Nuclear transmutation

Nuclear transmutation is the conversion of one chemical element or isotope into another, which occurs through nuclear reactions. Natural transmutation occurs when radioactive elements spontaneously decay over a long period of time and transform into other more stable elements. Artificial transmutation occurs in machinery that has enough energy to cause changes in nuclear structure of the elements. The machines that can cause artificial transmutation include the particle accelerator and tokamak reactor.

Origin

The term dates back to the search for the philosopher's stone. It was applied consciously to modern physics first by Frederick Soddy when he, along with Ernest Rutherford, discovered that radioactive thorium was converting itself into radium in 1901. At the moment of realization, Soddy later recalled, he shouted out: "Rutherford, this is transmutation!" Rutherford snapped back, "For Christ's sake, Soddy, don't call it transmutation. They'll have our heads off as alchemists."

Übersicht

Transmutation of transuranium elements (actinides) such as the isotopes of plutonium, neptunium, americium, and curium has the potential to help solve the problems posed by the management of radioactive waste, by reducing the proportion of long-lived isotopes it contains. When irradiated with fast neutrons in a nuclear reactor, these isotopes can be made to undergo nuclear fission, destroying the original actinide isotope and producing a spectrum of radioactive and nonradioactive fission products.

Reactor types

For instance, plutonium can be reprocessed into MOX fuels and transmuted in standard reactors. The heavier elements could be transmuted in fast reactors, but probably more effectively in a subcritical reactor[1] which is sometimes known as an energy amplifier and which was devised by Carlo Rubbia. Fusion neutron sources have also been proposed as well suited [2].

Reasoning behind transmutation

Isotopes of plutonium and other actinides tend to be long-lived with half-lives of many thousands of years, whereas radioactive fission products tend to be shorter-lived (most with half-lives of 30 years or less). From a waste management viewpoint, transmutation of actinides eliminates a very long-term radioactive hazard and replaces it with a much shorter-term one.

It is important to understand that the threat posed by a radioisotope is influenced by many factors including the chemical and biological properties of the element. For instance cesium has a relatively short biological halflife (1 to 4 months) while strontium and radium has a very long biological half-life. As a result strontium-90 and radium are much more able to cause harm than cesium-137 when a given activity is ingested.

Many of the actinides are very radiotoxic because they have long biological half-lives and are alpha emitters. In transmutation the intention is to convert the actinides into fission products. The fission products are very radioactive, but the majority of the activity will decay away within a short time. The most worrying shortlived fission products are those that accumulate in the body, such as iodine-131 which accumulates in the thyroid gland, but it is hoped that by good design of the nuclear fuel and transmutation plant that such fission products can be isolated from man and his environment and allowed to decay. In the medium term the fission products of highest concern are strontium-90 and cesium-137; both have a half life of about 30 years. The cesium-137 is responsible for the majority of the external gamma dose experienced by workers in nuclear reprocessing plants and at this time (2005) to workers at the Chernobyl site. When these medium-lived isotopes have decayed the remaining isotopes will pose a much smaller threat.

In stars

Gold is actually created by supernovae, which however transmute a lot of it into lead - a much easier process. Gold is valuable, precisely because it is a rather rare product. The alchemical belief in transmutation was based on a thoroughly wrong understanding of the underlying processes. Lavoisier first identified the chemical elements and Dalton restored the Greek notion of atoms to explain chemical processes. The disintegration of atoms is a distinct process involving much greater energies.

Genuine scientific transmutation is nicely described in Ken Croswell's book The Alchemy of the Heavens. He summarised the process as:

Burbidge, Burbidge, Fowler, Hoyle

Took the stars and made them toil:

Carbon, copper, gold, and lead

Formed in stars, is what they said

This summarises Synthesis of the Elements in Stars (Reviews of Modern Physics, vol. 29, Issue 4, pp. 547–650), by William Alfred Fowler, Margaret Burbidge, Geoffrey Burbidge, and Fred Hoyle, which was published in 1957. The paper explained how the abundances of essentially all but the lightest chemical elements could be explained by the process of nucleosynthesis in stars. Hoyle correctly predicted a previously unknown energy level of carbon on this basis.

Alchemy

In alchemy, it is believed that such transformations can be accomplished in table-top experiments and some researchers say they have found evidence of transmutation of elements in biological processes (see Kervran). These theories are regarded as pseudoscience by virtually all modern scientists.

Modern nuclear experiments have successfully transmuted lead into gold. The great expense of the procedure, however, far exceeds any gain[3]. In many ways it would be easier to convert gold into lead by nuclear means: by leaving gold in a high flux nuclear reactor for a long period of time.

¹⁹⁷Au + n --> ¹⁹⁸Au (half life 2.7 days) --> ¹⁹⁸Hg + n --> ¹⁹⁹Hg + n --> ²⁰⁰Hg --> + n --> ²⁰¹Hg --> + n --> ²⁰²Hg + n --> + n --> ²⁰³Hg (half life 47 days) --> ²⁰³Tl + n --> ²⁰⁴Tl (half life 3.8 years) --> ²⁰⁴Pb (half life 1.4 x 10¹⁷ years)