Around a third of the energy of an atomic bomb is released through thermal radiation, which travels at around the speed of light. This means that, even if you are standing thirteen miles away, you’ll still be temporarily blinded by the light of the blast.
If you were standing seven miles away, you might need to be treated for mild first-degree burns. But within five miles of the blast zone, third-degree burns are likely.
If you were any closer, death is almost certain. The centre of the Hiroshima bomb, dropped by the USA in the Second World War, was estimated to be around 300,000 Celsius. That’s about 250 times higher than the temperature at which most people are cremated.