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Occurrence and Production of Technetium
Occurrence
Since technetium is unstable, only minute traces occur naturally in the Earth's crust as a spontaneous fission product of uranium. In 1999 David Curtis (see above) estimated that a kilogram of uranium contains 1 nanogram (1 x 10-9 g) of technetium. Extraterrestrial technetium was found in some red giant stars (S-, M-, and N-types) that contain an absorption line in their spectrum indicating the presence of this element.
Artificial production - Byproduct production in fission wastes
In contrast with the rare natural occurrence, bulk quantities of technetium-99 are produced each year from spent nuclear fuel rods, which contain various fission products. The fission of a gram of the rare isotope uranium-235 in nuclear reactors yields 27 mg of 99Tc, giving technetium a fission yield of 6.1%. Other fissionable isotopes also produce similar yields of technetium.
It is estimated that up to 1994, about 49,000 TBq (78 metric tons) of technetium was produced in nuclear reactors, which is by far the dominant source of terrestrial technetium. However, only a fraction of the production is used commercially. As of 2005, technetium-99 is available to holders of an ORNL permit for US$83/g plus packing charges.
Since the yield of technetium-99 as a product of the nuclear fission of both uranium-235 and plutonium-239 is moderate, it is present in radioactive waste of fission reactors and is produced when a fission bomb is detonated. The amount of artificially produced technetium in the environment exceeds its natural occurrence to a large extent. This is due to release by atmospheric nuclear testing along with the disposal and processing of high-level radioactive waste. Due to its high fission yield and relatively high half-life, technetium-99 is one of the main components of nuclear waste. Its decay, measured in becquerels per amount of spent fuel, is dominant at about 104 to 106 years after the creation of the nuclear waste.
An estimated 160 TBq (about 250 kg) of technetium-99 was released into the environment up to 1994 by atmospheric nuclear tests. The amount of technetium-99 from nuclear reactors released into the environment up to 1986 is estimated to be on the order of 1000 TBq (about 1600 kg), primarily by nuclear fuel reprocessing; most of this was discharged into the sea. In recent years, reprocessing methods have improved to reduce emissions, but as of 2005 the primary release of technetium-99 into the environment is by the Sellafield plant, which released an estimated 550 TBq (about 900 kg) from 1995-1999 into the Irish Sea. From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year.
As a result of nuclear fuel reprocessing, technetium has been discharged into the sea in a number of locations, and some seafood contains tiny but measurable quantities. For example, lobster from west Cumbria contains small amounts of technetium. The anaerobic, spore-forming bacteria in the Clostridium genus are able to reduce Tc(VII) to Tc(IV). Clostridia bacteria play a role in reducing iron, manganese and uranium, thereby affecting these elements' solubility in soil and sediments. Their ability to reduce technetium may determine a large part of Tc's mobility in industrial wastes and other subsurface environments.
The long half-life of technetium-99 and its ability to form an anionic species makes it (along with 129I) a major concern when considering long-term disposal of high-level radioactive waste. In addition, many of the processes designed to remove fission products from medium-active process streams in reprocessing plants are designed to remove cationic species like caesium (e.g., 137Cs) and strontium (e.g., 90Sr). Hence the pertechnetate is able to escape through these treatment processes. Current disposal options favor burial in geologically stable rock. The primary danger with such a course is that the waste is likely to come into contact with water, which could leach radioactive contamination into the environment. The anionic pertechnetate and iodide are less able to absorb onto the surfaces of minerals so they are likely to be more mobile.
By comparison plutonium, uranium, and caesium are much more able to bind to soil particles. For this reason, the environmental chemistry of technetium is an active area of research. An alternative disposal method, transmutation, has been demonstrated at CERN for technetium-99. This transmutation process is one in which the technetium (99Tc as a metal target) is bombarded with neutrons to form the shortlived 100Tc (half life = 16 seconds) which decays by beta decay to ruthenium (100Ru). One disadvantage of this process is the need for a very pure technetium target, while small traces of other fission products are likely to slightly increase the activity of the irradated target if small traces of the minor actinoids (such as americium and curium) are present in the target then they are likely to undergo fission to form fission products. In this way, a small activity and amount of minor actinoids leads to a very high level of radioactivity in the irradated target. The formation of 106Ru (half life 374 days) from the fresh fission is likely to increase the activity of the final ruthenium metal, which will then require a longer cooling time after irradiation before the ruthenium can be used.
Artificial production - By deliberate separation from fission waste
The actual production of technetium-99 from spent nuclear fuel is a long process. During fuel reprocessing, it appears in the waste liquid, which is highly radioactive. After sitting for several years, the radioactivity has fallen to a point where extraction of the long-lived isotopes, including technetium-99, becomes feasible. Several chemical extraction processes are used yielding technetium-99 metal of high purity.
Artificial production - By neutron activiation of molybdenum or other pure elements
The meta stable (a state where the nucleus is in an excited state) isotope 99mTc is produced as a fission product from the fission of uranium or plutonium in nuclear reactors. Due to the fact that used fuel is allowed to stand for several years before reprocessing, all 99Mo and 99mTc will have decayed by the time that the fission products are separated from the major actinoids in conventional nuclear reprocessing. The PUREX raffinate will contain a high concentration of technetium as TcO4- but almost all of this will be 99Tc. The vast majority of the 99mTc used in medical work is formed from 99Mo which is formed by the neutron activation of 98Mo. 99Mo has a half-life of 67 hours, so short-lived 99mTc (half-life: 6 hours), which results from its decay, is being constantly produced. The hospital then chemically extracts the technetium from the solution by using a technetium-99m generator ("technetium cow," also occasionally called a molybdenum cow).
The normal technetium cow is an alumina column which contains molybdenum-98; inasmuch as aluminium has a small neutron cross section, it is convenient for an alumina column bearing inactive 98Mo to be irradated with neutrons to make the radioactive Mo-99 column for the technetium cow. By working in this way, there is no need for the complex chemical steps which would be required to separate molybdenum from a fission product mixture. This alternative method requires that an enriched uranium target be irradiated with neutrons to form 99Mo as a fission product, then separated.
Other technetium isotopes are not produced in significant quantities by fission; when needed, they are manufactured by neutron irradiation of parent isotopes (for example, 97Tc can be made by neutron irradiation of 96Ru).