JP7143029B2 - Manufacturing method of clearance metal - Google Patents

Description

The present invention separates and removes radioactive substances from radioactive metal waste contaminated with radioactive substances whose main components are radioactive nuclides such as radioactive cesium and radioactive strontium that tend to migrate to slag and dust when melted, thereby achieving clearance levels (commercially available). It relates to a method for producing metal (hereinafter referred to as "clearance metal") of a decontamination level that can be distributed equivalently to metal scrap of .

High-level, medium-level, and low-level radioactive waste is generated when radioactive facilities such as nuclear power plants are shut down and dismantled, and low-level radioactive waste accounts for 80% of the total. occupies more than Since it is assumed that radioactive waste is stored in a place that can be shielded, processing for storing low-level radioactive waste, which accounts for the majority of the waste in terms of quantity, poses problems of securing space and cost. Both are important.

At present, a method of storing in a storage container after processing such as melting and compression, which can greatly reduce the volume, is being applied (see Patent Document 1).
However, with regard to radioactive metal waste in particular, the metal itself is usually not activated, but is contaminated with radioactive substances attached to it. If a highly controlled melting technology is applied, it is possible to almost completely separate the metal and radioactive components (clearance). It can be used as a raw material for wire rods (free release).
For example, Japanese Patent Laid-Open No. 2002-200002 discloses the clearance by blasting.

However, at least in Japan at present, this decontamination metal is not utilized as a raw material (free release) in the same manner as metal scrap distributed in the market. Even if it is to be reused, it is assumed that it will be reused only by melting a small amount into shielding containers or ingots (shielding material) that are exclusively used on the premises of nuclear facilities.

The reason for this situation is that the history of the nuclear power facility itself is short, so the amount of waste generated when the facility reaches the end of its life is small, and the market distribution of detoxified metals is not successful. There are things that do not exist. For this reason, first, it is considered important to reliably detoxify a large amount of radioactive metal waste and to create a track record of reusing detoxified metals in nuclear facilities and the like.

To this end, (i) the establishment of a precisely controlled melting process capable of achieving a high degree of separation and removal of radioactive materials from radioactive metal waste and the detoxification of metals, (ii) the clearance metals for easy reuse. It is essential to establish a method for manufacturing As a result, residents' concerns about clearance metals will be eliminated, and they will be treated in the same way as scrap in the market and reused for general-purpose products such as shaped steel and wire rods by iron manufacturers in the market (free release). It is thought that it is possible to approach the desired figure.

JP 2013-40841 A JP 2007-248066 A

In this regard, the technology disclosed in Patent Document 1 is to melt metal scrap waste to be processed generated in a radiation utilization facility at high frequency and form an ingot into a flat plate shape that enables radiation measurement using a mold. Although it is possible to reduce the volume of metal dismantling waste, it has not yet reached the point of manufacturing clearance metals by detoxifying contaminated metal waste.
Moreover, if the clearance level is not satisfied as a result of radiation measurement after the formation of the ingot, there is also the problem that if the formed ingot is to be melted again, a large amount of energy is required for melting.
Furthermore, there is also the problem that ingot formation is difficult to handle such as radiation measurement and storage.

The present invention solves these problems, and aims to reduce the volume of radioactive metal waste contaminated with radioactive materials containing radionuclides that tend to migrate to slag or dust when molten, such as radioactive cesium and radioactive strontium. It is also an object of the present invention to provide a method for producing a clearance metal in which radioactive substances are separated and removed, and the clearance metal can be easily measured for radiation, stored, and remelted at a high yield.

(1) A method for producing a clearance metal according to the present invention is a method for producing a clearance metal from radioactive metal waste,
a radioactive metal waste charging step of charging into a melting furnace radioactive metal waste contaminated with a radioactive material containing, as a main component, a radionuclide that is likely to migrate to slag or dust during melting;
a slag raw material charging step of charging slag raw material, which is a raw material of slag, into the melting furnace as necessary;
a radioactive material transfer step of separating the radioactive material from the radioactive metal waste and transferring it to slag and dust by melting the input material in a melting furnace;
a dust treatment step of collecting and treating the dust;
The method is characterized by comprising a granular metal producing step of separating molten metal from the molten material melted in the melting furnace and using the separated molten metal as granular clearance metal.

(2) In addition, in the above (1), the granular metal generation step includes a granular metal radioactivity concentration measurement step of measuring the radioactivity concentration of the granulated molten metal, and the radioactivity concentration measurement step. The method is characterized by including a step of charging metal particles whose measured value exceeds the reference value into the melting furnace.

(3) In the method described in (1) or (2) above, the step of forming the metal granules is characterized by granulating the separated molten metal by water granulation or wind granulation.

(4) In addition, in the above (1) or (2), the granular metal generating step includes cutting or crushing the separated molten metal into rods, wires or plates. It is characterized by being granulated.

(5) In addition, in any one of the above (1) to (4), a molten residue generating step of recovering all or part of the molten residue from the melt melted in the melting furnace;
a remelting possibility determination step of determining whether or not the recovered molten residue can be reused based on the radioactivity concentration of the molten residue;
a molten residue charging step of charging the recovered molten residue into a melting furnace when it is determined that it can be reused in the remelting propriety determining step;
It is characterized by comprising a melted residue disposal step of disposing of the melted residue as radioactive waste when it is determined that the melted residue cannot be reused in the remelting propriety determining step.

(6) In the method described in (5) above, the step of determining whether or not remelting is possible is characterized by measuring the radioactivity concentration of the molten residue and making a determination based on the measured value.

