JPS6367599A - Method of washing spent fuel aggregate - Google Patents

Abstract

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

Description

[Detailed description of the invention] [Technical field to which the invention pertains]

The present invention relates to a spent fuel assembly cleaning apparatus for spent fuel assemblies of fast breeder reactors that use liquid metal such as sodium as a primary coolant.

[Prior art and its problems]

As is well known, spent fuel assemblies taken out from the above-mentioned nuclear reactors are generally cleaned of liquid metals such as sodium adhering to them in the process before being stored in a fuel storage pool. It is being done. In this case, the conventional method for cleaning spent fuel assemblies is to blow wet gas containing steam onto the spent fuel assemblies while they are housed in a cleaning tank, and then wash them with water. Generally, a steam-water cleaning method is employed in which the fuel assembly is dried with hot air or the like. However, the steam
The water cleaning method requires a long time of up to 1.5 hours to clean one spent fuel assembly, and the cleaning process also requires approximately 1.5 tons per spent fuel assembly. A large amount of radioactive cleaning waste liquid containing radioactive waste, radioactive corrosion products, etc. is generated, leading to troublesome problems in waste treatment such as increasing the size of waste liquid treatment equipment and increasing radioactive contamination. . On the other hand, as an alternative to the conventional steam-water cleaning method described above, a dry vacuum cleaning method has been proposed that removes liquid metal adhering to spent fuel assemblies without generating radioactive waste liquid. This dry vacuum cleaning method involves preheating the spent fuel assembly to a temperature higher than the melting temperature of liquid metal such as sodium, and placing the preheated spent fuel assembly in a cleaning tank. This method evaporates and eliminates the liquid metal adhering to the fuel assembly by reducing the pressure of the atmosphere in the cleaning tank below the saturated vapor pressure of the liquid metal at the preheating temperature. According to this dry vacuum cleaning method, no steam, water, etc. are used in the cleaning process, so there is no radioactive waste liquid generated during cleaning, and spent fuel assemblies can be disposed of by simply processing a small amount of exhaust gas in the gaseous waste treatment system. It becomes possible to wash effectively. On the other hand, as a method for preheating spent fuel assemblies in the dry vacuum cleaning method described above, the inventor proposed a method in which the temperature of the spent fuel assemblies is raised to the preheating temperature using decay heat generated by the spent fuel assemblies themselves, and A high-temperature gas preheating method blows inert gas heated from the outside to the spent fuel assembly into the spent fuel assembly, and a heater is installed around the cleaning tank, and the inert gas sealed in the cleaning tank is used as a heating medium. As a result, various preheating methods were tried, including a heater preheating method that indirectly heats the spent fuel assembly. As is well known, a fuel assembly is a structure consisting of a fuel pin that generates decay heat, surrounding upper and lower brackets, and non-heat generating parts such as a trumpet tube. In the preheating method using decay heat, the amount of heat generated by decay heat is small, and the heating rate is low at a few degrees Celsius/minute even near the fuel bottle, which is the heat generating part, and the fuel pin tends to have a stroke length of 4 minutes of the total length of the fuel assembly. Therefore, the temperature of the non-heat-generating portion away from the fuel pin is insufficiently raised. Furthermore, the high-temperature gas blower preheating method requires a large-scale gas circulation loop and also takes a relatively long time for preheating because the heat capacity of the gas as a heating medium is small. Furthermore, the heater preheating method has problems such as low heating efficiency and a long time required for preheating, and it has been found that it is difficult to efficiently and uniformly preheat the entire area of the spent fuel assembly within a short period of time. Moreover, if it takes a long time to preheat a spent fuel assembly, the time required for cleaning including the next depressurization operation process will be extended accordingly. The problem arises that it becomes impossible to keep up with the progress of the fuel exchange work.

[Purpose of the invention]

This invention has been made in consideration of the above points, and is a spent fuel assembly capable of uniformly heating the spent fuel assembly to a predetermined preheating temperature in a short period of time for a fully equipped dry vacuum cleaning device. The purpose of this invention is to provide a cleaning device for cleaning.

[Key points of the invention]

In order to achieve the above object, the present invention includes a preheating tank filled with the same liquid metal as the reactor primary coolant in a heated molten state as means for preheating a spent fuel assembly. The spent fuel assembly is immersed in liquid metal to preheat it, and then the spent fuel assembly is cleaned by evaporating the liquid metal adhering to the spent fuel assembly using a vacuum operation.

