What are the immediate effects of acute radiation exposure? such as in a nuclear reactor accident?

Depends on the dose: Massive exposure will cause immediate death, as in hiroshima, moderate exposure will cause various carcinomas and other complications.

Depends on how much.: A massive amount of radiation would cause death, but it is important to remember most scenarios have much less exposure than that. Even the workers at the fukushima plant have had very few health problems so far. Compare that with the 400 people who will die from tylenol (acetaminophen) overdose in the us this year. (and yet i still take tylenol (acetaminophen) myself if i need to, and we all still feel it is generally safe.).

Nuclear reactor: Core melt down spews a number of products skyward. The most prominent 2 are iodine isotopes, leading to increase in thyroid cancer. Iodide or iodine pills 'protect' by consuming the absorption traps, blocking the isotopic dangerous ones. The other is strontium isotopes, which as a divalent cation long acting cation like calcium, can deposit in bones.