Image caption The steel vessel will eventually weigh some 5,000 tonnes

The contract has been signed that will lead to the production of the biggest component in the Iter fusion reactor.

The multi-billion-euro facility being built in France will attempt to harvest energy by exploiting the same nuclear processes that power the Sun.

AMW, an Italian consortium, will construct most of the doughnut-shaped vessel at the centre of the reactor.

The Iter reactor facility due to be constructed is shaped like a doughnut - a Russian-conceived design referred to as a tokamak

Deuterium and tritium - two types, or isotopes, of hydrogen nuclei - are fed into the reactor and heated to 100 million Celsius

A powerful magnetic field holds this hot plasma, or gas, away from the walls of the reactor and squeezes it to initiate fusion

Iter hopes to do this in bursts of 500 seconds; a commercial reactor would have to run for prolonged periods

In a commercial reactor, energetic neutrons are absorbed in a surrounding 'blanket' to drive a steam-turbine system

Iter is not expected to begin operations until much later this decade.

Even then, these will be shake-down tests; full fusion power will not be achieved until the 2020s.

In a fusion reaction, energy is released when light atomic nuclei - the hydrogen isotopes deuterium and tritium - are fused together to form heavier atomic nuclei.

To use controlled fusion reactions on Earth as an energy source, it is necessary to heat these gases to temperatures exceeding 100 million Celsius - many times hotter than the centre of the Sun.

This will be done inside a vacuum vessel. AMW has now been given a 300m-euro contract to make the basic shell of this device.

It will be composed of nine steel sectors, each 13m high, 6.5m wide and 6.3m deep. When bolted and welded together, it will weigh some 5,000 tonnes. AMW will provide seven of the sectors; two will come from South Korea.

Iter is designed to produce 500MW of fusion power during pulses of at least 400 seconds. Critically, the machine is expected to demonstrate the principle that it possible to get far more energy out of the process than is used to initiate it.

The original plan was to build the experiment within 10 years for a budget of 5bn euros. But a range of issues, from technical to personnel matters, have conspired to inflate Iter's final price.

Many commentators now expect it to be in the region of 15bn euros. Europe is responsible for 45% of the total cost which the European Commission says will amount to 6.6bn euros.

Iter's six other partners - China, India, Japan, South Korea, Russia and the US - are each contributing 9% of the final cost.

Image caption Construction of the Iter reactor will take place at Cadarache in Southern France

Jonathan.Amos-INTERNET@bbc.co.uk