- thermal radiation is, in fact, light, which travels at the speed of light - but you should not imagine the entirety of the thermal energy to be released by a nuclear weapon to be released instantaneously in a radially travelling sphere. that being said, youre not going to outrun such an explosion. where you are at the time of detonation pretty much determines your odds of survival.
Maybe not. Richard Feynman claimed to have watched a nuclear explosion with nothing but regular glass between him and the blast. He assumed the glass would block any ultraviolet light and he was not blinded.
http://calteches.library.caltech.edu/34/3/FeynmanLosAlamos.h...
What kills the most people depends on the size of the bomb. For really small ones (like the US Army's nuclear bazooka from the 50s) the prompt radiation has the greatest lethal radius. For medium sized ones like Hiroshima, blast is the major one. For really big ones, thermal effects reach the farthest.
