Recontamination of decontaminated surfaces

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 

Normally, metallic clean surfaces will show a faster recontamination rate than those covered by an oxide layer. Therefore, several attempts have been made to create a suitable oxide layer either by treatment in an air-steam atmosphere or by 

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. 

Recontamination rates at the recirculation piping of a number of BWR plants have been compared by Duce (1990), in order to get an indication of the impact of different measures. These results show that the concentration of dissolved Co in the reactor water is the key parameter controlling the rate and extent of recontamination. When the measured recontamination rates were normalized with re- 

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. 

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