Air cooling was used in the vety first generation of nuclear reactors, namely, graphite-moderated natural uranium “piles,” which were built in both the

United Kingdom and the United States in the 1940s. The largest air-cooled reac­tors were at the U. K. Atomic Energy Authority’s Windscale establishment and were designed for plutonium production. The main problem with air as a coolant is that it is an oxidant, i. e., it supports combustion. In the case of the Windscale graphite-moderated piles, there was something of a dilemma: if the pile temperature was too high, the graphite oxidized, but if the pile temperature was too low, the graphite atoms could become permanently displaced from their natural positions by neutron bombardment. At higher temperatures, the atomic vibrations are sufficient to shake them back to their normal positions. Displacement of the atoms results in energy being stored, with possible acci­dent connotations, which we will discuss in the context of the Windscale acci­dent in Chapter 5.

Despite the ready availability of air, its oxidizing properties rule it out as a vi­able coolant in modern high-temperature reactors.