[Embodiments of the invention]

1 and 2 each show a cleaning device according to a different embodiment of the present invention. First, in the embodiment shown in FIG. 1, 1 is a fuel transfer device, 2 is a cleaning tank, and 3 is a cleaning tank 2. A preheating tank 4 serving as a preheating means independently provided in the preceding stage is a spent fuel assembly. Here, the cleaning tank 2 is a sealed container equipped with a door valve 2a at the top,
In addition, the cleaning tank 2 includes a vacuum tank 5. vapor trap 6
A vacuum pump 7 and a reduced pressure exhaust line 11 leading to a gaseous waste treatment system 10 via a valve 8.9 are connected. In addition, 12 is a drain collection tank 13 outside the tank to remove liquid metal sodium that has adhered to the cleaning tank 2 during the cleaning process.
A similar heater is also installed in the vacuum tank 5. On the other hand, the preheating tank 3 is a sealed container equipped with a door valve 3a at its top, and its interior is filled with sodium Na, which is the same as the primary coolant of the nuclear reactor. Also, add sodium Na to the area around the preheating tank 3! ! ! ! It is equipped with a heater 14 for melting. Note that the cleaning tank 2 is connected to an inert gas source 15 for replacing gas in the tank. Next, the procedure for cleaning the spent fuel assembly with the above configuration will be explained. First, in the standby state, the heater 14 is energized to heat the sodium Na stored in the preheating tank 3 to a predetermined temperature. It is carried into the preheating tank 3 through the door valve 3a and immersed in sodium Na which is in a molten and heated state. As a result, the spent fuel assembly 4, whose temperature has risen to approximately 150°C due to its own decay heat, is brought to a predetermined preheating temperature (preferably 450°C) within a short time, including the heat generating and non-heating parts. ~550℃
temperature). In addition, in parallel with this preheating process, the vacuum pump 7 is started with the valve 8 of the decompression exhaust line 11 closed to sufficiently reduce the pressure in the vacuum tank 5. After the above-mentioned preheating process is completed, the spent fuel assembly 4 is pulled out from the preheating tank 3 by operating the fuel transfer device 1 and transferred into the cleaning tank 2. The active substance is introduced into the cleaning tank 2 to perform gas replacement. Then, the valve 8 leading to the vacuum tank 5 is opened to rapidly reduce the pressure inside the cleaning tank 2, and the vacuum pump 7 continues to vacuum the inside of the cleaning tank 2. Then, the cleaning tank 2 is depressurized and evacuated so that the internal pressure of the cleaning tank 2 becomes less than the sodium saturated vapor pressure at that temperature, for example, about 4 to 10 Torr. As a result, the sodium adhering to the surface and inside of the spent fuel assembly 4 is promoted to evaporate, and is washed away from the spent fuel assembly 4 in the form of sodium vapor and mist. In addition, the sodium vapor generated in this process,
Most of the mist migrates and adheres to the inside of the cleaning tank 2 and the vacuum tank 5, and the remaining floating vapor and mist are absorbed and removed by the paper trap 6 inserted in the decompression exhaust line 11 along with the exhaust gas. Then, only the exhaust gas in an amount corresponding to the internal volume of the cleaning tank 2 is sent to the gaseous waste treatment system 10 to be purified. Also, along with the above-mentioned cleaning, the cleaning tank 2. The sodium adhering to the vacuum tank 5 is heated by a heater 12 installed in the cleaning tank and the vacuum tank to melt the sodium, and then the sodium is discharged to the drain tank 13 and recovered. Note that the recovered sodium is returned to the preheating tank 3 and used again if necessary. By preheating the spent fuel assembly 4 by immersing it in heated and molten sodium in this way, the spent fuel assembly 4 can be heated in a short time because the heat capacity of sodium is larger than that of inert gas. The temperature can be increased to a predetermined preheating temperature. In addition, the sodium used as a heating medium stored in the preheating tank 3 is the same as the primary coolant of the reactor, so there is no problem. Since the fuel assembly is heated to an elevated temperature, the adhering sodium can be easily removed, and the advantage is that the amount of adhering sodium is smaller than when the spent fuel assembly taken out from the reactor is placed directly in the cleaning tank 2. It will be done. Next, FIG. 2 shows another embodiment. In this embodiment, the same container 16 is used as the preheating tank and the cleaning tank described in FIG. A line 11 is connected thereto, and the container 16 and a separate sodium storage tank 17 arranged below the container 16 are interconnected via a sodium supply/discharge pipe IB. The container 16 is equipped with a heater 14 in the area surrounding the door valve 16a1 at the top, the sodium storage tank 17 is equipped with a heater 19 for heating and melting the stored sodium, and the sodium supply and discharge piping 18 is equipped with a heater 14. pulp 2
0. A tm pump 21 for feeding sodium is also installed. In the cleaning operation of a spent fuel assembly with this configuration, first, the electromagnetic pump 21 is operated to pump up the sodium stored in the sodium storage tank 17 into the container 16, and then the heater 14 is energized to fill the container 15. In this state, the spent fuel assembly 4 taken out from the reactor by operating the fuel transfer device 1 is carried into the container 16 through the door pulp and immersed in the heated and molten sodium. let When the spent fuel assembly 4 is heated to a predetermined preheating temperature, the pulp 2
0 is opened to discharge all the sodium in the container 16 to the sodium storage tank 17, and instead, inert gas is sent into the container 16 from the inert gas source 15 for gas replacement. Next, the process moves to a depressurization process, in which the pulp 8 between the vacuum tank 5 and the container 16, which had been depressurized in advance, is opened and the container 1 is removed.
By evacuating the inside of the spent fuel assembly 6 under reduced pressure, the sodium adhering to the spent fuel assembly 4 is evaporated and cleaned as in the previous embodiment. When cleaning is completed, the spent fuel assembly 4 is carried out, and then the electromagnetic pump 21 is started to pump sodium from the sodium storage tank 17 into the container 16 in preparation for the next cleaning.

