ELECTROLYTIC TREATMENT FOR NUCLEAR DECONTAMINATION

Abstract

A method of removing nuclear contamination from a surface comprising applying a DC-biased AC waveform in an electro-pickling process using nitric acid as the electrolyte.

Claims

1.13. (canceled)

14. A method of removing nuclear contamination from a surface comprising applying an asymmetric alternating current waveform with a net time-averaged current flow in one sense or the other to the surface in an electro-pickling process using nitric acid as the electrolyte in combination with an electrochemical oxidative destruction process, with electrolyte being recirculated between the two processes to create an effluent stream.

15. The method of removing nuclear contamination from a surface according to claim 14 in which the nitric acid has added chloride or organic chelating additives.

16. The method of removing nuclear contamination according to claim 14 in which a current density of between 0.1 and 1 amp inclusive per square centimetre is used.

17. The method of removing nuclear contamination according to claim 16 in which a current density of between 0.4 and 0.7 amps inclusive per square centimetre is used.

18. The method of removing nuclear contamination according to claim 14 wherein the asymmetric is periodically reversed, at a frequency less than that of the frequency of the alternating current waveform.

19. The method of removing nuclear contamination according to claim 14 in which the alternating current frequency is between 1 Hz and 1000 Hz inclusive.

20. The method of removing nuclear contamination according to claim 19 in which the alternating current frequency is between 2 Hz and 500 Hz inclusive.

21. The method of removing nuclear contamination according to claim 19 in which the alternating current frequency is between 5 Hz and 100 Hz inclusive.

22. The method according to claim 14 in which nitric acid has an added chloride or other oxidative species said species being selected from the group containing hydrochloric and hydrofluoric acid, citric acid, oxalic acid, ethylene diamine tetra acetic acid and hydrogen peroxide.

23. The method according to claim 14 employing a counter-electrode comprising or coated with at least one of iridium, platinum, ruthenium and metal oxide coatings on titanium.

24. The method according to claim 14 generating an effluent stream said stream being itreated electrochemically to remove chloride and organic additives.

25. The method according to claim 14 including applying ultrasonic energy to the pickling process.

26. A method of removing nuclear contamination from a surface comprising applying an asymmetric alternating current waveform with a net time-averaged current flow in one sense or the other to the surface in an electro-pickling process using nitric acid as the electrolyte in combination with an electrochemical oxidative destruction process, with electrolyte being recirculated between the two processes to create an effluent stream in which the nitric acid includes a material selected from the group comprising hydrochloric acid, hydrofluoric acid, oxalic acid, citric acid, ethylene diamine tetra acetic acid, and hydrogen peroxide; in which the current density is between 0.4 and 0.7 amps inclusive per square centimetre and the alternating current frequency is between 5 Hz and 100 Hz inclusive.

27. The method of removing nuclear contamination from a surface according to claim 26 in which the asymmetric bias of the applied alternating current waveform is periodically reversed, at a frequency less than that of the frequency of the alternating current waveform.

28. The method according to claim 26 employing a counter-electrode comprising or coated with at least one of iridium, platinum, ruthenium and metal oxide coatings on titanium.

29. A method of removing nuclear contamination from a surface comprising applying an asymmetric alternating current waveform with a net time-averaged current flow in one sense or the other to the surface in an electro-pickling process using nitric acid as the electrolyte in combination with an electrochemical oxidative destruction process, with electrolyte being recirculated between the two processes to create an effluent stream in which the nitric acid includes a material selected from the group comprising hydrochloric acid, hydrofluoric acid, oxalic acid, citric acid, ethylene diamine tetra acetic acid, and hydrogen peroxide; in which the current density is between 0.4 and 0.7 amps inclusive per square centimetre and the alternating current frequency is between 5 Hz and 100 Hz inclusive; in which the current density is between 0.4 and 0.7 amps inclusive per square centimetre and the alternating current frequency is between 5 Hz and 100 Hz inclusive; wherein the DC-bias of the applied alternating current waveform is periodically reversed, at a frequency less than that of the frequency of the alternating current waveform and wherein a counter-electrode comprising or coated with at least one of iridium, platinum, ruthenium and metal oxide coatings on titanium is employed.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0031] FIG. 1 shows an example of the voltage/time sequence used in this invention;

[0032] FIG. 2 shows schematically an example electrode set up used to demonstrate the present invention; and

[0033] FIG. 3 illustrates the impact adding hydrochloric acid to nitric acid in an electropickling decontamination process.

EXPERIMENTAL

[0034] Nitric acid is the base electrolyte. This is compatible with standard radio nucleotide recovery plants and does not corrode the materials of construction. Possible additives include hydrochloric acid, various organic acids including citric, oxalic, formic, ethylene diamine tetra acetic acid, other organic metal complexing or chelating agents including those based on nitrogen or phosphorous, as well as glycols and other organic compounds.

