The Japanese National Police Agency has officially confirmed 3,373 deaths, 1,897 injuries, and 6,746 people missing across sixteen prefectures, but estimated numbers are far higher, ranging from thousands to tens of thousands dead or missing.
The plants at Fukushima are Boiling Water Reactors (BWR for short). A BWR produces electricity by boiling water, and spinning a a turbine with that steam. The nuclear fuel heats water, the water boils and creates steam, the steam then drives turbines that create the electricity, and the steam is then cooled and condensed back to water, and the water returns to be heated by the nuclear fuel.
The reactor operates at about 285 °C.
The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with a very high melting point of about 2800 °C. The fuel is manufactured in pellets (cylinders that are about 1 cm tall and 1 com in diameter). These pellets are then put into a long tube made of Zircaloy (an alloy of zirconium) with a failure temperature of 1200 °C (caused by the auto-catalytic oxidation of water), and sealed tight. This tube is called a fuel rod. These fuel rods are then put together to form assemblies, of which several hundred make up the reactor core.
The solid fuel pellet (a ceramic oxide matrix) is the first barrier that retains many of the radioactive fission products produced by the fission process. The Zircaloy casing is the second barrier to release that separates the radioactive fuel from the rest of the reactor.
The core is then placed in the pressure vessel . . . designed to withstand the high pressures that may occur during an accident. The pressure vessel is the third barrier to radioactive material release.
The entire primary loop of the nuclear reactor – the pressure vessel, pipes, and pumps that contain the coolant (water) – are housed in the containment structure. This structure is the fourth barrier to radioactive material release . . . The containment structure is a hermetically (air tight) sealed, very thick structure made of steel and concrete. This structure is designed, built and tested for one single purpose: To contain, indefinitely, a complete core meltdown . . .
The earthquake that hit Japan was [about five] times more powerful than the worst earthquake the nuclear power plant was built for [8.2 vs 8.9] . . . .
Within seconds after the earthquake started, the control rods had [automatically] been inserted into the core and the nuclear chain reaction stopped. At this point, the cooling system has to carry away the residual heat, about 7% of the full power heat load under normal operating conditions.
The earthquake destroyed the external power supply of the nuclear reactor . . . For the first hour, the first set of multiple emergency diesel power generators started and provided the electricity that was needed. However, when the tsunami arrived (a very rare and larger than anticipated tsunami) it flooded the diesel generators, causing them to fail . . . . the reactor operators switched to emergency battery power . . . .
After 8 hours, the batteries ran out, and the residual heat could not be carried away any more. At this point the plant operators begin to follow emergency procedures that are in place for a “loss of cooling event” . . . .
[M]obile generators were transported to the site and some power was restored . . . . more water was boiling off and being vented than was being added to the reactor, thus decreasing the cooling ability of the remaining cooling systems. At some stage during this venting process, the water level may have dropped below the top of the fuel rods. Regardless, the temperature of some of the fuel rod cladding exceeded 1200 °C, initiating a reaction between the Zircaloy and water. This oxidizing reaction produces hydrogen gas, which mixes with the gas-steam mixture being vented.
One thing is certain: there are no easy options or magic cures, only intelligent, prudent choices on trade-offs and balances of risks. END
F/N: Daily Mail has an excellent summary here, with pictures and diagrams with explanations. For example: