1092 C. Llewellyn Smith and S. Cowley reasonably possible. But, leaving aside solar power for the moment, it is almost impossible to imagine renewables providing more than half of the energy currently supplied by fossil fuels. If we ask what is needed to take over the role of fossil fuels, there are only three candidates with sufﬁcient resource for the long term—solar, nuclear ﬁssion with uranium or thorium breeders and nuclear fusion. All three require substantial research and development before they are ready to be deployed on a large scale. Research on all three options should be a priority. In this paper, we focus on the promise, status and challenges of developing fusion as a power source. 2. The promise of fusion There are four key attributes that make fusion a desirable technology for power production. (a) Essentially unlimited fuel supply The fuels for the easiest fusion reaction are deuterium and tritium which can be ‘bred’ from lithium. There is enough deuterium in water for billions of years of world energy consumption and enough easily mined lithium for several hundreds of years. One kilogram of natural lithium yields about 3 × 1013 J or equivalent to 9 × 105 l of petrol. When lithium becomes scarce on land, it could be extracted from water, which contains enough to power the world for a few million years (Fasel & Tran 2005). (b) Safe operation The fusion reacting ‘plasma’ in a fusion power plant would contain insufﬁcient fuel to produce a dangerous runaway reaction or cause serious damage to the plant. Should the cooling circuit fail completely, radioactivity in the walls would continue to generate heat, but the temperature would peak well below the temperature at which the structure could melt. Fusion power plant designs minimize the inventory of radioactive isotopes (such as tritium) so that the worst imaginable accidents or incidents (such as earthquakes or aircraft crashes) would not require evacuation of the neighbouring population. (c) Low land use A fusion power plant would occupy roughly the same space as a ﬁssion or fossil fuel plant. Because of their intrinsic safety fusion plants could be placed on sites relatively close to population centres and other major consumers of energy. (d) Minimal waste and low environmental impact Although the by-products of fusion (helium and neutrons) are not radioactive, the structure will become activated when struck by the fusion neutrons. With appropriately chosen structural materials, however, the radioactive products will decay with half-lives of the order of 10 years, and all the components could be recycled within 100 years. Phil. Trans. R. Soc. A (2010)