[0035] 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 metal dissolution and hydroxyl ion and hydrogen production, the latter being beneficial for preventing passivation and scrubbing of the surface. The DC bias may optionally be varied in a continuous manner.

[0036] A low power waveform may be generated and then fed into a power amplifier, for example a potentiostat. Although the efficiency of such a process can be low (for example as low as 50%) it is a flexible process since any waveform may be specified.

[0037] 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 also 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 centimetre, and more preferably between 0.4 and 0.7 amps per square centimetre.

[0038] 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 on the electrolytes used and also on the composition of the metal surfaces.

[0039] The added halide ions or other oxidising additive are removed from the nitric acid solution using a second electrochemical treatment. This consists of a separate electrochemical cell through which the treatment solution is passed. This treatment may be carried out continuously or in a batch-wise process, with multiple paths of the solution through the cell to achieve the desired reduction in levels of halide or other additive.

[0040] An electrochemical surface treatment was carried out on test pieces of 304 stainless steel, followed by an electrochemical oxidation process to remove chloride ions. The effectiveness of the treatment was determined by measuring the thickness and weight loss of the sample pieces and by checking that the level of chloride ions in the electrolyte solution after electrochemical oxidation was below the 10 ppm required for subsequent treatment in conventional nuclear effluent treatment plant. The apparatus consisted of an electrical supply to provide a potential between working and counter-electrodes in an electrochemical pickling bath, an electrochemical pickling bath with 304 stainless steel working and counter electrodes, and a separate electrochemical oxidative step to remove additives (chloride ions in this example) from the nitric acid solution.

[0041] The electrical supply is composed of a driving circuit which creates an electrical waveform, an H bridge circuit which allows for the reversal of polarity of applied potential to form a square wave, and a DC power supply. The applied potential was 12 V and the power supply was capable of supplying 10 A. The frequency used was 10 Hz, though other frequencies may be used. The waveform used is shown in FIG. 1. A single cycle is shown as 72 ms of forward or positive bias, followed by 2 ms of zero potential, followed by 24 ms of negative or reverse bias, followed by 2 ms of zero bias, totalling 100 ms for a 10 Hz supply. The waveform shown has three times longer duration of forward bias than reverse bias and is therefore a DC-biased AC waveform. The 2 ms zero potential periods are provided to ensure reliable operation of the switching circuits and are not important for the electrochemical operation. The 2 ms periods may be reduced or omitted if the electrical circuit can operate robustly without them. The shape and detail of the waveform may be varied. It does not need to be square waves, and alternative waveforms would be suitable, so long as they conform to the constraint of being DC-biased AC waveforms. In this case the bias was positive so that the working electrode was anodically biased, but this need not be the case. The bias may usefully be reversed periodically.

[0042] A first experiment, tabulated in table 1 below, shows the very significantly greater rate of surface removal obtained when using an electrochemical process than is obtained when using a chemical process. Two 304 stainless steel coupons of exposed area 3 cm by 3 cm were immersed in a pickling solution that was 3 M in nitric acid and 0.3 M in hydrochloric acid. The rate of loss of metal from the surface of the exposed area was determined by weight loss. Two different treatments were applied. In the first there was no electric potential applied so that the process was a chemical one only. The coupons were exposed for 6 hours and the weight loss measured. In the second treatment the same chemical solution was used but now with the electrical potential applied, with the waveform as described above. The results are given in the table below and show that the electrochemical process gives a rate of removal more two orders of magnitude greater than the chemical process. This finding shows the advantageous rate of surface removal obtained with electrochemical processes compared to chemical processes.

TABLE-US-00001 TABLE 1 Rate of metal surface loss Treatment applied (grams per cm.sup.2 per 24 hours) 0.3M HNO.sub.3 + 0.3M HCl 0.013 No electrical potential 6 hours treatment duration 0.3M HNO3 + 0.3M HCl, 4.851 12 V, 10 Hz, 3:1 bias, 0.5 A/cm.sup.2, 10 minutes treatment duration

[0043] A second experiment shows the different throwing powers obtained with different solution properties when used in electrochemical surface treatments. The arrangement of electrodes is shown in FIG. 2.

