11.47pm GMT: In the comments, many people are asking what the implications of the nuclear reactor power failure is. Given how little we are being told by Tepco and the Japanese government, it's hard to say. The increase in background radiation near the plant isn't dangerous at this point, but of course that could change.
Reuters has a piece explaining what happens when a reactor loses coolant:
When all sources of power fail like at the Fukushima Daiichi plant, coolant begins to boil off, exposing the fuel rods. It would likely take several hours to boil off enough coolant before the core is hot enough to damage it.
If the fuel rods became damaged they would release radioactive material into the remaining coolant.
- Hours beyond that the metal surrounding the ceramic uranium fuel pellets could potentially start melting if temperatures reach well beyond 1000F as occurred at Three Mile Island. The ceramic fuel pellets would not melt until temperatures reached about 2000 degrees F.
- Hours after the fuel rods started to melt, the heat could potentially melt through the reactor vessel, which is made of high tensile steel four to eight inches thick
- It would take even more time before the containment fails, which could result in radioactive releases to the environment. The containment is an air tight steel or reinforced concrete structure with walls between four to eight feet thick.
As the Three Mile Island accident showed, operators can regain control of the situation if they can get the coolant flowing again.
