Method for neutralizing molten salts

Abstract

This invention provides a method for neutralizing molten salts that makes it possible to suppress corrosion of pipes and other components caused by nitrogen oxides generated by the neutralization reaction. [Solution] A method for neutralizing molten salt S in a molten salt handling facility 10 comprising a main tank 1 for storing molten salt S containing sodium nitrite and equipment 2 for using molten salt S as a heat transfer medium, wherein the molten salt S discharged from the main tank 1 is temporarily stored in a neutralization tank 11, nitric acid is added to the molten salt S stored in the neutralization tank 11, and when discharging the nitrogen gas oxide gas generated in the neutralization tank 11 to the outside of the neutralization tank 11, an inert gas is supplied into the neutralization tank 11 at the same time as the nitrogen gas oxide gas is discharged to the outside of the neutralization tank 11.

Description

Technical Field

[0001] The present invention relates to a method for neutralizing molten salt in a molten salt-using facility equipped with equipment that uses a molten salt containing sodium nitrite as a heat medium. More specifically, the present invention relates to a method for neutralizing molten salt that enables suppression of corrosion in pipes and the like caused by nitrogen oxides generated by a neutralization reaction.

Background Art

[0002] Molten salt is a salt composed of cations and anions and is usually solidified, but it is a substance that melts into a molten state at a temperature above a certain level. Among molten salts, there is a substance called HTS (Heat Transfer Salt) that is used to heat heat exchangers, reactors, etc. as a sensible heat type heat medium that stores and releases heat without undergoing a phase change. Examples of HTS include a mixture of sodium nitrite (NaNO2), sodium nitrate (NaNO3), and potassium nitrate (KNO3) (see, for example, Patent Document 1). This mixture is also called Niter, which is derived from the English name of potassium nitrate crystals.

[0003] Specific examples of HTS include (1) a mixture of 40% by weight of sodium nitrite (NaNO2), 7% by weight of sodium nitrate (NaNO3), and 53% by weight of potassium nitrate (KNO3); (2) 100% by weight of NaNO2; (3) 100% by weight of NaNO3; (4) 100% by weight of KNO3; (5) a mixture of 50% by weight of NaNO3 and 50% by weight of KNO3; (6) a mixture of 50% by weight of NaNO2 and 50% by weight of NaNO3; (7) a mixture of 34% by weight of NaNO2, 13% by weight of NaNO3, and 53% by weight of KNO3, and the like.

[0004] Although HTS is chemically stable, prolonged use at temperatures above 450°C causes thermal decomposition of NaNO2, producing NaNO3, Na2O, and N2. Above 600°C, the aforementioned decomposition reaction intensifies, and a decomposition reaction producing NO also occurs. While the decomposition reaction begins at 450°C, if iron rust or organic matter is present inside the equipment or piping, the reaction can start at around 300°C. The Na2O produced by this decomposition reaction corrodes steel. Furthermore, as the NaNO2 concentration decreases and the NaNO3 concentration increases, the freezing point of HTS also rises.

[0005] The decomposition reaction of NaNO2 is as follows: 5NaNO2 → 3NaNO3 + Na2O + N2 This decomposition reaction increases the amount of Na2O, which corrodes the steel, raises the freezing point of the HTS, and further reacts with carbon dioxide in the air to produce insoluble sodium carbonate (Na2CO3). The sodium carbonate crystals precipitate in the tank and, if supplied to the piping, can cause blockages.

[0006] Therefore, especially when using HTS at high temperatures, it is necessary to periodically measure and manage the alkalinity of the HTS. The management standard is set by selecting a specific concentration value as appropriate from the range of 0.5% to 1.0% of the weight of the HTS, for example, as the Na2O concentration. Based on such a management standard, the alkalinity of the HTS is measured periodically (for example, about once a year), and neutralization with nitric acid is performed before the Na2O concentration reaches the management standard.

