JP7391764B2 - Active metal processing method and active metal processing equipment - Google Patents

Description

Embodiments of the present invention relate to active metal processing technology.

In sodium-cooled fast reactors, metallic sodium or a sodium-potassium alloy is used as the coolant. These metals have the advantage of high heat removal performance due to their small thermal neutron absorption cross section and high thermal conductivity, but have the disadvantage of reacting violently with oxygen or water and causing ignition or explosion. When a sodium-cooled fast reactor is decommissioned, a large amount of metallic sodium is generated. The amount of sodium used in sodium-cooled fast reactors is extremely large: 4,700 tons at France's Super Phoenix and 1,520 tons at Japan's Monju. In particular, since metallic sodium is a dangerous substance (a pyrophoric substance and a water-restrictive substance), it is desirable to treat the metallic sodium to a stable state at the time of decommissioning.

In countries where sodium-cooled fast reactors are being decommissioned, large amounts of metallic sodium are already being processed using a method called the NOAH method. In this NOAH method, metallic sodium is reacted with a large amount of water to form an aqueous sodium hydroxide solution, which is neutralized and discarded. However, because a large amount of hydrogen gas is generated during this process, there is a risk of explosion. Furthermore, since a large amount of waste liquid or strong acid for neutralization is required, the amount of secondary waste increases. The NOAH method is simple in principle and has a proven track record of use, but it has the problems of being inherently dangerous because it brings water-inhibiting substances into contact with water, and generating large amounts of liquid and gaseous waste. be. Future decommissioning of fast reactors will require safer treatment methods that generate less waste.

In order to solve these problems, a technology has been proposed to treat sodium metal without using water. In this technique, a water-free molten salt is used as a reaction field, and metallic sodium and a metal oxide as a treatment agent are reacted in the molten salt according to the following reaction formula. Note that M in the reaction formula is a predetermined metal material.

2Na + MO = Na2O + M

Na2O produced according to this reaction formula has lower activity than metallic sodium and is dissolved in molten salt, so it can be safely stored as waste. However, the metal oxide used as the treatment agent is solid, and if it has a higher density than the molten salt, it may precipitate in the molten salt, and the metal oxide may not be brought into sufficient contact with the metal sodium. Therefore, further improvement in processing efficiency is required.

JP2019-47881A JP2020-8322A

Fast Reactor Database 2006 Update (IAEA)

The problem that the present invention seeks to solve is to improve the efficiency of processes for safely stabilizing active metals using molten salts.

An active metal treatment method according to an embodiment of the present invention includes a step of converting the active metal into a non-metal form by reacting the active metal with a treatment gas containing oxygen atoms in a molten salt, and treating the active metal. combusting the reducing gas obtained in an atmosphere containing air or oxygen molecules to regenerate it into the processing gas .

Embodiments of the present invention provide active metal processing techniques that can improve the efficiency of processing to safely stabilize active metals using molten salts.

FIG. 1 is a configuration diagram showing an active metal processing apparatus. FIG. 2 is a processing flow diagram showing an active metal processing method. Graph showing the reaction of Example 1. Graph showing the reaction of Example 2. Graph showing the reaction of Example 3. Graph showing the reaction of Example 4.

Hereinafter, embodiments of an active metal processing method and an active metal processing apparatus will be described in detail with reference to the drawings.

Reference numeral 1 in FIG. 1 is an active metal processing apparatus of this embodiment. This active metal processing apparatus 1 is an apparatus for stabilizing an active metal 2 in a molten salt 3. A processing gas 4 containing oxygen atoms is used as the processing agent. This processing gas 4 oxidizes the active metal 2 in the molten salt 3 to form a more stable oxide.

In this embodiment, metallic sodium is exemplified as the active metal 2 to be subjected to stabilization treatment. Moreover, the molten salt 3 is a liquid made of salt molten at a high temperature. The molten salt 3 can dissolve metal salts, and because of its high temperature, the solute reaction rate is fast. Furthermore, the molten salt 3 has the advantage that it has high radiation resistance and can be reused by removing impurities.

As the molten salt 3, sodium chloride-lithium chloride, sodium chloride-potassium chloride, sodium chloride-calcium chloride, or sodium chloride-barium chloride can be used. Note that these salts may be used in combination.

As the active metal 2, the alkali metal sodium (Na) is exemplified. Further, other alkali metals or alkaline earth metals may be used as the active metal 2. For example, lithium, potassium, rubidium, cesium, magnesium, calcium, strontium, or barium may be used as the active metal 2. Note that a combination of these metals may be used. That is, the active metal 2 of this embodiment is at least one of an alkali metal and an alkaline earth metal. In this way, even if the active metal 2 is an alkali metal or an alkaline earth metal, it can be safely stabilized in the molten salt 3.

The active metal processing apparatus 1 includes a reaction vessel 5, a gas introduction pipe 6, a gas discharge pipe 7, a gas supply pump 8, a gas regeneration unit 9, and a gas storage tank 10.

Reaction vessel 5 holds active metal 2 and molten salt 3. The reaction container 5 is composed of a container body and a lid that closes the opening of the container body. Then, the inside of the reaction container 5 can be sealed. Stabilization treatment is performed inside this reaction vessel 5.

Note that the molten salt 3 used has a higher density than the active metal 2. In this way, the layer of molten salt 3 in the liquid phase can be located at a lower position than the layer of active metal 2 in the liquid phase inside the reaction vessel 5.

The gas introduction pipe 6 is a cylindrical member extending from the outside of the reaction container 5 toward the inside. Processing gas 4 is introduced into the reaction vessel 5 through this gas introduction pipe 6 . The gas introduction pipe 6 extends downward from the lid of the reaction vessel 5, and has an introduction port 11 opened at its lower end. This introduction port 11 is provided inside the reaction vessel 5 at a height position where the molten salt 3 exists. That is, the introduction port 11 is provided at a position lower than the interface 12 between the molten salt 3 and the active metal 2. Note that the position where the molten salt 3 exists inside the reaction vessel 5 is specified in advance according to the amount of the molten salt 3 to be introduced.

The gas discharge pipe 7 is a cylindrical member extending from the outside of the reaction container 5 toward the inside. The reducing gas 13 generated inside the reaction vessel 5 is discharged to the outside of the reaction vessel 5 through the gas discharge pipe 7 . The gas exhaust pipe 7 extends downward from the lid of the reaction vessel 5, and has an exhaust port 14 opened at its lower end. This exhaust port 14 is provided inside the reaction vessel 5 at a position higher than the liquid level 15 of the active metal 2 . The liquid level 15 of the active metal 2 inside the reaction vessel 5 is specified in advance according to the amount of the active metal 2 to be charged.

The gas supply pump 8 supplies the processing gas 4 into the reaction vessel 5 via the gas introduction pipe 6 . When the gas supply pump 8 is driven, the processing gas 4 is blown into the inlet 11 . The processing gas 4 blown through the inlet 11 becomes fine bubbles and rises. Note that an air diffuser that generates air bubbles may be connected to the tip of the gas supply pump 8.

The gas regeneration unit 9 regenerates the reducing gas 13 discharged via the gas discharge pipe 7 into the processing gas 4. The gas storage tank 10 stores the processing gas 4. The gas supply pump 8 supplies the processing gas 4 contained in the gas storage tank 10 to the reaction vessel 5 . Further, the gas supply pump 8 supplies the processing gas 4 regenerated by the gas regeneration unit 9 to the reaction vessel 5 .

The processing gas 4 is an oxide gas containing at least oxygen atoms. Carbon dioxide (CO ₂ ) is exemplified as the processing gas 4 of this embodiment. Further, the processing gas 4 may be sulfur dioxide (SO ₂ ) or nitrogen dioxide (NO ₂ ). That is, the processing gas 4 of this embodiment is at least one of COx, SOx, and NOx. In this way, the processing gas 4 containing oxygen atoms can be easily obtained. Note that a combination of these gases may be used.