According to the present invention, the clearance metal can be produced with a high yield, and since the produced clearance metal is a granular metal, the degree of freedom of usage is high.
In addition, in the granular metal production step, a radioactivity level measurement step of measuring the radioactivity level of granulated molten metal, and a granular metal that does not satisfy the clearance level in the radioactivity level measurement step is charged into the melting furnace. By providing a metal injection step, the measurement object is granular, so it is easy to measure the radioactivity level. There is also an effect that the amount may be small.
Furthermore, it is possible to reduce the volume of molten residues contaminated with radioactive substances that are generated in the production of clearance metals.

3 is a flow chart of a method for manufacturing clearance metal according to an embodiment of the present invention; FIG. 4 is an explanatory diagram illustrating a radioactive metal waste charging step (closed charging) in the method for producing clearance metal according to the embodiment of the present invention; FIG. 4 is an explanatory diagram illustrating a radioactive metal waste charging step (open charging) in the method for producing clearance metal according to the embodiment of the present invention; FIG. 4 is an explanatory diagram illustrating a radioactive metal waste charging step (pusher-type sealed charging method) in the method for manufacturing clearance metal according to the embodiment of the present invention. FIG. 3 is an explanatory diagram illustrating a radioactive metal waste charging step (suspended sealed charging method) in the method for manufacturing clearance metal according to the embodiment of the present invention. 4 is a graph showing the relationship between the time from the start of tapping and the temperature change of molten metal in the method for producing clearance metal according to the embodiment of the present invention. FIG. 4 is an explanatory diagram of a granular metal producing step (mode of using a ladle) in the method for producing clearance metal according to the embodiment of the present invention; FIG. 2 is an explanatory diagram of a granular metal producing step (a form of using a double-sided tilting melting furnace) in the method for producing clearance metal according to the embodiment of the present invention; It is a flowchart in the case of collecting and reusing slag as another aspect of the embodiment of the present invention.

The method for producing clearance metal according to the present embodiment is a method for producing clearance metal from radioactive metal waste, and as shown in FIG. It comprises a processing step and a particulate metal forming step.
1 do not necessarily proceed in the order shown in FIG. 1. For example, the radioactive substance transfer process and the dust treatment process may proceed simultaneously.
Hereinafter, the meanings of the terms used in each step will be explained, and then the details of each step and the processing associated with each step will be explained.

<Description of terms>
[Radioactive metal waste]
"Radioactive metal waste" refers to radioactive waste consisting primarily of metals. Radioactive metal waste generated at a nuclear power plant is, for example, what was installed in the facility and discarded due to the abolition, repair, change, accident, etc. of the facility. Usage forms include tanks, pipes, corridors, ducts, supports, pumps, motors, etc. Radionuclides such as radioactive cesium and radioactive strontium are attached to these.

It should be noted that the behavior of the radioactive material adhering to the radioactive metal waste in the molten state differs depending on the type. Since cesium is highly volatile, it migrates to dust, and strontium migrates to slag because of its high affinity with slag, but cobalt does not exhibit such behavior. The present invention is directed to radioactive metal wastes contaminated with radioactive materials, the main components of which are radioactive cesium, radioactive strontium, or other radionuclides that tend to migrate to dust or slag when melted.

[Clearance metal]
"Clearance metal" refers to a metal whose radioactivity concentration is below the clearance level. Here, the clearance level is the level at which the International Atomic Energy Agency (IAEA) recognizes that the effects on the human body can be ignored no matter how metals, concrete, etc. are reused or landfilled as waste. Cesium is 0.1 Bq/g or less, and radioactive strontium is 1 Bq/g or less.

The waste generated by the decommissioning of nuclear power plants is classified as follows according to the radioactivity concentration.
L1: Low-level radioactive waste for intermediate depth disposal
L2: Low-level radioactive waste to be disposed of in the shallow ground pit
L3: Low-level radioactive waste to be disposed of in the shallow underground trench
CL: Clearance object
NR: non-radioactive waste

Next, each step in the method for producing a clearance metal according to this embodiment will be described in detail.

<Radioactive metal waste input process>
The radioactive metal waste input process is a process in which radioactive metal waste contaminated with radioactive substances, which are mainly composed of radioactive nuclides such as radioactive cesium and radioactive strontium that easily migrate to dust and slag during melting, is charged into the melting furnace.
It is preferable to separate and pretreat the radioactive metal waste before the radioactive metal waste charging step.

《Discretion》
It is assumed that carbon steel accounts for more than 90% of the radioactive metal waste generated from decommissioned nuclear power plants, etc. However, in addition to carbon steel, copper, chromium, etc. are mixed. Although it depends on the ratio of components other than iron, it is possible to produce a clearance metal by melt-treating a mixture of these components. Moreover, even such a clearance metal can be used as a counterweight or the like.

However, clearance metals containing a large amount of impurities have limited reuse applications, so it is desirable to exclude metals other than carbon steel from processing. Materials other than carbon steel should be separated into combustible materials, inorganic materials, copper, aluminum alloys, stainless steel, etc., and treated separately before being charged into the melting furnace.

A method for sorting radioactive metal waste will be explained.
As a sorting method, it is preferable to combine magnetic sorting and manual sorting.
Stainless steel can be melted even if it is mixed and melted, but when recycling the metal after melting, it is preferable to exclude it from melting by sorting because there are compositional restrictions on alloy components such as chromium.
Furthermore, from the viewpoint of work safety and manufacturing efficiency of clearance metal, it is preferable to perform dose rate measurement and exclude high-dose metal waste.

