EP0071020A2

EP0071020A2 - Method for removing radioactive material from devices and apparatus therefor

Info

Publication number: EP0071020A2
Application number: EP82105586A
Authority: EP
Inventors: Nobuyoshi Miyaji
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1981-06-30
Filing date: 1982-06-24
Publication date: 1983-02-09
Also published as: EP0071020A3

Abstract

A method for removing radioactive material fixed to a device (4), comprising the steps of immersing the contaminated device (4) in a sodium flow (13), thereby dissolving the radioactive material into the sodium flow (13), and thereafter collecting the radioactive material in a trap (7). An apparatus for carrying out above method, wherein a receptacle (1) for receiving a device (13) contaminated with radioactive material and a trap (7) for the radioactive material are disposed in a sodium-circulating system provided with a heating means (9).

Description

This invention relates to a method and apparatus for removing radionuclides, such as 54Mn, 58Co, 60Co, 5l_Cr or 58_Fe, from the devices of the primary cooling system of a nuclear power plant.
A nuclear power plant having a fast breeder reactor is known. During the operation of the reactor, the tubular fuel sheath and reactor core are bombarded with neutrons. As a result, radionuclides having a long half-life, such as ⁵⁸Co, 6⁰Co or 54Mn, are formed in large quantities. The radionuclides are dissolved into sodium coolant as the tubular fuel sheath and the reactor corrode.
The sodium coolant containing the radionuclides is circulated in the primary cooling system of the power plant. As' the coolant repeatedly flows through the primary cooling system, the radionuclides precipitate on the surfaces of the devices which are disposed in the coolant passage. The precipitated radionuclides emit radiation. The radiation may endanger the people who are engaged in maintenance and repair of the fast breeder reactor.
In the case of a large fast breeder reactor for use in a demonstration power plant, such radiation as mentioned above is estimated to be about 10 R/hr after the reactor has been used two to three years. Therefore, it is necessary to remove radionuclides from the surfaces of the cooling pipes and the devices disposed in the coolant passage when the reactor is repaired. If some of these pipes and devices are replaced by new ones, they should not be discarded before as much radioactive material as possible is removed from them.
It is more important to remove 54Mn than to remove the other radionuclides mentioned above. Precipitation of 60Co can be reduced by raising the purity of stainless steel, i.e., the material of the tubular fuel sheath and reactor core, which is the source of this radionuclide 54_Mn. ⁵8Co has a relatively short half-life and does not dissolve into sodium coolant as easily as 54Mn does 58Co is therefore slightly responsible for the radiation from the cooling pipes and the devices disposed in the coolant passage. By contrast, 54mn is generated from iron, the chief component of stainless steel. Purification of stainless steel does not help to reduce the precipitation of ⁵⁴Mn. If generated from the reactor core or the tubular fuel sheath, ⁵⁴Mn can easily be dissolved into the sodium coolant. As a result, 54_Mn is greatly responsible for the radiation from the cooling pipes and the devices disposed in the coolant passage.
Three methods are generally used to remove radionuclides from devices which are disposed in the coolant passage and which thus come into contact with the coolant.
The three methods are:

(1) Brushing the surfaces of the devices.
(2) Cleaning the devices with ultrasonic waves.
(3) Cleaning the devices with chemicals.

Methods (1) and (2) do not work well. The radionuclides precipitated on the devices remain in contact with the sodium coolant heated to 500°C or more while the fast breeder reactor is in action. Heated at such a high temperature, the radionuclides diffuse into the walls or grain boundaries of the devices. They can then hardly be removed from the devices by cleaning the devices with water vapor or alcohol. Nor can they be removed by brushing the devices or by cleaning the devices with ultrasonic waves.
Method (3), which uses an acid, such as nitric acid, phosphorous acid or citric acid, is more promising. Method (3) is disadvantageous, nonetheless. As the devices are cleaned with an aicd, they may corrode to such extent that they become no longer reusable. Further, there remains the question of how the used acid can be disposed of, without causing troubles. Still further, various problems will arise when method (3) is used to clean large devices.
Water or chemicals should not be used to clean the devices taken from the primary cooling system of a nuclear power plant. They will violently react with sodium coolant, generating much hydrogen. They must not be applied on the devices before the sodium coolant is completely removed from the devices.
A so-called "system cleaning" is proposed and developed at present. This technique is to remove radioactive material from all or some of the devices used in a light water reactor. The system cleaning cannot be successfully by method (3).
The object of the present invention is to provide a method and apparatus for removing radionuclides, such as 54_Mn, 60_Co, 58_Co, 5lCr or 59Fe, by remote control from the devices taken from the primary cooling system of a nuclear power plant, so that the cleaned devices can be used again.
A method according to the invention comprises the step of immersing contaminated devices in flowing sodium, thereby dissolving the radioactive material into the flowing sodium, and the step of guiding the sodium to a trap, thereby collecting the radioactive material.
An apparatus according to the invention comprises a system for circulating liquid sodium, a receptacle disposed in the sodium circulating system for receiving devices contaminated with radioactive material, a trap for collecting the radioactive material, and means for heating the liquid sodium.
The method according to the invention is characterized in that liquid sodium is heated to a high temperature, thereby dissolving radioactive material precipitated on devices, and then is cooled, thereby depositing and collecting the radioactive material. The flowing coolant is heated to 550°C or more, preferably 600°C or more. The coolant is cooled to 400°C or less, preferably 150°C or less. The method of this invention is further characterized in that nickel or a nickel alloy (preferably, containing 70 % or more of nickel) issued as a getter to absorb 54Mn dissolved in liquid sodium. The higher the temperature of sodium, the more effectively the getter absorbs 54Mn. The temperature of the liquid sodium should therefore be preferably 450°C or more, more preferably 550°C or more. The getter is used in such a form that its contact surface with the sodium is sufficiently large.
This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