[0007] The neutralization reaction of Na2O with nitric acid is as follows: Na2O + 2HNO3 → 2NaNO3 + H2O Furthermore, the reaction between NaNO2 and nitric acid, which occurs simultaneously with the neutralization reaction, is shown in the following equation. 2NaNO2+2HNO3→2NaNO3+NO+NO2+H2O As described above, neutralizing HTS with nitric acid inevitably generates nitrogen oxides. However, when handling HTS, water dilution is performed to allow for storage and startup at milder temperatures, so the generated nitrogen oxides react with the water in the HTS system to produce nitric acid again.

[0008] Therefore, when neutralizing HTS with nitric acid, it is necessary to quickly remove the nitrogen oxides generated by the neutralization reaction from the system, and equipment for this purpose is required. If there is no equipment to remove the generated nitrogen oxides, nitric acid will be produced as described above, and this nitric acid will cause corrosion in the piping and other components. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2015-67670 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] The object of the present invention is to provide a method for neutralizing molten salt that makes it possible to suppress corrosion of pipes and the like caused by nitrogen oxides generated by the neutralization reaction. [Means for solving the problem]

[0011] The molten salt neutralization apparatus used in the present invention is an apparatus for neutralizing a molten salt in a molten salt utilization facility that includes a main tank for storing a molten salt containing sodium nitrite and equipment for using the molten salt as a heat transfer medium, and is characterized by comprising a neutralization tank for temporarily storing the molten salt discharged from the main tank, a nitric acid addition device for adding nitric acid to the molten salt stored in the neutralization tank, an exhaust device installed in the neutralization tank, and a supply device for supplying inert gas into the neutralization tank.

[0012] The present invention provides a method for neutralizing a molten salt to achieve the above objective, comprising a main tank for storing a molten salt containing sodium nitrite and equipment for using the molten salt as a heat transfer medium, wherein the method for neutralizing the molten salt is characterized by temporarily storing the molten salt discharged from the main tank in a neutralization tank, adding nitric acid to the molten salt stored in the neutralization tank, and discharging the nitrogen oxide gas generated in the neutralization tank to the outside of the neutralization tank, while simultaneously supplying an inert gas into the neutralization tank. [Effects of the Invention]

[0013] In this invention, when neutralizing a molten salt in a molten salt handling facility that includes a main tank for storing molten salt (HTS) containing sodium nitrite (NaNO2) and equipment that uses the molten salt as a heat transfer medium, a neutralization tank for temporarily storing the molten salt discharged from the main tank, a nitric acid addition device for adding nitric acid to the molten salt stored in the neutralization tank, and an exhaust device installed on the neutralization tank are attached. By temporarily storing the molten salt discharged from the main tank in the neutralization tank, adding nitric acid to the molten salt stored in the neutralization tank using the nitric acid addition device, and exhausting the nitrogen oxide gas generated in the neutralization tank to the outside of the neutralization tank using the exhaust device, the neutralization reaction is carried out in the neutralization tank, which is separate from the main tank, and the nitrogen oxides generated by the neutralization reaction can be quickly removed from the system. As a result, it is possible to suppress the reaction of nitrogen oxides with water contained in the molten salt (HTS) system to generate nitric acid again. This makes it possible to suppress corrosion of pipes, etc., caused by nitrogen oxides generated by the neutralization reaction.

[0014] The molten salt neutralization apparatus according to the present invention has a transfer pipe for transferring molten salt discharged from the main tank to a neutralization tank, a circulation pipe for circulating molten salt discharged from the neutralization tank back to the neutralization tank, and a return pipe for returning molten salt discharged from the neutralization tank back to the main tank, and it is preferable that a nitric acid addition device is connected to the circulation pipe. Alternatively, the molten salt neutralization apparatus according to the present invention has a transfer pipe for transferring molten salt discharged from the main tank to a neutralization tank, and a return pipe for returning molten salt discharged from the neutralization tank back to the main tank, and it is preferable that a nitric acid addition device is connected to the transfer pipe. With such a practical structure, corrosion of pipes and the like caused by nitrogen oxides generated by the neutralization reaction can be effectively suppressed.

