JPS6256479B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a fast breeder reactor (loop type, tank type, etc.) that uses liquid sodium as a coolant in the cover gas phase within the reactor vessel. The generation of high-concentration sodium mist or sodium aerosol is controlled by controlling the temperature of the surrounding walls of the cover gas phase, especially the lower surfaces of the rotating and fixed plugs installed at the top of the furnace vessel, which corresponds to the ceiling of the cover gas phase. The present invention relates to a method for preventing such damage by

In a fast breeder reactor, for example, as shown in Fig. 1, there is a reactor core 2 inside a reactor vessel 1, which is filled with sodium to cool it, and is fed through inlet and outlet piping (not shown). It circulates and circulates. The sodium free liquid level 3 is covered with a cover gas phase 4, and the upper part of the furnace vessel 1 is further provided with a large rotating plug 5 and a small rotating plug 6 to isolate them from the outside air.
It is covered with etc. When the nuclear reactor is operating, the sodium temperature reaches a high temperature of 450 to 550°C due to heat generation from the reactor core 2. On the other hand, the rotary plugs 5 and 6 at the top of the reactor vessel have various mounted equipment such as control rod drive mechanisms on their upper parts, and protect the instruments and electronic parts built into these, as well as improve the work efficiency of operators. It is cooled to keep it in good condition.
For this cooling, the sodium free liquid level in the reactor vessel is 3
A temperature difference with a vertically upward negative gradient occurs in the cover gas phase between the bottom surface 7 and the lower surface 7 of the rotating plug, and due to this temperature difference, the sodium evaporated from the sodium free liquid level into the cover gas flows into the cover gas phase. It condenses inside, generating fine sodium mist or sodium aerosol with a particle size of 0.5 to 20 μm and a high concentration of 1 to 30 g/m ³ per unit volume of the cover gas phase. When such fine and highly concentrated sodium mist is generated, it is carried by the natural convection of the cover gas and adheres to the low-temperature walls of various equipment installed in the upper part of the reactor vessel. If it adheres to a wall surface at a temperature below 0.9°C, the deposits containing impurities will stick, and the rotational and vertical movements of the rotating plug and control rod drive mechanism will be significantly inhibited.

So far, we have worked on the fast reactor Rhapsody in France, the sodium cooled reactor KNK in West Germany, and the fast reactor FBR in the United States.
Due to the adhesion of sodium mist to the low-temperature walls and its adhesion, torque became abnormally large when rotating the rotary plug or moving the control rod up and down, making these operations impossible.
This required complicated work such as heating the stuck parts to melt off the stuck sodium, or removing the stuck substances with a manipulator, which seriously hindered the operation of the nuclear reactor. Problems caused by the adhesion of sodium mist occur in places where there is a large temperature difference between the top and bottom of the narrow vertical double cylinder gap 8 of nuclear reactor equipment as described above, and natural convection of the cover gas occurs inside the gap. This is because sodium mist is transported by this.

In recent years, various cover gas natural convection prevention mechanisms have been devised to prevent natural convection within such a narrow vertical double cylinder gap and prevent sodium mist adhesion. However, since there are many vertical double cylindrical gaps in the cover gas phase at the top of the reactor vessel, equipping all of them with natural convection prevention mechanisms is not only economical but also increases the reliability of each natural convection prevention mechanism. It is almost impossible and unrealistic since the system has not been established yet.

In addition to the problems caused by the fixation of sodium mist, the following problems also occur when high concentration mist is generated in the cover gas phase. For example, in vapor traps and mist traps installed in cover gas supply and exhaust piping, a large amount of sodium mist flows in when exhausting, but in order to efficiently remove such sodium mist, 99.9%
It is necessary to develop a mist trap with the above-mentioned high collection efficiency. In particular, in the gas piping that flows into the reactor primary system cover gas purification equipment, the contamination of a small amount of sodium mist will greatly reduce the performance of the purification equipment, so the concentration of sodium mist in the cover gas after passing through the mist trap is It must be less than several ppm, and for this purpose, the collection efficiency of the mist trap must be lower than the mist concentration 10.
g/ ^m3 , it must be extremely high, 99.9999% or higher.

The purpose of the present invention is to essentially solve the various difficulties associated with the generation of sodium mist as described above.
The object of the present invention is to provide a method that can suppress the generation of sodium mist and sodium aerosol in the cover gas phase in a nuclear reactor vessel.

While measuring the sodium mist in the cover gas, the present inventors learned that there is a temperature condition under which the sodium mist disappears, and if this was applied to a nuclear reactor, the effect would be We focused on the fact that it would be extremely large, and completed the present invention.

