Static Fusion

Fusion reactions occur only at very high temperatures and most

readily between deuterium and tritium ("heavy" and "super-heavy"

hydrogen) producing a helium nucleus and a neutron (Figure 1.1).

The reaction releases energy, mainly carried by the neutron, and this

energy would be used to generate electricity and possibly hydrogen in

a fusion power station. Deuterium (D) is easily extracted from water,

and tritium (T) would be made by the neutrons hitting a blanket

containing a lithium compound around the very hot, fully ionised D-T

gas ("plasma"). As the energy released is about ten million times as

much as from a chemical reaction, the amount of fuel required is

correspondingly less; half a bath of water plus the lithium in one

laptop battery would produce 200,000kW-hours of electricity - the

same as 70 tonnes of coal, and equal to the UK's per capita

electricity consumption for 30 years. Fusion reactions do not produce

greenhouse gases, and the materials used in fusion power stations

could largely be recycled, thus minimising waste production.

Deuterium

Tritium Neutron

Helium

+ energy (17.6 MeV)

Figure 1.1 The deuterium - tritium fusion reaction

The stars use gravity to confine the plasma needed for fusion. On

earth, the most promising method uses a "magnetic bottle" to keep

the plasma away from material surfaces, which if contact were made

would cool and pollute it. (Another approach, "inertial fusion", is also

pursued, with contributions from scientists at the Rutherford Appleton

Laboratory described in chapter 2.) The temperature required is

about 150 million oC, ten times hotter than the centre of the sun,

which is routinely achieved in JET. The most developed type of

"magnetic bottle" is called a tokamak. JET, MAST and ITER are all

tokamaks, though MAST is a more compact ("low aspect ratio")

version called a "spherical tokamak" (Figure 1.2).

Annual Report of the EURATOM/UKAEA Fusion Programme 2004/05

1.3

1 Executive Summary

In the tokamak strong magnetic fields are produced by currents in

coils surrounding the plasma, and by a current flowing in the plasma

itself. Other essential ingredients are high vacuum conditions,

powerful heating systems (using high energy beams of neutral atoms,

radiofrequency waves and microwaves) and, to measure the plasma

performance, a wide range of instrumentation ("diagnostics"). The

importance of a number of key issues for fusion plasma performance

are summarised in Appendix A. These are:

* Confinement: the losses of energy and particles from the

plasma must be minimised for an efficient system

* Stability: when operating limits are approached the plasma

can become unstable. It is important to avoid this while

maximising performance, especially the pressure of the

plasma as the fusion power released is proportional to this

squared.

* Exhaust: the edge of the plasma must be sufficiently cool

where it meets material surfaces to ensure that damage to

these surfaces is minimal and pollution of the plasma by

impurities is low.

* Steady-state: ideally, a fusion power station would operate

continuously (in "steady-state"), and so advanced operating

modes of the tokamak are being investigated which might

allow this.

* Optimum configuration: while the JET/ITER-like tokamak is

the most developed system, other magnetic configurations

have advantages. In Europe, the more compact "spherical"

variant of the Tokamak is investigated at Culham, and the

stellarator is studied in Germany and Spain.

Figure 1.2 MAST

Annual Report of the EURATOM/UKAEA Fusion Programme 2004/05

1.4

1 Executive Summary

As well as plasma physics, fusion research addresses the wide range

of technology and materials required for the many components that

would surround the plasma in a power station (Figure 1.3). The

issues on which UKAEA concentrates are how the choice of

technology and materials affects safety, environmental and economic

performance, and how the fusion neutrons affect the properties of

these materials and therefore their useful lifetime.

Figure 1.3 Schematic of a Fusion Power Plant including the production of neutrons

(n)

...