http://www.bbc.co.uk/news/world-asia-pacific-12911190

7 April 2011 Last updated at 11:35 GMT

Japan's nuclear reactor: radioactive leaks

As work continues to bring the Fukushima Daiichi nuclear plant back under control,Reactors 2 and 3, are the most likely sources for the main leaks of radioactivity.

Reactor 2: On 6 April, Tepco announced a leak of radioactive water had been stopped. They saidengineers had injected chemical agents to solidify soil near a leaking crack, 20cm long.

Earlier attempts to plug the leak had failed. This leak was believed to have been the source of high levelsof radiation found in seawater near the plant.

It was thought that radioactive steam had been flowing from the core into the reactor housing and throughcracks in the water-filled suppression chamber beneath the reactor.

Radioactive water in a tunnel underneath the reactor and the turbine building (see diagram below) hadprevented workers from gaining access. This tunnel emerges at the front of the building as a trench - just55 metres from the sea.

Reactor 3: Uses more plutonium in its fuel rod mix than the other damaged reactors. Small amounts ofplutonium have been found in soil at the plant indicating this reactor may have suffered a partialmeltdown. Highly radioactive water has been detected here; engineers are planning to remove it forstorage in waste buildings.

Decommissioning the plant

This involves removing the fuel rods, storing them and then reprocessing them at another plant. Then allnon-core buildings will be demolished. Core buildings (which house the reactors) have to remain in tactfor at least 40 years because dismantling them would release more radioactive material. Some Japanesenuclear experts are considering whether to cover those core buildings with a special material to stop thespread of radioactive substances.

Reactors 5 and 6 would re-open at some point after consultation with local residents.