JPS6019478B2 - Vitrification method and vitrification equipment for radioactive waste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the treatment of radioactive waste, and in particular to a method of solidifying radioactive waste as a glass material using a glass-forming substance (hereinafter referred to as "vitrification method"). abbreviated as ) and an apparatus for carrying out the method.

In the vitrified material obtained by the present invention, the calcined radioactive waste is encapsulated and fixed in a physically and chemically stable state by a glass-forming substance, and the vitrified material has medium-low concentration and high concentration. Suitable for semi-permanent storage of high concentration radioactive waste. As the contribution of nuclear power generation to the electricity supply increases, the amount of highly concentrated radioactive waste fluid generated, especially from spent nuclear fuel reprocessing plants, tends to increase year by year.

When storing these waste liquids in tanks, storage space becomes a problem not only in terms of safety and management, but also in terms of quantity and volume.Conversion technology (solidification technology) to solidify the liquid for easy storage is needed. There is a strong need for the establishment of Conventionally, various methods have been proposed for solidifying highly concentrated radioactive waste, such as ceramic solidification method, metal composite solidification method, etc., but at present, vitrification method is technically insufficient. It is also said to be the most economically advantageous.

Various vitrification methods have been proposed, but a method that includes a step of melting calcined radioactive waste and glass-forming material placed in an electrically insulating crucible using high-frequency induction heating is the most popular. It is viewed as promising because of its advantages such as a simple device and good thermal efficiency.

This method involves continuously or intermittently charging radioactive waste calcined material and glass-forming material from the top of the crucible, while intermittently discharging the molten mixture of the materials from an outflow nozzle provided at the bottom of the crucible and storing it. It is introduced into a container and solidified. Therefore, continuous operation of the device is possible. In addition to the advantages mentioned above, this method has the following advantages: the electrode does not come into contact with the molten glass, the molten glass can be heated almost uniformly regardless of its position in the crucible, and the electrically insulating crucible is not heated directly. There are advantages such as there is no risk of overheating and the crucible is less likely to be eroded by molten glass. However, despite the above-mentioned advantages, the conventional vitrification method using high-frequency induction heating has the following drawbacks.

First, 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, the glass-forming material may be pre-applied by other means to approximately 80%
It is necessary to heat it to a temperature of 0.00 to 1200 qo to melt it and make it a good electrical conductor with an electrical resistance of about 100 qo or less. Conventional preheating methods include the following methods.
(1i)3 When initially bagging radioactive waste calcined material and glass-forming material at the bottom of the crucible, metals (e.g. A
A method of inducing induction heating at the start of operation by adding a good electrical conductor, such as a chip of I, Fe).

(ii) A vitreous material that has higher crab conductivity and melts more easily than the glass-forming material used to encapsulate and solidify radioactive waste is placed in the bottom of the crucible, and when the crucible starts operating, A method of inducing induction heating.

However, the above methods (i) and (ii) each had the following drawbacks.

In method (i), the metal chips placed in a bag at the bottom of the crucible are extremely heated due to their high conductivity, resulting in the risk of the crucible overheating and being destroyed by thermal shock or being eroded and damaged by the glass. The sex is great. Further, after the induction heating starts, the metal chips gradually dissolve into the glass, and there is a great possibility that a non-uniform deposited layer will be formed in the glass. In particular, the compositional design of the glass-forming materials used is often such that heavy metals such as molybdenum can be contained in the fission products close to the limit, and the formation of an immiscible layer from these glasses is difficult to achieve in the vitrified product. properties, especially the leaching rate, which indicates long-term chemical durability. (Method ``-'' uses glass with a high content of alkali metals such as Na as a glassy substance with high electrical conductivity, so the leaching rate of the resulting vitrified material is high, and the radioactive substances encapsulated and fixed are The second disadvantage of conventional glass melting technology is that while the molten mixture of calcined radioactive waste and glass-forming material is removed from the outflow nozzle of the crucible to the containment vessel in a timely manner, Since the process assumes continuous operation in which newly calcined material and glass-forming material are charged continuously or intermittently from the input port, the crucible may be damaged during temporary interruptions in operation that are sometimes required. There is a great risk of this happening.