[0015] The molten salt neutralization apparatus according to the present invention preferably has a pneumatic device that supplies inert gas into the neutralization tank. By simultaneously discharging nitrogen oxide gas to the outside of the neutralization tank and supplying inert gas into the neutralization tank by the pneumatic device, nitrogen oxides generated by the neutralization reaction can be quickly removed from the system.

[0016] In the molten salt neutralization apparatus according to the present invention, it is preferable that the nitric acid addition apparatus adds nitric acid at a rate of 0.01% to 0.05% relative to the flow rate of the molten salt. This allows the neutralization reaction to proceed smoothly and minimizes the nitrogen oxides generated by the neutralization reaction. [Brief explanation of the drawing]

[0017] [Figure 1] This is a schematic diagram showing a neutralization apparatus for molten salt according to an embodiment of the present invention. [Figure 2] This is a schematic diagram showing a neutralization apparatus for molten salt according to another embodiment of the present invention. [Modes for carrying out the invention]

[0018] The configuration of the present invention will be described in detail below with reference to the attached drawings. Figure 1 shows a molten salt neutralization apparatus according to an embodiment of the present invention.

[0019] As shown in FIG. 1, the molten salt neutralization device according to the present embodiment is a device attached to a molten salt usage facility 10 including a main tank 1 for storing a molten salt S and a device 2 using the molten salt S as a heat medium. The main tank 1 includes a heating device (not shown) and is configured to heat the molten salt in the main tank 1 to an arbitrary temperature. The device 2 is not particularly limited, and is, for example, a device that uses the molten salt S as a heat medium, such as a heat exchanger or a reactor. The molten salt usage facility 10 includes a pump 3 for sending out the molten salt S in the main tank 1, a pipe 4 connecting the pump 3 and the inlet of the device 2, and a pipe 5 connecting the outlet of the device 2 and the main tank 1. Of course, the molten salt usage facility 10 may include additional heating devices, cooling devices, exhaust devices, and the like.

[0020] The molten salt S (HTS) used in the present invention is composed of at least containing sodium nitrite (NaNO2). For example, a three-component mixture of sodium nitrite (NaNO2), sodium nitrate (NaNO3), and potassium nitrate (KNO3), a two-component mixture of sodium nitrite (NaNO2) and potassium nitrate (KNO3), and the like can be mentioned. More specifically, a three-component mixture containing 40% by weight of sodium nitrite (NaNO2), 7% by weight of sodium nitrate (NaNO3), and 53% by weight of potassium nitrate (KNO3), a two-component mixture containing 50% by weight of sodium nitrite (NaNO2) and 50% by weight of potassium nitrate (KNO3), and the like are exemplified.

[0021] When handling molten salt S, pure water is added to the molten salt S during shutdown to lower its freezing point. This allows for storage and startup at milder temperatures. The concentration of water added to the molten salt S is preferably 6% to 50% by weight. For example, the freezing point of molten salt S containing 40% by weight sodium nitrite, 7% by weight sodium nitrate, and 53% by weight potassium nitrate is approximately 142°C, but adding 15% by weight diluent water lowers the freezing point to about 70°C. In this case, the temperature of the molten salt S when the molten salt equipment 10 is shut down is approximately 80°C, and the temperature of the molten salt S during neutralization is also approximately 80°C. The freezing point tends to decrease as the amount of diluent water added increases. Considering that the boiling point of nitric acid is around 120°C, the temperature of the molten salt S during neutralization should ideally be in the range of 25°C to 110°C.

[0022] In the molten salt usage equipment 10, the molten salt S in the main tank 1 is heated to a high temperature of 450°C or higher and continuously supplied to the equipment 2. The equipment 2 uses the molten salt S as a heat transfer medium, and the molten salt S returns from the equipment 2 to the main tank 1. When the molten salt S is used at a high temperature for a long period of time in this manner, a decomposition reaction occurs in the molten salt S, generating sodium oxide (Na2O). An increase in Na2O due to the decomposition reaction corrodes the steel, raises the freezing point of the molten salt S, and furthermore, reacts with carbon dioxide in the air to produce insoluble sodium carbonate (Na2CO3), which is undesirable. Therefore, it is necessary to periodically measure the alkalinity of the molten salt S (for example, about once a year) and neutralize it with nitric acid before it reaches the permissible limit of Na2O (Na2O concentration per unit weight of HTS of 0.5% to 1.0%).