"Preprocessing"
In order to perform precisely controlled and smooth melting, radioactive metal waste should be shredded before entering the melting furnace, and the shredded material should be lashed or placed in a bucket before being put into the melting furnace. .
There are restrictions on the size of the opening for charging into the furnace, depending on the processing scale of the melting furnace, charging method, and melting method (melting furnace structure), and the maximum charging size is defined based on this. For example, large objects among tanks and metal scraps need to be downsized to the input size by pretreatment such as cutting.

Although it is advantageous for the size to be fed into the melting furnace to be finer to some extent for melting, the energy and cost required for shredding become larger as the size is finer, so a practically suitable size should be selected in consideration of both.
Also, even when lashing objects or buckets are thrown in, they may get caught in the entrance of the furnace or the hopper, and some of them may fall off and interfere with the opening and closing of the lid or door. There is

In addition, since it is difficult for people to enter and work in the melting facility, a charging section with the same structure as the actual charging section was created in a place where people can easily enter, and a trial charging was conducted to facilitate smooth operation. It is more preferable to actually perform the loading operation after confirming the loading.

In order to carry out pretreatment, pretreatment equipment is installed, and in the pretreatment equipment, objects to be treated are received and stored from incoming vehicles, and radioactive metal waste is sorted by material and downsized to the input size. It is preferable to store it temporarily and put it into a melting furnace (packaging, packaging, etc.) so that it can be transported to the melting facility.

The processes considered to be necessary from acceptance to storage before charging into the melting furnace include a receiving process, a sorting process, and a cutting process, and the requirements for each process are as shown below.

[Acceptance process]
In the process of receiving radioactive metal waste, it is desirable to detect non-target high-dose objects with a dose detection device (gate monitor, etc.) at the entrance of the vehicle, remove them, and carry them out.

[Sorting process]
In the sorting process, a radiation monitor or the like is used to sort by dose or material/shape. In the dose selection process, the items evaluated as having high doses by the radiation monitor, etc. are isolated, and after temporarily storing the items with high doses, they are moved to storage facilities on the premises. It is possible to prevent contamination of the processed material.

In material selection, by preventing the mixing of unmeltable substances such as aluminum alloys, copper and resins, it is possible to achieve stable melting and ensure the quality of metal castings. It is preferable to perform shape selection after material selection. In shape selection, a cutting method, which is a post-process, is selected by sorting complex structures and simple structures, sorting by metal thickness, and the like.

[Cutting process]
The cutting step is a step of downsizing the object to be processed, but in principle, mechanical cutting may be performed and manual cutting may be performed as an auxiliary.

Next, the melting furnace into which the radioactive metal waste is charged will be described.
《Melting Furnace》
The functions performed in the melting furnace in the present invention include (i) separation of radioactive elements and metals, (ii) promotion of separation (addition of slag to promote separation), (iii) component adjustment according to the application, ( iv) liquefaction for molding according to the application.

Low, medium and high frequency induction furnaces, plasma furnaces, arc furnaces, gas furnaces, cupola furnaces, etc. are applicable as melting furnaces that require such functions. can be selected.
Moreover, it is desirable to apply a structure that has a high airtightness and little leakage of dust and gas from the furnace. Moreover, the melting heat source is preferably an electric type, which can easily obtain a high temperature, has good operability, and is suitable for melting radioactive metal waste. low, medium and high frequency induction furnaces, plasma furnaces and arc furnaces.

Plasma furnaces are capable of melting minerals such as miscellaneous solids and ash. Also, the torch insertion part can be sealed. Arc furnaces, like plasma furnaces, are capable of melting inorganic substances and are often used to melt iron scraps. However, since the electrode portion becomes hot, the electrode insertion portion cannot be sealed. The induction furnace heats the conductor by generating eddy currents on the principle of heating, so if the object to be processed contains a mixture of objects with different shapes or inorganic substances, the melting efficiency will decrease (melting time is required or heating is not possible). ).

Furnace shapes and features of typical electric melting furnaces are shown below. Plasma furnaces and arc furnaces heat from the surface, so the frontage (furnace diameter) is wide and the bath depth is shallow. In addition, since the frontage (furnace diameter) can be widened, the size of the material to be charged can be increased.
The induction furnace has a narrow frontage (furnace diameter) and a deep bath because it indirectly heats the molten material by means of a heating coil arranged on the side wall of the furnace. In addition, since the frontage (furnace diameter) is narrow, the size of the charged material is small.

[Injection method]
There are closed charging and open charging as charging methods to the melting furnace, and the structure and characteristics of each will be explained.
FIG. 2 is an explanatory diagram for explaining the sealed charge. In FIG. 2, 1 indicates a furnace body, 3 indicates a furnace lid, and 5 indicates a charging/feeding device provided on the side wall of the furnace. The space between the furnace body 1 and the furnace lid 3 is sealed. The input supply device 5 has a hopper 7 for receiving radioactive metal waste, an input chute 8 , a double damper 9 provided on the input chute 8 , and a pusher 11 .

As a feature of the sealed charging performed by the charging supply device 5, continuous charging is possible and off-gas sealing is possible. However, since the size of the input material is restricted by the size of the frontage of the feeder, it is difficult to increase the size of the input material.
Plasma furnaces and arc furnaces are applicable to closed charging.