In this embodiment, active metal 2 (metallic sodium) is treated in molten salt 3 and converted to a stable compound in non-metallic form without using complicated equipment and processes. The processing gas 4 containing oxygen atoms that the inventors have focused on is a new processing agent that can replace the metal oxides of the prior art. The processing gas 4 is blown into the molten salt 3 to cause the active metal 2 and the processing gas 4 to react, thereby converting the active metal 2 into a stable chemical form.

As shown in FIG. 2, the active metal treatment method includes a blowing step 16, a stabilization step 17, and a regeneration step 18. These steps are executed in parallel.

The blowing step 16 is a step of blowing the processing gas 4 through the inlet 11 provided in the molten salt 3 located at a lower position than the active metal 2 inside the reaction vessel 5 . In this way, the processing gas 4 blown into the molten salt 3 from the inlet 11 rises, and the active metal 2, the molten salt 3, and the processing gas 4 are mixed at the interface 12 between the active metal 2 and the molten salt 3. Since they are mixed repeatedly, processing efficiency can be improved.

The stabilization step 17 is a step in which the active metal 2 is reacted with a treatment gas 4 containing oxygen atoms in the molten salt 3 inside the reaction vessel 5 to convert the active metal 2 into a non-metallic form. In this stabilization step 17, first, the active metal 2 and the molten salt 3 are charged into the reaction vessel 5. Then, the active metal 2 is converted into a non-metallic form by the processing gas 4 blown into the reaction vessel 5 in the blowing step 16 .

In this embodiment, since the molten salt 3 has a higher density than the active metal 2, the layer of the molten salt 3 in the liquid phase is formed at a position lower than the layer of the active metal 2 in the liquid phase inside the reaction vessel 5. Further, a processing gas 4 is blown into the molten salt 3 located at a position lower than the active metal 2 using a gas introduction pipe 6. Bubbles of the processing gas 4 rising from the inlet 11 of the gas introduction pipe 6 mix at the interface 12 between the molten salt 3 and the active metal 2, promoting the reaction. Further, the bubbles of the processing gas 4 stir the molten salt 3 and the active metal 2 so that they are mixed at the interface 12 .

In the stabilization step 17, a treated compound 19, a spent molten salt 20, and a reducing gas 13 are generated. For example, when metallic sodium is treated as the active metal 2, the treated compound 19 includes sodium oxide. Further, when CO ₂ is used as the processing gas 4, carbon monoxide (CO) is generated as the reducing gas 13.

The regeneration step 18 is a step of regenerating the processing gas 4 by burning the reducing gas 13 obtained by processing the active metal 2 in an atmosphere containing air or oxygen molecules. In this way, the reducing gas 13 discharged as the active metal 2 is processed can be reused as the processing gas 4.

In this embodiment, an oxidation-reduction reaction between the active metal 2 and the processing gas 4 as a processing agent is used, and the density difference between the active metal 2, the molten salt 3, and the processing gas 4 is utilized. In this way, the active metal 2 can be converted into various forms such as oxides, chlorides, and carbonates. In addition, it is possible to improve the reaction efficiency and the separability between the treated compound 19 and the spent molten salt 20.

An example of a specific reaction will be explained. Note that, as a specific example, the active metal 2 is sodium (Na) and the processing gas 4 is carbon dioxide (CO ₂ ), but other substances may be applied to this embodiment.

First, the active metal 2 is added to the molten salt 3 held as a liquid inside the reaction vessel 5. As the molten salt 3, NaCl-CaCl ₂ , NaCl-KCl, NaCl-LiCl, etc. can be used. At this time, the active metal 2 is present at a higher concentration than the molten salt 3 due to the difference in specific gravity. A processing gas 4 is blown into this molten salt 3 through a gas introduction pipe 6 or the like. A redox reaction between the active metal 2 and the treatment gas 4 converts the active metal 2 into a treated compound 19 in non-metallic form. The redox reaction proceeds according to reaction formula (1).

