1514 GMT, 11 March 2011
Paul Marks, senior technology reporter
The Japanese government declared its first ever "state of nuclear emergency" today after two of its nuclear power stations suffered major failures in the aftermath of today's earthquake.
Quake damage cost one plant its reactor cooling system - an absolutely critical capability for a nuclear facility - while another suffered a fire in its turbine hall. Residents in a 3-kilometre radius of the coolant-afflicted plant have been evacuated.
Both incidents were considered risky enough to require the issuing of emergency alerts to the International Atomic Energy Authority in Vienna, Austria. However, the Japanese government insists this was precautionary and that no radiation release has occurred.
As the quake struck, seismic sensors triggered shutdowns at 15 of Japan's nuclear power stations. Eleven are thought to have resumed operation, but the four nearest the epicentre have remained shut - and the problems occurred at two of those.
After a nuclear plant is shut down, control rods are normally inserted into the reactor core to quench the fission reaction. But the reactor remains hot and still needs cooling.
At the Fukushima Daiichi nuclear power plant, located just to the south of the Miyagi district, which was worst hit by the quake and tsunami, electrical systems powering the plant's cooling system failed - and a backup diesel generator powering water pumps didn't cut in. The shut-down reactor remains hot.