Radioactive waste drying system and radioactive waste drying method

Abstract

Even when large amounts of water are attached to the radioactive waste, it is possible to dry the radioactive waste in a short time without causing the water to freeze. [Solution] The system includes a chamber 11 that houses a waste container 2 containing radioactive waste 1 with water attached to it, a heater 12 that heats the radioactive waste when it operates, heating the water attached to the radioactive waste and exhausting it as steam to the outside of the chamber, a vacuum pump 14 that reduces the pressure inside the chamber when it operates to remove water remaining in the radioactive waste, and a control device 17 that controls the heater and the vacuum pump. The control device is configured to operate the heater to heat the chamber until the inside of the chamber reaches atmospheric pressure, exhausting the water attached to the radioactive waste as steam to the outside of the chamber, and when the inside of the chamber reaches atmospheric pressure, to operate the heater to heat the chamber while gradually reducing the pressure of the vacuum pump, reducing the pressure inside the chamber to 610 Pa to remove water remaining in the radioactive waste.

Description

Technical Field

[0001] Embodiments of the present invention relate to a radioactive waste drying system and a radioactive waste drying method.

Background Art

[0002] Among the radioactive wastes generated by the operation or decommissioning of nuclear facilities, high-level radioactive wastes are stored in water. Therefore, when the waste is lifted into the air and stored in a waste container, the container contains a large amount of water. In order to prevent this water from generating hydrogen gas by radiolysis, it is necessary to sufficiently dry the radioactive waste. As drying methods for this radioactive waste, there are a hot air drying method, a vacuum drying method, and the like. Also, an index for completion of drying of radioactive waste is to maintain the radioactive waste at a vacuum of 100 Pa or less for a predetermined time.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0004] Incidentally, when drying radioactive waste with hot air, especially if the radioactive waste is stored in a container, the reduced air permeability can make drying difficult. On the other hand, when vacuum drying radioactive waste, if the radioactive waste contains a large amount of water (for example, 20 liters of water in a waste container of about 1 cubic meter), drying will take a long time, and may exceed the target drying time (for example, 5 hours). Furthermore, in this vacuum drying method, there is a risk that the water contained in the radioactive waste or the water stored at the bottom of the chamber may freeze due to the temperature drop caused by the release of heat of vaporization of the water.

[0005] The embodiments of the present invention have been made in consideration of the above circumstances, and aim to provide a radioactive waste drying system and a radioactive waste drying method that can dry radioactive waste in a short time without freezing the water, even when a large amount of water is attached to the radioactive waste. [Means for solving the problem]

[0006] The radioactive waste drying system in an embodiment of the present invention comprises: a chamber for directly or in a container to contain radioactive waste to which water has adhered; a heating means installed at the bottom of the chamber and operating to heat the radioactive waste, heat the water adhering to the radioactive waste, and exhaust it as steam to the outside of the chamber; a depressurization means connected to the chamber and operating to reduce the pressure inside the chamber to remove water remaining in the radioactive waste; and a control means for controlling the heating means and the depressurization means, wherein the control means is configured to operate the heating means until the inside of the chamber reaches a first predetermined pressure, exhausting the water adhering to the radioactive waste as steam to the outside of the chamber; and when the inside of the chamber reaches the first predetermined pressure, to operate the heating means while gradually reducing the pressure of the depressurization means, reducing the pressure inside the chamber to a second predetermined pressure to remove water remaining in the radioactive waste.

[0007] The radioactive waste drying method in an embodiment of the present invention is characterized by sequentially performing the following steps: a chamber containing a container holding radioactive waste to which water has adhered; a heating means installed at the bottom of the chamber, which operates to heat the radioactive waste and heat the water adhering to the radioactive waste, exhausting it as steam to the outside of the chamber; and a depressurization means connected to the chamber, which operates to reduce the pressure inside the chamber and remove any water remaining in the radioactive waste; a heating step in which the heating means is operated to heat the chamber until the inside of the chamber reaches a first predetermined pressure, and the water adhering to the radioactive waste is exhausted as steam to the outside of the chamber; and a heating and depressurization step in which, when the inside of the chamber reaches a first predetermined pressure, the heating means is operated to heat the chamber while the depressurization means is gradually depressurized, reducing the pressure inside the chamber to a second predetermined pressure and removing any water remaining in the radioactive waste. [Effects of the Invention]