Fig. 1 schematically shows an apparatus according to the invention for removing radioactive material;
Fig. 2 schematically shows another apparatus according to the invention for removing radioactive material;
Figs. 3 and 4 are partially sectional views of two trapping tanks used in this invention for collecting radioactive material; and
Fig. 5 schematically shows a further apparatus according to the invention for removing radioactive material.

_Fig. 1 shows an apparatus according to the present invention. The apparatus has a tank 1 with an upper open end which is made of stainless steel. The upper end of the tank 1 is closed by a flange 2. A perforated table 3 is disposed in the lower portion of the tank 1. A device 4 contaminated with radioactive material is mounted on the table 3 and thus enclosed in the tank 1. The tank 1 is connected to a heat exchanger 6 by a pipe 5. The pipe 5 is connected at one end to the upper portion of the tank 1 and at the other end to the upper end of the heat exchanger 6. The heat exchanger 6 is connected to a trapping tank 7 by another pipe 5. The trapping tank 7 is made of stainless steel. In the tank 7 a filter 7a is disposed which is made up of either metal meshes or Rasching rings. The trapping tank 7 is located in a wind tunnel 8. A fan 8' is provided at one end of the tunnel 8 to supply air into the tunnel 8 and cool the trapping tank 7. The-trapping tank 7 is connected to a pump 10, which is connected to the heat exchanger 6 by a pipe 5. The heat exchanger 6 is connected to a heater 9 by another pipe 5. The heater 9 is connected to the lower end of the tank 1 by still another pipe 5. Connected by the pipes 5, the tank 1, heat exchanger 6, trapping tank 7, heater 9 and pump 10 form a system for circulating liquid sodium.
The lower end of the tank 1 is connected to a valve 17. The valve 17 is connected to a drain tank 12 by a pipe 11. The tank 1 is filled with liquid sodium 13 in order to remove the radioactive material from the device 4. The liquid sodium 13 is discharged from the tank 1 into the drain tank 12 after the device 4 has been cleaned. The tank 1 is not completely filled. The remaining room is filled with a cover gas or argon gas 14. Similarly, the drain tank 12 is not completely filled with liquid sodium 15, and the remaining room is filled with argon gas 16.
In operation, the flange 2 is removed from the tank 1, and the device 4 is put on the table 3. The tank 1 is then closed by the flange 2. The sodium-circulating system is evacuated by a cover gas charging/discharging unit (not shown). The sodium-circulating system is then filled with argon gas supplied from the cover gas charging/discharging unit. The valve 17 is opened, thereby supplying the liquid sodium from the drain tank 12 to the tank 1 until the tank 1 is completely filled. The value 17 is then closed. The pump 10 is operted, thus allowing the liquid sodium to flow through the system at a desired rate. Thereafter, the fan 8' and the heater 9 are set to such conditions that the sodium is maintained at a desired temperature in the tank 1 and at another desired temperature in the trapping tank 7. The fan 8' and the heater 9 are so operated as to maintain the temperature of the sodium at 600°C or more in the tank 1 and at about 400°C in the trapping tank 7.
The fan 8' and the heater 9 are started. The liquid sodium is heated by the heater 9. The heated sodium is supplied to the tank 1. As the liquid sodium flows upwards in the tank 1, the radioactive material precipitated on the device 4 is dissolved into the sodium. The liquid sodium containing the ratioactive material is supplied to the heat exchanger 6 through the pipe 5 and cooled by the heat exchanger 6. The cooled sodium is supplied to the trapping tank 7 and is further cooled.
The solubilities of Fe, Cr, Mn, Co and the like in the liquid sodium increase with the temperature of the liquid sodium. Hence, as the sodium is cooled in the trappling tank 7, these metals precipitate on the filter 7a simultaneously with the precipitation of the radioactive material. As a result, the content of the radioactive material and these metals in the sodium is reduced. The liquid sodium is then supplied by the pump 10 to the heat exchanger 6 and then to the heater 9. The sodium is heated by the heater 9. The heated sodium is supplied to the tank 1.
The process mentioned above is repeated until the radioactive material is removed to a desired extent and collected in the trapping tank 7. Thereafter, the valve 17 is opened, thereby discharging the liquid sodium from the tank 1 to the drain tank 12. The empty tank 1 is cooled to room temperature. The flange 2 is then removed, and the cleaned device 4 is taken out of the tank 1.
The degree of cleaning depends on the temperature of the sodium in the tank 1 and that of the sodium in the trapping tank 7. The degree of cleaning also depends on the concentration of oxygen in the liquid sodium and, needless to say, the cleaning time. When the cleaning was conducted for 240 hours while maintaining the temperature of the sodium at 600°C in the tank 1 and at 400°C in the trapping tank 7 and the oxygen concentration at 10 ppm, 85 % of 5⁴Mn, 80 % of 58Co, 80 % of 60Co, 90 % of 51Cr and 89 % of 59Fe were removed.
Fig. 2 shows another apparatus according to the invention. The apparatus has a tank 21 made of stainless steel. The tank 1 has an upper open end which is closed by a flange 23. In the lower portion of the tank 21 a 54Mn-removing unit 24 made of nickel getter material is disposed. A perforated table 25 is mounted on the unit 24. A device 26 contaminated with 54Mn is mounted on the table 25. A pipe-27 is connected at one end to the upper portion of the tank 21 and at the other end to an electromagnetic pump 28. Another pipe 29 is connected at one end to the pump 28. The other end of the pipe 29 is connected to the lower end of the tank 21. A valve 30 is connected-to the pipe 29. The valve 30 is connected also to a drain tank 32 by another pipe 31.