2Na + CO ₂ = Na ₂ O + CO (1)

In this way, metallic sodium (Na) is oxidized to sodium oxide (Na ₂ O) and CO ₂ gas is reduced to CO gas.

When Na is reacted with a solid metal oxide as in the prior art, the metal oxide is reduced to metal and remains in the molten salt 3, but as in the present embodiment, it is used as the processing gas 4. When CO ₂ gas is used, CO gas is generated as the reducing gas 13, so it is possible to continuously discharge the CO gas to the outside of the reaction vessel 5 without using special equipment. When the CO gas discharged to the outside of the reaction vessel 5 is heated and mixed with air or the like in the gas regeneration unit 9, it becomes _CO2 through a combustion reaction, and thus can be reused as the processing gas 4.

In addition, since the CO _{2 gas blown into the molten salt 3 from the inlet 11 of the gas introduction pipe 6 always moves upward, the CO 2} gas injected into the molten salt 3 from the inlet 11 of the gas introduction pipe 6 always moves upward, so that at the interface 12 between the molten salt 3 layer and the Na (active metal 2) layer, , a reaction occurs with CO ₂ gas. Since the Na ₂ O generated at this time is dissolved and removed by the layer of molten salt 3, a new Na surface is always formed at the interface 12, and the reaction by the CO ₂ gas continues to proceed.

Figure 3 shows the reaction behavior when 1 mol of metallic Na was treated using NaCl-KCl (50:50 mol%) at 700°C as the molten salt 3 and CO ₂ as the processing gas 4, based on thermodynamic equilibrium calculations. This is the graph I found. Note that (l) in the graph (for example, NaCl (l)) means a liquid, and (g) (for example, CO ₂ (g)) means a gas.

As shown in this graph, it can be seen that around 0.6 mol of CO ₂ gas was blown in, metal Na disappeared from the system and Na was converted to NaCl. The injected _CO2 gas generates C (carbon) at the beginning of the reaction, but when a sufficient amount of _CO2 gas is injected, most of it becomes a mixed gas of CO and _CO2 and is discharged outside the system. be done. Note that a part of it reacts with the molten salt 3 to form Na ₂ CO ₃ and K ₂ CO ₃ . The spent molten salt 20 (including the treated compound 19) becomes NaCl-KCl-Na ₂ CO ₃ -K ₂ CO ₃ . Note that the used molten salt 20 may contain other substances.

Figure 4 shows a thermodynamic equilibrium calculation of the reaction behavior when 1 mol of metallic Na is treated using NaCl-CaCl ₂ (50:50 mol%) at 700°C as the molten salt 3 and CO ₂ as the processing gas 4. This is a graph obtained by

As shown in this graph, it can be seen that metallic Na disappears from the system and Na is converted to NaCl around the time when about 0.2 mol of CO ₂ gas is blown into the system. The injected _CO2 gas generates C (carbon) at the beginning of the reaction, but when a sufficient amount of _CO2 gas is injected, most of it becomes a mixed gas of CO and _CO2 and is discharged outside the system. be done. Note that a part of it reacts with the molten salt 3 to form CaCO ₃ . The spent molten salt 20 (including the treated compound 19) becomes NaCl-CaCl ₂ -CaCO ₃ . Note that the used molten salt 20 may contain other substances.

Figure 5 shows a thermodynamic equilibrium calculation of the reaction behavior when 1 mol of metallic Na is treated using NaCl-CaCl ₂ (50:50 mol%) at 700°C as the molten salt 3 and SO ₂ as the processing gas 4. This is a graph obtained by

As shown in this graph, it can be seen that around 0.2 mol of SO ₂ gas was blown in, metal Na disappeared from the system and Na was converted to NaCl. _SO2 exists in the molten salt 3 as a sulfide, but when _SO2 gas is further blown into _the molten salt, the sulfide disappears and sulfates such as _Na2SO4 and _CaSO4 are formed. The spent molten salt 20 (including the treated compound 19) becomes NaCl-CaCl ₂ -Na ₂ SO ₄ -CaSO ₄ . Note that the used molten salt 20 may contain other substances.

