What is a millisievert compared to a rad?
100 Rad = 1 Gy
1 Gy = 1 J/kg
1 Sv = 1 J/kg * w (w is radiation type scaling for alpha/beta/gamma high/low energy particle/wave)
So for gamma ray w is 1. Milli is 1/1000th
100 Rad = 1 Gy = 1 Sv * 1
1 millisievert of gamma ray is 0.1 Rad
International Commission on Radiological Protection recommended limit for volunteers averting major nuclear escalation: 500 mSv[9]
1 mSv = 100 mrem = 0.1 rem
500 mSv = 500 * 100 mrem = 50,000 mrem
1000 mrem = 1 Rem
So if they only (slightly) exceeded 50 Rem, that is not a serious life threatening dose with medical care.
As a US Nuclear Worker, my proceedures governing an emergence dose suggest I should not exceed a dose about 1/2 of what international guidance is for an emergency planned situation.
For argument sake, 100 Rem dose to put water back in the spent fuel pool would be a reasonable risk/reward for what I know to be true. With medical care, I would expect to have no long term effects from that dose.
Until you get up above 500 Rem dose you are not talking about a death sentence. This is VERY SERIOUS and would statistically kill some.
With high-level medical intervention, the LD100 (lethal dose 100% of the population) can be
increased to ~850 R.
http://www.nucmedconsultants.com/tutorials/nrc/gen1.htm