The tank 21 is filled with liquid sodium 22 in order to remove the radioactive material from the device 26. The liquid sodium 22 is discharged from the tank 21 into the drain tank 32 after the device 26 has been cleaned to a desired extent. The tank 1 is not completely filled. The remaining room is filled with argon gas. Since no oxygen is present in the tank 21, the liquid sodium will not burn in the tank 21. Similarly, the drain tank 32 is not completely filled with liquid sodium 34, and the remaining room is filled with argon gs 35 for the same purpose.
Connected by the pipes 27 and 29, the tank 21 and the pump 28 form a system for circulating the liquid sodium.
In operation, the flange 23 is removed from the tank 21. The device 26 is then put on the table 25. The tank 21 is then closed by the flange 23. The sodium-circulating system is evacuated by an evacuating unit (not shown). This done, the valve 30 is opened. As a result, the liquid sodium flows upwards from the drain tank 32 into the tank 21. When the tank 21 is filled with a desired amount of liquid sodium, the valve 30 is closed. Argon gas is then introduced into the space 33 provided in the tank 21 from a cover gas supply unit (not shown) until the gas pressure in the space 33 reaches about atmospheric pressure.
The liquid sodium in the tank 21 is heated to 450°C or more by a heater (not shown). The heated sodium is circulated in the sodium-circulating system by the pump 28. The sodium is repeatedly heated and circulated for a predetermined time. During this process, 54Mn precipitated on the surface of the device 26 is dissolved into the liquid sodium. The sodium containing 54_Mn flows from the upper end portion of the tank 21 to the lower portion of the tank 21 through the pump 28. As the liquid sodium flows through the 54Mn-removing unit 24, 54Mn is trapped by the unit 24. The content of ⁵4_Mn in the liquid sodium is thus reduced. As the sodium flows upwards from the unit 24, ⁵⁴Mn is dissolved into the sodium. ⁵⁴Mn is further dissolved and filtered many times. Thus, more and more 54Mn is removed from the device 26.
Upon lapse of said predetermined time the valve 30 is opened, thus discharging the liquid sodium from the tank 21 into the drain tank 32. The empty tank 21 is cooled to room temperature. The flange 23 is then removed, and the cleaned device 26 is taken out of the tank 21.
Fig. 3 and Fig. 4 each shows a trappling tank which is used as the ⁵⁴Mn-removing unit 24 of the apparatus shown in Fig. 2. The trapping tank of Fig. 3 comprises a hollow cylinder 36 made of metal, a perforated disc 37 fixed to the upper end of the cylinder 36 and another perforated disc (now shown) fixed to the lower end of the cylinder 36. The cylinder 36 contains a getter 38 which is made of a thin nickel plate coiled into a cylinder. The trapping tank shown in Fig. 4 comprises a hollow cylinder 36 made of metal, a mesh 38 attached to the upper end of the cylinder 36 and another mesh (not shown) attached to the lower end of the cylinder. The cylinder 36 contains a getter 40 which is made up of nickel Rasching rings. Both getters 38 and 49 are so designed as to have as large a surface as possible that contacts the liquid sodium flowing through the trapping tanks. Hence, the getters 38 and 49 can effectively trap 54Mn dissolved in the liquid sodium.
The degree of cleaning which the apparatus of Fig. 2 performs depends on the temperature of the liquid sodium in the tank 21 and, of course, the cleaning time. When a device made of stainless stell SUS-304 which was contaminated with 54Mn was cleaned for 240 hours while maintaining the temperature of the liquid sodium at 550°C in the tank 21, 85 % of 54Mn was removed. The higher the temperature of the sodium, the faster the cleaning proceeded. The cleaning was proved effective when the temperature of the liquid sodium was kept at 450°C or more. The 54Mn-removing unit 24 remained effective after repeated use over a long time.
Another embodiment will now be described with reference to Fig. 5. Unlike in the apparatus of Fig. 2, a 54Mn-removing unit 24 is disposed in a tank 41 rather than in a main tank 21. A pipe 27 connects the upper end portion of the main tank 21 to the upper end portion of the auxiliary tank 41. The lower end of the auxiliary tank 41 is connected to a pump 28. The pump 28 is connected to the lower end portion of the main tank 21 by a pipe 29. Hence the tank 21, pipe 27, tank 41, pump 28 and pipe 29 form a system for circulating liquid sodium 22. In all other respects the apparatus of Fig. 5 is identical with the apparatus shown in Fig. 2.
If the main tank 21 is replaced by a tank of the primary cooling system of a nuclear reactor and the 54_Mn-removing unit 24 is disposed in the tank of the primary cooling system, the "system cleaning" will then be possible for all or some of the devices which are provided in the primary cooling system.
As described above in detail, according to the present invention it is possible to easily and effectively remove radioactive material from devices merely by immersing the devices in a liquid sodium, i.e., the coolant used in a fast breeder reactor, without using a conventionally used detergent, such as an acid.
The method and apparatus according to the invention can therefore save time ad labour and can clean the devices without corroding or damaging them. The devices cleaned by the method and apparatus can be used again.
The method and apparatus of the invention can be easily conducted and operated. They can thus be carried out and operated by remote control. This would help to reduce the risk of workers being exposed to radioactive rays.

