Study of atmospheric dispersion of radioactive effluents under inversion condition at coastal station Kalpakkam for radiological impact

This study investigates the impact of inversion layers on the dose due to air-born radioactive effluents at tropical coastal site Kalpakkam. Observations from a meteorological tower, GPS Sonde and SODAR are used to identify the presence of inversion conditions. A high resolution mesoscale model WRF is used to simulate the boundary layer meteorological fields. Sensitivity tests with two PBL schemes indicated that the non-local scheme YSU better produced the observed inversion characteristics. Both simulations and observations indicated that inversion layers are marked with strongly stable atmosphere characterized by steep positive temperature gradients and high values of bulk Richardson number. Simulations using FLEXPART dispersion model for hypothetical tracer release on typical days indicated that inversion condition leads to roughly 3 times higher ground level concentration compared to daytime unstable conditions due to confining the plume to a shallow surface layer. FLEXPART simulations using WRF-YSU meteorological data produced roughly 2 to 3 times higher concentrations during inversion condition and roughly 0.5 to 0.7 times lower concentration during daytime unstable condition compared to MYNN. Simulations conducted with FLEXPART for routine Ar-41 releases from an operational reactor at the coastal site suggest that the plume gamma dose rate during inversion condition is almost 2 to 3 times more compared to daytime convective unstable atmospheric condition. Analysis of simulated particle positions suggested that inversion layers trap the radioactive effluents, thereby lead to relatively higher dose rates compared to unstable daytime conditions. The study shows that inversion layers cause unfavourable conditions for dispersion of radioactive effluents and increases the radiological impact.