Radioenvironmental survey of the Megalopolis power plants fly-ash deposits

The Megalopolis lignite field basin is located in the centre of the Peloponese peninsula in southern Greece. Two lignite-fired power plants are in operation in this region: Megalopolis-A (600MW 3 units) since the early 70's and Megalopolis-B (300MW 1 unit) since the early 90's. An extensive research project for the determination of the natural radioactivity of lignite and ashes from the Megalopolis power plants started in the Nuclear Engineering Section of the National Technical University of Athens (NES-NTUA) in 1983. The project has evolved to an integrated radioenvironmental survey of the Megalopolis lignite field basin area. The present work aims at the presentation of the radioenvironmental survey of the fly-ash deposits in the vicinity of the power plants. The power plants produce over 2 million metric tonns of fly-ash annually, which is primarily disposed off in deposits located mainly at exhausted lignite mines. These deposits are in the form of ash layers several meters thick, which are then covered by a layer of soil. The disposed fly-ash has a 226Ra content which sometimes exceeds 1 kBqkg-1, which is very high compared to the mean 226Ra radioactivity of surface soils in Greece (25 Bqkg-1). Results regarding (a) γ-dose rate, (b) radon concentration in the ambient air, (c) soil radon exhalation rate and (d) soil gas radon concentration are reported from systematic field measurements at three deposit sites. Repeated measurements under various environmental conditions have been carried out, in order to estimate the variability of the radon related quantities. In addition, soil sampling of the surface and 0-80 cm layer has been conducted at the deposits to allow for the determination of the activity of natural radionuclides, namely 226Ra, 232Th, 40K, 234Th and 210Pb. The radiological characterization of the fly-ash deposits obtained through these results is compared to the natural radioactivity background in the wider Megalopolis area, using additional results of a previous similar survey by NES-NTUA [1]. It can be concluded that, from the radiological point of view, the Megalopolis fly-ash deposits do not differ significantly from the rest of the Megalopolis lignite field basin, despite the fact that most of the underground soil layers consist of fly-ash with high 226Ra content. A possible explanation for this might be that radon emanation from the fly-ash layer is as low as radon emanation from natural soil due to fly-ash crystallization process in the power plant furnace. This explanation is currently under experimental investigation in the NES-NTUA Laboratory. Furthermore, the comparison of results from different aged deposition fields concludes that aging and restoration processes in the deposition fields, such as tree cultivation and agriculture have a positive effect to radon related radiological parameters.