So, given that human body area viewed from the front is around 0.5 m^2, then a 70 kg human in space 1000 miles from the explosion would get 6.4 kJ * 0.5 / 70 kg

So, given that human body area viewed from the front is around 0.5 m^2, then a 70 kg human in space 1000 miles from the explosion would get 6.4 kJ * 0.5 / 70 kg = 45 gray. That would mean that even being a couple thousand miles away could be fatal. But that's assuming 100% of Tsar Bomba energy would be ionizing radiation, I don't know what % would that be in reality.