[en] The materials that are used in the core of a nuclear reactor are subjected to more severe conditions than are normally encountered in other plant. They may be expected to survive in a corrosive environment, at very high temperatures which may fluctuate within a wide range, and they may be subjected, therefore, to high and varying heat fluxes which give rise to thermal stresses and shocks. At the same time, the materials are under irradiation by neutrons and by gamma rays, which may not only change their physical structure but also materially alter their constitution. In proving any new reactor system, or the materials that may be used in it, there is an obvious need to study the behaviour of the materials in question under conditions closely simulating those which might be encountered in the reactor. The last stage in a programme to test materials to this end, may involve the construction and operation of a reactor experiment or a prototype built purely for experimental purposes. Such a reactor, however, will be very costly, and before it is constructed and operated there may still be a very great need to carry out preliminary tests on the materials that are proposed for its construction. This will involve the use of irradiation facilities in so-called research reactors, where specimens may be subjected to all the conditions which are likely to be encountered in the reactor.