ELECTROLYTIC TREATMENT FOR NUCLEAR DECONTAMINATION

Abstract

An electrolytic treatment system to decontaminate the surface of a radioactively contaminated metallic workpiece has at least two electrodes in close proximity to the surface but not in direct electrical contact. The electrodes are separated from the surface by an electrolyte. Insulation is provided in the electrolyte between the electrodes to avoid or minimize a direct current path between the electrodes though the electrolyte.

Claims

1. An electrolytic treatment system to decontaminate the surface of a radioactively contaminated metallic workpiece comprising: at least two electrodes, each in close proximity to the surface but not in direct electrical contact and separated from the surface by an electrolyte by a distance sufficient to allow movement of the electrodes along the surface; and insulation in the electrolyte between the electrodes, said insulation minimizing any direct current path between the electrodes otherwise than through the workpiece.

2. The electrolytic treatment system according to claim wherein the electrodes are alternatingly polarized as cathodes and anodes using a DC supply.

3. The electrolytic treatment system according to claim wherein the electrodes are alternatingly polarized as cathodes and anodes when an AC voltage is applied.

4. The electrolytic treatment system according to claim where the electrodes are alternatingly polarized as cathodes and anodes when a DC biased AC voltage is applied.

5. The electrolytic treatment system according to claim 1, further including seals that contain the electrolyte solution to a region of the article surface being treated, and wherein the electrodes may be configured with variable spacing and geometry and conveyed across the article surface.

6-7. (canceled)

8. The electrolytic treatment system according to claim 3, wherein the alternating current frequency is between 1 Hz and 1000 Hz inclusive.

9. The electrolytic treatment system according to claim 8, wherein the alternating current frequency is between 2 Hz and 500 Hz inclusive.

10. The electrolytic treatment system according to claim 9, wherein the alternating current frequency is between 5 Hz and 100 Hz inclusive.

11. (canceled)

12. The electrolytic treatment according to claim 10, wherein the electrolyte contains less than 10% v/v nitric acid.

13. The electrolytic treatment according to claim 1, wherein the electrolyte contains 2% v/v nitric acid.

14. The electrolytic treatment system according to claim 1, wherein the electrolyte contains one or more of a chloride salt and an organic complex.

15. The electrolytic treatment system according claim 1, wherein the electrolyte is totally or partially retained in a porous structure with connected pores.

16. The electrolytic treatment system according to claim 14, further comprising a pump to recirculate the electrolyte fluid to the electrodes and a treatment station in which the eluent stream is treated electrochemically to remove chloride and organic complexes.

17. The electrolytic treatment system according to claim 1, further comprising an ultrasonic energy source to improve the efficiency and effectiveness of the electrochemical process.

18. The electrolytic treatment system according claim 5, wherein the electrode assembly includes one or more sensors comprising radiation sensors, ultrasound x-ray or other non-destructive sensors, pH sensors, conductivity sensors, Raman or infrared probes.

19. The electrolytic treatment system according to claim 18, wherein the one or more sensors are mounted ahead or behind the electrodes wherein the sensor data is used to control at least one of movement rate, processing time, current density, voltage control, fluid flow rate or other control actions.

20. The electrolytic treatment system according to claim 1, further comprising a power supply in an isolated circuit wherein the voltage applied to the electrodes has no reference to ground potential.

21. (canceled)

22. The electrolytic treatment system as in claim 20, wherein the voltage bias on the electrodes are alternatingly reversed.

23. The electrolytic treatment system according to claim 22, in which the ratio of the resistance of the current path through the non-contacted workpiece to the direct electrode to electrode path is 0.6 or less.

