Fresh uranium is mostly U238, with only 0.7% of U235. Unless neutrons are very carefully preserved, there are not enough of them to sustain a chain reaction without increasing the amount of U235. Normally, uranium has to be enriched to 3-5% U235 by separating out the U235 and adding it to normal uranium. Because the two iso­topes differ in mass by only 1.3%, separation is slow; and large installations are needed to fuel power plants. The two main methods are gas diffusion, used in the USA and France, and gas centrifuge, used in Russia and the rest of Europe [41]. In gas diffusion, uranium hexafluoride (UF6) is passed multiple times through porous barriers through which U235 passes 0.43% faster than U238. Gas centrifuges are tall cylinders spinning at high speeds in vacuum. The centrifugal force pushes the heavier isotope out faster. Though gas centrifuges are more efficient, using only 0.09% of the energy generated by the plant compared with 3.6% for gas diffusion, it is a newer technology and it would be costly for the USA to convert to it. The operative word here is not “convert” but “covert.” Centrifuges are discussed further in the Nuclear Proliferation section.

Advanced technology has not overtaken these brute-force methods. Accelerating uranium ions in beams in which the isotopes would have different momenta was tried initially. During WWII, a plasma discharge was tried in the USA, but instabili­ties arose. This was the origin of Bohm diffusion (see Chap. 6). In the 1970s, a laser method was developed at the Livermore laboratory in California in which a laser beam could preferentially put U235 into an excited state, and this could allow it to be extracted separately. At the same time, another laser method was applied to UF6 at the Los Alamos laboratory in New Mexico. A scheme by John Dawson to use two-ion hybrid cyclotron waves in a uranium plasma was implemented at TRW Inc. in Redondo Beach, California [42]. Though this produced palpable amounts of U235, the project was canceled in favor of the Livermore project for political reasons.