3A and 3B are explanatory diagrams for explaining the open charging. In open charging, as shown in FIG. 3, the furnace lid 3 is opened to charge the radioactive metal waste.
A feature of open charging is that continuous charging is not possible, so batch (intermittent) charging is required. When the furnace lid is opened, gas scatters and off-gas sealing is impossible, so a double enclosure (surrounding the furnace body 1, environmental dust collection) is required. becomes. However, since the size of the charged material is not restricted by the furnace diameter, it is easy to increase the size of the charged material.
Plasma furnaces, arc furnaces and induction furnaces are applicable for open charging.

In the case of closed charging, the amount of exhaust gas is small because there is no suction other than generated gas from the furnace. On the other hand, in the case of open feeding, in addition to the generated gas from the furnace, there is suction from the furnace cover seal surface, and the amount of exhaust gas is large. In addition, there is a lot of dust collected in the double housing part, and the total amount of exhaust gas can be as much as 70 times that of closed input.

As for the mechanism for charging metal into the furnace, a pusher type (direct charging) or a hanging type (container charging) can be applied as a sealed charging method.
FIG. 4 is an explanatory diagram of a pusher-type sealed charging method for a plasma furnace. In FIG. 4, the same reference numerals are given to the same parts as in FIG. In the figure, 12 is a torch, and 13 is a closed charging gate that isolates between the furnace body 1 and the pusher 11 except when charging.
Incidentally, two supply ports can be installed for each furnace so that the radioactive metal waste can be continuously supplied into the furnace. In FIG. 4, only one group is illustrated.

The charging flow will be described with reference to FIG. 4. First, with the charging gate 13 closed, the charged material 15 to be melted is charged from the hopper 7 and received by the upper damper 9a (FIG. 4(a)). Next, the upper damper 9a is opened to move the input 15 to the lower damper 9b (FIG. 4(b)), the upper damper 9a is closed, and the lower damper 9b is opened to move the input 15 to the pusher 11. (Fig. 4(c)).
The lower damper 9b is closed, the charging gate 13 is opened, and the pusher 11 is operated to charge the charging material 15 into the furnace (FIG. 4(d)).

Since the supply port is installed on the side wall of the furnace, the size of the supply port is limited. In general, when an irregular shaped object is pushed by the pusher 11, the size of the input material 15 should be about 1/3 to 1/4 (experimental value) of the supply opening in order to eliminate clogging.

FIG. 5 is an explanatory diagram of a suspended closed charging system for a plasma furnace. In FIG. 5, the same parts as in FIG. 4 are given the same reference numerals. In FIG. 5, 17 is a hanging chute, 19 is a hanging device, 21 is a mobile cart, 23 is an input gate, 24 is a gate driving unit for driving the input gate, and 25 is a container (box pallet) containing the input 15. , and common reference numerals are given to parts common to those in FIG.

The input flow will be described with reference to FIG. 5. With the input gate 23 closed, the container 25 containing the input material is moved to the vicinity of the suspension chute 17 by the mobile carriage 21 (FIG. 5(a)). The container 25 is hung by the hanging device 19 and moved into the hanging chute 17 (FIG. 5(b)). The charging gate 23 is opened by the gate drive unit 24, and the container 25 is charged into the furnace by the hanging device 19.
The hanging type feed is batch feeding, but the maximum feeding size can be larger than the pusher type.

From the above, the suspension type is preferable to the pusher type in that there is no restriction on the size of the input material 15 .

<Slag raw material input process>
The slag raw material charging step is a step of charging a slag raw material (lime, silica sand, etc.) that becomes slag by melting into a melting furnace.

The slag layer suppresses deterioration of the furnace lid and furnace body due to radiation from the surface of the molten metal during melting, and if there is a part that becomes extremely hot with a plasma torch, for example, the deterioration due to radiation is also greatly affected. to For this reason, a slag layer is formed even in a general steelmaking operation. The amount of slag required is several percent of the metal in the case of an induction furnace, and about 10 to 15 percent of the metal in the case of a plasma furnace or an arc furnace.

However, since radioactive metal waste usually contains about 2 to 3% of slag components, the amount of slag raw material that needs to be charged into the melting furnace is deducted by that amount. Furthermore, part of the iron (Fe) in the radioactive metal waste is also oxidized and transferred to the slag during melting.
Therefore, in the case of an induction furnace that requires a small amount of slag, it may be possible to process the slag without charging the slag raw material. However, when it is necessary to separate the slag and the metal more precisely, such as when the radioactivity concentration of the radioactive metal waste is high, it is desirable to add slag raw material.
On the other hand, plasma furnaces and arc furnaces require a certain amount of slag to maintain the furnace body and melting energy efficiency. Adding a large amount of slag raw material promotes the reduction of the radioactivity concentration of the metal, but the amount of slag generated increases, making operation difficult and requiring more energy, so an appropriate amount should be added. is preferred.

In addition, when the molten residue determined to be remeltable in the remelting possibility determination step described later is put into the melting furnace and remelted, the molten residue plays the same role as the slag raw material. It is possible to reduce or eliminate the input amount of slag raw material.

As described above, in order to balance the transfer of radioactive substances, prevent deterioration of the furnace body, and effectively utilize heat, at least the residual melt is put into the melting furnace and is not remelted. It is desirable to adjust the migration balance of slag and dust by adding slag raw materials (lime, silica sand) as necessary. For this reason, the present embodiment includes a slag raw material charging step.

<Radioactive material transfer process>
The radioactive substance transfer step is a step of melting the input material 15 in a melting furnace to separate the radioactive substances from the radioactive metal waste and transfer them to slag and dust.
Radioactive substances (mainly strontium and cesium) adhering to radioactive metal waste can be separated from the metal and transferred to slag and dust by melting the radioactive metal waste.