24. The electrolytic treatment system according to claim 1, in which the ratio of the resistance of the current path through the non-contacted workpiece to the direct electrode to electrode path is 0.3 or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 is a schematic drawing of a system according to the invention for treating the internal surface of a pipe;

[0041] FIG. 2 is a graphical illustration of the impact of the invention piece when metal is etched from the workpiece, and

[0042] FIG. 3 is a schematic drawing of a system according to the invention for treating a planer surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Treating the Internal Surface of a Pipe

[0043] FIG. 1 shows an embodiment of the invention intended for the treatment of the internal surfaces 2, of a pipe 1. The working region of the pipe is flooded with an electrolyte solution 4. There may optionally be provided seals 3 which contain the electrolyte to the immediate vicinity of the electrodes or in the absence of such seals the entire volume of the pipe may be flooded. Insulating materials 7 are present between a pair of electrodes 6. An arrangement 5 is provided to support the electrode assembly and propel it along the pipe. The electrodes 6 are kept at a defined distance from the inside surface of the pipe by means of the support arrangement 5. The electrical current path is from electrodes 6, across the electrolyte-filled gap to the surface of the pipe, along the pipe and then across the electrolyte filled gap to the other electrode 6 and then through the external power circuit and back to the first electrode. The various parts of the electrode treatment device are connected by connecting structures 9. An umbilical connection 8 takes power, data and liquid connections along the pipe to a remote power and control unit outside the pipe.

Example of Embodiment 1

[0044] Sections of 304 stainless steel pipe were EASD treated in nitric acid solutions of two different concentrations at an applied current of 8 A for 30 minutes. The etching power supply produced a 50 Hz waveform of consisting of 13 mS in one polarity and 7 mS with reversed polarity.

[0045] The pipe had an internal diameter of 104 mm, was immersed vertically in a bath of nitric acid and had no direct electrical contact with the etching power supply. The internal surface of the pipe was electrochemically treated with a non-contacting pair of electrodes which were directly connected to the opposite polarities of the etching power supply and were mounted in the center of the pipe with a mechanism to allow them to be moved up and down the pipe either continuously or stepwise.

[0046] The effect of changes to the geometry of internal energized electrodes and the electrolyte resistivity on the relative electrical resistance of the direct electrode to electrode current path and the path via the workpiece on the mass loss on the internal surface of a section of 304 stainless steel pipe subject to electrochemical treatment are shown below. A current of 8 A was applied for 30 minutes and the etching power supply produced a 50 Hz waveform of consisting of 13 mS in one polarity and 7 mS with reversed polarity.

[0047] The directly energized electrodes had diameters (7.1, 8.1 9.1 cm), were 2 cm long and were separated by various distances (1.0, 2.0, 3.2, 3.9 and 6.3 cm) by electrical insulating discs of the same diameter as the electrodes.

[0048] The applied voltage causes a current to pass between the directly energized electrodes through the electrically conducting electrolyte via electron exchange reactions at the electrode electrolyte interfaces. The electron exchange reactions do the electrochemical work including metal dissolution. There is a direct current path between the energized electrodes just through the conducting electrolyte which does no electrochemical work on the pipe and an indirect path which does electrochemical work (etching) on the inside of the pipe. The path is energized electrode to electrolyte to internal surface of the pipe opposite the electrode then along the highly conducting pipe wall to the internal surface opposite the other electrode then into the electrolyte and back into opposite polarity electrode.

[0049] The electrical resistance of the direct path and the path through the workpiece where metal is etched from the workpiece were calculated and the metal loss plotted as a function of their ratio in FIG. 2. This incorporates both distance and electrolyte changes.

[0050] Metal loss from the workpiece decreases as the ratio of the workpiece path to the short circuit path directly from electrode to electrode increases. This is logical as more current will flow down the lower resistance path. Thus, the metal removal rate or process efficiency can be controlled by adjusting the geometry and electrolyte conductivity.

[0051] The results show that:

[0052] The pipe which had no direct electrical contact to the power supply lost mass when electrochemically treated demonstrating that non-contact electrochemical etching can be effectively carried out on a pipe.

[0053] The relative metal removal efficiency as determined by sample mass loss of the tube was substantially the same whether the electrodes are stationary or scanned down the sample pipe.

[0054] The relative metal removal efficiency as determined by sample loss of the tube demonstrates that the proportion of the current used in the non-contact etching process is increased if the electrical resistance of the direct path between the directly energized electrodes is increased by decreasing the electrolytic conductivity.