[0008] According to embodiments of the present invention, even when a large amount of water is attached to the radioactive waste, the radioactive waste can be dried in a short time without freezing the water. [Brief explanation of the drawing]

[0009] [Figure 1] A diagram showing the configuration of a radioactive waste drying system according to one embodiment. [Figure 2] Figure 1 shows the chamber, with a portion of the side cut away to reveal the interior. [Figure 3] Figure 2 is a perspective view showing the bottom of the chamber as seen from below. [Figure 4] A graph showing the state of water. [Figure 5] A graph showing the actual operating curves for a vacuum pump to perform a gradual depressurization operation. [Figure 6] A flowchart illustrating the operation of the radioactive waste drying system in Figure 1. [Figure 7] A graph showing the change in chamber pressure when the radioactive waste drying system in Figure 6 is put into operation. [Modes for carrying out the invention]

[0010] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. Figure 1 is a diagram showing the configuration of a radioactive waste drying system according to one embodiment. The radioactive waste drying system 10 shown in Figure 1 dries radioactive waste 1 to which a large amount of water is attached, and consists of a chamber 11, a heater 12 as a heating means, an exhaust unit 13, a vacuum pump unit 15 equipped with a vacuum pump 14 as a depressurization means, a cold trap unit 16, and a control device 17 as a control means.

[0011] Here, radioactive waste 1 is generated, for example, from the operation or decommissioning of a nuclear facility, and consists of radioactive metal waste such as spent channel boxes, control rods, and in-reactor structures. Since this radioactive metal waste is cut after being stored in water, it contains a large amount of water (e.g., 20 liters) as initial water. In order to prevent this water from generating hydrogen gas through radiolysis, radioactive waste 1 needs to be thoroughly dried. This drying completion is indicated by maintaining a vacuum of 100 Pa or less for a predetermined time (e.g., 1 hour).

[0012] As shown in Figures 1 and 2, the chamber 11 is designed to house radioactive waste 1 contaminated with water, either directly or in a waste container 2. In this embodiment, the radioactive waste 1 is housed in a waste container 2, and this waste container 2 is housed inside the chamber 11. The chamber 11 is a rectangular or cylindrical shape with a bottom portion 20, and a top cover portion 21 is provided opposite the bottom portion 20. An atmospheric vent pipe 22 equipped with an opening valve V0 is provided on this top cover portion 21, and an exhaust pipe 23 from the exhaust unit 13 is connected to it. The atmospheric vent pipe 22 and the exhaust pipe 23 may be provided on the side of the chamber 11.

[0013] Furthermore, a pressure gauge 24 is provided on the top cover portion 21 to measure the pressure P inside the chamber 11. The pressure measurement value of this pressure gauge 24 is transmitted to the control device 17. Also, a thermometer 25 is provided near the bottom surface portion 20 of the chamber 11 to measure the temperature T inside the chamber 11. The temperature measurement value of this thermometer 25 is transmitted to the control device 17. In a state where the radioactive waste 1 is accommodated in the chamber 11 via the waste container 2, water (indicated by the symbol W in FIG. 2) adhering to the radioactive waste 1 may be stored on the bottom surface portion 20 of the chamber 11.

[0014] As shown in FIGS. 1 and 3, a plurality of heaters 12 are installed on the lower surface of the bottom surface portion 20 which is the lower part of the chamber 11 or on the lower side surface of the chamber 11. This heater 12 is preferably an electric heating wire heater or an IH (Induction Heating) heater, etc., but in this embodiment, an IH heater is adopted. By the heating operation of this heater 12, the bottom surface portion 20 of the chamber 11 is heated to, for example, 300°C, and the water adhering to the radioactive waste 1 and further stored on the bottom surface portion 20 of the chamber 11 is heated to steam and exhausted through the exhaust unit 13.

