http://www.huffingtonpost.com/marvin-resnikoff/fukushima-nuclear-meltdown-japan_b_835932.htmlThe slow motion events occurring at Japan's (or GE's) Fukushima reactor cannot be sugar-coated. It is a doomsday scenario unfolding.
Nuclear reactors are not the same as coal/oil/gas electricity plants. Unlike conventional plants, they cannot be turned off. So while brave workers were tending to Units 1, 2 and 3 reactors, attempting against all odds to keep the reactor from overheating, the fuel pool at Unit 4 was left untended; without makeup water to cool them, the fuel rods overheated. Above 1800 oF, an exothermic reaction, a fire, took place with the zirconium cladding around the uranium pellets. Zirconium burned, forming zirconium oxide and hydrogen gas, which then exploded and released radioactive cesium, a semi-volatile metal, to the atmosphere.
Near the plant, the radiation levels dangerously escalated to 400 milliseiverts/hour (or 40 rems/hour in U.S. parlance). Considering background is on the order of 1 milliseivert per YEAR, this means a yearly background dose every 9 seconds. Put plainly, workers at the Fukushima reactors are putting their lives in immediate jeopardy.
What is a fuel pool? MORE AT LINK
Each year a commercial reactor operates, approximately 30 tons of fuel are irradiated. Every year or year and a half, this fuel is moved to a fuel pool for safe storage. Under 20 feet of circulating and replenished water, the fuel is stored. Water shields the radioactivity and cools the fuel, which still gives off heat. If water is not resupplied, which apparently was the case at unit 4, the water levels decline, the fuel is uncovered and it overheats, leading to a hydrogen explosion.