What is happening in quake-stricken Japan nuclear plant?

Technicians are battling to stabilise a third reactor at a quake-stricken Japanese nuclear plant, which has been rocked by a second blast in three days.

The fuel rods inside reactor 2 at the Fukushima Daiichi plant have been fully exposed on two separate occasions, raising fears of a meltdown.

Seawater is being pumped into the reactor to try to stop the rods overheating and the authorities have asked the US for assistance in managing the crisis.

David Shukman explains what is going on inside the plant, and assesses the potential risks from radiation.