In order to achieve the above object, in the present invention, when the sodium temperature near the sodium free liquid level in the reactor vessel is 450 to 550°C, which is the normal operating condition of a nuclear reactor, the bottom surface temperature of the rotating plug or fixed plug is approximately
It is configured to maintain the temperature above 440℃. The reason why the bottom surface temperature of rotating plugs and stationary plugs is set at approximately 440° C. or higher is based on the following experimental facts. The equipment used in this experiment was
It consists of a sealed container with heating means on the walls including the ceiling, a temperature measuring device on the ceiling as well, and high temperature liquid sodium circulating inside.
The experiment was conducted by maintaining the temperature of the liquid sodium and the temperature of the ceiling at predetermined temperatures, respectively, and measuring the concentration of sodium mist in the cover gas phase at that time. sodium temperature
The concentration of sodium mist in the cover gas phase is measured by keeping the temperature above 430℃, for example 450℃ or 530℃, and varying the temperature of the ceiling of the cover gas phase, which corresponds to the bottom surface of the rotating plug in the reactor vessel. , almost independent of the sodium temperature, the ceiling temperature approximately
At temperatures above 440°C, the formation of sodium mist is uniformly suppressed, and the cover gas phase is mainly composed of sodium vapor. FIG. 2 shows experimental data showing the relationship between the sodium mist concentration and the cover gas ceiling temperature. In the same figure, the sodium temperature is 450℃.
The mist concentration is shown when the ceiling temperature of the cover gas phase is changed at 530℃ and 530℃, but the sodium mist becomes uniformly and rapidly diluted when the ceiling temperature is about 440℃ or higher, and almost It can be seen that no mist is generated. This ceiling temperature is approximately 440℃
The reason why sodium mist is no longer generated is considered to be that the mist generation nuclei that promote the generation of sodium mist disappear due to thermal decomposition when the cover gas has a temperature higher than this temperature.

Therefore, even in the cover gas phase in the actual reactor vessel, mist generation can be prevented by setting the same ceiling temperature, that is, the bottom surface temperature of the rotating plug or fixed plug, to about 440° C. or higher. Specifically, the ceiling temperature of the cover gas phase inside the reactor vessel is set to approximately
To achieve a temperature of 440°C or higher, for example, the heat insulating performance of the bottom of the rotating plug or fixed plug can be improved, or if necessary, a preheater can be installed in the bottom of the plug to perform heating. In this way, by setting the cover gas ceiling temperature to about 440°C or higher, it is possible to suppress the generation of sodium mist in the cover gas. It becomes necessary to strengthen the cooling to protect the temperature, which is not favorable from a thermoeconomic point of view, so the lower limit is at most several tens of degrees Celsius, for example, 10 to 20 degrees Celsius. That is, a preferable temperature range is 440°C or higher and 450 to 460°C or lower.

Now, if the ceiling temperature of the cover gas phase in the reactor vessel is maintained under the above-mentioned temperature conditions, the sodium content in the cover gas phase will mainly be only sodium vapor. However, in the case of only sodium vapor, for example in the case shown in FIG. Even if the sodium vapor is transported, most of the sodium vapor condenses on the relatively high-temperature surrounding wall and becomes liquid and is captured, and the sodium content is lost before reaching the low-temperature part at the upper end of the gap. Because of this, almost no sodium adheres or precipitates.

In addition, the mist trap attached to the cover gas exhaust system does not need to function as a filter for conventional mist, but only serves as a steam condenser, so its sodium vapor trapping efficiency increases dramatically. do. This means that in sodium mist, the diffusion coefficient is
10 ^-6 to 10 ^-8 cm ² /s, while for steam it is 0.5
This is because the mass transfer rate of the vapor increases significantly, to ~1.2 cm ² /s. Furthermore,
Radioactive substances with high vapor pressure, such as cesium, are no longer carried by the sodium mist, so they can be effectively removed by mist trapping by vapor condensation, and they can also be removed by trapping the low-temperature wall of the cover gas phase. Adhesion is also significantly reduced because there is no sodium mist to serve as a carrier.

As is clear from the above detailed description, according to the present invention, it is possible to suppress the generation of sodium mist and sodium aerosol in the cover gas phase inside the reactor vessel, and therefore, various difficulties associated with the generation of sodium mist can be suppressed. Since the problem can be solved in a number of ways, it can produce many excellent effects as mentioned above.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of a sodium-cooled fast breeder reactor, and FIG. 2 is a graph of experimental data showing the relationship between the sodium mist concentration and the ceiling temperature of the cover gas phase. 1... Reactor vessel, 2... Core, 3... Sodium liquid level, 4... Cover gas phase, 5... Large rotating plug, 6... Small rotating plug, 7... Bottom surface of rotating plug, 8... Gap .

Claims (1)

[Claims] 1. A method for suppressing the generation of sodium mist or sodium aerosol in the cover gas in a reactor vessel of a sodium-cooled fast breeder reactor, characterized by maintaining the temperature of the lower surface of the reactor upper plug in the reactor vessel at approximately 440°C or higher. .