In other words, an insulating refractory crucible can hold molten glass,
Once cooled to near room temperature, stress is generated due to the difference in thermal expansion coefficient between the crucible material and the glass, causing it to break to the point where it cannot be reused. However, from time to time, it may be necessary to temporarily suspend the melting apparatus for various reasons such as maintenance of the apparatus or measures against accidental accidents. Therefore, the melting equipment must be equipped with means to deal with such situations. In order to prevent damage to the crucible during shutdown, the glass must be heated to a temperature higher than the temperature at which the molten glass solidifies and stress begins to occur at the crucible-glass interface, that is, the slow cooling temperature of the glass (approximately 500 oo). need to be retained.

On the other hand, prevention of erosion of the crucible and volatilization of harmful radioactive materials,
Considering demands such as power saving, it is desirable to maintain the temperature slightly higher than the annealing temperature of the glass.
Regarding this point, when considering the conventional techniques mentioned above as preheating methods, in methods (i) and (ii), when the temperature of the glass is lowered to around 700 to 800 qo, the electrical resistance of the glass increases rapidly. As a result, it becomes difficult to perform direct conductive heating, and as a result, it is nearly impossible to maintain a constant temperature near the slow cooling temperature.

Therefore, in order to restart melting without damaging the crucible after temporarily suspending operation, the glass must be maintained at a considerably high temperature, which prevents the volatilization of harmful radioactive substances.
Erosion of the melting pot is inevitable. As detailed above,
To summarize the shortcomings of the conventional technology in the method of placing calcined radioactive waste and glass materials in bags in an insulating refractory crucible and melting and solidifying them using high-frequency induction heating, the first is the preheating method, and the second is the melting method. There is a risk of damage to the crucible when the calendar is temporarily suspended.

The object of the present invention is to provide a vitrification method in which calcined radioactive waste and a glass-forming material are placed in an insulating refractory crucible, melted and solidified by high-frequency concessional heating, and an apparatus for carrying out the method, as described above. By eliminating the drawbacks of the conventional technology,
Any special preheating aids (metal chips, other glasses, etc.)
The temperature of the molten mixture can be lowered and maintained at any time to near the annealing temperature of the glass, and the temperature can be raised again to continue melting and solidification. The object of the present invention is to provide a vitrification method and equipment for carrying out the method.

The method of the present invention involves charging calcined radioactive waste and glass-forming material into an electrically insulating and fireproof crucible, turning it into molten glass by high-frequency induction heating, and then removing the molten glass from the crucible and storing it. In the vitrification method of radioactive waste, which is introduced into a container and solidified: The calcined material and glass-forming material charged in the crucible are collected from silicon carbide, etc. provided inside or outside the crucible. A step of preheating and melting a movable resistance heating element by high-frequency induction heating; A step of raising and holding the molten glass above the liquid level; a step of gradually or intermittently feeding the calcined product of radioactive waste and a glass-forming substance into the crucible; A step of melting the mixture separated in the step of 2 and digesting it as molten glass: The glassy mass, which normally closes the outflow nozzle at the bottom of the crucible, is temporarily melted by high-frequency induction heating to close the nozzle. It is characterized by having a step of shutting off the glass, intermittently taking out a certain amount of molten glass, filling it into a storage container, and solidifying it.

According to the method of the present invention, a resistance heating element made of silicon carbide or the like provided inside or outside the crucible is conductively heated by high frequency waves, and this is used as a medium to preheat the amount of radioactive waste and glass forming material to be initially placed in the bag. In order to achieve this, there is no need to charge any other special materials for preheating (metal chips, highly conductive, low melting point vitreous materials, etc.).

