There are two separate issues here. One is the safety of radiation. This is a medical question. It can be tested in the laboratory, but we have enough "field

There are two separate issues here. One is the safety of radiation. This is a medical question. It can be tested in the laboratory, but we have enough "field experience" with it by now to know that it is indeed Very Bad News(tm) in high doses. The second is the safety of nuclear power. This has many factors including the first issue and also including things such as evacuation response. Our ability to recognize threats (such as plainly unsafe radiation) and react effectively and rapidly is itself a reason that nuclear power is safe. Properly regulated nuclear power is safer than nuclear power that is not. Sort of like how cardiac arrest in Seattle is safer than it is in Detroit, since the EMS response time is about 1/3rd the time.