Routine testing of absolute filters in nuclear power stations evolution of the methods used in Belgium

The standardized DOP-methodology for in-situ testing was used since 1965 in Belgium for Q.A. of the filter systems in the nuclear facilities. The appearance of a second filtration stage together with more compact ventilation systems required a more sensitive methodology for in-situ testing of the filter system as a whole unit, without interruption of the process ventilation. Since 1981 investigation of the methodology with the single particle laser particle-size spectrometer has been undertaken. The paper under consideration reports the description of this methodology together with considerations on the gained experience. (author)