Figure 6 shows a thermodynamic equilibrium calculation of the reaction behavior when 1 mol of metallic Na is treated using NaCl-CaCl ₂ (50:50 mol%) at 700°C as the molten salt 3 and NO ₂ as the processing gas 4. This is a graph obtained by

As shown in this graph, it can be seen that around 0.4 mol of NO ₂ gas was blown in, metal Na disappeared from the system and Na was converted to NaCl. Note that it can be seen that the NO ₂ gas starts reacting immediately from the start of the blowing of the NO ₂ gas, and N ₂ and O ₂ are generated. Therefore, there is no NO ₂ line in the graph. The spent molten salt 20 (including the treated compound 19) becomes NaCl-CaCl ₂ -CaO. Note that the used molten salt 20 may contain other substances.

In this embodiment, the reaction between the active metal 2 and the processing gas 4 can be promoted by dissolving metallic sodium, which is the active metal 2, in the molten salt 3. If an attempt was made to react the active metal 2 with the processing gas 4 without dissolving it in a prescribed solvent, the active metal 2 would become a fine lump, and only the surface of the lump would react, and the inside would not be reacted. It stops progressing. By dissolving the active metal 2 in a solvent, the molecules of the active metal 2 can always come into contact with the processing gas 4, so that the reaction can be promoted.

In this embodiment, the active metal processing apparatus 1 is used as a part of the waste treatment process, but it can be used not only for waste treatment but also as a part of the treatment process for reusing the active metal 2. Also good.

Furthermore, when processing sodium that directly cools the fast reactor core, there is a possibility that the sodium may contain radioactive substances. Therefore, the active metal processing apparatus 1 may further include a shielding wall that serves as a biological shield.

Note that as the processing gas 4, it is preferable to use a gas other than oxygen gas (oxygen molecules). In this way, it is possible to prevent the processing gas 4 from violently reacting with the active metal 2.

According to the embodiments described above, a molten salt is used to convert an active metal into a non-metallic form by reacting the active metal with a treatment gas containing oxygen atoms in the molten salt. The efficiency of the process for safe stabilization can be improved.

Although several embodiments of the invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, changes, and combinations can be made without departing from the gist of the invention. These embodiments and their modifications are included within the scope and gist of the invention as well as within the scope of the invention described in the claims and its equivalents.

DESCRIPTION OF SYMBOLS 1...Active metal processing equipment, 2...Active metal, 3...Molten salt, 4...Processing gas, 5...Reaction vessel, 6...Gas introduction pipe, 7...Gas discharge pipe, 8...Gas supply pump, 9...Gas regeneration Unit, 10...Gas storage tank, 11...Inlet, 12...Interface, 13...Reducing gas, 14...Exhaust port, 15...Liquid level, 16...Blowing process, 17...Stabilizing process, 18...Regeneration process, 19 ... Treated compound, 20 ... Spent molten salt.

Claims (6)

reacting an active metal with a treatment gas containing oxygen atoms in a molten salt to convert the active metal to a non-metallic form ;
Regenerating the processing gas by burning the reducing gas obtained by processing the active metal in an atmosphere containing air or oxygen molecules;
including,
Active metal processing method. The processing gas is at least one of COx, SOx, and NOx,
The active metal treatment method according to claim 1. The active metal is at least one of an alkali metal and an alkaline earth metal.
The active metal treatment method according to claim 1 or claim 2. The molten salt has a higher density than the active metal,
The active metal treatment method according to any one of claims 1 to 3. a step of blowing the processing gas through an inlet provided in the molten salt at a position lower than the active metal;
The active metal treatment method according to claim 4. a reaction vessel for reacting an active metal with a treatment gas containing oxygen atoms in a molten salt to convert the active metal to a non-metallic form ;
a gas regeneration unit that burns a reducing gas obtained by processing the active metal in an atmosphere containing air or oxygen molecules to regenerate the processing gas;
Equipped with
Active metal processing equipment.

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