Wasn't it mostly Nina screaming for that though? It only recently occurred to me that she was telling Suzaku to use it in an area with a high percentage of Brittannian citizens.
As for the physics, it looked like a nuclear explosion quickly followed by implosion, which I seem to remember actually taking place in some nuclear detonations. Just not to that extent. It seems like perhaps the writers had a bit of commentary on dirty bombs and the problem with nuclear weapons, saying that if this happened it would have been better for Hiroshima's people.