From Views of the World, the blog of Benjamin Hennig, Senior Research Fellow in the School of Geography and the Environment at Oxford University:
The explanation, from his blog:
The cartogram shown above displays locations of nuclear power plants in Europe taken from an IAEA database of nuclear reactors published by the Center for International Earth Science Information Network (CIESIN) at Columbia University. This database includes facilities which are at varying stages of decommissioning which is a time-intensive and expensive process due to its continuing hazards.
In addition to the locations of the nuclear plants, circles of 20/30 and 80 kilometre distances are drawn around as the immediate risk zones. The underlying basemap uses a gridded cartogram based on equal population projection to put the differing exposures of populations into perspective. Each circle of equal distance is resized relative to the number of people living in the vicinity of each nuclear power plant. The locations of the most severe incidents above INES level 5 that happened in Europe are highlighted in the map, including the Chernobyl disaster which happened 30 years ago. An additional inset map shows the distribution of nuclear sites in the United Kingdom on a conventional map in comparison to a gridded population cartogram in more detail.