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Experts say it's likely the uranium fuel in Units 1 and 2 partially melted down after coolant pumps stopped working. Reactors going into service now have cooling systems designed to work for days even if pumps fail. In advanced reactors, not yet on the market, even a total loss of cooling would not cause a meltdown—the fuels are designed to withstand intense heat.
As pressure rose, operators vented some radioactive steam into the atmosphere to prevent the primary containment chamber from exploding. New reactors have stronger containment vessels that can withstand pressure from reactor overheating.
The 40-foot-deep pool in which spent fuel rods are stored in Unit 4 apparently ran dry. It is located outside the primary containment vessel at a quake-vulnerable elevation. Hydrogen, apparently produced by chemical reactions with the exposed rods, exploded on Mar. 15, and a fire broke out. New designs place fuel pools lower, but still outside of containment. Many futuristic designs don't use fuel rods at all. Pebble bed reactors, for example, store used fuel in a lead-lined chamber that doesn't need cooling.