This reconstruction of the radioactive plume was created by the Institut de Radioprotection et Sûreté Nucléaire (IRSN), the French Government's official agency on radiation and nuclear matters.
It is a graphic illustration of the vast extent of radioactive contamination of Europe (and eventually the rest of the Northern Hemisphere) by the developing Chernobyl catastrophe.
IRSN produced “The Chernobyl Plume: Modelling atmospheric dispersion of caesium-137 across Europe following the Chernobyl accident,” an updated simulation made in March, 2011 (French with English subtitles). It explains the path of the radioactive cloud over Europe between 26th April and 6th May 1986. The following text is mirrored from this film simulation:
In 2005, IRSN produced a simulation of the path travelled across Europe by the radioactive cloud folowing the Chernobyl accident.
This simulation was produced using a new generation of operational atmospheric dispersion models developed for use in case of nuclear emergency. To qualify this new long-distance dispersion model, IRSN experts applied it to the atmospheric release of caesium-137 caused by the Chernobyl accident, based on a reconstruction of the meteorological conditions observed in Europe in the days following the accident. The model calculated the distribution of air contamination at ground level on a European scale at 15-minute intervals, from 26th April to 10th May 1986. The computed results were then compared with actual measurements taken throughout Europe over the same period, showing satisfactory agreement between calculations and measured data.
See Also: graphic from Lawrence Livermore National Laboratory showing dispersions of Chernobyl radioactive cloud on 27 April and 6 May 1986. Graphic reproduced from page two of “The Chernobyl Catastrophe – Consequences on Human Health,” Greenpeace, 18 April 2006.