Aerosol and fallout monitoring in France from 1959 to 2014: 55 years of improvement of the surveillance, from the sampling to the measurement

As a support for the public authorities the French Institute of Radioprotection and Nuclear Safety (IRSN) is in charge of the environment surveillance in France. The OPERA-Air (Observatoire Permanent de la Radioactivite) sampling network is part of this monitoring activity and includes, in addition to 40 low flow (10 to 80 m3.h-1) samplers, 8 high and very high flow (namely 300 and 700 m3.h-1) aerosol samplers. This network also comprises rain and cloud water samplers in order to observe and study the atmospheric fallout in a general way. This paper will focus on these last kinds of samples and the aerosol filters taken by the high flow and very high flow samplers and measured at trace levels in the laboratory of environmental radioactivity measurement by low level gamma ray spectrometry. The developments made during the last decades both in the sampling process and in the measurement techniques will be described. The improvement on the air samplers with higher flow, larger surface or thinner filters for a smaller counting geometry went in concert with the metrology improvements, with new measurement equipments such as low background detectors, well-type detectors or anti cosmic set up. All these implementations were and are still made in order to deal with the decreasing anthropogenic radioactivity levels in the environment, especially in its atmospheric part, and to keep the OPERA-Air network meeting its dual purpose of surveillance as well as research. The improvements in sampling and measuring will be presented as well as some results obtained thanks to them: - Determination of 137Cs at trace level in fog/clouds waters in order to study the rainout scavenging mechanism and to improve our capability in modeling radionuclide deposition. - Determination over France of trace elements at a few μBq.m-3 during the Fukushima Daiichi accident such as 129mTe-129Te, 132Te-132I, 136Cs and 140La. - Detection of 134Cs due to the Fukushima accident in the rain water samples until November 2012 giving information on the scavenging efficiency of precipitations below the cloud base (washout mechanism). - Detection of 131I traces in the air due to abnormal releases from a Hungarian facility. - Determination of the size distribution of airborne radionuclides soon after the Fukushima Daiichi nuclear accident in France or during normal operation in the vicinity of French nuclear facility in order to improve our modeling capability and deposition calculations. Document available in abstract form only. (authors)