[0015] As shown in FIG. 1, the exhaust unit 13 has the aforementioned exhaust pipe 23, and an on - off valve V1, a filter 26, a cooling chiller 27, and a water storage tank 28 are sequentially arranged in this exhaust pipe 23 from the chamber 11 side. The on - off valve V1 is opened by the control device 17 when exhausting steam from the chamber 11. The filter 26 removes dust (including radionuclides) in the air containing steam. The cooling chiller 27 cools the air containing steam to turn the steam into water, and this water is stored in the water storage tank 28, and the air is discharged into the atmosphere. The water stored in the water storage tank 28 is drained by appropriately opening the drain valve V11 provided in the water storage tank 28.

[0016] The heating operation of the heater 12 and the operation of the exhaust unit 13 are carried out in a state where the pressure P in the chamber 11 is greater than the atmospheric pressure. Further, by the heating operation of the heater 12 and the operation of the exhaust unit 13, most (for example, 99%) of the water adhering to the radioactive waste 1 and further stored on the bottom surface portion 20 of the chamber 11 is recovered and removed.

[0017] As shown in FIG. 1, the vacuum pump unit 15 and the cold trap unit 16 have a continuous vacuum pipe 30, and the tip of the vacuum pipe 30 on the cold trap unit 16 side is connected to the upstream side (chamber 11 side) of the on-off valve V1 in the exhaust pipe 23 of the exhaust unit 13. However, the above tip of the vacuum pipe 30 may be directly connected to the top cover portion 21 of the chamber 11 instead of the exhaust pipe 23. In the vacuum pipe 30, an on-off valve V2, V3, a cooling chiller 31, an on-off valve V4, V5, a cold trap 32, and an on-off valve V6 are sequentially arranged from the above tip side (chamber 11 side) to constitute the cold trap unit 16.

[0018] In this cold trap unit 16, a cooling chiller bypass pipe 33 provided with an on-off valve V8 that bypasses the on-off valve V3, the cooling chiller 31, and the on-off valve V4 is provided. Further, in the cold trap unit 16, a cold trap bypass pipe 34 provided with on-off valves V9 and V10 that bypass the on-off valve V5, the cold trap 32, and the on-off valve V6 is provided. The cooling chiller 31 cools the air containing water. Also, the cold trap 32 exchanges heat by cooling with the cooler 35 to further cool the air containing water cooled by the cooling chiller 31 and recover moisture. The opening and closing of the on-off valves V2 to V6, V8 to V10 are controlled by the control device 17. Further, the water stored in the cold trap 32 is drained by appropriately opening the drain valve V12 provided in the cold trap 32.

[0019] The vacuum pump unit 15 comprises an on-off valve V7, a filter 36, a vacuum pump 14, and a water storage tank 37, which are sequentially arranged downstream of the on-off valve V6 in the vacuum piping 30. The filter 36 removes dust (including radionuclides) from the air that contains moisture. The water storage tank 37 stores moisture from the air and exhausts the air into the atmosphere. The water stored in the water storage tank 37 is drained by appropriately opening the drain valve V13 provided in the water storage tank 37.

[0020] The operation of the vacuum pump 14 is controlled by the control device 17, similar to the opening and closing of the on-off valve V7. The vacuum pump 14 is connected to the chamber 11 via vacuum piping 30 and reduces the pressure inside the chamber 11 to a vacuum below atmospheric pressure by depressurizing it, thereby recovering and removing any water remaining in the radioactive waste 1 (for example, the remaining 1% water) mainly by the cooling chiller 31 and cold trap 32 of the cold trap unit 16. The depressurization operation of this vacuum pump 14 is divided into two parts: operation during the heating and depressurization process, where it operates together with the heater 12 to reduce the pressure inside the chamber 11 from atmospheric pressure (a first predetermined pressure) to 610 Pa (a second predetermined pressure), and operation during the drying confirmation process, where the depressurization operation is performed with the heater 12 stopped to reduce the pressure inside the chamber 11 to 100 Pa or less (a third predetermined pressure).

