It's real. It looks like a model or some sort of CGI trick but it is utterly and terrifyingly real.
This is an Atomic Cannon test from 1953 in Nevada on 25 May 1953 and was the same type of bomb detonated over Hiroshima.
Those little dots in the foreground are actually vehicles - tanks and jeeps deployed to see how the blast would affect them.
Needless to say they were utterly destroyed...