[0023] In Figure 1, the molten salt usage equipment 10 is equipped with a neutralization tank 11 for temporarily storing the molten salt S discharged from the main tank 1, a nitric acid addition device 20 for adding nitric acid to the molten salt S stored in the neutralization tank 11, and an exhaust device 17 installed in the neutralization tank 11. More specifically, a switching valve 6 is provided in the piping 4 of the molten salt usage equipment 10, and a transfer pipe 12 is connected between the switching valve 6 and the neutralization tank 11. The switching valve 6 selectively switches between a flow path to equipment 2 and a flow path to the neutralization tank 11. When the flow path to the neutralization tank 11 is selected, the molten salt S discharged from the main tank 1 is transferred to the neutralization tank 11 via the transfer pipe 12. The neutralization tank 11 has a capacity to store the total amount of molten salt S and the total amount of water added for water dilution. The neutralization tank 11 is equipped with a circulation pipe 13 that circulates the molten salt S discharged from the neutralization tank 11 back into the neutralization tank 11. A pump 14 is installed in the middle of the circulation pipe 13. A switching valve 15 is provided in the circulation pipe 13, and a return pipe 16 is connected between the switching valve 15 and the main tank 1. The switching valve 15 selectively switches between the flow path to the neutralization tank 11 and the flow path to the main tank 1. When the flow path to the main tank 1 is selected, the molten salt S discharged from the neutralization tank 11 returns to the main tank 1 via the return pipe 16. The nitric acid addition device 20 is connected to the circulation pipe 13.

[0024] An exhaust device 17 is installed above the neutralization tank 11, near the return port of the circulation piping 13. The exhaust device 17 has the function of discharging nitrogen oxide gas generated inside the neutralization tank 11 to the outside of the neutralization tank 11. The exhaust device 17 may simply be a structure that allows exhaust, but it may also be equipped with a device for detoxifying nitrogen oxides (e.g., a scrubber). In addition, an air supply device 18 is installed above the neutralization tank 11 to supply an inert gas (e.g., nitrogen gas) into the neutralization tank 11. The air supply device 18 purges the space inside the neutralization tank 11 with an inert gas and is used when discharging residual air inside the neutralization tank 11 to the outside of the neutralization tank 11, and when discharging nitrogen oxide gas generated inside the neutralization tank 11 to the outside of the neutralization tank 11. It is preferable that the air supply device 18 be installed on the opposite side of the return port of the circulation piping 13 from the exhaust device 17. This arrangement allows nitrogen oxide gas released from the circulation piping 13 into the gas phase of the neutralization tank 11 to be immediately sent to the exhaust device 17.

[0025] The nitric acid addition device 20 consists of a nitric acid tank 21 for storing nitric acid, a pipe 22 connected between the nitric acid tank 21 and the circulation pipe 13, a pump 23 installed in the middle of the pipe 22, a valve 24 installed in the pipe 22 between the pump 23 and the circulation pipe 13, and a flow meter 25 disposed between the pump 23 and the valve 24. For example, a needle valve can be used as the valve 24. Nitric acid is delivered by the pump 23, and its flow rate is adjusted by the valve 24. It is preferable to install a mixer or the like at the nitric acid addition location so that the molten salt S and nitric acid are efficiently mixed. If a mixer or the like is not installed, it is preferable to insert the pipe 22 into the circulation pipe 13 and bend the pipe 22 in the direction of flow of the molten salt S, as shown in the enlarged portion of Figure 1. Even when such a double-pipe structure is adopted, the molten salt S and nitric acid are efficiently mixed. Furthermore, it is preferable that the connection point between pipe 22 and the circulation pipe 13 be closer to the point where the circulation pipe 13 connects to the neutralization tank 11 (the return port of the circulation pipe 13). This arrangement allows the generated nitrogen oxides to be quickly moved to the gas phase of the neutralization tank 11 and promptly discharged from the exhaust device 17.

