Near-field radioactive particle dynamics and empirical fallout patterns in Chernobylʹs Western and Northern Plumes

We compared geographically referenced ground-based measurements of gamma and beta radiation to model predictions of particle dispersion and estimate the influences of particle sizes, wind speeds, and vertical and lateral turbulence on the near-field (<12 km) fallout patterns resulting from Chernobylʹs first two releases of radioactive materials. Excellent conformity between empirical measures and model predictions was found when reasonable atmospheric parameters were assigned. Whereas particle size contributions to the two plumes were similar, atmospheric influences on particle flow differed markedly, resulting in rapid widening, meander, and diminished radiation doses in the Northern Plume relative to that for the narrower and intensely radioactive Western Plume. The Northern Plume is probably the result of multiple particle characteristics released simultaneously. Near-field depositions were due to particulate nuclear fuel probably ejected during helicopter drops of sand onto the burning reactor. Depositions much farther north of the reactor complex were likely due to smaller (1–3 μm) aerosols of volatile radionuclides. Predominant particle sizes in the near-field traces of both plumes were >40 μm in diameter. These large particulates eliminate passive dispersion of oxidized nuclear fuel and aerosol releases of volatile elements as the primary sources of contamination in the Northern Plume at distances <12 km from the Chernobyl reactor.