Electrochemical decontamination procedures

During the last years a special electrochemical procedure was developed for the decontamination of large or inmobil equipments using movable cathodes. 

Redox Processes. Among the most serious impurity problems for electrochemical applications is the contamination of electrolytes with halides. Since they easily react anodicaUy they can be expected to reduce the size of the electrochemical window drastically but the readiness of their anodic decomposition can be used for a decontamination procedure. This was recently described by Li et al. [133] for chloride impurities. They found that, in combination with a subsequent removal of the gaseous product Qi by absorption, electrochemically pure ionic liquids can be obtained. Ethylene was bubbled through the solution to absorb the chlorine gas. Without such an absorption step, the soluble complex CI3 - was formed which could not be removed by vacuum distillation. Both formation and subsequent removal of the complex Cl j can be easily followed spectrometrically due to a strong band of this species at 302 nm. 

From the above it can be concluded that a selection of various procedures is needed to cope with all the required decontamination tasks. As an example, in Table 4.14. the chemical, electrochemical and mechanical processes developed and applied by Siemens/KWU are shown, together with the different fields of application and the materials for which they have been qualified (according to Wille and Bertholdt, 1989). 

As was described in the preceding section, the chemical decontamination processes result in a chemical dissolution of the oxide contamination layers present on the surfaces of the materials. In contrast, the electrochemical decontamination procedures are based on the anodic dissolution of a thin surface layer of the base material, as a consequence of which the oxide layer is peeled off from the surfaces of the materials. As a prerequisite for successful application, the electrolyte used has to be in electrical contact with the base material. Basically, the oxide layers are electrical insulators, but their porosity is usually large enough to ensure a sufficiently high electrical conductivity. On some occasions, very tight Fe203 layers have been observed in BWR circuits which rendered the direct application of the... 

In the practical application of electrochemical decontamination procedures, different electrode arrangements are possible, depending on the geometric form of the component to be treated. Two basic possibilities, namely the bath (or immersion) technique and the in-situ technique, are schematically shown in Fig. 4.53. When the bath technique is used, which is mainly applicable for disassembled parts with... 

When PWR steam generators are to be replaced, the radionuclides deposited on the inner surfaces of the main coolant pipes represent a radiation source which may severely handicap the work therefore, the end sections at least have to be cleaned before starting the work. Decontamination of the relevant pipe sections by electrochemical techniques is a very effective means of removing the contamination layers and has been repeatedly applied. Mechanical procedures such as glass bead blasting have also proven highly effective in reducing the radiation levels in such limited areas. 

Electrochemical decontamination is the reverse procedure of electroplating. The upper oxide-layers containing the contaminants can be removed from the metal surface using a direct electric current between the workpiece and a cathode in an electrolyte. The method can be applied by immersing the workpiece to be decontaminated in an electrolyte tank as an anode. 

The first electrochemical equipment using movable cathode was developed for the decontamination of the control rods (Fig.l. y but the movable cathode was handled by a worker. High (200-400) DFs were achieved by this procedure, but when the inital dose rate was high,personnel s exposure was high as well. 

The advantages of the remotely operated electrochemical decontamination procedure are as follows ... 

Considering the obtained results with the electrochemical decontamination technology, the development of the procedure has been extended for the decontamination of the main pipeline of the primary circuit. 

The electrochemical decontamination procedures developed so far use strong acides - pH l - as electrolytes, which would cause corrosion on the metal surface if they can not be removed in a short time. Therefore the following criteria were established. 

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