>= ~5 Sv in an acute dose is generally fatal, but a man named Albert Stevens was injected with a small dose of plutonium in a human radiation experiment, and suffered a net dose of 64 Sv over a 21 year period before dying of heart disease. [1]
"Whatever doesn't kill you makes you stronger" has _some_ truth to it. The body adapts and learns from various harmful things, hence it's less likely to affect you as strongly in the future.
The same dose delivered fast does much more damage than the dose delivered slowly.
At least that's the theory. The NRC works on the principle that the speed doesn't matter - it's just the cumulative that counts.
It's not a settled matter, although the research does seem to point in that direction.