Essay About Fusion Reactions And Annual Report Of The Euratom

Static Fusion
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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 UKs 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 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 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