JPS6112238B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the treatment of radioactive waste, and more particularly, to a method of solidifying calcined radioactive waste into a ceramic object using a ceramic-forming substance (hereinafter abbreviated as "ceramic solidification method") and It relates to an apparatus for carrying out the method. With the spread of nuclear power generation, the amount of highly concentrated radioactive waste fluid generated from spent nuclear fuel reprocessing plants is increasing year by year. Storing these radioactive waste liquids in liquid form in tanks poses problems not only in terms of safety and management, but also in storage space in terms of quantity and volume, so we are developing technology to convert them into solidified materials that are easier to store (solidification technology). There is a strong need for the establishment of Conventionally, various methods have been proposed for solidifying highly concentrated radioactive waste, such as vitrification and metal composite solidification, but at present, vitrification is technically insufficient. It is said to be the most economically advantageous. Furthermore, various methods have been proposed for this vitrification method, but one method involves melting the calcined radioactive waste and glass-forming material charged in an electrically insulating crucible using high-frequency induction heating. This method is viewed as promising due to its advantages such as a simple device and good thermal efficiency. That is, this method involves continuously or intermittently charging radioactive waste calcined material and glass-forming material from the top of the crucible, and intermittently removing the molten mixture of the materials from an outflow nozzle provided at the bottom of the crucible. It is guided to a containment vessel and solidified. Therefore, according to this method, continuous operation of the apparatus is possible, and in addition to the above-mentioned advantages, the molten glass electrodes do not contact each other, and the molten glass can be heated almost uniformly regardless of its position in the crucible. Since the electrically insulating crucible is not directly heated, there is no risk of overheating, and it has advantages such as being less likely to be eroded by the molten glass in the crucible. However, despite the above-mentioned advantages, the conventional vitrification method using high-frequency induction heating has the following drawbacks. That is, calcined radioactive waste and glass-forming materials usually have extremely high electrical resistance at room temperature, and it is difficult to induce induction heating directly from room temperature because it is impossible to generate an induced current using high frequency waves. Therefore, it is necessary to heat and melt the glass-forming substance in advance to a temperature of about 800 to 1200°C by other means to make it a good electrical conductor with an electrical resistivity of about 10 Ωcm or less. The above-mentioned drawbacks in producing a vitrified body by high-frequency induction heating also pose problems in a method of solidifying ceramics by high-frequency induction heating. In other words, in the conventional ceramic solidification method, the calcined powder of radioactive waste and the additives used to solidify the ceramic have high electrical resistance at room temperature, so it is necessary to use a part or all of the mixture. It is difficult to heat by high frequency induction without preheating using some auxiliary means. The purpose of the present invention is to improve the above-mentioned drawbacks of the conventional ceramic solidification method and facilitate high-frequency induction heating, thereby improving thermal efficiency and simplifying and speeding up the solidification process of radioactive waste. An object of the present invention is to provide a method and an apparatus for carrying out the method. As a result of research to achieve the above-mentioned results, the present inventors found that adding silicon carbide powder as a third component to calcined radioactive waste and ceramic forming materials is extremely effective in improving heating efficiency in high-frequency induction heating. I found that. That is, in the method of solidifying radioactive waste into ceramics according to the present invention, a calcined product of radioactive waste and a ceramic-forming material are charged into a crucible made of an electrically insulating refractory material, and after being made into a molten body by high-frequency induction heating, the molten material is heated. When treating radioactive waste by removing the body from the crucible and introducing it into a containment vessel to solidify it, silicon carbide powder is added as a third component to the calcined material and ceramic forming material charged in the crucible.
