Technetium occurs naturally in the Earth's
crust in minute concentrations of about 0.003 parts per trillion. Technetium is so rare because the
half-lives of 97Tc and 98Tc are only More than a thousand of such periods have passed since the formation of the
Earth, so the probability of survival of even one atom of
primordial technetium is effectively zero. However, small amounts exist as spontaneous
fission products in
uranium ores. A kilogram of uranium contains an estimated 1
nanogram , equivalent to ten trillion atoms, of technetium. Some
red giant stars with the spectral types S, M, and N display a spectral absorption line indicating the presence of technetium. These red giants are known informally as
technetium stars.
Fission product In contrast to 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
uranium-235 in
nuclear reactors yields 27 mg of technetium-99, giving technetium a
fission product yield of 6.1%. The amount of technetium-99 from nuclear reactors released into the environment up to 1986 is on the order of 1000 TBq (about 1600 kg), primarily by
nuclear fuel reprocessing; most of this was discharged into the sea. Reprocessing methods have reduced emissions since then, 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 to 1999 into the
Irish Sea. From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year. Discharge of technetium into the sea resulted in contamination of some seafood with minuscule quantities of this element. For example,
European lobster and fish from west
Cumbria contain about 1 Bq/kg of technetium.
Fission product for commercial use The
metastable isotope technetium-99m is continuously produced as a
fission product from the fission of uranium or
plutonium in
nuclear reactors: ^{238}_{92}U ->[\ce{sf}] ^{137}_{53}I + ^{99}_{39}Y + 2^{1}_{0}n ^{99}_{39}Y ->[\beta^-][1.47\,\ce{s}] ^{99}_{40}Zr ->[\beta^-][2.1\,\ce{s}] ^{99}_{41}Nb ->[\beta^-][15.0\,\ce{s}] ^{99}_{42}Mo ->[\beta^-][65.94\,\ce{h}] ^{99}_{43}Tc ->[\beta^-][211,100\,\ce{y}] ^{99}_{44}Ru Because used fuel is allowed to stand for several years before reprocessing, all molybdenum-99 and technetium-99m is decayed by the time that the fission products are separated from the major
actinides in conventional
nuclear reprocessing. The liquid left after plutonium–uranium extraction (
PUREX) contains a high concentration of technetium as but almost all of this is technetium-99, not technetium-99m. The vast majority of the technetium-99m used in medical work is produced by irradiating dedicated
highly enriched uranium targets in a reactor, extracting molybdenum-99 from the targets in reprocessing facilities, and recovering at the diagnostic center the technetium-99m produced upon decay of molybdenum-99. Molybdenum-99 in the form of molybdate is
adsorbed onto acid alumina () in a
shielded column chromatograph inside a
technetium-99m generator ("technetium cow", also occasionally called a "molybdenum cow"). Molybdenum-99 has a half-life of 67 hours, so short-lived technetium-99m (half-life: 6 hours), which results from its decay, is being constantly produced. , unshielded, 1958. A Tc-99m
pertechnetate solution is being eluted from Mo-99
molybdate bound to a chromatographic substrate Almost two-thirds of the world's supply comes from two reactors; the
National Research Universal Reactor at
Chalk River Laboratories in Ontario, Canada, and the
High Flux Reactor at
Nuclear Research and Consultancy Group in Petten, Netherlands. All major reactors that produce technetium-99m were built in the 1960s and are close to the
end of life. The two new Canadian
Multipurpose Applied Physics Lattice Experiment reactors planned and built to produce 200% of the demand of technetium-99m relieved all other producers from building their own reactors. With the cancellation of the already tested reactors in 2008, the future supply of technetium-99m became problematic.
Waste disposal The long half-life of technetium-99 and its potential to form
anionic species creates a major concern for long-term
disposal of radioactive waste. Many of the processes designed to remove fission products in reprocessing plants aim at
cationic species such as
caesium (e.g.,
caesium-137) and
strontium (e.g.,
strontium-90). Hence the pertechnetate escapes through those processes. Current disposal options favor
burial in continental, geologically stable rock. The primary danger with such practice is the likelihood that the waste will contact water, which could leach radioactive contamination into the environment. The anionic pertechnetate and
iodide tend not to adsorb into the surfaces of minerals, and are likely to be washed away. By comparison
plutonium,
uranium, and
caesium tend to bind to soil particles. Technetium could be immobilized by some environments, such as microbial activity in lake bottom sediments, and the
environmental chemistry of technetium is an area of active research. An alternative disposal method,
transmutation, has been demonstrated at
CERN for technetium-99. In this process, the technetium (technetium-99 as a metal target) is bombarded with
neutrons to form the short-lived technetium-100 (half-life = 16 seconds) which decays by beta decay to stable
ruthenium-100. If recovery of usable ruthenium is a goal, an extremely pure technetium target is needed; if small traces of the
minor actinides such as
americium and
curium are present in the target, they are likely to undergo fission and form more
fission products which increase the radioactivity of the irradiated target. The formation of ruthenium-106 (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. The actual separation of technetium-99 from spent nuclear fuel is a long process. During
fuel reprocessing, it comes out as a component of the highly radioactive waste liquid. After sitting for several years, the radioactivity reduces to a level where extraction of the long-lived isotopes, including technetium-99, becomes feasible. A series of chemical processes yields technetium-99 metal of high purity.
Neutron activation Molybdenum-99, which decays to form technetium-99m, can be formed by the
neutron activation of molybdenum-98. When needed, other technetium isotopes are not produced in significant quantities by fission, but are manufactured by neutron irradiation of parent isotopes (for example, technetium-97 can be made by neutron irradiation of
ruthenium-96).
Particle accelerators The feasibility of technetium-99m production with the 22-MeV-proton bombardment of a molybdenum-100 target in medical cyclotrons following the reaction 100Mo(p,2n)99mTc was demonstrated in 1971. The recent shortages of medical technetium-99m reignited the interest in its production by proton bombardment of isotopically enriched (>99.5%) molybdenum-100 targets. Other techniques are being investigated for obtaining molybdenum-99 from molybdenum-100 via (n,2n) or (γ,n) reactions in particle accelerators. ==Applications==