TRANSMUTATION of the elements was the goal of the medieval
alchemists. They dreamed of the riches to be won by the man who
could find the philosopher's stone—a substance that, among other
wonderful properties, would convert base metals such as lead
into gold. Actual transmutation, though, had to await those
modern alchemists, the atomic physicists. Nuclear reactors
transmute elements routinely. They break uranium atoms, which
are heavy, into lighter so-called fission products, such as
technetium. This releases energy, along with sub-atomic
particles called neutrons. Some of these neutrons go on to hit
further uranium nuclei so hard that they, too, shatter and
release yet further neutrons. It is this chain reaction that
sustains the process. Other neutrons, however, are captured by
uranium nuclei. That makes those nuclei heavier still,
converting them into neptunium, plutonium, americium and curium.
All these by-products of nuclear fission are radioactive, and
many will remain so for thousands—sometimes milli