[0044] The two rectangular electrodes shown are completely submerged in the electrolyte solution. The holding vessel is omitted for clarity. Wires, 1, connect the electrodes to the power supply. A pickling bath of 800 cm.sup.3 was used. 1mm thick stainless steel 304 test pieces to form electrodes 2 and 3 were cut to size, each 50 mm wide, parallel to one another and 10 mm apart. The working electrode 2 is the piece representing the object that needs to be decontaminated and is 100 mm long. The counter-electrode 3 is also of 304 stainless steel and is 40 mm long. Suitable materials for the counter-electrode include titanium coated with one of a number of metals and oxides including platinum, platinum oxide, iridium oxide, ruthenium oxide or electrodes composed of carbon composite materials including graphite based materials. The electrical supply to the electrodes is as described above with reference to FIG. 1. Experiments were carried out for a fixed time of ten minutes. After that time the reduction in thickness of the working electrode 2 was measured at intervals along its length. The end of the working electrode that is adjacent to one end of the counter electrode is marked as 5 on FIG. 2, the end of the working electrode furthest from the counter electrode is marked as 4 on FIG. 2. The thickness of the working electrode was measured at ten points one centimetre apart.

[0045] The results are shown in graphical from in FIG. 3. The horizontal axis shows the distance from the end 4 of the working electrode 2 (of FIG. 2). Thus the end 4 is at the origin of the graph labelled 0 cm. The part of the working electrode labelled 5 in FIG. 2 is 10 cms from the end labelled 4 and is shown as the 10 cm mark on the horizontal axis of the graph in FIG. 3. Thus points on the right hand side of the graph correspond to the end of the working electrode near 4 in FIG. 2, the points on the left hand side correspond to the end of the working electrode near 5 in FIG. 2.

[0046] The two thick horizontal lines at the top of graph show schematically the relative positions of the working and counter-electrodes. The square points show the effect of an electrochemical process when a nitric acid solution was used. The percentage reduction in thickness of the working electrode is approximately constant at 6% for that part of the working electrode that is parallel to the counter-electrode, and then falls off to a low value further away from the counter-electrode, as is expected. The diamond shaped points show the effect of adding 0.3 M HCl to the solution. There is an overall increase in the thickness loss, equivalent to an increase in the rate of loss. The throwing power 7, shown as the distance between the dashed lines, is effectively increased and this is marked on the graph, showing that reduction in thickness (and rate of material loss) at the 4 cm point 8 in the case with HCl added, is approximately equal to the reduction in thickness at the 6 cm point 6 in the case without HCl. The result of adding the HCl can be seen as extending the effective reach of the electrodes by 2 cm.

[0047] In a third experiment the added HCl was removed in a separate electrochemical cell. This cell had the following features. The active electrode areas were 25 cm by 25 cm. The cell was divided using a Nafion cation selective ion exchange membrane (other membrane separators and membranes can be used). The purpose of the ion exchange membrane is to minimises metal deposition on the cathode and prevent parasitic Fe (II)/Fe(III) redox couples arising from the presence of iron ions from the dissolved stainless steel which would otherwise reduce current efficiency. The cathode material was stainless steel which is preferred when treating nitric acid solutions and the anode was titanium coated with a mixture of iridium and tantalum oxides. The cell had a narrow inter-electrode gap of 5 mm and included turbulence promotors in the form of mesh in the electrode gap to give good mass transport and the capability to operate at a high current density. Other inter-electrode spacings are possible depending on the details of the cell construction. The cell was operated with a current density of 0.4 A/cm.sup.2. The outcome of treating the electrolyte with this cell was an evolution of chlorine gas and a reduction in the concentration of chloride ions from 0.3 M to below 10 ppm (or approximately 310.sup.4 M). This lower level is acceptable for subsequent processing in conventional nuclear effluent treatment plant of 304 stainless steel.

[0048] The foregoing show that rapid electrochemical surface removal with good throwing power can be achieved by the use of hydrochloric acid as an additive to nitric acid in an electro-pickling process, and that the chloride ions can be removed subsequently using an electrochemical oxidative step. It has therefore been shown, surprisingly, that a nitric/hydrochloric acid mix can meet all of the objectives required for an nuclear surface decontamination process.

[0049] Other embodiments of the invention involve the use of different electrode materials and additives to nitric acid. Possible additives to nitric acid include hydrochloric acid, formic acid, ethylene diamine tetra-acetic acid, and hydrogen peroxide amongst others. Any suitable oxidising agent which can be destroyed electrochemically is suitable. Other anode materials in the oxidative destruction step may be used depending on the added reagent that needs to be removed. Iridium, platinum and ruthenium metal and metal oxide coatings on titanium are suitable for chloride ion removal (chlorine gas evolution) and for the oxidation to carbon dioxide of organic acids such as formic acid, by means of the creation of intermediate oxy-chloride ions. Boron-doped diamond electrodes are also effective at evolving chlorine and generating radical ions for oxidation of organic species. The complete system may optionally use a mixture of anode types. A suitable level of chemical oxygen demand (COD) such that solutions can be processed in 304 stainless steel treatment plants is 10 ppm COD.