[0026] Next, the method of using the molten salt neutralization apparatus described above will be explained. First, the operation of the molten salt usage equipment 10 is stopped, and after the temperature of the molten salt S has sufficiently decreased, the switching valve 6 is switched to the flow path to the neutralization tank 11, and the pump 3 is driven to temporarily store the molten salt S discharged from the main tank 1 in the neutralization tank 11 via the transfer pipe 12. Next, the molten salt S stored in the neutralization tank 11 is circulated through the circulation pipe 13. At this time, nitric acid is added to the molten salt S from the nitric acid addition device 20 connected to the circulation pipe 13. When nitric acid is added to the molten salt S in the neutralization tank 11 in this way, nitrogen oxide gas is generated in the neutralization tank 11 due to the neutralization reaction of the molten salt S. Such nitrogen oxide gas is separated into gas and liquid within the neutralization tank 11 and discharged to the outside of the neutralization tank 11 by the exhaust device 17. The amount of nitrogen oxides generated is calculated from the amount of nitric acid added, and the nitrogen oxide gas is discharged while controlling the concentration of nitrogen oxides discharged into the atmosphere to be below the environmental standard. When an aeration device 18 is provided, nitrogen oxide gas is discharged to the outside of the neutralization tank 11, and at the same time, inert gas is supplied into the neutralization tank 11 by the aeration device 18, thereby quickly removing nitrogen oxides generated by the neutralization reaction from the system. The flow rate of the inert gas supplied by the aeration device 18 is determined based on the flow rate of nitric acid added, thereby effectively preventing the accumulation of nitrogen oxides in the neutralization tank 11. In particular, the equipment shown in Figure 1 has the advantage that the neutralization work can be performed with the molten salt equipment 10 and the neutralization equipment completely separated, thus completely eliminating the inflow of nitrogen oxide gas into the molten salt equipment 10.

[0027] In the molten salt neutralization apparatus described above, the nitric acid addition device 20 should add nitric acid at a rate of 0.01% to 0.05% relative to the flow rate of the molten salt S. For example, adding nitric acid at a rate of 0.03% relative to the flow rate of the molten salt S is optimal. The flow rate of the molten salt S is set to a rate at which the entire amount of molten salt S in the main tank 1 (including the amount of water added) circulates once in 30 minutes to 1 hour. For example, if the total capacity of the main tank 1 is 10 m³ 3 In that case, the flow rate of molten salt S is 10 m 3 / hour~20m3 This is calculated as / hour. By adding nitric acid in the above proportion, the neutralization reaction can be carried out smoothly, and the nitrogen oxides generated by the neutralization reaction can be minimized. In other words, if the ratio of nitric acid added to the flow rate of molten salt S is less than 0.01%, the neutralization process will take longer than necessary, and conversely, if it exceeds 0.05%, the amount of nitrogen oxides generated will increase, and the excess nitric acid produced in the molten salt S system will cause corrosion of pipes and other components, as well as accelerate the deterioration of the molten salt S. Nitric acid may be added at a constant flow rate, but it is also possible to change the flow rate according to the amount added. For example, it is optimal to set the initial flow rate to the maximum and then gradually decrease the flow rate thereafter. Such control can be performed by a computer program.

[0028] According to the molten salt neutralization device described above, the neutralization reaction is carried out in a neutralization tank 11 separate from the main tank 1, and nitrogen oxides generated by the neutralization reaction can be quickly removed from the system. Therefore, it is possible to suppress the reaction of nitrogen oxides with water contained in the molten salt S system to generate nitric acid again. This makes it possible to suppress corrosion of pipes and other components caused by nitrogen oxides generated by the neutralization reaction. In particular, since carbon steel or chromium-molybdenum steel is generally used in molten salt equipment 10 to withstand high temperatures, corrosion by nitric acid is likely to occur, but by attaching the neutralization device described above, corrosion of the molten salt equipment 10 can be effectively suppressed. As for the material of the neutralization device, stainless steel, such as SUS304 steel, is used to suppress corrosion by nitric acid.