It is characterized in that it facilitates high-frequency induction heating by adding it in an amount of 20 to 50% by weight based on the total amount. The ceramic solidification device for radioactive waste of the present invention also includes a crucible made of an insulating refractory material, an induction coil arranged around the crucible with a plurality of windings, and connected to a high-frequency oscillator for heating the crucible. , comprising a device for charging calcined radioactive waste, a ceramic forming material, and silicon carbide powder into the crucible, and an outflow nozzle provided at the bottom of the crucible for taking out the melt. It is. The present invention will be explained in more detail below. The method of solidifying radioactive waste into ceramics of the present invention consists of the following steps. That is, first, a step of continuously or intermittently feeding into a crucible made of an electrically insulating refractory material a mixture containing 20 to 50% by weight of a calcined radioactive waste, a ceramic forming material, and a silicon carbide powder. Next, the mixture supplied in the above step is made into molten ceramics by high-frequency induction heating, and finally, a certain amount of the molten ceramics is intermittently taken out from the outflow nozzle and filled into a storage container and solidified. . Furthermore, in the last step above,
If necessary, even if the crucible is out of operation,
By performing high-frequency induction heating only on the outflow nozzle portion, it is possible to prevent the outflow nozzle from closing due to solidification of the molten ceramic and facilitate the outflow operation. Silicon carbide added in the first step described above has good heating efficiency, excellent oxidation resistance, and corrosion resistance, and even if it is decomposed by oxidation, it does not release CO ₂
and SiO ₂ , which is also a part of ceramic forming substances.
Therefore, even if the decomposition products mix into the molten ceramics, no problem will occur. In addition, induction heating of silicon carbide using high frequency is extremely easy, and by adjusting the high frequency output, it is possible to heat silicon carbide from room temperature to 1300℃.
The temperature can be adjusted to any temperature within the range. Therefore, by adding silicon carbide powder, not only is it not necessary to preheat the ceramic-forming material by other means to a temperature that allows high-frequency induction heating, but it is also possible to There is no need to separately charge a preheating material such as a melting point glass material. The content of silicon carbide is preferably 20 to 50% by weight, and 20 to 50% by weight based on the total amount.
If it is less than % by weight, the temperature rise will be slow and disadvantageous;
If it exceeds 50% by weight, it is inappropriate because the amount of waste calcined bodies to be processed will decrease. In addition, the ceramic sulfur-forming substance is used to solidify radioactive waste calcined bodies,
The amount added is preferably 60 to 90% by weight, including silicon carbide, based on the total amount. As the ceramic-forming substance, ordinary oxide-based ceramic materials can be widely used, but materials containing at least one substance selected from Al ₂ O ₃ , SiO ₂ and alkaline earth metal oxides are particularly preferably used. . Next, in order to further clarify the method of the present invention, one embodiment will be described along with an example of an apparatus for implementing the method. The apparatus shown in Figure 1 consists of an insulating refractory crucible 1
It is equipped with This insulating refractory crucible must be able to withstand the corrosive effects of ceramics and must not be susceptible to breakage even at the maximum design temperature of approximately 1500°C. For example, quartz glass materials with excellent thermal shock resistance, quartz glass sintered refractories (Glass Rock), and Al ₂ O ₃ with excellent corrosion resistance
--ZrO ₂ --SiO _2- based or Al ₂ O ₃ -based electrocast refractories are preferred. These have the properties shown in the table below.

