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Nuclear reactors become obsolete by wearing out or by becoming outclassed or out of date. The lifetime of a conventional power reactor is probably less that 30 years, but the life of its potentially dangerous radioactivity is now known to be of a geological time scale. When a reactor is shutdown permanently, the fuel rods, cooling water, and radioactively ‘hot’ pieces are removed for long-term storage. High-level and low-level radioactive isotopes contained in the liquids and solids removed present a special problem (see Eos, Feb. 9, 1982, p. 147). It is the main frame and construction of worn out reactors that are the major problem, however. Mainly because of neutron activation processes that occur during reactor operation, components have been found to contain nickel-59 and niobium-94, both of which have very long half-lives: 80,000 and 20,300 years, respectively. The two isotopes are only present in trace amounts in metals incorporated in reactor components, but they emit levels of radiation above acceptable levels. The result is that the entire reactors must be dismantled, reduced to shippable size pieces, and the pieces stored for geologic time periods (several half-lives).