressure vessel 1 . The following two materials are considered for the core holding device 3.

(1) Reflection type: The molten core is held by a heat-resistant material with a high melting point such as Mg0 (melting point 2800 C).

(2) Absorption type: The heat of the molten core is absorbed by the heat of fusion of glass fragments, etc.

But if the core melts, its weight, temperature.

The amount of heat generated is 580 tons, 30271 r at 170 minutes after the accident penetrating the pressure vessel, and 2.3 x 10 fuW, which is a huge amount, and it is impossible to maintain it permanently with the above-mentioned core holding device. For example, when MgO retaining material is used, it takes four days to penetrate a 1.6 m thick core retainer. Since this device does not release heat to the outside of the system, after penetrating the furnace Ib holding device, gas is generated due to reaction with conc. In the end, conventional methods delay the failure of the containment vessel, which is the last barrier for confining radioactive materials, and hope that the radioactive materials decay during this time, essentially maintaining the integrity of the containment vessel. I can't.

As a countermeasure to this problem, a method of cooling the molten core can be considered, but it is impossible to use the method of cooling directly with a coolant because the reaction generates water vapor, gas, etc., and the pressure in the containment vessel increases.

[Purpose of the invention]

In order to solve the above problems, the present invention utilizes a heat pipe capable of non-direct contact cooling to cool and solidify the molten core by discharging the heat of the molten core to the outside of the containment vessel. The purpose is to provide a molten core cooling system that maintains the integrity of the containment vessel.

[Embodiments of the invention]

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

FIG. 2 shows a molten core cooling system according to the present invention. The molten core 4 that has passed through the pressure vessel 1 is held by a core holding device 3. The heat pipe 5 has a heat absorption part 6 immersed in the molten core 4, and a heat radiation part 7 provided outside the containment vessel. Because it is non-direct contact cooling, the pressure inside the containment vessel 2 does not rise, and the molten core 4 is discharged outside the thermal containment vessel through the heat pipes 5, where it is cooled and solidified on the core holding device 3. Sisters,
The integrity of the containment vessel 2 is ensured.

Here, the material and scale of the core holding device 3 will be considered.

The material for the core holding device 3 needs to be a heat-resistant material with a high melting point. Possible materials include W ('Imelt)
= 3377 C), C(Tmelt) 350
0C) TaC (Tme It = 3880C). In addition, since the inner diameter of the pressure vessel is 6.4 m, the radius of the circle r = 3.0 m is required to receive the entire molten core.
A concave holding device with a cross section of 2 m or more is required.

In addition, if all the reactor internals were to fall, their weight would be 58
0t, and using UO2 density 1o and sg/cd as a representative, the capacity is 55m1. Considering that this is received by a hemispherical core holding device, r = 3.0m. If the shape of the molten core holding device 3 is hemispherical, it is appropriate that the radius r = about 3.5 m. Next, we will consider the material of the heat pipe's outer tube and medium. The material for the outer tube must have a high melting point and high thermal conductivity, and the following materials can be considered.

W: Melting point Tmel t = 3377C Thermal conductivity λ =
1.5 W/ On-deg (at200cE'
) C: Tme It > 3500 CJ = 1
．． ．． 05W/Cm -deg (at 1000C
) Among these, tungsten (W) has a proven track record, so it is better to use Wt. As the cooling medium, since the molten core is at a high temperature of about 3000 C, a high temperature cooling medium such as Na, which can obtain a large heat flux as shown in Table 1, can be used. As shown in Table 2, these metals are solid at room temperature and automatically melt when the molten core falls.
Evaporate. Therefore, during normal operation, it is highly safe because it is solid, and in the event of an accident, it automatically starts without the need for any starting mechanism.

Table 1 Table 2 Furthermore, the scale of the heat pipe 5 will be considered.

First, the diameter f of the heat exchanger tube and the case where Na is used as the cooling medium will be calculated. The heat transfer area S of the heat transfer tube can be calculated using the following formula.

Q = (1 axis x S where Q; generated heat i = 3.7
= 0.38m", converted to radius, r = 0.35m,
Although a cylindrical heat exchanger tube with a radius greater than this is required, it is completely possible.

Next, the heat transfer surface 21 of the heat pipe heat absorption section 6 will be examined. The heat transfer area can be calculated using the following equation.

