What are the immediate effects of acute radiation exposure?

I was reading about Chernobyl and the fire fighters who first contained the flames in the reactor. What are the effects of radiation exposure to an exposed core like they were exposed to? What happens to the body under EXTREME radiation. On a macro and micro level.So your body just can not replicate itself because of chromosomal damage? What about the deaths that occur in a couple of hours, what does the radiation do to the body to kill it in such a quick time?

It literally burns you at very high doses, and within a half hour, it causes extreme nausea and diarrhea. The radiation destroys nucleic in the cells, so tissues start to die off because cells can't replicate without DNA. Since most cells in the body only live for a few weeks, its a slow death as each type of tissue reaches the end of its cellular life span.