THE OTHER WAY

In any case, both of the now feasible enrichment methods rely on the minute difference in mass between the fissile U-235 and the non-fissile U-238.

Since the two isotopes differ in nothing but mass, it would seem that large amounts of energy must forever be used up in the enrichment process.

Not so, said some nuclear scientists. If the two isotopes have no other property to distinguish them, let's give them one: electric charge. Each isotope has certain resonant frequencies (corresponding to lines in its absorption spectrum); when stimulated by light of that frequency (color), one of its electrons whirling round the nucleus is excited to a higher energy level, and it can get so excited as to tear itself out of the atom altogether (just like a glass may shatter when hit by sound of its resonant pitch or a wooden bridge may collapse when a platoon of soldiers marches across it and the frequency of their step coincides with the natural frequency at which the bridge vibrates). Once that happens, the remaining atom becomes a positive ion, i.e., it acquires a positive charge.

Laser light has made it possible to stimulate atoms at their exact resonant frequencies, and since these frequencies differ slightly for U-235 and U-238, it should theoretically be possible to irradiate a gas containing both isotopes so as to ionize only one of them. It is then an easy matter to separate them electrically.

We wrote about this possibility last March. At the time we said that nobody knew whether the method would work for uranium (it had already been demonstrated for some other elements).

Since then, it has been achieved. Scientists have been working on laser isotope separation at Exxon Nuclear, the Avco Everett Research Labs, the Los Alamos Labs, and other institutions in the US and abroad (particularly Israel and Russia). The first to announce success, in a paper given at the International Quantum Electronics Conference in San Francisco last summer, was the Lawrence Livermore Lab, operated for the AEC by the U. of California. The method was described in detail by LLL research physicist Sam A. Tuccio. The good news: Laser isotope separation can enrich the concentration of U 235 in a single stage from 0.7% to a whopping 60%. The bad news: At present, this can only be done on a microscopic scale.

The LLL method uses a one-two punch to ionize U 235 atoms. A thin stream of vapor from a heated uranium sample is irradiated by very finely tuned laser light, capable of resolving the "hyperfine structure" of the spectra of U 235 and U 238. This will selectively excite (or, roughly speaking, "loosen") electrons only in U 235. The second punch is to kick these loosened electrons completely free, and this is done by ultraviolet light from a mercury arc lamp. The U 235 atoms are thereby positively charged, and they are then deflected from the uranium vapor stream by a negatively charged collection cup. The amount collected in the cup was so small that a sensitive mass spectrometer had to be used to confirm its presence; but nobody, at this stage, expects researchers to separate uranium isotopes by the shovelful.

"The technology we are working with," says Dr Ticcio, "is really in its infancy. We should be able to verify a number of other approaches experimentally and determine their economic attractiveness."

The enrichment work at LLL, performed by a staff of 30, began in 1972. The LLL's success is, of course, only a very first step on a long road. Direct utilization of the principle on a large scale would require, for example, lasers I,000 more powerful than now available.

But miniscule as the amount of U 235 separated may be, it is vastly greater than the energy supplies mobilized by the politicians since the Arab oil embargo.