14CO2 dispersion around two PWR nuclear power plants in Brazil

Atmospheric air samples were taken within 3 km from power plants encompassing five different distances and wind directions. Samples were taken between 2002 and 2005 aiming to evaluate the environmental 14C enrichment due to the operation of Brazilian nuclear power plants. The sampling system consisted of a pump connected to a trapping column filled with a 3 M NaOH solution. The trapped CO2 was analyzed for 14C by using a single stage accelerator mass spectrometry (SSAMS). All sampling sites revealed measurable 14C excess values. The maximum excesses were of 15 and 14 mBq/m3 for sampling sites placed at NE of the power plants, which is the main wind direction in the area. The mean excesses values were 12 mBq/m3 to the NE direction, 8 mBq/m3 to the E, 10 mBq/m3 to the N, 8 mBq/m3 to the WNW and 7 mBq/m3 to the W direction (increasing distances from NE to W). Compared to other Light Water Reactors (LWR) data, these means' values are significantly higher than the average worldwide reported value of 3 mBq/m3. Available data indicate that the observed values are not related to 14C emission by the power plants vent stack. Other factors, such as topography, seem to have an important role because it affects wind dispersion thus favoring 14C accumulation in the sampled area. Moreover, the high elevations around the power plants enhance the chances to measure high values of 14C since the plume can be intercepted before it is drawn to the ground. Modeling of the plume has shown that its dispersion does not follow a Gaussian model and that agreement between atmospheric CO2 and vegetation 14C activities occurs only for sampling sites placed at NE of the power plants.