One of the limitations of fission power is that it depends on uranium (and possibly thorium) reserves, which are a finite resource. The utilisation of fast reactors and accelerator-driven reactors, especially if used in a thorium fuel cycle (since the reserves of thorium are greater those of uranium) would substantially increase the energy available from this resource, but nonetheless the statement remains true at least in principle. The goal of generating almost limitless energy from the fusion of appropriate light isotopes of hydrogen or lithium has been a dream of scientists for many years. This dream is not yet realised but it is deemed that sufficient progress has been made towards achieving controlled fusion, that fusion reactors deserve a mention in this introductory chapter on present generation reactors.

A significant problem in the development of a fusion reactor has been the confinement of the nuclei in order that the fusion reactor can proceed in a controlled manner. Fusion with a positive energy balance is only possible at very high temperatures. These must be so high that the thermal agitation of the atoms is sufficiently energetic that the electrostatic repulsion of the positively charged nuclei can be overcome, enabling collisions to occur.

A number of different fusion reactions have been postulated between the isotopes of hydrogen, helium and lithium. However, the majority of research efforts have concentrated on the deuterium-tritium reaction. This is the easiest reaction to achieve. Nevertheless, temperatures must be of the order of 100 million degrees.

There is also a confinement criterion, which requires that the period of the confinement time and the neutron density must exceed a stringent limit (Lawson Criterion).

Focus has concentrated on essentially two types of confinement, magnetic and inertial. Of these, magnetic confinement has received the most attention.

In magnetic confinement, a strong external magnetic field consisting of a high density of field lines is imposed. In a toroidal system, the field is circular such that the nuclei in the deuterium-tritium mixture travel in helical paths around the magnetic lines of force. This gives rise to the shape of a torus. In an ‘open’ system, the field lines are not closed but a series of magnetic coils are arranged to reflect particles back into the centre of the field. These are referred to as ‘magnetic mirrors’. The challenge with either of these methods is that the plasma should not contact the confining vessel, otherwise the temperature will fall.

In inertial confinement, pellets are made from a mixture of deuterium and tritium in a mixture frozen at about 15 K. These are then irradiated either by very powerful laser beams or by electron (or ion) beams. These compress and heat the material to fusion level temperatures; inertia results in very high densities for very short periods of time (order of a nanosecond). However, there are practical difficulties with this approach associated with the laser efficiency and engineering problems in achieving a continuous power output.

With regard to on-going research, the Tokamak system has probably attracted the most attention. The ideas were originally conceived in the former Soviet Union. The system is based on the closed magnetic field configuration in the shape of a torus. The Joint European Torus (JET) project in the UK has made progress in generating significant amounts of power, in 1991, 2 MW were achieved. However, the break-even point, i. e. the generation of as much fusion power as is required in heating up the plasma has not yet been achieved. The trend is generally for larger Tokamaks in the quest to achieve higher and higher temperatures and conditions that will satisfy the Lawson criterion.

In the US, the Lawrence Livermore Laboratory and the Los Alamos Laboratory are carrying out work on inertial confinement. Lower fractions of energy produced against input have been produced in comparison with the Tokamak approach.

Significant progress in fusion technology has been achieved to date and these have been described in this chapter. For the next generation of Tokamaks, the resources of the interested nations are likely to be pooled in the International Tokamak Experimental Reactor (ITER) Project.