The development and qualification of structural and plasma materials capable of withstanding the intense irradiation of a fusion reactor, without generating long-lived waste, is another major challenge for the post-ITER era.
The very energetic neutrons from the fusion reaction (14 MeV – Mega electron volts) that bombard the materials in the reactor wall disrupt the material’s crystalline lattice, creating structural defects. They can also react with the nuclei of the materials, producing gases (hydrogen and helium) within them. The quantities produced are a hundred times greater than those produced by neutrons from fission reactors, which are seven times less energetic. The accumulation of helium in materials weakens them and causes them to swell.
While the irradiation of the materials will lead to less than 3 dpa (displacements per atom) for the entire duration of ITER’s operation, it could produce more than 30 dpa per year during the permanent operation of a DEMO-type power reactor: each atom in the material will have been displaced an average of 30 times per year from its initial position in the crystalline lattice, causing major structural defects.
Today, some nuclear power plant components are subject to up to 80 dpa at the end of the installation’s life. The use of steels qualified for nuclear power plants is therefore largely appropriate for ITER. On the other hand, it will be necessary to increase the level of resistance for the materials expected in a DEMO-type installation, and in particular to minimise the effect of helium accumulation.
This issue represents a clear technological challenge for the scientific community, motivating its own research programme.
IFMIF-EVEDA: characterising fusion materials
The qualification of materials capable of withstanding the intense irradiation of 14 MeV neutrons is the main objective of the IFMIF (International Fusion Materials Irradiation Facility) irradiation facility project, the engineering phase of which (IFMIF-EVEDA, Engineering Validation and Engineering Design Activities) is being carried out under the “Broader Approach” agreement between Japan and the EU.
The aim of this instrument is to qualify advanced materials that can withstand the extreme conditions specific to future fusion reactors. IFMIF will consist of two deuteron accelerators, delivering beams with a total power of 10 MW in parallel and continuously onto a liquid lithium target, to generate an intense neutron flux (1017 neutrons/s) of 14 MeV. This value corresponds to the annual flux of future reactors.
To implement this ambitious project, we first need to build prototypes of the main sub-systems. This is the aim of IFMIF-EVEDA which, in addition to providing an engineering report, must validate the operation of the accelerator, the lithium target and the high-flux test modules. The activities, planned over a ten-year period, are shared between the coordination team based in Rokkasho (Japan), where the prototype accelerator (LIPAc) will be installed, and other teams spread between Europe and Japan.
Europe is supplying the vast majority of the prototype accelerator, with Japan essentially providing the testing infrastructure. Four European countries are involved: France (CEA-Irfu in Saclay), Spain (CIEMAT in Madrid), Italy (INFN in Legnaro) and Belgium (SCK-CEN in Mol). A European team based at Garching (Germany) within F4E is responsible for coordinating the design and construction of LIPAc.
