Iter takes shape as major contracts look set to deliver

28 October 2010

A contract for the vacuum vessel and ramped up production rates for the super conducting magnet strands means that the next-step magnetic fusion reactor Iter is beginning to take shape. 


 Iter vacuum vessel 
A cutaway of the Iter Vacuum vessel (Image: Iter) 
The European domestic agency for Iter, Fusion for Energy, has signed a contract with European consortium AMW (Ansaldo Nucleare, Mangiarotti and Walter Tosto) for the supply of seven sectors of the tokamak's vacuum chamber. The 16 October contract is expected to run for six years and is worth €300 million ($417 million). It amounts to the single biggest work package Europe is required to contribute to Iter under the construction framework – both monetarily and in terms of sheer size.


In a tokamak the vacuum vessel is the toroidal (D-shaped doughnut) chamber in which the fusion reaction takes place. The walls of the chamber must be able to maintain a high quality vacuum pressure and must be able to support various important components, such as the blanket, divertor, vacuum systems and ports for various control systems. The vacuum wall is complex structure requiring precision engineering, and the contract awarded represents one of the most challenging and technically important of the whole Iter project.


The Iter vacuum vessel will have double steel walls and passages for essential cooling water. The chamber will measure a little over 19 metres across by 11 metres high, and house a volume of about 1400 cubic metres. Altogether, the nine required segments of the vessel will weigh about 5000 tonnes, which Fusion for Energy compared to the Eiffel Tower.


This vacuum vessel is twice the size and 16 times the mass of any other tokamak previously built. In terms of reactor physics, the size increase is a major part of the reason why Iter hopes to be able to deliver significant net energy returns when plasma testing begins in earnest about a decade from now.


Super conducting magnet milestone 


In further good news for Iter development, last month saw a significant milestone crossed in the production effort for the super conducting magnet as 100 tonnes of the Nb3Sn strands were tallied. Lengthwise this measures over 21,000 kilometres. In total the reactor will require about 400 tonnes of these multi-filament composite wires, which will be responsible for creating the multi-Tesla magnetic field required to contain the burning plasma.


Very few existing tokamaks have used super conducting magnets to date. They allow for a much longer plasma confinement time than is possible with other types of magnets which quickly overheat, but are also very expensive to produce. The Iter magnets are a very significant contributor to overall construction costs. 


Production of the strands is taking place in six of the Iter contributing regions including China, Japan, Korea, Russia, the EU and the USA. The first procurement arrangement was signed in 2007, but it took over 19 months to get the signatures of every domestic agency.


Increasing production rates and ensuring quality control has proven challenging. Prior to Iter the world capacity for Nb3Sn was only about 15 tonnes per year and none of the necessary quality assurance procedures were in place. Now, however, all of the suppliers have implemented an almost identical quality control regime, and production is on track to finish by 2013.


Researched and written
by World Nuclear News