[Table] The device for heating the charge in the crucible 1 consists of an induction coil 2 placed around the crucible 1 and away from the crucible, and a high-frequency oscillator 3 that supplies a high-frequency current to the induction coil 2. . The induction coil 2 is made of a water-cooled hollow copper pipe. Furthermore, the high frequency oscillator 3 is capable of supplying high frequency alternating current capable of induction heating the molten ceramic made of the calcined radioactive waste, the ceramic forming substance, and silicon carbide charged in the crucible 1, Moreover, it is preferable that the frequency can be adjusted to the optimum frequency according to each process. Considering the efficiency of induction heating, it is thought that a frequency of at least 100 KHz is required, but it is generally carried out at a frequency on the order of several MHz to several tens of MHz, and the output of the high frequency oscillator 3 is
Adjust the melt to the desired temperature as variable depending on the load. The upper part of the crucible has a superstructure consisting of a metallic shoulder 4 and a cylindrical part 5 and is connected to an exhaust gas device 6. The inner surface of the shoulder 4 reflects radiant heat from the charge in the crucible 1 to reduce heat losses within the crucible. The exhaust gas device 6 is designed to prevent harmful gases such as NOx and easily volatile harmful components such as cesium oxide, which are generated when the radioactive waste, ceramic forming material, and silicon carbide react to form ceramics, from diffusing into the surroundings. This is a device that sucks in, removes harmful substances using a filter, etc., and exhausts them into the atmosphere. An outflow nozzle 7 is provided at the bottom of the crucible to take out the molten ceramics out of the crucible 1. In the embodiment shown in FIG. 1, the outlet nozzle 7 is made of the same refractory material, continuous with the crucible 1, but it can also be made of other refractory materials. Around the outflow nozzle 7, there is an induction coil 8 and a high frequency oscillator 9 that supplies high frequency waves to the induction coil, which serves as a heating device to heat and melt the ceramic lump that solidifies due to the stoppage of operation of the device and closes the outflow nozzle. is provided. Furthermore, as an auxiliary means for rapidly heating and melting the ceramic lump closing the outflow nozzle 7, a heating element which is directly inductively heated by an induction coil may be provided as necessary. As this heating element, for example, as shown in FIG.
Heat generating ring 7 made of a resistance heating element such as silicon carbide
A is provided between the outflow nozzle 7 and the induction coil 8, and in the example shown in FIG. 3, the inner wall of the outflow nozzle 7 is provided with a heat-resistant metal layer 7b made of stainless steel, Inconel, or the like. The thickness of the heat-resistant metal layer 7b needs to be 1 mm or less in consideration of the difference in coefficient of thermal expansion with the refractory that is the constituent material of the outflow nozzle 7. A part of the upper structure of the crucible is provided with a hopper, a constant feeder 11, and a vibrator 12 as means for charging calcined radioactive waste, ceramic forming material, and silicon carbide powder into the crucible 1. Further, a metal containment vessel 14 is arranged below the outflow nozzle 7, and is provided with means (not shown) for heating the containment vessel and its contents. This heating means serves to fill the container with molten ceramics without any gaps by heating the storage container when receiving the molten ceramics from the outflow nozzle 7. Next, using the device of the present invention described above,
An example of a method for solidifying radioactive waste into ceramics will be explained. First, an initial charge of a mixture of calcined radioactive waste, ceramic-forming material, and silicon carbide powder is mixed at a fixed ratio by a hopper and a constant feeder 11 at a set constant speed, and then passed through a vibrator 12 into a crucible. Supply into 1. Next, a high frequency current is applied to the induction coil 2 by the high frequency oscillator 3 to induction heat the silicon carbide powder. When the initial charge of the mixture becomes molten ceramic and the molten ceramic itself reaches a state where it can be directly induction heated, if induction heating is to be continued, a new mixture is added using the chute 13 extended to the center line of the crucible 1. , either continuously or intermittently. This freshly supplied mixture is melted and digested to combine with the previously melted material as molten ceramics. Then, when the liquid level of the molten ceramic reaches near the maximum allowable level of the crucible 1 and is sufficiently melted, the outflow nozzle 7 is opened and a fixed amount of molten ceramic is filled into the storage container 14. At this time, if the outflow nozzle 7 is closed by a solidified ceramic lump, a high frequency wave is supplied from a high frequency oscillator 9 to an induction coil 8 placed around the outflow nozzle 7 in advance, and the ceramic lump is Must be melted by induction heating. Next, the mixture of the calcined product, ceramic forming material, and silicon carbide powder is continuously added to
Alternatively, it is intermittently supplied into the crucible 1 and the above-mentioned steps are repeated. Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples. Example A ceramic-forming substance having the composition shown in Table 1,
Calcined simulated radioactive waste having the composition shown in Table 2 was prepared, and silicon carbide powder was mixed therewith in the proportions shown in Table 3 as raw materials.

