4.3.2 Nuclear fusion
When very high speed nuclei of light atoms collide and coalesce to form larger nuclei — nuclear fusion, any surplus mass is converted into energy according to the Einstein relation, E = mc2. Uncontrolled nuclear fusion is the basis of the hydrogen bomb and powers the Sun and stars, but can it be controlled and harnessed? From the 1950s onwards, lavishly funded research has pursued this goal, achieving the first controlled release of fusion energy from the Joint European Torus (JET) experiment in 1991.
The fusion reactions which have been studied are those of deuterium and tritium (the heavy isotopes of hydrogen, containing, respectively, one and two neutrons plus a proton) which fuse to produce either helium-3 or helium-4. For example:
Deuterium (hydrogen-2, ) comprises 0.015% of the hydrogen atoms in natural waters; the oceans contain 4.2×1013 tonnes of deuterium. In theory, this could produce 3.4×1012 EJ of energy if extracted and fused, a factor of 107 greater than the entire fossil fuel bank.
Tritium (hydrogen-3, H ) does not occur naturally, but is produced in nuclear fission reactors that use water as a moderator or coolant. It is also a by-product of the nuclear fusion process itself, when lithium atoms that form part of the containment system of the reactor capture neutrons liberated by fusion; tritium use in nuclear fusion could conceivably extend this source of energy almost indefinitely. However, tritium is radioactive with a half-life of 12 years, and bombardment of fusion reactor vessels by neutrons and other subatomic particles would give rise to a radioactive waste problem — though on a much smaller scale than that associated with fission reactors. The main technical problems in sustaining fusion reactions are:
- nuclei must be brought to within 10-15 m of each other before the strong nuclear attractive forces can overcome electrostatic repulsions
- to achieve the necessary kinetic energies, temperatures of about 108 °C are required.
The current design for a proposed International Thermonuclear Experimental Reactor (ITER) is for a high-temperature plasma confined by suspension in an intense doughnut-shaped magnetic field (known as a torus). Six partners (China, the EU, Japan, Russia, South Korea and the US) will fund ITER, with India expressing strong interest, but in early 2005 the reactor site had still to be agreed upon. Fusion research has three major objectives:
- break-even: when total output power = total input power. This was demonstrated at the JET experiment in the UK in 1997
- burning plasma: where the plasma is mainly self-heated by particle collisions, with little external power needed
- ignition: when the plasma generates so much energy that no input power is required.
A reactor would probably need only to achieve the first two aims to be commercially viable, and ITER is intended to demonstrate burning plasma for the first time. However, a commercial prototype reactor is unlikely to be operational until at least 2050, assuming that the research funding is maintained at current levels.