In graphics: What went wrong at Fukushima?




How the problems unfolded

a. A magnitude 8.9 earthquake hit northern Japan on Friday 11 March and the nuclear reactors in operation at Fukushima Daiichi were shut down. Control rods were activated to stop the nuclear reactions that generate power.

b. However, power supplies were affected by the quake and resulting tsunami, meaning the cooling systems, which stop the fuel in the core heating up to unsafe levels, failed. Cooling is still needed even after a reactor is shut down because the fuel continues to give off enormous heat.

c. The lack of power meant water stopped circulating and began to boil, creating steam. As the water began to boil away, it is believed the rods were exposed to air above the water line, causing them to heat further, melting or splitting their zirconium alloy casings.

d. As the zirconium alloy reacted with the steam, it created hydrogen gas, which is highly volatile. As engineers tried to vent this gas outside the containment chamber to relieve the pressure, the gas exploded, blowing off the roof of the outer building.

e. Officials are now swamping the units with seawater as an emergency coolant, and are using boronic acid, which hampers nuclear reactions.


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