Therefore, the above-mentioned disadvantages resulting from this are not present. Note that as embodiments of the method of the present invention, there are cases in which the heating element is provided inside the crucible and cases in which it is provided outside the crucible, but the former embodiment in which the heating element is provided inside the crucible is better for the following reasons. It is superior to the latter embodiment. That is,
If the heating element is installed outside the crucible, the charge inside the crucible will be indirectly heated through the crucible wall, resulting in low thermal efficiency. As a result of being directly heated by the crucible, the temperature becomes higher than that of the bag contents inside the crucible, resulting in greater erosion by the glass. On the other hand, when a heating element is provided inside the crucible, the charging material comes into direct contact with the heating element, so thermal efficiency is high, and the crucible itself is heated indirectly through the charging material. Therefore, the corrosivity of the glass does not become particularly large due to overheating. To further clarify the method of the present invention, one embodiment will be described along with an example of an apparatus for carrying out the method.

This embodiment is an example of a case where a heating element is provided inside the crucible. The apparatus shown in FIG. 1 includes an insulating refractory crucible 1. The apparatus shown in FIG. This insulating refractory crucible can withstand the corrosive effects of glass and has a maximum operating design temperature of approximately 15
It must be such that there is no risk of damage even at 00:00. For example, quartz glass or quartz glass refractory (Glass Rock), which simulates thermal shock resistance, and AI203-Zr02-Si, which has excellent erosion resistance due to glass.
02 series or Yama 203 series electrocast refractories are preferred. These have properties as shown in Table 1. The device for heating the charge in the crucible 1 (Table 1) consists of an induction coil 2 arranged around the crucible 1 at a distance from the crucible, and a high-frequency oscillator 3 supplying a high-frequency current to the induction coil 2.

The induction coil 2 consists of a water-cooled hollow steel pipe. The high-frequency oscillator 3 can supply high-frequency alternating current that can conductively heat the molten glass made of the calcined radioactive waste and the glass-forming substance packed in the crucible 1, and It is preferable that the frequency can be adjusted to the optimum frequency according to the process. Considering the efficiency of induction heating, it is thought that a frequency of at least 100 KHZ or more is required, but it is likely to be carried out at a frequency on the order of several MHZ to several tens of MHZ. Of course, the present invention does not limit the frequency of the high frequency to be used. Further, the high frequency oscillator 3 needs to have a variable output. The upper part of the crucible has an upper structure consisting of a metal scrap part 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 cylindrical portion 5 has a structure capable of housing a raised heating element 12 as described later. The exhaust gas device 6 is equipped with a suction filter, etc. to prevent harmful gases such as NQ and volatile harmful components such as cesium oxide, which are generated when the radioactive waste reacts with the glass-forming material and vitrifies, from dispersing into the surroundings. This is a device that removes harmful substances and exhausts them into the atmosphere. An outflow nozzle 7 for taking out the molten glass out of the crucible 1 is provided at the bottom of the crucible.

In the embodiment shown in FIG. 1, the outlet nozzle 7 consists of the same continuous refractory material as the crucible 1, but it can also consist of other refractory materials. As shown in detail in Figures 2 and 3,
Except when taking out the molten glass, the outflow nozzle is closed.As a heating device for heating and melting the glass gob, there is an induction coil 8, a high frequency oscillator 9 that supplies high frequency to this induction coil, and a direct transfer by the induction coil. It is equipped with a heating element that is heated. As this heating element, in the example shown in FIG. 2, a heating ring 1 made of a resistance heating element such as silicon carbide is used.
In the example shown in FIG. 3, a heat-resistant metal layer 11 made of stainless steel, Inconel, etc. is provided on the inner wall of the outflow nozzle 7. The thickness of this heat-resistant metal layer 11 is determined by taking into account the difference in the thermal modulus of the heat-resistant metal layer 11 from the refractory material that is the constituent material of the outflow nozzle 7.
It must be less than 1 rib. The crucible 1 is provided with a resistance heating element 12 in the center thereof.
It is preferable to have a rod-like shape, and it can be raised and lowered in the vertical direction by a mechanism using pulleys, gears, etc.

FIG. 1 shows the heating element 12 lowered to its lowest position and immersed in the molten glass 13. But keep it away from the bottom of the crucible to avoid overheating the crucible. In this state, a subheating process is performed. FIG. 4 shows a state in which the heating element 12 is raised to the highest position and is away from the liquid level of the molten glass 13. After the secondary heating step is completed, the step of directly induction heating the glass is carried out in this state. The heating element 12 is
Any material may be used as long as it can be easily heated by high frequency induction, has oxidation resistance and glass erosion resistance, and can withstand the maximum design temperature, but silicon carbide, molybdenum silicide, etc. are preferred. It is. In particular, silicon carbide has good heating efficiency, excellent oxidation resistance, and glass erosion resistance, and even if it is decomposed by oxidation, it becomes CQ and Sj02, and even if the decomposition products mix into molten glass, it will not cause any trouble. does not occur. In addition, induction heating of silicon carbide by high frequency is extremely easy, and by adjusting the high frequency output,
The temperature can be adjusted to any temperature within the range from room temperature to 130,000 ℃. - When considering other metal materials as the material for the heating element 12, we found that molybdenum, which is normally used as an electrode for glass melting, is easily oxidized, and that iron-based metals such as stainless steel have a low allowable operating temperature. In addition, the carbon rod is subject to severe wear at the glass metal line part.
As a result of research conducted by the present inventors, it was found that this method has the following drawbacks and cannot meet the purpose of the present invention. As shown in FIG. 5, the heating element 12 is connected to a stainless steel support rod 15 via an insulator 14 such as silicon nitride, and has a mechanism capable of moving up and down as described above.

Part of the superstructure of the crucible includes a hopper and constant feeder 16, a pipelator 17, and a heating element 12 as means for charging the calcined radioactive waste and glass-forming material into the crucible 1. When the crucible is raised to its highest position, a chute 18 is provided which can extend to the center line of the crucible.

Also, below the outflow nozzle 7 is a metal containment vessel 19.
and includes means (not shown) for heating the containment vessel and its contents.

This heating means serves to fill the container with molten glass without any gaps by heating the storage container 19 when receiving the molten glass from the outflow nozzle 7. next,
An example of a method for vitrifying radioactive waste using the apparatus of the present invention described above will be described.

First, the heating element 12 is lowered into the crucible 1 to its lowest position, and the initial charge amount of a mixture of calcined radioactive waste and glass-forming material is mixed at a constant ratio using the hopper and the constant feeder 16. , is fed into the collar 1 through the pipelator 17 at a set constant speed.

Next, a high frequency current is applied to the induction coil 2 by the high frequency oscillator 3 to conductively heat the heating element 12. When the initial charging amount of the mixture becomes molten glass by the heating element 12 which has become red-hot and reaches a state where the molten glass can be directly induction heated, the heating element 12 is pulled out of the crucible,
Hold in the highest position. Next, the chute 1 is extended to the center line of the crucible 1 while directly inductively heating the molten glass.
8, the mixture of calcined material and glass-forming material is fed gradually or intermittently. The newly supplied mixture is melted, digested, and integrated with the previously melted mixture as molten glass. Next, the liquid level of the molten glass is
When the maximum permissible level has been reached and the melting and clarification are sufficient, the heating ring 10 or the heat-resistant metal layer 11 is heated by high frequency conduction to melt the glass lump closing the outflow nozzle 7, thereby preventing the outflow. Open the nozzle 7. After filling the containment vessel 19 with a certain amount of molten glass, the heating ring 1
0 or the heat-resistant metal layer 11 is stopped, and the outflow nozzle 7 is closed again. At this time, if the molten glass is completely taken out, it will be necessary to perform the secondary heating process again using the heating element 12, so it is necessary to leave some molten glass in the crucible. The mixture of calcined material and glass-forming substance is then fed again gradually or intermittently into the crucible 1 and the above-described steps are repeated. According to the apparatus of the present invention, when it becomes necessary to temporarily suspend the melting of glass, the heating element 12 is lowered into the molten glass in the crucible to bring it into the state shown in FIG.

When the high frequency output is lowered, the direct induction heating of the molten glass stops, but the temperature of the glass can be easily maintained near the slow cooling temperature by the induction heating of the heating element 12. Therefore, the crucible will not be damaged. When the cause of the interruption in melting is resolved and the melting operation is to be resumed, the high frequency output is increased, and when the temperature of the molten glass reaches a temperature at which it can be directly heated by induction, the heating element 12 is pulled out of the crucible and the process is resumed to a steady state. You can return to driving. The following points can be mentioned as effects of the present invention. [1} Compared to conventional high-frequency induction heating devices, only a resistive heating element is added, and no complicated separate heating layer is required.

Therefore, since no special additives are required other than the calcined radioactive waste and the glass-forming material to be treated, the operation can be simplified. (2) The time when preheating by the heating element is finished and direct induction heating of the molten glass becomes possible without the heating element can be easily determined by the change in the anode input current of the oscillator tube that is continuously recorded.

In particular, when the heating element is installed inside the crucible, the heating element is placed directly into the charge, so the thermal efficiency is extremely high and the temperature can be raised to a temperature that allows direct induction heating of molten glass in a single hour. can do.

{41 With conventional devices, it was difficult to maintain the glass in the crucible at a relatively low temperature near the annealing temperature and temporarily suspend the melting process, but with the present invention, it is easy to maintain the glass in the crucible and temporarily suspend the melting process. Therefore, there will be no problems such as destruction of the crucible or volatilization of harmful radioactive materials.

Then, if necessary, the temperature can be raised again to restart melting. Next, as another embodiment of the present invention, a method and apparatus will be described in which a heating element made of silicon carbide or the like is provided outside an insulating refractory crucible and the heating element is heated by high frequency. FIG. 6 shows an example of the apparatus according to this embodiment, in which only the crucible and the heating device provided outside are shown, and the others are omitted. That is, this device includes a ring-shaped heating element 22 that can be raised and lowered by a mechanism using pulleys, gears, etc. between an insulating refractory crucible 20 and an induction coil 31 placed around the outside thereof, and generates heat. An insulating quartz glass tube 23 is provided between the body 22 and the guiding coil 21.
It is equipped with This embodiment also allows the preheating process to proceed without adding special materials for child heating (metal chips, highly conductive, low melting point vitreous materials, etc.) to the initial charge, and It is also possible to maintain the temperature of the glass in the crucible near the slow cooling temperature during the pause in melting. EXAMPLE The mixture melted in this example is a mixture of a glass-forming material and a calcined simulated fission product having the following compositions in weight ratios of 8 parts and 20 parts, respectively.

Glass forming substance (wt%) Si02
58.7 &03 17.5 Li20 38 K20 2.5 Ca0 2.5 Zn0 2.5 Na20 12.5 N203 5.0 Simulated fission product calcined product (wt%) R et 0 1.1 S^] 3.2 Y203 2.1 Zr02 14.2 Moo3 15.8 Cs20 80 B20 5.2 La203 4.7 Ce02 9.0 Nmo03 22.2 Fe203 10.6 Cr203 0.9 Ni0 3.0 The insulating refractory crucible is It is a quartz glass crucible with an inner diameter of 90 mm, a height of 130 mm, and an outlet nozzle with an inner diameter of 8 mm.The induction coil is a 4-turn hollow copper pipe coil with an inner diameter of 11 mm and a height of 7 mm. .

A heating element made of silicon carbide with a diameter of 3 mm and a length of 7 mm is supported in the center of the upper brim, and the mixture is put in and started at a frequency of 3 MHZ and an anode input by an anode-tuned eye-excited oscillation tube. went. It will completely melt in about 2 hours, and it will melt completely from the bottom of the crucible.
When more molten glass was present, it was guided directly to the glass and continued to melt even when the heating element was pulled up.

Then, the anode input is set to 5. Add the raw material mixture at a rate of 500 g/hr, and when the molten glass reaches the maximum level, i.e., 70 g/h from the bottom,
The lower outlet nozzle was heated to cause the molten glass to flow out.

As mentioned above, melting could be continued by leaving about 3 feet of glass from the bottom of the crucible.

In addition, when temporarily suspending melting, insert the heating element for starting into the glass and lower the anode input to 2.5 to 60 W.
00) was able to be maintained.

