A SHATTERING VOICE

Most of the uranium found in nature comes as U 238; less than 1% is the fissile U 235 used in nuclear fission reactors. (The U 238 can be bred into plutonium, but it is not itself fissile.) Since the two are isotopes of the same element, they cannot be separated by chemical reactions; the U 235 has to be extracted by purely physical methods, relying on the very slightly lighter weight of the U 235 nucleus. Since this process raises the percentage of fissile U 235 in the total amount of uranium, it is called "enrichment."

Much effort of the Manhattan Project in World War II was directed toward finding a way to achieve this, and two ways were found: gaseous diffusion and centrifugal separation. The more efficient process is the former, which is now used in all US enrichment plants. The French Atomic Energy Commission has recently decided to use the centrifugal process, setting it apart from the other Common Market countries. The reasons are probably political: If the rest of Europe had opted for the centrifugal process, la grandeur frangaise would presumably have dictated the use of diffusion.

There is a way of evading the costly and complicated enrichment process altogether, and that is to use the uranium in its natural, unenriched mixture, relying on the tiny fraction of U 235 to do the job where it is, with the U 238 looking on and going to waste. As a matter of fact, that is what Enrico Fermi did in the historic reactor that went into operation with a power of half a watt in Chicago on December 2,1942. Commercially, natural uranium oxide is now used only in the Canadian 250 MW Gentilly Station, a highly sophisticated plant built with a view to Canada's large supplies of uranium ore.

But now scientists of the Lawrence Livermore Lab and the Los Alamos Scientific Labs are looking into a totally new way of achieving separation of isotopes, which could one day make the present cumbersome enrichment process obsolete.

Some singers are said to be able to shatter a glass with their voice. The trick is not merely to have a strong voice, but to sing at the natural frequency of the glass (the pitch heard when the glass is tapped). Each wave of the impinging sound will then reinforce the natural oscillation of the glass, and the vibrations will build up until the mechanical stress may shatter the glass.

An analogous principle can be used on atoms, in which the electrons whirl round the nucleus at natural frequencies. If they are illuminated by strong laser light of the same frequency, an electron may be torn out of its shell, leaving behind a positively charged ion. The resonant frequencies of the uranium isotopes are different, and it may be possible to ionize one of them in this way. If only one of the isotopes has an electric charge, it is then child's play to separate it in an electric field.

Reportedly such laser separation has been achieved for isotopes of mercury. Can it be done for uranium?

Nobody knows. But AEC-sponsored research teams are working on it.