Nuclear fission technetium

Technetium then became available in a weighable quantity because of uranium nuclear fission leading to the production of "Tc in nuclear reactors. The total amount of "Tc in the world at the end of 1993 is estimated to be 78 tons, more abundant than rhenium on the earth. 

The congeners of manganese are very rare. The middle element of the group, technetium (Z = 43), has not been isolated from mineral sources, and there is good reason to suppose that detectable quantities do not occur naturally (Chap. 27, Exercise 7). Weighable amounts of this element and its compounds have been made from molybdenum by nuclear displacement and from uranium by nuclear fission (p. 474). Rhenium (Z =5 75) occurs naturally but in only tiny amounts. It has been estimated that there is one atom of rhenium present in the earth s crust for each two billion (2 X 109) atoms of silicon. Rhenium was discovered in 1925, technetium in 1937. 

The discovery of technetium in 1937 by the Italian scientists Carlo Perrier and Emilio Segre was an important affirmation of the configuration of the Periodic Table. The table had predicted the existence of an element with 43 protons in its nucleus, but no such element had ever been found. (In fact, technetium does not occur naturally on Earth, as all of its known isotopes are radioactive and decay to other elements on a timescale that is relatively small when compared with the age of the earth.) Perrier and Segre were able to observe technetium from molybdenum that had been bombarded with deuterons. They named the element technetium, from the Greek word technetos, meaning artificial. Technetium is produced in relatively large quantities during nuclear fission, so there is currently an ample supply of the element from nuclear reactors and nuclear weapons production. 

Technetium is one such constituent of radioactive waste where the need for a chemical means of detection exists, but a sensor does not. Technetium is not found in appreciable quantities in nature however, the isotope c is a byproduct of the thermal nuclear fission of U, U, and Pu at 6.1%, 4.8%, and 5.9% yields, respectively (7), and significant quantities of c exist at many DOE sites. Tc exhibits rather weak P decay (Ebuk = 0.292 keV), but it is of particular concern for two reasons its long half life (2.13 x 10 y) and the high solubility of its most common form in aqueous environmental media, pertechnetate (TcO/) (2). Pertechnetate does not readily adsorb to most minerals, and therefore in aqueous form and under suitable conditions, it may rapidly present itself to subsurface waters (3,4). 

Following the discovery of radioactivity and nuclear fission, and the development of techniques in radiochemistry, it became possible to fill the remaining few gaps in the periodic table. The last gap to be filled was that corresponding to element 43, which became known as technetium from the Greek techne, meaning artificial or manufactured. It was manufactured in the course of some radiochemical reactions that would not have been feasible before the advent of nuclear physics. Until recently, it was believed that this element did not occur naturally, but a reexamination of old evidence has now suggested that it does in fact occur naturally and that early reports of its discovery made in 1925 may have been unjustly discredited. ... 

Among the long-lived isotopes of technetium, only Tc can be obtained in weigh-able amounts. It may be produced by either neutron irradiation of highly purified molybdenum or neutron-induced fission of uraniimi-235. The nuclides Tc and Tc are exclusively produced in traces by nuclear reations. Because of the high fission yield of more than 6%, appreciable quantities of technetimn-99 are isolated from uranium fission product mixtures. Nuclear reactors with a power of 100 MW produce about 2.5 g of Tc per day . 

The major characteristic of technetium is that it is the only element within the 29 transition metal-to-nonmetal elements that is artificially produced as a uranium-fission product in nuclear power plants. It is also the tightest (in atomic weight) of all elements with no stable isotopes. Since all of technetiums isotopes emit harmful radiation, they are stored for some time before being processed by solvent extraction and ion-exchange techniques. The two long-lived radioactive isotopes, Tc-98 and Tc-99, are relatively safe to handle in a well-equipped laboratory. 

The technetium isotope of interest for nuclear fuel waste disposal is Tc. It is a pure 3-emitter (E = 0.293 MeV) with a half-life of 2.13x10 years. Its high fission yield of 6% accounts for the relatively high concentration 0.02% by weight) (1) in fuel discharged from a CANDU (CANada Deuterium Uranium) reactor (burnup 650 GJ/kg U). 

Technetium-99m ("Tcm) is a radionuclide that finds many applications in nuclear medicine. Virtually all technetium used in nuclear medicine labs is prepared synthetically from other radioactive materials. "Tcm is produced by the (3 decay of "Mo as illustrated in the reaction below. "Mo is produced through fission of 235U or via the capture of a neutron by "Mo. 

From 1937, when technetium was re-introduced into the terrestrial environment as the first man-made element1,2, until the ready availability of "mTc for medical purposes3, the chemistry of this element was regarded largely as an academic curiosity. Since it is formed in good yield in fission reactors fueled by 235U4, much early work was devoted to its separation from spent nuclear wastes4,5. ... 

Technetium was the first of the artificially produced elements (1937) when it was obtained, as the isotopes 95Tc and Tc, by bombarding Mo with deuterons. Today twenty-one isotopes, all radioactive, are known with mass numbers 90-111. The longest lived isotope is Tc (ha 4 x 106 y), but the commonest is "Tc (tm — 2 x 105 y). It is isolated in fairly large quantities from spent nuclear fuel, where it constitutes ca. 6% of the fission products. The total amount of "Tc is about 78 tons, which exceeds the known amount of rhenium in the earth s crust. "Tc emits a soft (293.6 Kev) /3 particle and can be handled with only very modest precautions. 

Technetium has not been found in nature. It can be obtained readily as a product of uranium fission in nuclear power plants, however, and is now produced in quantities of many kilograms per year. One medical use relies on the tendency... 

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