[0021] When the vacuum pump 14 is operated to reduce pressure and create a vacuum below atmospheric pressure inside the chamber 11, the temperature inside the chamber 11 drops due to the release of heat of vaporization of water, making it easy for water adhering to the inner surface of the chamber 11 to freeze. To prevent this freezing, during the heating and depressurization process, the vacuum pump 14 reduces the pressure inside the chamber 11 to a low vacuum region (region α in Figure 3) that is below atmospheric pressure and above 610 Pa, thereby drying the water remaining in the radioactive waste 1 inside the chamber 11. Furthermore, during the drying confirmation process, the vacuum pump 14 reduces the pressure inside the chamber 11 to a high vacuum region (region β in Figure 3) of below 100 Pa, and maintains this state for a predetermined time (for example, 1 hour) to complete the drying of the radioactive waste 1 inside the chamber 11.

[0022] Incidentally, when the vacuum pump 14 reduces the pressure inside the chamber 11 from atmospheric pressure, which is a low vacuum, to 610 Pa, which is close to the triple point of water, it gradually reduces the pressure inside the chamber 11 from atmospheric pressure to 610 Pa, as shown in the actual operating curve M, based on the predicted curve B (Figure 5), which is a pressure drop profile calculated in advance along the gas-liquid equilibrium line in the phase diagram of water (Figure 4).

[0023] The predicted curve B is obtained by solving the differential equation (1) which must be satisfied by the pressure P (in-chamber pressure) inside chamber 11, which is a function of elapsed time t, in equation (2), where the released gas constant Q0 (Pa·m) 3 This can be obtained by changing the ( / s). This emission gas constant Q0 corresponds to the amount of vapor to be released and removed from the air in the chamber 11. In equation (2), the case where the emission gas constant Q0 > 0 represents the expected curve B, and the case where Q0 = 0 represents the ideal curve A.

[0024] V(dP / dt)=-S(P-P0)+Q0…(1) t = (V / S)log(P - Q0 / S) + C ... (2)

[0025] In equations (1) and (2), P is the chamber pressure (Pa), P0 is the ultimate pressure (Pa), and S is the pumping speed (m 3 / s), V is the chamber volume (m³ 3 ), where C is the integration constant. The above-mentioned pumping speed S is calculated from the pumping capacity of the vacuum pump 14, taking into account the piping resistance of the vacuum piping 30. The chamber volume V is the volume inside the chamber 11 excluding the volume of the radioactive waste 1 and waste container 2 present inside the chamber 11. This chamber volume V can be calculated if the internal space volume of the chamber 11 itself, the volumes of the radioactive waste 1 and waste container 2, and the pumping speed S are known, since the volumes of the radioactive waste 1 and waste container 2 can be determined by measuring their weight.

[0026] The actual operating curve M shown in Figure 5 is based on the predicted curve B, and involves performing the same amount of pressure reduction by the vacuum pump 14 at the same or different time intervals. This makes it possible to gradually reduce the pressure P inside the chamber 11 by gradually reducing the pressure of the vacuum pump 14 in order to create a low vacuum of 610 Pa from atmospheric pressure inside the chamber 11.

[0027] The control device 17 shown in Figure 1 controls the operation of the heater 12 and vacuum pump 14 and the opening and closing of the on-off valves V1 to V10 in order to dry the radioactive waste 1 which is stored in the waste container 2 and contained in the chamber 11. The procedure is shown in Figure 6. The radioactive waste 1 and the waste container 2 are brought into the chamber 11 by removing the top cover 21 of the chamber 11, and after they are brought in, the chamber 11 is closed by the top cover 21. At this time, the opening valve V0 of the atmospheric vent pipe 22 and the drain valve V14 provided on the bottom surface 20 of the chamber 11 are closed, for example, manually.

[0028] With the radioactive waste 1 and waste container 2 contained in the chamber 11, the control device 17 opens valve V1, closes valve V2, and starts heating the heater 12 (S1). As a result, the water adhering to the radioactive waste 1 and further stored in the chamber 11 is heated by the heater 12, turns into steam, and flows out to the exhaust unit 13 outside the chamber 11. There, it is cooled by the cooling chiller 27 of the exhaust unit 13, turns back into water, and is stored in the water storage tank 28, where most of it (e.g., 99%) is recovered. In the heating process of step S1, the pressure P inside the chamber 11 rises above atmospheric pressure and then falls, as shown in Figure 7. The control device 17 continues step S1 until the pressure reading on the pressure gauge 24 drops to atmospheric pressure (S2).