Claims

1. A method for removing radioactive material from devices, comprising the steps of:

immersing contaminated devices in flowing sodium, thereby dissolving the radioactive material into the flowing sodium; and

guiding the sodium containing the radioactive material to a trap, thereby collecting the radioactive material.

2. A method according to claim 1, wherein the sodium in which the devices are immersed in maintained at a relatively high temperature and the sodium which flows in the trap is maintained at a relatively low temperature, thereby depositing the radioactive material in the trap.

3. A method according to claim 2, wherein said relatively high temperature is at least 550°C, and said relatively low temperature is at most 400°C.

4. A method according to claim 3, wherein said relatively low temperature is at most 150°C and above the melting point of sodium.

5. A method according to claim 2, wherein said trap includes metal meshes which are disposed in the sodium flow.

6. A method according to claim 2, wherein said trap includes Rasching rings which are disposed in the sodium flow.

7. A method according to claim 1, wherein said trap include a getter made of nickel or a nickel alloy whose chief component is nickel.

8. A method according to claim 7, wherein said sodium flow is maintained at 450°C at least.

9. A apparatus for removing radioactive material from devices, comprising:

a system for circulating liquid sodium;

a receptacle disposed in the sodium-circulating system for receiving devices contaminated with radioactive material;

a trap for collecting the radioactive material; and

means for heating the liquid sodium.

10. An apparatus according to claim 9, wherein said trap includes a getter made of nickel or a nickel alloy whose chief component is nickel.

ll. An apparatus according to claim 9, wherein said trap is formed integral with said receptacle.