Pool of used fuel is stored on an upper floor of the reactor building in Fukushima

A design flaw by the Japanese builders appear to be at the heart of the crisis at Fukushima:

The extraordinary practice of putting the pool where the highly radioactive used fuel is stored on an upper floor of the reactor building, instead of, as is normal, having it at ground level.

This has made it particularly hard for workers to try to fill it with the water needed to isolate the fuel, keep it cool and stop it from melting – and is one reason why helicopters have had to be used.

General Electric "Mark I" Boiling Water Reactors comprise five of the six installed at the stricken complex in Japan, and all those so far hit by the world's first multi-reactor nuclear disaster.

General Electric offers in its instruction manual that the pool "is not above the reactor vessel and not designed to drain to the reactor vessel".

In addition to the danger caused by the building design, there are safety problems in the Mark 1 reactor type:

In 1976, three General Electric engineers resigned because: "the effects of a loss of coolant to the reactor core had not been fully taken into account. The result could tear the containment apart and create an uncontrolled release.

General Electric says that The Mark I meets all regulatory requirements and has performed well for over 40 years.

New reactors have been designed to be much safer, with the European Pressurised Reactor (EPR), planned to be built here over the next few years, especially so.

But official documents show that the EPR will produce several times more of the radioactive iodine and caesium that would be rapidly released in an accident than do present-day reactors.

Source: The Telegraph