the desired purity. Commercial materials To date, large-scale commercial isotope separation of only three elements has occurred. In each case, the rarer of the two most common isotopes of an element has been concentrated for use in nuclear technology: Uranium isotopes have been separated to prepare enriched uranium for use as nuclear reactor fuel and in nuclear weapons. Hydrogen isotopes have been separated to prepare heavy water for use as a moderator in nuclear reactors. Tritium is both a nuisance in the coolant / moderator of water moderated reactors and a valuable product, it is thus sometimes separated from the coolant Lithium-6 has been concentrated for use in thermonuclear weapons. Tritium is commonly produced from Lithium-6 which is often enriched for this purpose, too. Some isotopically purified elements are used in smaller quantities for specialist applications, especially in the semiconductor industry, where purified silicon is used to improve crystal structure and thermal conductivity, and carbon with greater isotopic purity to make diamonds with greater thermal conductivity. Isotope separation is an important process for both peaceful and military nuclear technology, and therefore the capability that a nation has for isotope separation is of extreme interest to the intelligence community. Alternatives The only alternative to isotope separation is to manufacture the required isotope in its pure form. This may be done by irradiation of a suitable target, but care is needed in target selection and other factors to ensure that only the required isotope of the element of interest is produced. Isotopes of other elements are not so great a problem as they can be removed by chemical means. This is particularly relevant in the preparation of high-grade plutonium-239 for use in weapons. It is not practical to separate Pu-239 from Pu-240 or Pu-241. Fissile Pu-239 is produced following neutron capture by uranium-238, but further neutron capture will produce Pu-240 which is less fissile and worse, is a fairly strong neutron emitter, and Pu-241 which decays to Am-241, a strong alpha emitter that poses self-heating and radiotoxicity problems. Therefore, the uranium targets used to produce military plutonium must be irradiated for only a short time, to minimise the production of these unwanted isotopes. Conversely, blending plutonium with Pu-240 renders it less suitable for nuclear weapons. If the desired goal is not an atom bomb but running a nuclear power plant, the alternative to enrichment of uranium for use in a Light Water