I was looking at the game info and was actually interested in figuring out how this would actually end IRL. Supposing this to be a huge nuclear reactor gone critical, it wouldn't look like a giant H-Bomb going off vaporizing everyone in an instant. Instead, the explosion would likely occur when the core fell through the cracked exterior and hit the underground water supply, irradiating the local water and making a massive explosion of radioactive water vapor.
Wikipedia is generally a good source for RP style investigations:
http://en.wikipedia.org/wiki/Radiation_poisoningThat I think would be the most likely cause of death and would most likely kill off about 50% of the people (so a coin would actually do) in severe cases. Sickness would induce 30 minutes after the event occurred (so we'd all think we were free and clear for a short time) and would consist of severe nausea and vomiting, disorientation, dizziness, fatigue, low blood pressure and head-aches.
The effects would severely hit everyone, but some would survive and some would die (perhaps the numbers would be higher in terms of percentage do to a lack of medical staff).
This still doesn't eliminate death by tumors or cancer. So ultimately, I think the reactor might take a while to kill us the thought... of course, that would be IRL.
It is expected though that all players would come down with severe Acute Radiation Exposure, resulting in those symptoms and a nasty ride. Surviving the explosion doesn't begin to put you in the free and clear.