[0029] After the neutralization process is complete, the switching valve 15 is switched to the flow path to the main tank 1, and the pump 14 is driven to return the molten salt S in the neutralization tank 11 to the main tank 1 via the return pipe 16. This makes the molten salt usage equipment 10 operational again.

[0030] Figure 2 shows a molten salt neutralization apparatus according to another embodiment of the present invention. In Figure 2, the same reference numerals are used for parts identical to those in Figure 1, and detailed descriptions of those parts are omitted.

[0031] In Figure 2, the molten salt handling equipment 10 is equipped with a neutralization tank 11 for temporarily storing the molten salt S discharged from the main tank 1, a nitric acid addition device 20 for adding nitric acid to the molten salt S stored in the neutralization tank 11, and an exhaust device 17 installed in the neutralization tank 11. More specifically, a switching valve 6 is provided in the piping 4 of the molten salt handling equipment 10, and a transfer pipe 12 is connected between the switching valve 6 and the bottom of the neutralization tank 11. A return pipe 16 is connected between the top of the neutralization tank 11 and the main tank 1. The nozzle of the return pipe 16 on the neutralization tank 11 side is bent downward inside the neutralization tank 11, and its opening end is positioned below the liquid level of the molten salt S. This makes it possible to create a structure in which the horizontal portion of the pipe 16 is filled with molten salt S when the molten salt S is withdrawn, and nitrogen oxide gas does not enter. On the other hand, the nozzle on the main tank 1 side of the return pipe 16 is positioned so as to be submerged in the liquid surface of the molten salt S, and a gas vent hole is provided in the nozzle in the gas zone above the liquid surface of the molten salt S within the main tank 1. By providing such a gas vent hole, even if nitrogen oxide gas is mixed into the return pipe 16, that gas can be separated. The nitric acid addition device 20 is connected to the transfer pipe 12.

[0032] As described above, the nitric acid addition device 20 consists of a nitric acid tank 21, piping 22, a pump 23, a valve 24, and a flow meter 25. For example, a needle valve can be used as the valve 24. In this case as well, as shown in the enlarged portion of Figure 2, it is preferable to insert the piping 22 into the transfer piping 12 and bend the piping 22 in the direction of flow of the molten salt S. When such a double-pipe structure is adopted, the molten salt S and nitric acid are mixed efficiently. Furthermore, it is desirable that the joint between the piping 22 and the transfer piping 12 be closer to the point where the transfer piping 12 connects to the bottom of the neutralization tank 11. With this arrangement, the generated nitrogen oxides can be quickly moved to the gas phase of the neutralization tank 11 and rapidly discharged from the exhaust device 17.

[0033] Next, the method of using the molten salt neutralization apparatus described above will be explained. First, the operation of the molten salt usage equipment 10 is stopped, and after the temperature of the molten salt S has sufficiently decreased, the switching valve 6 is switched to the flow path to the neutralization tank 11, and the pump 3 is driven to discharge a portion of the molten salt S from the main tank 1 and temporarily store it in the neutralization tank 11 via the transfer piping 12. At this time, nitric acid is added to the molten salt S from the nitric acid addition device 20 connected to the transfer piping 12. When nitric acid is added to the molten salt S in the neutralization tank 11 in this manner, nitrogen oxide gas is generated in the neutralization tank 11 due to the neutralization reaction of the molten salt S. Such nitrogen oxide gas is separated into gas and liquid within the neutralization tank 11 and discharged to the outside of the neutralization tank 11 by the exhaust device 17. If there is an air supply device 18, by discharging the nitrogen oxide gas to the outside of the neutralization tank 11 and simultaneously supplying inert gas into the neutralization tank 11 by the air supply device 18, the nitrogen oxides generated by the neutralization reaction can be quickly removed from the system. When the molten salt S in the neutralization tank 11 reaches the height of the return pipe 16, the molten salt S returns to the main tank 1 via the return pipe 16 due to overflow. While this circulation continues for a while, nitric acid is added to the molten salt S from the nitric acid addition device 20 to perform the neutralization work.

