Experimental investigation and numerical modelling of tritium wash-out by precipitation in the area of the nuclear power plant of Paks, Hungary

Complete text of publication follows. Tritium is an important radioactive isotope because its natural occurrence in the air and precipitation due to natural and artificial sources. In order to investigate the natural changes in tritium concentration, the artificial component has to be known. Some field experiments were carried out before to investigate the washout of tritium by precipitation emitted from artificial sources, but none of them were carried out around a real power plant, measuring the deposition pattern. We collected rainwater around the nuclear power plant of Paks (Paks NPP), and analyzed them for tritium. The rainwater samplers were constructed at ATOMKI. Their structure consists of a funnel, support parts and a long tube, acting as the storage vessel for the rainwater. One end of the tube is connected to the funnel, the other end is open. This way when 'new' rainwater falls, it pushes out the 'old' water, causing that always the last rainwater is in the tube (if enough rain is falling). In total 54 samplers were placed out around the Paks NPP in two half circles, with radiuses 400 and 800 m, pointing east. We collected samples after a rain period on 7-8 June 2009. They were prepared and measured with liquid scintillation counting (LSC) for tritium. We measured some samples also with the 3He-ingrowth method, to ensure better accuracy. The measurement results in Fig. 1 clearly show the trace of the tritium plume emitted from the plant. However, the highest values are not very high, compared to environmental levels, and considering the fact that all the samples were collected from the area of the plant. A numerical model was coded to calculate the washout of tritium theoretically from the meteorological and emission data, and to estimate the effect of the plant in larger distances. In Fig. 1 it is apparent that the model, however does not describe the data precisely, but gives reasonable results, especially for the outer circle. The calculations also show that the tritium levels are comparable with the environmental ones over some kilometers of distance from the plant.