Fusion as a future energy source

Abstract

FEATURES 32 Fusion Characteristics as an Energy SourceResources The potential energy resource from fusion is enormous, primarily because the energy produced per unit mass of fuel is so large, and this is the principal reason why fusion R&D has been pursued. If we were able to harness energy simply from the fusion of two deuterium atoms (D-D fusion), which is possible in theory but difficult in practise, we could supply mankind's energy needs for billions of years. At present, however, we have a much less ambitious target, the fusion of deuterium with tritium (D-T fusion), which is roughly 100 times higher in cross-section. As tritium decays with a half-life of 12.3 years, it is not abundant on Earth so the plan is for a fusion power plant to be self-sufficient in tritium, using the neutrons which result from the fusion reaction to convert lithium into tritium. With present estimated fuel reserves of lithium, there is enough to provide mankind's energy needs for thousands of years, by which time we may have solved the problem of D-D fusion.Designing a fusion plant to be self-sufficient in tritium is not a trivial problem. Although simply surrounding a fusion neutron source with natural lithium can, in theory, produce up to 70% more tritium than required, the practical reality is more difficult [1]. There needs to be a box structure to contain the lithium compound, there needs to be coolant flowing to take away the large amounts of energy deposited by the neutrons and we want to restrict the thickness of the structure to minimise the size of the containing magnets. In existing designs, changing the isotopic mix of the lithium and including a neutron multiplier such as beryllium or lead are used as tools to produce an optimised design, but this is far from complete and demonstrated. Tests of these fusion blankets are proposed for the ITER device.One aspect of tritium supply will relate to the growth phase of fusion power. To start up a new power plant, an initial inventory of tritium is required and this will have to come either from another fusion plant or from a fission plant. Particularly in the early implementation phase, it is important to minimise the inventory of tritium needed to start up a new plant whilst ensuring sufficient tritium is available. WasteOne of the differentiating factors of fusion when compared to fission is the waste products. Unlike fission, fusion reactions do not produce radioactive waste directly but the neutrons can cause radioactivity in the surrounding materials -hence the emphasis on developing and testing of low activation materials, those that do not generate long-lived radioactive waste [2]. The same happens in fission plants of course but there the structural activation is a very small part of the waste produced -in fusion it is the dominant part. One goal of the fusion R&D programme is to optimise the use of materials, for instance to minimise the need for repository storage, perhaps to zero, but this is not guaranteed. Optimising material properties is a key, and somewhat neg...