Retention of radioactive gases and aerosols in saturated reprocessing effluents

The process gases resulting from reprocessing operations are generally saturated with moisture. For final filtering of the radioactive aerosols and gases contained in them, the relative humidity of the effluents must have been reduced to 70% or less and they must have undergone one or more cleaning operations before very-high-efficiency (VHE) filtering. In order to optimize the system for processing this type of effluent, a knowledge is required of the physico-chemical transformations which occur between the point of emission and final filtering. The first part of the paper describes the present state of knowledge concerning the behaviour of volatile aerosols and compounds, such as ruthenium, which are borne by the process gases during their passage through the plant. The results are based on published data and measurements carried out by the authors in an active medium. Reference is made to sampling problems and to the systems which have been developed. On the basis of this knowledge, the best methods for cleaning prior to VHE filtering are defined for typical gaseous effluents. Several systems (washing, mist separators, pre-filtering) are examined and their relative performance compared. Methods for testing their efficiency are described. The advantages and disadvantages of using test aerosols are also discussed, together with the problems of testing in a radioactive environment. (author)