Q=□ ・S 1 /q radius + 1/hd・ΔT where q radius: radial heat flux = 30 ow/cm' (see Table 1) hd: heat transfer coefficient between molten core and heat vibration heat absorption part = 5000 W/m' ψdeg ΔT: Temperature difference between the molten core and the heat pipe heat absorption part = 20001; From this, S = 10 m'. In order to achieve this, a circular heat absorbing plate with a radius of 1.8 m may be used as shown in FIG. It is also possible to use a square heat absorbing plate with one side of 3.2 m. These can be fully installed in the hemispherical core holding device 3 with a radius of about 3.5 m. In this method, since the heat absorbing part 6 of the heat pipe is in direct contact with each other, it was possible to achieve this with a heat transfer surface of 10 mz. This cannot be achieved with a structure in which the heat pipe is embedded in the core holding device 3 or placed below it.

Furthermore, since the heat dissipation section 7 of the heat pipe is located outside the containment vessel 2, a sufficient area can be taken up.

Furthermore, it is also possible to attach heat radiation fins 8 to improve efficiency.

FIG. 4 shows another embodiment of the present invention.

The core holding device 3 is immersed in the suppression pool water 9, and the cooling effect of the pool water 9 is added. Core holding device thickness t = 0.1 m
If so, it has the ability to sufficiently cool the molten core 4 only by cooling the pool water. Also, the heat dissipation part 7 of the heat pipe
is immersed in fuel pool water 8) to improve the efficiency of heat dissipation.

FIG. 5 shows another embodiment of the present invention. In this embodiment, the heat pipe 5 is loop-shaped. According to this structure, the evaporation/rising flow path and the condensation/falling flow path are separated, so there is no interfacial resistance due to opposing flows, and the efficiency of heat transport can be improved. Furthermore, since the heat pipe 5 is immersed in pool water, the condensation effect is increased.

〔Effect of the invention〕

As explained above, according to the present invention, since there is no direct contact cooling, 1) there is no pressure increase in the containment vessel due to the generation of water vapor or gas, 11) there is no heat generation due to chemical reactions, and since a heat pipe is used, in) inexpensive and high cost; Reliability: Since the heat absorption part is in direct contact with the reactor core, the heat transfer coefficient is 1.V) Since the heat radiation part is provided outside the containment vessel, increases in temperature and pressure can be suppressed, Na, etc. Because it uses a cooling medium with a high melting point and a high boiling point, ■1) Automatic operation, vi
i) High heat transfer coefficient, using ultra-high melting point heat pipe outer tube such as WXC, and molten core holding material, viii) No gas generation due to melting, Ix) Rapid reduction due to melting/penetration We can provide a complete system for cooling and solidifying the molten core, which has many features such as no reaction, and thereby maintains the integrity of the containment vessel.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a conventional molten core holding device, FIG. 2 is a sectional view of a molten core cooling device according to an embodiment of the present invention, and FIG.
The figure is an enlarged view of the heat absorption part of the heat pipe in the molten core cooling system according to one embodiment of the present invention, FIG. 4 is a sectional view of the molten core cooling system according to another embodiment of the present invention, and FIG. FIG. 3 is a cross-sectional view of a molten core cooling device according to yet another embodiment of the invention. DESCRIPTION OF SYMBOLS 1... Pressure vessel, 2... Containment vessel, 3... Core holding device, 4... Molten core, 5... Pete no ζ Eve,
6...Heat Tsukibu heat absorption section, 7...Heat Tsukibu heat dissipation section, 8...Continued from page 1 of heat dissipation0 Inventor Akira Matsumoto Kami 1 Sakusho, Chiyoda-ku, Tokyo

Claims (1)

[Claims] 1. A vessel for receiving the molten core located directly below the pressure vessel and having a wider area than the horizontal cross section of the pressure vessel, and a heat absorbing part immersed in the molten core and a heat radiating part provided outside the containment vessel. 1. A molten core cooling system comprising a heat pipe and a molten core cooling system, which is characterized in that the heat of the molten core can be released to the outside of the containment vessel by non-direct contact cooling. 2. The molten core cooling system according to claim 1, characterized in that a material having a melting point of 300 or more is used as the medium of the heat pipe. 3. The molten core cooling device according to claim 1, wherein a material having a melting point of 3000C or more is used for the outer pipe heat absorption part and the container of the heat pipe. 4. The molten core cooling system according to claim 1, wherein the container receiving the molten core is immersed in the cooling water of the suppression suction chamber. 5. The molten core cooling device as set forth in claim 1, wherein the heat radiating portion of the heat pipe is immersed in fuel pool water. 6. The molten core cooling device according to claim 1, wherein the heat pipe is of a loop type to improve thermal efficiency.