By increasing the radio frequency power, glass melting could be restarted very easily.

[Brief explanation of the drawing]

FIG. 1 shows a partial outline of the radioactive waste vitrification apparatus according to the present invention. FIGS. 2 and 3 are cross-sectional views of the outflow nozzle used in the apparatus according to the present invention, and FIG. 4 is a cross-sectional view showing the heating element in the radioactive waste vitrification apparatus according to the present invention. A sectional view showing the lifted state, Figure 5 is the same as Figures 1 and 4.
FIG. 6 is a longitudinal cross-sectional view showing the heating element in the figure, and FIG. 6 is a cross-sectional view showing a part of the radioactive waste vitrification apparatus according to the present invention in which the heating element is provided outside the crucible. 1...
Insulating fireproof crucible, 2... Guiding coil, 3...
・...High frequency oscillator, 7... Outflow nozzle, 8...
...Concession coil, 9...High frequency oscillator, 10.
... Heat generating ring, 11 ... Heat resistant metal layer,
12... Resistance heating element, 13... Molten glass, 16... Hopper and constant feeder, 17... Piper 18... Chute,
19...Containment vessel, 20...Insulating fireproof crucible, 21...Induction coil. Figure 1 Figure 2 Figure 3 Figure 5 Figure 4 Figure 6

Claims (1)

[Scope of Claims] 1. Calculated radioactive waste and a glass forming material are charged into a crucible made of an electrically insulating refractory material, and after being made into molten glass by high-frequency induction heating, the molten glass is taken out from the crucible and stored. In the vitrification method for radioactive waste, which is guided into a container and solidified: The initial charge of the calcined material and glass-forming material charged into the crucible is preheated by high-frequency induction heating using a resistance heating element. a step of heating and melting; a step of raising and holding the resistance heating element above the liquid surface of the molten glass when the initial charge has been melted and reaches a stage where direct induction heating can be performed; and a step of placing radioactive waste in the crucible. supplying the calcined material and the glass-forming substance; by direct induction heating of the molten glass by means of high frequency;
melting the mixture supplied in the previous step and digesting it as molten glass; temporarily melting the glassy mass that normally closes the outflow nozzle at the bottom of the crucible by high-frequency induction heating to open the nozzle; Stopper it, take out the molten glass,
The method is characterized by comprising a step of filling a storage container and solidifying the container. Vitrification method for radioactive waste. 2 Claim No. 1
The vitrification method for radioactive waste as described in 2. The vitrification method is carried out by providing a resistance heating element inside a crucible. 3. The vitrification method for radioactive waste as set forth in claim 1, which is carried out by providing a resistance heating element outside the crucible. 4. A crucible made of an insulating refractory material, an induction coil arranged with a plurality of turns around the crucible and connected to a high-frequency oscillator, and a calcined product of radioactive waste and a glass-forming material placed in the crucible. an outflow nozzle provided at the bottom of the crucible for taking out the molten glass, and a heating device for melting the glass lump closing the outflow nozzle and opening the outflow nozzle. In a waste vitrification device: A radioactive waste vitrification device characterized by having a resistance heating element that can be raised and lowered. 5. The vitrification apparatus for radioactive waste according to claim 4, wherein a resistance heating element is provided inside the crucible. 6. The vitrification apparatus for radioactive waste according to claim 4, wherein the resistance heating element is provided outside the crucible and between the crucible and the induction coil. 7. The apparatus for vitrification of radioactive waste as set forth in claim 4, wherein the outflow nozzle is made of an electrically insulating refractory material, and the heating device for melting the glass lump closing the outflow nozzle is The vitrification device comprises a heat-resistant metal layer provided on the inner wall of the nozzle and an induction coil connected to a high frequency oscillator and arranged around the outflow nozzle. 8. The apparatus for vitrification of radioactive waste according to claim 4, wherein the outflow nozzle is made of an electrically insulating refractory material, and the heating device for melting the glass lump closing the outflow nozzle, The vitrification device comprises a resistive heating element arranged around the outflow nozzle and an induction coil connected to a high frequency oscillator and arranged outside the resistive heating element.