[0034] According to the molten salt neutralization apparatus described above, the neutralization reaction is carried out in a neutralization tank 11 separate from the main tank 1, and the nitrogen oxides generated by the neutralization reaction can be quickly removed from the system. Therefore, it is possible to suppress the reaction of nitrogen oxides with water contained in the molten salt S system to generate nitric acid again. This makes it possible to suppress corrosion of piping and other components caused by nitrogen oxides generated by the neutralization reaction. In particular, in the equipment shown in Figure 2, the size (capacity) of the neutralization tank 11 can be reduced to less than 1 / 10 of the main tank 1. Also, compared to the equipment shown in Figure 1, the pump 14 is not required. Therefore, the equipment shown in Figure 2 has the advantage of keeping costs low.

[0035] After the neutralization process is complete, when pump 3 is stopped, the molten salt S in the neutralization tank 11 returns to the main tank 1 via the transfer piping 12 due to its own weight. This makes the molten salt usage equipment 10 operational again.

[0036] In each of the embodiments described above, a neutralization device is attached to the molten salt usage equipment 10. However, equipment such as the neutralization tank 11 and the pipes 12, 13, and 16 through which the molten salt S flows are required to be heated to a temperature equivalent to that of the molten salt S during neutralization by performing steam tracing or the like. Such heating means should be capable of adjusting the temperature within a range of, for example, 25°C to 110°C.

[0037] Furthermore, while the specific piping connection structures of the neutralization tank 11 and the nitric acid addition device 20 to the molten salt usage equipment 10 are described in the embodiments described above, the piping connection structures of the neutralization tank 11 and the nitric acid addition device 20 are not limited to the embodiments described above, and the present invention also encompasses other piping connection structures. [Explanation of symbols]

[0038] 1 Main tank 2 equipment 3,14,23 pumps 4,5,22 Piping 6.15 Diverter Valve 11 Neutralization tank 12 Transfer piping 13 Circulation piping 16. Return piping 17 Exhaust system 18. Air supply device 20 Nitric acid addition device 21 Nitrate tank 24 valves 25 Flow meter S Molten salt

Claims

1. A method for neutralizing a molten salt in a molten salt handling facility comprising a main tank for storing a molten salt containing sodium nitrite and equipment for using the molten salt as a heat transfer medium, characterized in that the molten salt discharged from the main tank is temporarily stored in a neutralization tank, nitric acid is added to the molten salt stored in the neutralization tank, and when discharging the nitrogen oxide gas generated in the neutralization tank to the outside of the neutralization tank, an inert gas is supplied into the neutralization tank at the same time as the nitrogen oxide gas is discharged to the outside of the neutralization tank.

2. The method for neutralizing a molten salt according to claim 1, characterized in that when adding nitric acid to the molten salt, the nitric acid is added in a ratio of 0.01% to 0.05% relative to the flow rate of the molten salt.

3. A method for neutralizing molten salt according to claim 1 or 2, characterized in that a neutralization apparatus is used which comprises a neutralization tank for temporarily containing the molten salt discharged from the main tank, a nitric acid addition device for adding nitric acid to the molten salt contained in the neutralization tank, an exhaust device installed in the neutralization tank, and a pneumatic supply device for supplying inert gas into the neutralization tank.

4. The neutralization device comprises a transfer pipe for transferring molten salt discharged from the main tank to the neutralization tank, a circulation pipe for circulating molten salt discharged from the neutralization tank back to the neutralization tank, and a return pipe for returning molten salt discharged from the neutralization tank back to the main tank, wherein the nitric acid addition device is connected to the circulation pipe, as described in claim 3.

5. The neutralization method for molten salt according to claim 3, wherein the neutralization apparatus includes a transfer pipe for transferring molten salt discharged from the main tank to the neutralization tank, and a return pipe for returning molten salt discharged from the neutralization tank to the main tank, and the nitric acid addition apparatus is connected to the transfer pipe.

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