Recontamination, decontaminated surface

In decontaminating a reactor component, there is always the problem that after a relatively short period of continued plant operation redistribution of radionuclides from the non-decontaminated surfaces of the other circuits and components will result in a recontamination of the cleaned regions. This effect can only be reliably precluded by decontamination of the whole reactor system, possibly with the fuel in place. Such decontamination operations require additional preparatory work as compared with the decontamination of components, due to the large volumes to be treated which require large amounts of decontamination solution, resulting in large amounts of radioactive waste. Further, additional preparatory work is necessary because different materials are exposed simultaneously to the decontamination solution. Since the mid-1960 s, a number of reactors have been decontaminated in this way more details will be given in Section 4.5.3. 

In summary, the most promising means of reducing the rate of recontamination of decontaminated surfaces seems to be minimization of the °Co concentration in the coolant. All the attempts made to reduce recontamination of decontaminated surfaces by appropriate pretreatment did not show a clear beneficial result. When, in the course of a replacement action, new pipes or components are installed, a pretreatment of these surfaces by electropolishing plus pre-oxidation (hot moist air) seems to be effective. However, because of the complex process of Co incorporation into the superficial oxide layers it is often difficult to distinguish clearly between incidental correlations and those which are in fact caused by the measures taken. 

The decontamination effectiveness of these non-chemical procedures varies greatly depending on the specific technique and installations applied. Usually they are applied for the decontamination of limited and well-defined areas, such as the end regions of the main coolant pipes in steam generator replacement actions. The application of abrasive methods often suffers from the hazard of surface damaging of the materials, so that rapid recontamination of the decontaminated areas during... 

Several authors have reported that recontamination rates are usually less severe when dilute chemical decontamination processes have been applied. The most likely explanation of the high recontamination rates observed after the use of concentrated reagents is that these solutions were corrosive, removing the entire protective film from the surfaces and roughening the underlying base material. Nevertheless, even when applying soft decontamination procedures, the phenomenon of recontamination is a severe handicap for further work. Numerous attempts, therefore, have been made to reduce the rate of recontamination in order to reduce... 

Experiments performed in an SGHWR loop showed that specimens of stainless steel SS 304, Inconel 600 and Zircaloy-4, previously decontaminated with the Lomi or the NP/Lomi processes, recontaminate within a few months to their preceding activity levels (Bridle et al., 1986). Surfaces which had been decontaminated with the Lomi process alone experienced a faster recontamination than those treated with the NP/Lomi process. This difference may be explained by the fact that application of the NP/Lomi process results in a complete removal of the surface oxide layer, leaving behind a surface which shows less tendency to attract suspended corrosion product particles. 

The obtained DF-values were favourable (20-500), and after several subsequent decontamination the increasing of recontamination has not been observed because of the smooth surface prodeuced by the electropolishing effect. 

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