<p>The Nuclear Engineering Laboratory of the National Technical University of Athens (NEL-NTUA) is among the oldest laboratories conducting radioactivity mesurements in Greece, founded in the early sixties. One of the main activities at NEL-NTUA is environmental radioactivity studies, mainly based on gamma spectroscopic analysis. For this purpose NEL-NTUA is equipped with a variety of Germanium detectors for in-vitro and in-situ measurements. Starting back in the early eighties, environmental radioactivity studies at NEL-NTUA were significantly boosted after the Chernobyl accident in 1986&#160; when they focused on the Chernobyl fallout radionuclides, as well as some natural radionuclides typically determined in environmental studies, namely <sup>232</sup>Th, <sup>226</sup>Ra and <sup>40</sup>K. As a result of these studies maps of nine Chernobyl fallout radionuclides and the three natural radionuclides in continental Greece surface soils were produced.&#160;</p><p>Since natural radioactivity in soil is in most cases relatively low, high volume samples had to be analyzed. Over the years, the acquisition of detectors capable of detecting low energy photons (LEGe) along with the development of techniques to correct for self-absorption of low energy photons within the sample, allowed for the accurately determination of radionuclides emitting such photons, like <sup>234</sup>Th (63.29keV), <sup>210</sup>Pb (46.52keV) and <sup>241</sup>Am (59.54keV). These newer studies showed that a significant disruption of radioactive equilibrium in surface soil between <sup>226</sup>Ra and <sup>210</sup>Pb is very common, while radioactive equilibrium disruption between <sup>238</sup>U and <sup>226</sup>Ra is common as well. It is interesting to notice that the mean activity ratio <sup>210</sup>Pb/<sup>226</sup>Ra as obtained from ~300 sample measurements is of the order of ~4, while the mean activity ratio<sup> 226</sup>Ra/<sup>238</sup>U was estimated to be around one. A mapping of radioactive equilibrium disruption that followed provided interesting results.</p><p>In the years to follow studies focused on the vertical distribution of natural (<sup>210</sup>Pb) and artificial (<sup>137</sup>Cs) radionuclides in soil and sea sediments and the study of radionuclides fractionation in soil as well as NORM. Both types of studies require the analysis of small volume samples &#8211; of the order of 20-50g or even less. Therefore, the development of techniques for sampling of soil vertical profile and the accurate analysis of small samples was of great importance. These analyses require high efficiency detectors, such as XtRa detectors, background reduction techniques, such as Compton Suppression Systems, optimized sample geometries for higher full energy peak efficiency. Sophisticated techniques for background determination and subtraction, in order to obtain accurate results for natural radionuclides which are often detected in the background, are also required.</p><p>Another field of research at NEL-NTUA is the development and improvement of techniques for monitoring of <sup>222</sup>Rn daughters outdoors using on-line detector systems, as well as for monitoring of natural and artificial radionuclides in atmospheric precipitations and aerosols (<sup>7</sup>Be, <sup>210</sup>Pb, <sup>22</sup>Na) using high volume air samplers.</p><p>Aim of this work is to present the research conducted at NEL-NTUA over the years with regard to the environmental radioactivity, as well as the current activities&#160;in the field and those planned for the future.</p>
