Engineers planning the world’s largest nuclear fusion reactor have completed designs for the system’s most technically challenging component, known as the ‘blanket’.
The team at ITER based in southern France are hoping to build the first experimental nuclear fusion reactor to generate more energy than it consumes, with the aim of creating a power source that doesn’t produce carbon dioxide or large amounts of long-term radioactive waste.
The proposed blanket system that will line the inside of ITER’s doughnut-shaped 500MW tokamak reactor chamber overcomes the major challenge of how to absorb some of the 150 million °C heat that will be generated by the fusion reaction while containing the radiation produced.
ITER’s engineers say the blanket is the last major component to be designed and completion will allow the project to move to the main manufacturing stage, with procurement due to start later this year and eventual assembly of the blanket scheduled to begin in May 2021.
‘The blanket modules have to be designed to withstand electromagnetic loads on top of the already quite heavy thermal loads,’ ITER’s head of internal components, Mario Merola, told The Engineer. ‘This makes the design of the blanket system one of the most challenging of the whole ITER machine.’
The reactor will mirror the process that generates energy in the Sun: two isotopes of hydrogen are heated to extreme temperatures so they become ions (plasma) and then collided and fused together, releasing a fast-travelling neutron that transfers energy as heat.
The blanket is what will capture this energy. It will consist of 440 four-tonne modules covering a total surface of 600m2, each comprising a beryllium ‘first wall’ containing a water-cooling system to contain the plasma and absorb the heat, and a water and steel shield block to absorb the neutrons themselves.
Each first wall panel will be made from a number of ‘fingers’ attached to a central backbone through which the pressurised cooling water will flow, passing over an array of cooling fins and reaching temperatures of up to 150°C.
‘[This means] if during the manufacturing something goes wrong, at worst we reject the finger and not the whole part, which would be quite valuable,’ said Merola.
Six additional modules will be located in ports around the middle of the chamber that will test ways of releasing or ‘breeding’ tritium, one of the hydrogen isotopes used in the fusion reactor, by reacting the plasma with lithium.
Test ideas include breeding tritium from lithium held in ceramic material or from a liquid lithium lead compound. Future commercial-scale reactors would likely include tritium breeding on each of the blanket’s modules.
Previous designs for the ITER blanket featured two dedicated components for limiting the plasma but the engineers decided this would not provide enough flexibility and so redesigned the system so the entire blanket would act as a physical boundary to the plasma.
This meant each module had to be shaped to produce a curved surface so if there were any manufacturing misalignment the edges would not protrude with a sharp leading edge into the plasma and risk damage.
This issue also led the engineers to separate the shield block from the first wall so the more vulnerable, plasma-facing element could be replaced if needed without removing the entire module. Each panel is expected to be replaced at least once in ITER’s lifetime.
The first wall panels will need to absorb both the radiative heat on the surface at an intensity of up to 5MW/m2 – around 5,000 times more than that felt on a summer’s day outside in the south of France – and the volumetric heat transferred by the neutrons, which carry around 80 per cent of the energy produced by the fusion reaction.
The blanket also needs to cope with electromagnetic loads, forces exerted by magnetic fields interacting with the currents induced when the reactor shuts down abruptly.
Once the first wall has captured the neutrons’ heat, hydrogen and iron atoms contained, respectively, in the shield block’s optimal combination of water and steel will absorb the neutrons themselves.