Why new materials are vital for the future of fusion power

Materials for fusion will have to cope with intense neutron irradiation knocking every atom off its lattice site several times a year.  Displaced atoms (green) and vacant lattice sites (red) at the end of the collision phase of displacement cascades created by 5, 10 and 20keV atomic recoils at 600K.

Before commercial fusion reactors can be built complex problems in engineering and materials science must be solved. "So far, high-power plasmas have been run for tens of seconds at most," says Dr Steve Roberts of the University of Oxford, "in a power plant these have to be contained in a stable structure for tens of years." Dr Roberts is part of a research collaboration which involves the Universities of Oxford, Edinburgh, Cambridge, Liverpool, Queen’s University Belfast and UKAEA Culham. "One of the main problems we face is finding materials for the parts of a reactor that are subject to very severe neutron irradiation," he explains. The reactor walls and the ‘divertor’ that removes helium from the plasma will be bombarded with neutrons of higher energy than those found in fission reactors. "The damage produced will be such that every atom in these components will be knocked off its lattice site several times a year," Dr Roberts tells us, "without materials that remain mechanically stable under these conditions fusion power will not happen."


Because neutron irradiation turns many elements into dangerously radioactive isotopes only some materials are suitable: “Luckily, among the few that can be used are iron, carbon and chromium – so special steels are a possibility,” Dr Roberts tells us. Tungsten and vanadium alloys and ceramics based on silicon carbide are also being investigated. The damage produced by neutron irradiation is compounded by reactions between neutrons and the wall materials causing sub-micron bubbles of helium to form – the combination makes the materials more brittle. Nano-scale oxide dispersions may offer a solution to both problems but more basic research is needed in this area. Until the International Fusion Materials Irradiation Facility is built there is no way of physically testing materials in fusion power station conditions "we have to infer the behaviour of candidate materials from the behaviour of more lightly irradiated materials," explains Dr Roberts, "we use materials modelling techniques over a wide range of length scales from quantum mechanics to fracture mechanics."

Iron is one of the few elements that can cope with the harsh conditions inside a fusion reactor.  Here atoms in the core of a disclocation are shown interacting with a loop of interstitial atoms in iron under sress at 300K.

So far Dr Roberts and colleagues have focused on iron-chromium systems as the basis for steels to form ITER’s structural components. "The solubility of chromium in iron is not known with any certainty below about 400 Celsius – which is important as radiation damage will allow easy reversion of the alloys towards equilibrium" he comments. Recent quantum mechanical modelling indicates a dip in the bonding energy of alloys with a low chromium content and the project will be using this to predict the low-temperature region of the phase diagram. Experimental studies, funded by UKAEA Culham, are also being conducted on the flow and fracture of iron and binary iron-chromium alloys.