[0029] When the pressure inside the chamber 11 reaches atmospheric pressure, the control device 17 closes the on-off valves V1, V8, V9, and V10 with the opening valve V0 closed, and opens the on-off valves V2, V3, V4, V5, V6, and V7, thereby heating the heater 12 and gradually depressurizing the vacuum pump 14 (S3). As a result, the pressure P inside the chamber 11 gradually decreases from atmospheric pressure to 610 Pa, as shown in Figure 7, and the inside of the chamber 11 becomes a low vacuum. Therefore, the moisture-containing air inside the chamber 11 flows to the cold trap unit 16 and the vacuum pump unit 15, and the moisture in the air is recovered in the cooling chiller 31 and cold trap 32 of the cold trap unit 16 and the water storage tank 37 of the vacuum pump unit 15. In this heating and depressurization process of step S3, any water remaining in the radioactive waste 1 inside the chamber 11 (e.g., the remaining 1% water) is removed without freezing.

[0030] In step S3, the switching of the heater 12 to the heating operation and the vacuum pump 14 to the depressurization operation was described as being performed when the pressure gauge 24 indicates that the pressure inside the chamber 11 has reached atmospheric pressure. However, in step S1, a predetermined time may be confirmed in advance for the amount of water (initial water) adhering to the radioactive waste 1 and stored in the chamber 11 to be sufficiently boiled and evacuated, and the operation may be performed after this predetermined time has elapsed.

[0031] The control device 17 determines whether the temperature T inside the chamber 11, measured by the thermometer 25, is T>0°C and the pressure P inside the chamber 11, measured by the pressure gauge 24, is P<610 Pa (S4). If T≦0°C or P≧610 Pa, the control device 17 continues step S3.

[0032] When the temperature T and pressure P in the chamber 11 reach T>0°C and P<610 Pa, the control device 17 closes valves V1, V3, V4, V5, and V6 with the opening valve V0 closed, opens valves V2, V8, V9, V10, and V7, stops the heater 12, and causes the vacuum pump 14 to depressurize. Due to this depressurization operation of the vacuum pump 14, the air in the chamber 11 does not pass through the cooling chiller 31 and cold trap 32 of the cold trap unit 16, but instead passes through the cooling chiller bypass pipe 33 and cold trap bypass pipe 34 to the vacuum pump 14, where it is further depressurized until the chamber 11 reaches a high vacuum of P<100 Pa. Then, as shown in Figure 7, the control device 17 performs the drying confirmation process by maintaining the pressure P in the chamber 11 at P<100 Pa for a predetermined time (for example, 1 hour) (S5).

[0033] After the drying confirmation process in step S5 is completed, for example, the release valve V0 is opened by a worker to release the chamber 11 into the atmosphere, the top cover 21 of the chamber 11 is removed, and the waste container 2 containing the radioactive waste 1 is removed from the chamber 11 (S6).

[0034] As configured as described above, this embodiment provides the following effects (1) and (2). (1) The heater 12 is operated to heat the chamber 11 until the pressure inside the chamber 11 reaches atmospheric pressure, and the water adhering to the radioactive waste 1 is exhausted as steam to the exhaust unit 13 outside the chamber 11, where it is recovered. When the pressure inside the chamber 11 reaches atmospheric pressure, the vacuum pump 14 of the vacuum pump unit 15 is operated to gradually reduce the pressure inside the chamber 11 from atmospheric pressure to 610 Pa, creating a low vacuum state. The water in the air inside the chamber 11 is then recovered mainly by the cooling chiller 31 and cold trap 32 of the cold trap unit 16, thereby removing any water remaining on the radioactive waste 1.

[0035] As a result, most of the large amount of water attached to the radioactive waste 1 (e.g., 99% water) can be removed by exhausting it as steam outside the chamber 11 using the heater 12. Furthermore, any remaining water on the radioactive waste 1 (e.g., 1% water) can be removed without freezing in the low vacuum state of the chamber 11, where the pressure is reduced from atmospheric pressure to 610 Pa. Consequently, even if a large amount of water is attached to the radioactive waste 1, it can be dried in a short time without the water freezing.

