Fukushima2

What went wrong at Fukushima?

Officials in Japan are scrambling to cool a second nuclear reactor at Fukushima 1 power plant following a huge blast on Saturday.

The explosion blew apart the building housing reactor No. 1, where technicians had been venting steam. Reactor No. 3 is now the subject of concern and a state of emergency has been declared.

Cooling systems to both reactors failed after the 8.9-magnitude quake struck off the north-east coast on Friday.

A state of emergency has also been declared at a second nuclear facility, at Onagawa, after excessive radiation levels were recorded. But Japan's nuclear energy agency said the rise in local radiation levels might have been caused by the Fukushima leak.

So, what went wrong at Fukushima?

1. The cooling systems, which stop the fuel heating up to unsafe levels, failed at two of Fukushima's reactors - 1 and 3.

2. This meant water stopped circulating and began to boil, leading to a rise in pressure.

3. At reactor 1, it is believed this rise in pressure caused the casing around the fuel rods to become damaged. When they came into contact with water, it created hydrogen gas.

4. As engineers tried to vent this gas outside the containment chamber to relieve the pressure, the gas exploded when it came into contact with oxygen. The containment chamber was not damaged, but part of the outer concrete shell was blown off.

5. Fuel rods inside reactors 1 and 3 continue to heat the water and engineers are using sea water as an emergency coolant.