Most of the strontium (Sr) (97% or more) migrates to slag, and the remainder migrates to dust. Cesium (Cs) volatilizes during the melting process, and part of it (about 10 to 70%) shifts to dust and the rest to slag.
Moreover, it is known that cesium, which has properties similar to potassium, is captured by slag at a higher rate when the slag basicity (CaO/SiO ₂ ) is decreased.

Since strontium and cesium migrate to slag, in this case, if the amount of slag is small, for example, the strontium concentration in the slag becomes relatively high, and there is a possibility that the amount of strontium involved in the metal when the slag and metal are separated increases. be.
Furthermore, cesium volatilizes during the radioactive material transfer process and is separated into slag and fly ash. Inconvenience may occur due to restrictions on manual work.
As described above, in the radioactive material transfer step, it is desirable to appropriately adjust the amount and basicity of the slag to promote the transfer of the radioactive material from the radioactive metal waste to the slag and dust.

In addition, when the radioactive material is transferred to the slag, the slag and the molten metal have different densities of 2.7 t/m ³ and 7 t/m ³ , respectively. .

In melting in a melting furnace, it is effective to lower the melting temperature in order to improve energy efficiency.
In the ironmaking process, there are pig iron and steel, but the difference between them is mainly in the carbon concentration. 1490 ℃ and the difference is very large. On the other hand, the viscosity tends to decrease as the molten iron temperature rises, and decreases as the carbon concentration increases.

Therefore, when the metal and the slag are separated by standing still in a ladle or tundish device in the later-described granular metal production process, the melting temperature must be sufficiently higher than the melting point of the molten metal in order to improve the separation of the molten metal and the slag. or increasing the carbon concentration (up to 4.5%) to lower the viscosity and melting temperature of the molten metal. Since the metal is molded and processed after separation, it is desirable to avoid reheating or keeping warm by applying energy from the outside.

Generally, the temperature of the molten metal drops by 30°C to 70°C when it is transferred to a ladle or the like. Furthermore, it takes about 4 minutes for the product to stand and separate with a ladle, etc. Considering the temperature drop during that time and the temperature drop during the casting process, the melting temperature in the melting furnace can be set at least 100°C higher than the melting point. desirable. As a result, the separation is effectively promoted, and a higher degree of separation is possible than by heating or keeping the temperature close to the melting point.

FIG. 6 shows the result of examining the temperature drop of the molten metal when a ladle is used. In FIG. 6, 26 is a melting furnace, 27 is a ladle, 29 is a mold, 31 is molten metal, and 33 is slag.
The separable time of the slag 33 is a value obtained by subtracting the tapping time, transfer time and casting time from the time from tapping to the end of casting. In either case, it is possible to ensure a slag separation time of 4 minutes or more, and it is possible to separate the slag 33 and the molten metal 31.

Carburization is relatively easy to achieve by adding carbonaceous materials such as coke and charcoal to the melting furnace when the radioactive metal waste is charged into the melting furnace, and melting them at the same time. The carburization efficiency is high in the case of In the case of a melting method using a carbonaceous material such as a cupola as a melting heat source, the material is naturally recarburized. When molten metal is obtained by carburizing and separating slag, it may be necessary to adjust the composition according to the final application. When used in steel, it is necessary to decarburize, but by burning the carbon with air or oxygen blowing, it is possible to achieve both decarburization and prevention of temperature drop. As for the decarburization process of the molten metal after separation, the possibility of contamination is very small if it is performed under closed conditions because it basically satisfies or almost satisfies the clearance. .

<Dust treatment process>
The dust treatment step is a step of collecting and treating dust generated from the melting furnace in the radioactive substance transfer step.
In the radioactive substance transfer step, as described above, among the radioactive substances, cesium in particular has a high rate of transfer to dust.
The dust generated from the melting furnace is captured by a dust collector and treated as radioactive waste.

<Particulate metal generation process>
The granular metal production step is a step of separating metal from a molten material melted in a melting furnace and turning the separated molten metal into granular clearance metal.
The granular metal production process is roughly divided into two processes: a separation process for separating the metal from the molten material and a granulation process for making the separated molten metal into granules. Each process will be described below.

《Separation process》
A specific embodiment of the separation step includes an embodiment using a ladle or a tundish device.
A specific method of using a ladle is shown in FIG. In FIG. 7, the same parts as in FIG. 6 are given the same reference numerals. It should be noted that the same method can be applied even if a tundish device is used instead of the ladle.

In the case of the embodiment using a ladle, as shown in FIG. 7, a transfer step of transferring the molten material (molten metal 31 and slag 33) melted in the melting furnace 26 to the ladle 27, and the ladle 27 is left to stand still. The ladle 27 comprises a static separation step of statically separating the molten material in the ladle 27 into a molten metal layer and a slag layer, and a molten metal extraction step of extracting the molten metal 31 from the bottom of the ladle 27 .

[Transfer process]
In the transfer step, as shown in FIG. 7, the melting furnace 26 is tilted to transfer the molten metal 31 and the slag 33 to the ladle 27 .

In general, the depth of the molten metal in the melting furnace is shallow, so the molten metal may involve slag during tapping, and the molten metal and slag are discharged separately so that they are separated into two layers and do not mix. it is not easy to do.
Therefore, it is desirable to secure a high separation accuracy by receiving it in a deep ladle and taking a standing time for separation again.

The melting furnace 26 shown in FIG. 7 is a one-side tilting melting furnace, but a melting furnace (double-tilting melting furnace 26) that can be tilted on both sides and has outlets on both sides in the tilting direction can also be used. (See Figure 8). The structure and mechanism of the double-sided tilting melting furnace are disclosed, for example, in Japanese Patent Application Laid-Open No. 9-264522.
The double-side tilting melting furnace 26 is tilted using a hydraulic cylinder or the like so that the right side in the figure is lowered, and the upper layer of the melt (mainly slag 33) is discharged out of the furnace from one discharge port. The remaining slag 33 and molten metal 31 are transferred to the ladle 27 from the other discharge port by tilting using a hydraulic cylinder or the like so that the left side is lowered. As a result, the amount of slag 33 flowing into the ladle 27 is reduced, so that the separation time in the stationary separation step, which will be described later, can be shortened, and the separation accuracy is improved.

[Standing separation process]
The stationary separation step is a step of transferring the molten material to the ladle 27 in the transfer step and then leaving the ladle 27 stationary to separate the molten material in the ladle 27 into a molten metal layer and a slag layer.
As described above, the slag 33 and the molten metal 31 have significantly different densities of 2.7 t/m ³ and 7 t/m ³ , respectively. It can be separated into two layers of metal layers.
As described above, it takes about 4 minutes to stand and separate in the ladle 27 . In addition to being used for static separation, the ladle can also be used when molding molten metal.

[Molten metal withdrawal process]
As shown in FIG. 7, the molten metal extracting step is a step of extracting the molten metal layer 31 from the ladle 27 so as not to mix the slag layer 33 into the clearance metal.
After the stationary separation, when the opening at the bottom of the ladle 27 is opened, the molten metal 31 falls by gravity. Generally, at the beginning of the opening, only the molten metal 31 falls, but as time passes and the depth of the molten metal 31 becomes shallower, at a certain timing a vortex is generated in which the slag 33 on the surface is drawn directly into the opening. .
Since the radioactive material has migrated to the slag layer, it is necessary to prevent the slag 33 from being entangled in the molten metal 31 to be extracted.

The slag 33 and a part of the molten metal 31 are allowed to remain in the ladle 27, and the molten metal 31 is extracted. It may be remelted. In this way, it is possible to achieve both reliable clearance and improved yield of molten metal recovery.

When the opening provided at the bottom of the ladle 27 becomes larger, the speed of withdrawing the molten metal 31 increases and the efficiency improves, but on the other hand, since the eddy current is generated at the stage where the interface is high, the amount of withdrawal at one time is small. As a result, the amount of melted residue increases.
Conversely, if the opening provided at the bottom of the ladle 27 becomes smaller, the speed of withdrawing the molten metal 31 will be slower and the efficiency will be lower. increases, and the amount re-melted as molten residue decreases. As described above, the opening provided at the bottom of the ladle 27 affects the efficiency of the particulate metal production process, so it is preferable to set the optimum value based on various conditions of actual operation.

《Granulation process》
Specific aspects of the granulation process include water granulation to granulate the molten metal by injecting a large amount of pressurized water to the separated molten metal, and rapid cooling by injecting a gas such as air into the molten metal. Air granulation can be mentioned.
Alternatively, the molten metal may be shaped into rods, wires, or plates, cut or crushed, and processed into granules.

Regarding the size and shape of the granulated metal particles, when using a granular clearance metal as a scrap equivalent as a raw material for ironmaking, it is about several mm to several tens of cm, and may be selected according to the equipment to be used.
In addition, when the granular clearance metal is filled and used, the size of the container to be filled and the ease of filling may be taken into consideration.
However, considering that the metal granules are filled into a container at the time of radioactivity measurement, or supplied to the public and used after being filled, it is desirable that the metal granules can be easily moved manually. In this regard, in the case of roadbed material, the size of the aggregate used is stipulated to be 53 mm or less, and when paving the road, workers are performing work such as leveling with a shovel. In this case, it is considered that the weight of the particles affects workability. From this point of view, the weight per piece when workability is taken into consideration is 417 g or less for a roadbed material with a maximum size of 53 mm and a specific gravity of 2.8. By the way, if this is converted into an iron cube with a specific gravity of 7.8, the size will be 38 mm or less. Since the actual shape is different from a cube and has variations, it is difficult to select a desirable shape. On the other hand, since weight is directly related to workability, it is meaningful to specify it.

When the particles are packed and used, it may be necessary to process the particles so that the specific gravity and shape meet predetermined conditions. In addition, it is expected that leakage of metal granules when the storage container or the like is damaged during use may be regarded as a problem. As a countermeasure, by coating with rubber, plastic, cement, etc. with high weather resistance and chemical resistance, it is possible to satisfy the required specifications as a filler and improve reliability.
Regarding the adjustment of the specific gravity and shape, it is conceivable to use sand, gravel, glass, etc. in addition to the coating material.

<Step of measuring particulate metal radioactivity concentration>
The granular metal radioactivity concentration measurement step is a step of measuring the radioactivity concentration of the granular clearance metal. Verification of clearance levels in Japan consists of (i) establishment of measurement and evaluation methods by operators, (ii) authorization by the Nuclear Regulation Authority, (iii) implementation of measurement and evaluation methods by operators, and ( iv) The Nuclear Regulation Authority confirms the results of measurement and evaluation. In order to prevent corrosion before and after shipping the molded product, it is necessary to take anti-corrosion measures such as applying a rust inhibitor after molding and processing.
However, it is not a radioactivity concentration measurement for the purpose of shipping as clearance metal, but a preliminary measurement for the purpose of determining whether the metal extracted from the ladle or tundish needs to be returned to the melting furnace again. If it is a measurement, whether the radioactivity concentration is below the reference value (for example, it can be set arbitrarily, such as the value corresponding to the clearance level or the value obtained by multiplying it by 0.8 in anticipation of the safety factor) It can also be a simple measurement for confirmation only.