【Effect of the invention】

As described above, according to the present invention, a preheating tank filled with the same liquid metal as the reactor primary coolant in a heated molten state is provided as a means for preheating a spent fuel assembly. By immersing the spent fuel assembly in liquid metal to preheat it, and then cleaning the spent fuel assembly using a decompression operation, the spent fuel assembly can be heated to a predetermined preheating temperature within a short period of time. It is possible to uniformly heat and raise the temperature, and the cleaning time can be significantly shortened compared to conventional preheating methods using decay heat, high-temperature gas blower preheating methods, heater preheating methods using single-volume indirect heating, and the like.

[Brief explanation of drawings]

FIGS. 1 and 2 are system diagrams of cleaning apparatuses according to different embodiments of the present invention, respectively. In each figure, 2: cleaning tank, 3: preheating tank, 4: spent fuel assembly,
5: Vacuum tank, 7: Vacuum pump, 11: Decompression exhaust line, 14: Heater, 16: Container serving as preheating tank and cleaning tank, 17: Sodium storage tank, 18: Sodium supply and discharge piping. Figure 1

Claims (1)

[Claims] 1) A spent fuel assembly cleaning device for a nuclear reactor using liquid metal such as sodium as a primary coolant, which heats and raises the spent fuel assembly to a predetermined temperature using a preheating means. and preheat it,
Thereafter, with the spent fuel assembly housed in the cleaning tank, the atmosphere in the cleaning tank is depressurized to evaporate and remove liquid metal adhering to the spent fuel assembly, thereby cleaning the spent fuel assembly. The preheating means is provided with a preheating tank filled with the same liquid metal as the reactor primary coolant in a heated molten state, and the spent fuel assembly is immersed in the liquid metal in the preheating tank. A spent fuel assembly cleaning device characterized in that the spent fuel assembly is preheated by heating the spent fuel assembly. 2) In the cleaning device according to claim 1, the preheating tank is a tank filled with liquid metal that is separate and independent from the cleaning tank, and the preheating tank is equipped with a heater for heating the liquid metal. A spent fuel assembly cleaning device characterized by: 3) In the cleaning device according to claim 1, the same container serves as both the preheating tank and the cleaning tank, and the container is equipped with a heater for heating the liquid metal, and a liquid metal storage space is provided separately from the container. A spent fuel assembly cleaning device, characterized in that it is connected to a tank via a liquid metal supply and discharge pipe.