[0036] (2) When the chamber 11 is reduced in pressure from atmospheric pressure to 610 Pa by the vacuum pump 14 and the chamber 11 is brought into a low vacuum state, the control device 17 gradually reduces the pressure of the vacuum pump 14. Therefore, when the vacuum pump 14 reduces the pressure inside the chamber 11 from atmospheric pressure to 610 Pa, the control device 17 does not need to provide feedback control of the vacuum pump 14 based on the pressure measurement value of the pressure gauge 24, thus simplifying the control system of the vacuum pump 14 in the control device 17.

[0037] Although embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. Such substitutions, modifications, and combinations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of Symbols]

[0038] 1...Radioactive waste, 2...Waste container, 10...Radioactive waste drying system, 11...Chamber, 12...Heater (heating means), 13...Exhaust unit, 14...Vacuum pump (depressurization means), 15...Vacuum pump unit, 17...Control device (control means), 20...Bottom section, 23...Exhaust piping, 24...Pressure gauge, 25...Thermometer, 27...Cooling chiller, 28...Water storage tank, 30...Vacuum piping, 31...Cooling chiller, 32...Cold trap, 33...Cooling chiller bypass pipe, 34...Cold trap bypass pipe, B...Predicted curve (pressure drop profile), M...Actual operating curve, P...Pressure, T...Temperature.

Claims

1. A chamber for directly or in containers containing radioactive waste with water attached, A heating means is installed at the bottom of the chamber and, by operating, heats the radioactive waste, heats the water adhering to the radioactive waste, and exhausts it as steam outside the chamber. A depressurization means connected to the chamber and operating to reduce the pressure inside the chamber to remove water remaining in the radioactive waste, The system includes a control means for controlling the heating means and the depressurization means, A radioactive waste drying system characterized in that the control means is configured to operate the heating means until the chamber reaches a first predetermined pressure, thereby exhausting water adhering to the radioactive waste as steam to the outside of the chamber, and when the chamber reaches the first predetermined pressure, to operate the heating means while gradually depressurizing the depressurizing means, thereby depressurizing the chamber to a second predetermined pressure, thereby removing water remaining in the radioactive waste.

2. The radioactive waste drying system according to claim 1, characterized in that the control means controls the heating means to stop when the pressure inside the chamber falls below a second predetermined pressure and rises above 0°C, and to further reduce the pressure inside the chamber to a third predetermined pressure by depressurization operation of the depressurization means and maintain it for a predetermined time.

3. The radioactive waste drying system according to claim 1, characterized in that the first predetermined pressure is atmospheric pressure and the second predetermined pressure is 610 Pa.

4. The radioactive waste drying system according to claim 2, characterized in that the second predetermined pressure is 610 Pa and the third predetermined pressure is 100 Pa.

5. The radioactive waste drying system according to claim 1, characterized in that the stepwise depressurization in the chamber until a second predetermined pressure is reached is carried out based on a pressure drop profile that has been pre-calculated along the gas-liquid equilibrium line in the phase diagram of water.

6. A chamber containing a container holding radioactive waste with water attached, A heating means is installed at the bottom of the chamber and, by operating, heats the radioactive waste, heats the water adhering to the radioactive waste, and exhausts it as steam outside the chamber. A depressurization means is provided, which is connected to the chamber and operates to reduce the pressure inside the chamber to remove water remaining in the radioactive waste. A heating step in which the heating means is operated to heat the chamber until the chamber reaches a first predetermined pressure, and the water adhering to the radioactive waste is exhausted outside the chamber as steam, A method for drying radioactive waste, characterized by sequentially performing a heating and depressurization step, in which, when the pressure inside the chamber reaches a first predetermined pressure, the heating means is operated to heat the pressure while the depressurization means is operated to gradually depressurize the pressure inside the chamber until the pressure inside the chamber reaches a second predetermined pressure, thereby removing water remaining in the radioactive waste.

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