As another note, when a reactor melts down, its not the same as a nuclear bomb. It doesn't catastrophically cascade and explode (Chernobyl was a steam explosion, Fukushima was from hydrogen gas.) It degrades and releases radioactive isotopes, much like a dirty bomb, minus the explosive. It is dangerous, but not completely-level-everything-in-X-miles dangerous. Considering the amount of fail-safes that have to simultaneously fail, or the level of sabotage that would have to be done to even cause a meltdown, nuclear power plants are safer than warp cores in Star Trek.
Now for A. The radiation most often used in a fission reaction is gamma radiation, the most penetrative, which is the reason for massive amounts of concrete and/or lead. But, the most dangerous by far is ingestion of either alpha particles or (as previously pointed out), radioactive isotopes. Gamma produced by NP is avoidable (the farther the better), preventable (the thicker and denser the shielding, the better), and temporary (once the source is gone, no further exposure.) Any ingested isotope or alpha particle stays and continues to cause damage, and considering it is continuing to release radiation, the only way to prevent getting worse is somehow to remove it from the body. Now, before people get completely scared, many scientific studies show that you naturally have some damn-near harmless isotopes in you already (Carbon 14 for example,), and some even suggest small amounts of radiation over an extended period can be beneficial.