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Pierre-Gilles de Gennes, winner of the Nobel prize for Physics, famously declared ‘We say that we will put the sun into a box. The idea is pretty. The problem is, we don't know how to make the box.’ He was talking about nuclear fusion reactors, and the ‘box’ is the casing of the reactor. But what should the box be made of? This is the problem that materials scientists around the world are working on in preparation for the construction of ITER (International Thermonuclear Experimental Reactor).
ITER will be the next stage of research into nuclear fusion, and aims to pave the way for commercial fusion reactors by developing the necessary technologies and processes. The reactor will reproduce the nuclear reactions that occur in the sun using super-heated gas, or plasma, in a tokamak. The tokamak generates an intense doughnut-shaped magnetic field which confines the plasma and allows fusion to take place.
ITER will use two isotopes of hydrogen: deuterium and tritium. When these isotopes fuse, they produce helium nuclei and high energy neutrons. The problem is that the resulting severe neutron irradiation turns many elements into dangerously radioactive isotopes. The challenge is to find materials for the reactor walls that will withstand this bombardment and remain stable.
Researchers have already discovered that iron, carbon and chromium remain relatively stable, and are looking into the possibility of using special steels. It is not, however, possible to recreate the conditions inside a nuclear fusion reactor in the lab, so researchers are having to infer the behaviour of materials, using data from lightly irradiated samples and a variety of modelling techniques.
ITER will be located in Caderache, in southern France. Construction of the site is due to begin in 2008, and the tokamak itself is scheduled to be built in 2011. Scientists will need to be confident that the materials chosen to build the reactor can indeed withstand the extraordinary conditions needed for nuclear fusion.
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