[0055] When the ratio of the electrical resistance of the current path through the non-contact workpiece to the direct electrode to electrode path is greater than 0.6 there is already a significant improvement in metal removal efficiency; this is even more marked reduction in metal loss; but as the ratio falls further to 0.3 there is an even greater improvement.

Embodiment 2. Treating a Planar Surface

[0056] FIG. 3 shows a surface layer 12 that is to be removed. The working area is surrounded by an enclosure 13 which is filled with an electrolyte solution 18. Optionally a sealing arrangement 14 prevents leakage of the electrolyte from within the sealed area. Insulating pieces 16 separate one electrode 15 from another electrode 15 opposite polarity. Electrodes 15 are held at a defined distance from the surface 2. The insulating pieces 16 separate the electrodes 15 and minimize current flowing through the electrolyte directly between the electrodes 15. An umbilical 17 carries power, data and electrolyte supplies to the device. An external means of moving the whole assembly across the surface to be treated is provided.

[0057] Nitric acid is the preferred base electrolyte. This is compatible with standard radionuclide recovery plants and does not corrode the materials of construction. The dissolved metal in nitric acid can be subsequently precipitated or crystallized by evaporation of the water, providing an abatement route for the spent electrolyte and metals/radionuclides.

[0058] In each of FIGS. 1 and 3 only one pair of electrodes is shown, in practice, the carrier 9 or cover 13 could contain a number of pairs of electrodes all electrically isolated from one with insulating material 7 or pieces 16.

[0059] General for all Embodiments

[0060] The electrical waveform for use in the decontamination process is preferably a DC-biased AC waveform. It is also desirable to have the possibility to reverse the polarity of the DC bias periodically. This has the effect of changing the balance between amounts of hydroxyl ion and hydrogen produced, which is beneficial for preventing passivation and helps scrub the surface. The DC bias may optionally be varied in a continuous manner.

[0061] The current density is an important aspect of the invention as it affects the concentration of hydroxyl ions. Hydroxyl ions are important as they help to combat passivation and hydrogen generation. Greater current densities are beneficial therefore, but only up to a point, since at higher current there is a loss of efficiency due to resistive heating that is proportional to the square of the current. In practice there is an optimum current density. The preferred current density is between 0.1 and 1 amp per square centimeter, and more preferably between 0.4 and 0.2 amps per square centimeter.

[0062] The frequency of the AC component of the waveform used may be in the range 1-1000 Hz. The preferred frequency is in the range 5-100 Hz. As frequency increases less of the electrical energy is used in the desired electrochemical conversion, because of the capacitance of the interface, but the alternating current aids removal of passivation via scrubbing and other mechanisms, and in practice a frequency of between 5-100 Hz is preferred. The preferred frequency is dependent to some extent on the electrolytes used.

[0063] The electrodes can be of variable spacing and geometry to suit the application. Insulators may be included between and around the electrodes or may be included around electrodes to prevent or reduce the electrical short circuiting but allow fluid to pass via either/or both internal external path with internal compartments.

[0064] The AC frequency is between 1 Hz and 1000 Hz inclusive, but normally the range is between 2 Hz and 500 Hz inclusive, but usually with best results being obtained between 5 Hz and 100 Hz inclusive.

[0065] The electrolyte used may also contains one or more of a chloride salt, a fluoride salt, and an organic acid or a complexing agent.

[0066] The eluent stream resulting from the surface treatment can be subsequently be electrochemically treated remove chloride and organic molecules.

[0067] During the treatment, ultrasonic energy can be applied to the system to improve the efficiency and effectiveness of the electrochemical process.

[0068] To help monitor the system in use, the spaces between the electrodes contains instrumentation such as, but not limited to, radiation sensor, ultrasound x-ray or other non-destructive evaluation techniques, pH sensor conductivity sensor, Raman or infrared probe. In addition, or as an alternative, instrumentation is mounted ahead or behind the electrodes and used to control one or all the following; movement rate, position, location, processing time, current density, voltage control, fluid flow rate or other control actions.