Impact of radon on personal contamination monitors at the exit of the restricted controlled areas in EDF nuclear power plant

Abstract

Background: Between 2008 and 2017, EDF has progressively replaced its old Personal Contamination Monitors (PCM) with a new generation to strengthen the control of the workers at the exit of the restricted controlled areas. One of the novelties of the new PCM generation is the introduction of gamma channels which, coupled to the beta channel, allows a more reliable and more precise measurement. The new PCM also includes a better monitoring of the C2 portals alarms triggering system, a greater detection surface to minimize the dead zone and carries out morphological compensation to take into account the attenuation of the gamma background by the body. This new PCM is very efficient and capable of detecting low contamination activities. The sensitivity of this monitoring system is such that the solid progeny of 222Rn deposited on clothes (such as the 214Pb and the 214Po) could in some cases be detected and triggered the C2 portals alarm. Objective: In order to quantify the impact of radon (222Rn) and its progeny on the C2 portals triggers, a specific study has been conducted. Beyond the physical aspect, several purposes are targeted: (1) to provide quantitative explanations about the C2 alarms attributed to radon and determine which channels are more affected by the 222Rn progeny; (2) to determine the existence of a relation between C2 alarms and high 222Rn volume activity; (3) to optimize practices at the exit of the Restricted Controlled Areas (RCA), for example, adapt waiting time before control in PCM. The study consisted in measuring the activity of radon progeny deposited on several types of suits used in EDF Nuclear Power Plants (NPPs). To assess the impact of the 222Rn, several factors have been studied such as the 222Rn exposure time, the 222Rn concentration and the nature of textile fibers constituting clothes. For this study, several facilities from the Laboratoire de Physique des deux infinis de Bordeaux (LP2i Bordeaux) have been used, including a 222Rn emanation standard source and various emanation chambers between 60 L and 750 L. A specific calibration method to characterize radon progenies deposited has been developed (see Sects. 2.4 and 3). The 222Rn concentration in the sample exposure environment is verified both using a commercial (see Sects. 2.4 and 3) and experimental high sensitivity radon detectors. The measurements of the 222Rn progeny on clothes are made immediately after exposure by gamma spectrometry control on a High Purity Germanium (HPGe) detector. This experiment has been carried out at the PRISNA measurement platform (Plateforme Régionale Interdisciplinaire de Spectrométrie Nucléaire en Aquitaine) located in LP2I Bordeaux. Conclusions: In this study, several phenomena have been highlighted for a better understanding of the C2 portal triggering. We validate that in presence of 222Rn volume concentration activity lower than 300 Bq/m3, an alarm in beta channels of C2 portals can be triggered because of the presence of the radon progeny on clothes. The triggering of the gamma channels could appear only if the volume concentration activity of radon is higher than 3000 Bq/m3. The results obtained in this study could led to optimize and harmonize the practices at the exit of Restricted Controlled Area (RCA) like adapt waiting time before control in PCM.