For the measurement, the metal particles are coated with an anticorrosive agent for antirust treatment, and then filled in a measuring container of a certain size (for example, an iron box with a side length of about 1 m). By doing so, it becomes easier to automate the taking in and out of the storage facility. In addition, since the measurement conditions are almost constant, efficient radioactivity measurement is possible. Measurements are made in a manner that measures all six sides of the container. The measurement time is about 10 minutes.
Once the measurement confirms that the clearance level has been reached and the country's confirmation is given, it can be stored and shipped under conditions without legal restrictions.

<Injection process of metal not reaching clearance>
The clearance non-achieving metal charging step is a step of returning the metal judged to exceed the standard value in the radioactive concentration measuring step to the melting furnace in the radioactive material transfer step and remelting it.
The maximum surface dose rate of radioactive strontium (Sr90) in the radioactive metal waste to be treated is approximately 1 mSv/h, and the overall weighted average is 0.1 mSv/h. This weighted average value shall correspond to a radioactivity concentration of 160 Bq/g. Assuming a high-frequency furnace, the amount of slag generated is 2% (including slag components from scrap), the slag rate during melting is 70%, and the capture rate of radioactive strontium (Sr90) in slag. is 100%.
Based on this embodiment, if the processed metal does not satisfy the clearance level (1 Bq/g or less), the main cause is operational error (such as when performing work that deviates from regular work) and equipment malfunction. For example, the slag content is mixed into the metal. The maximum amount of slag mixed is empirically about 700 g/t. When the maximum slag content is 700g/t, the radioactivity concentration is 8Bq/g (*1) calculated from the hypothetical values mentioned above. This is considered the maximum radioactivity concentration of non-clearance metals, which is sufficiently low compared to the weighted average radioactivity concentration of 160 Bq/g of radioactive strontium (Sr90) in radioactive metal waste, Well worth the investment.
*1. {(160×1000×1000×1)/(0.02×1000×1000×0.7)}×700/1000000

Similarly, the maximum surface dose rate of radioactive cesium (Cs137) in radioactive metal waste is approximately 1 mSv/h, and the overall weighted average is 0.1 mSv/h. This weighted average value is assumed to correspond to a radioactivity concentration of 145 Bq/g. Assuming a high-frequency furnace, the amount of slag generated is 2% (including slag components from scrap), the slag rate during melting is 70%, and the capture rate of radioactive cesium (Cs137) in slag. is 60%.
The radioactivity concentration when the maximum amount of mixed slag is 700g/t is 4.4Bq/g (*2) from the same calculation as above. Although this is considered the highest maximum activity concentration for non-clearance metals, it is sufficiently low compared to the weighted average activity concentration of 145 Bq/g for radioactive cesium (Cs137) in radioactive metal waste. , well worth the reintroduction.
*2. (160×1000×1000×1)/(0.02×1000×1000×0.7)}×700/1000000

Therefore, when radioactive metal waste is mixed with non-clearance metals and treated, the average radioactivity concentration of the entire object to be treated is clearly reduced (diluted), so the concentration of radioactive substances in the metal in the radioactive transfer process is also reduced. definitely going down. In addition, the recovery rate of clearance metals recovered from radioactive metal waste is improved, and the need to dispose of non-clearance metals as waste is eliminated.
The timing and method of returning metals that have not reached clearance to the melting furnace should be determined by considering the amount of metals that have not reached clearance and the amount of radioactive metal waste that needs to be disposed of. In this case, any method can be used, such as determining the amount and timing of return based on the measurement results of the radioactivity concentration of the radioactive metal waste and empirical rules.
Further, by granulating the metal, it is easier to charge the metal into the melting furnace 26, and the energy required for melting can be reduced.

As described above, in the present embodiment, by melting the radioactive metal waste, the radioactive substances adhering to the radioactive metal waste are transferred to the slag, the molten metal is separated from the melt, and the separation Since the molten metal thus obtained is turned into granular clearance metal, the radioactive substances adhering to the radioactive metal waste can be reliably separated, and since the clearance metal has a granular shape, it is suitable for storage and use. .
It is also easy to remelt after granulation, even if the clearance level has not been reached.

In the above embodiment, no particular reference is made to the treatment of slag, but since slag has an important function for transferring radioactive substances, it is possible to effectively utilize slag by reusing it. At the same time, it is possible to reduce the amount of slag to be treated.
When slag is reused, after the granular metal production step or in parallel with the granular metal production step, as shown in FIG. A molten residue disposal step may be performed.
Each step will be described in detail below.

<Molten residue generation process>
The molten residue generation step is a step of using the molten material after the molten metal is separated in the granular metal generation step as the molten residue.
As a specific method for producing the molten residue, in the case of the embodiment using the ladle 27, the molten material (mainly slag and also containing the molten metal) after the molten metal is extracted in the molten metal extraction process in the granular metal production process ) can be discharged to a slag pan or the like by tilting the ladle 27 or the like. The same applies to the embodiment using a tundish device.