【table】

【table】

[Table] The insulating refractory crucible into which the above mixture is charged has an inner diameter of 90 mmφ, a height of 130 mm, and an inner diameter of the outflow nozzle of 8 mmφ.
A quartz glass crucible with an internal diameter of 110 mm
An induction coil consisting of a water-cooled hollow copper pipe of mmφ was wound four times from the bottom of the crucible to a height of 70 mm.
Then, the mixture shown in Table 3 was charged into the crucible to a height of about 100 mm from the bottom of the crucible, and the crucible was started using an anode-tuned self-excited oscillator tube at a frequency of 3 MHz and an anode input of 6 KW. In the case of Examples 1-4, the mixture was completely dissolved in 2-4 hours. After the above initial charge is melted, the anode input is set to 5.5 KW and the mixture is further charged at a rate of 500 g per hour, and when the molten ceramic reaches the maximum level, i.e. 70 mm from the bottom,
The lower outlet nozzle was heated to cause the molten ceramic to flow out. In Comparative Example 1, high-frequency induction heating was performed in the same manner as in Examples 1 to 4, but no increase in temperature of the mixture was observed. In Comparative Example 2, high-frequency induction heating was performed in the same manner as in Examples 1 to 4, but the temperature of the mixture rose extremely slowly, and the mixture did not dissolve even after 24 hours of heating. Furthermore, in the case of Comparative Example 3, the amount of radioactive waste calcined bodies to be processed decreased, and excess silicon carbide powder that did not mix with the melt clogged the outflow nozzle, causing an obstruction when sulfurizing the molten ceramics. Summer. As is clear from the above examples and comparative examples, the ceramic solidification method of the present invention facilitates high-frequency induction heating, improves thermal efficiency, simplifies and speeds up the ceramic solidification process, and improves radioactive waste. It is very useful for processing.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view partially showing an outline of a ceramic solidification device for radioactive waste according to the present invention, and FIGS. 2 and 3 are cross-sectional views of an outflow nozzle used in the device according to the present invention. . 1... Insulating fireproof crucible, 2... Induction coil, 3...
High frequency oscillator, 4... Shoulder part, 5... Cylindrical part, 6... Exhaust gas device, 7... Outflow nozzle, 7a... Heat generating ring, 7
b... Heat-resistant metal layer, 8... Induction coil, 9... High frequency oscillator, 10... Molten ceramics, 11... Hopper and constant feeder, 12... Vibrator, 13... Chute, 14... Containment vessel.

Claims (1)

[Claims] 1. Calcined radioactive waste and ceramic forming material are charged into a crucible made of an electrically insulating refractory material, and after being made into a melt by high-frequency induction heating, the melt is taken out from the crucible and stored. When treating radioactive waste by introducing it into a container and solidifying it, the calcined material charged in the crucible and at least one substance selected from Al ₂ O ₃ , SiO ₂ and alkaline earth metal oxides are used. Ceramic solidification of radioactive waste, characterized in that high-frequency induction heating is facilitated by adding silicon carbide powder as a third component in a range of 20 to 50% by weight based on the total amount to a ceramic-forming material. Law. 2. A crucible made of an insulating refractory material, an induction coil arranged around the crucible with a plurality of turns and connected to a high-frequency oscillator for heating the crucible, and into the crucible a calcined product of radioactive waste;
A device for charging a ceramic forming material made of at least one substance selected from Al ₂ O ₃ , SiO ₂ and alkaline earth metal oxides and silicon carbide powder, and a device for taking out the melt provided at the bottom of the crucible. an outflow nozzle;
Ceramics solidification equipment for radioactive waste. 3. A patent claim in which the outflow nozzle is made of an electrically insulating refractory material, and around the outflow nozzle an induction coil is arranged, which is connected to an independent high-frequency oscillator different from the high-frequency oscillator for heating the crucible. The device according to item 2 of the scope of the invention.