The question of what material or structure could withstand the intense radiation in a fusion reactor is just one of the challenges to be faced on the road to fusion-based energy production. The answer could be a wall of liquid lithium.
Nuclear fusion could provide an effectively limitless source of safe, clean energy, but there are a number of formidable technical challenges to be met before a fusion-based power plant can be built. Writing in Nuclear Fusion, Richard Majeski and colleagues1 report progress in the use of molten lithium metal to absorb the large flux of high-energy neutrons given off by a fusion plasma.
When two hydrogen atoms fuse to produce a helium atom, the net difference between the combined mass of the hydrogen atoms and the resulting helium atom is converted directly into energy. But in order to make two hydrogen atoms fuse, the significant Coulomb repulsion between their nuclei must be overcome. In the Sun, for example, this is achieved by the immense temperatures and pressures at its core. For an Earth-based fusion reactor, one possibility to recreate these conditions is in a magnetically confined hydrogen plasma.
To reduce the energy required to fuse two atoms in such a plasma, hydrogen is replaced by its isotopes deuterium and tritium. In addition to producing helium, fusion of deuterium and tritium also produces free neutrons, which carry off much of the excess energy produced in the reaction. This provides a convenient means of extracting energy from the plasma, but raises the problem of how to handle the flux of about 1019 high-energy neutrons per square metre that a commercial reactor is expected to emit during just one second of operation.
There have been many suggestions for how to build the first wall of a fusion reactor to withstand the intense flux of neutrons. These solutions commonly involve making the wall out of solid tungsten or carbon, owing to their strength and high melting points. However, radiation-induced structural degradation would mean than any solid wall is likely to need replacing regularly, which could be hazardous and expensive.
To avoid such problems, Majeski et al. investigate not a solid plasma-facing structure, but a liquid — specifically, liquid lithium. Lithium will be an important ingredient in the operation of any commercial reactor, as it is used to produce tritium (which does not occur naturally) through its reaction with neutrons. In their latest research the authors study the effects that introducing a pool of lithium held in a tray has on a hot deuterium plasma. Their results are encouraging: they see improvements in the plasma's characteristics including a 25% increase in its electron temperature and a fourfold reduction in the loop voltage required to maintain the plasma.
Majeski, R. et al. Recent liquid lithium limiter experiments in CDX-U. Nuclear fusion 45, 519–523 (2005) doi:10.1088/0029-5515/45/6/014
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Gerstner, E. Molten radiation shield. Nature Phys (2005). https://doi.org/10.1038/nphys003