<Process for judging whether remelting is possible>
The remelting propriety determination step is a step of determining whether or not the melted residue can be remelted based on the radiation dose of the melted residue.
When radioactive metal waste is melted, the radioactive material migrates to slag. Then, when the slag to which the radioactive substances have migrated is recovered as a molten residue and is melted again with the radioactive metal waste, the radioactive substances are gradually concentrated in the slag, and the radioactive concentration of the molten residue containing the slag also increases. get higher

Disposal of molten residue is carried out according to disposal standards, but if the radioactivity level exceeds L1, extra depth disposal is required and disposal becomes very difficult.
On the other hand, when the radioactivity levels are L2 and L3, disposal in shallow areas is relatively easy.
Specifically, if the activity level is L3, trench disposal is required. Trench disposal refers to shallow underground disposal of radioactive waste that is not solidified in a container at a waste burial site without an engineered barrier.
Also, if the radioactivity level is L2, it will be disposed of in a pit. Pit disposal refers to shallow-ground disposal of the materials to be treated that have been sealed or solidified in a container in a waste disposal site with an engineered barrier installed.

Thus, if the radioactivity level is L1 or higher, disposal of the molten residue becomes difficult, so the radioactivity level should be less than L1.
Therefore, if the radioactivity level is L2 and there is no margin up to the upper limit of L2, and if the melted residue is remelted, it will reach the L1 level in the next remelting possibility judgment step, and it will be determined that remelting is not possible. to decide. This is because, as described above, when the L1 level is reached, labor and costs increase.

On the other hand, if the radioactivity level is L3 or L2, but there is a margin up to the upper limit of L2, even if the molten residue is put into the melting furnace and remelted, the level measurement in the next remelting possibility judgment step However, if it does not reach the L1 level, it is determined that remelting is possible. This allows the radioactivity level of the residue to be raised to the upper limit of the L2 level, thus reducing the amount of residue to be disposed of.
It should be noted that the radiation dose of the molten residue does not have to be measured strictly like the clearance metal.

<Molten residue input process>
The molten residue charging step is a step of charging the molten residue into the melting furnace together with the radioactive metal waste when it is determined that the residue can be reused in the remelting possibility determination step.
The melted residue is put into the melting furnace, processed from the radioactive material transfer process to the melted residue generation process, and the remelting possibility judgment process judges whether it can be remelted again. , the molten residue is put into the melting furnace together with the radioactive metal waste. In this way, the processing from the melted residue charging step to the remelting possibility determination step is repeated, and the greater the number of repetitions, the higher the radioactivity concentration of the slag in the melted residue.
In addition, the molten residue determined to be remeltable may be immediately returned to the melting furnace and used for the subsequent treatment process, and after the treatment process is performed several times using other slag raw materials, it is melted. You can put it back in the furnace. Moreover, when a plurality of lines of melting furnaces are installed, it is not necessary to return to the melting furnace of the same line, and the melting furnace of another line may be returned.

<Molten residue disposal process>
The melted residue disposal step is a step of disposing of the melted residue in a predetermined manner based on the radioactivity level when it is determined that the remelting is not possible in the remelting possibility determining step.
As described above, in this embodiment, the radioactivity level of the molten residue is L2 or less, so the above-described pit disposal may be performed.

As described above, in the present embodiment, if part or all of the molten residue is recovered and reused, it becomes unnecessary to newly add slag raw material, and the disposal amount of the molten residue can be reduced. Soaking can be realized.

1 Furnace Body 3 Furnace Cover 5 Input Supply Device 7 Hopper 8 Input Chute 9 Double Damper 9a Upper Damper 9b Lower Damper 11 Pusher 13 Input Gate 12 Torch 15 Input Material 17 Suspension Chute 19 Suspension Device 21 Mobile Cart 23 Input Gate 24 gate drive 25 vessel 26 melting furnace 27 ladle 29 mold 31 molten metal 33 slag

Claims (5)

A method of producing clearance metals from radioactive metal waste, comprising:
a radioactive metal waste charging step of charging radioactive metal waste contaminated with a radioactive material mainly composed of radioactive cesium or radioactive strontium into a melting furnace;
a slag raw material charging step of charging slag raw material, which is a raw material of slag, into the melting furnace as necessary;
a radioactive material transfer step of separating the radioactive material from the radioactive metal waste and transferring it to slag and dust by melting the input material in a melting furnace;
a dust treatment step of collecting and treating the dust;
a granular metal producing step of separating molten metal from the molten material melted in the melting furnace and using the separated molten metal as granular clearance metal ;
a molten residue generating step of recovering all or part of the molten residue from the melt melted in the melting furnace;
a remelting possibility determination step of determining whether or not the recovered molten residue can be reused based on the radioactivity concentration of the molten residue;
a molten residue charging step of charging the recovered molten residue into a melting furnace when it is determined that it can be reused in the remelting propriety determining step;
A method for producing a clearance metal, comprising a molten residue disposal step of disposing of the molten residue as radioactive waste when it is determined in the remeltability determination step that the metal cannot be reused . 2. The method of manufacturing a clearance metal according to claim 1, wherein said remelting determination step measures the radioactivity concentration of said molten residue and determines based on the measured value. The granular metal generation step includes a granular metal radioactivity concentration measurement step of measuring the radioactivity concentration of the granulated molten metal, and the granular metal whose measured value exceeds the reference value in the radioactivity concentration measurement step is fed to the melting furnace. 3. The method for producing a clearance metal according to claim 1, further comprising a step of charging the unreached clearance metal to be charged. 3. The method for producing a clearance metal according to claim 1, wherein said granular metal producing step granulates the separated molten metal by water granulation or wind granulation. 3. The clearance metal according to claim 1 or 2, wherein said granular metal producing step forms the separated molten metal into rods, wires or plates and then cuts or crushes them into granules. Production method.

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