Technetium-99
Encyclopedia
Technetium-99 is an isotope of technetium
which decays with a half-life
of 211,000 years to stable ruthenium-99, emitting soft beta rayshttp://www.hps.org/publicinformation/ate/q4510.html, but no gamma rays.
Technetium-99 has a fission product yield
of 6.0507% for thermal neutron fission of uranium-235
, making it the most significant long-lived fission product
of uranium fission, with a half-life over 2000 times as long as the next longest-lived fission product.
Technetium-99m
is a short-lived (half-life about 6 hours) metastable nuclear isomer
used in nuclear medicine
, produced from molybdenum-99. It decays by isomeric transition
to technetium-99, a desirable characteristic, since the very long half-life and type of decay of technetium-99 imposes little further radiation burden on the body.
have halflives in the intermediate range.
(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 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.
) 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, 134Cs) 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 adsorb 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.
, 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 short-lived 100Tc (half-life = 16 seconds) which decays by beta decay
to ruthenium
(100Ru).
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
which decays with a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 211,000 years to stable ruthenium-99, emitting soft beta rayshttp://www.hps.org/publicinformation/ate/q4510.html, but no gamma rays.
Technetium-99 has a fission product yield
Fission product yield
Nuclear fission splits a heavy nucleus such as uranium or plutonium into two lighter nuclei, which are called fission products. Yield refers to the fraction of a fission product produced per fission.Yield can be broken down by:#Individual isotope...
of 6.0507% for thermal neutron fission of uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
, making it the most significant long-lived fission product
Long-lived fission product
Long-lived fission products are radioactive materials with a long half-life produced by nuclear fission.-Evolution of radioactivity in nuclear waste:...
of uranium fission, with a half-life over 2000 times as long as the next longest-lived fission product.
Technetium-99m
Technetium-99m
Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as 99mTc. The "m" indicates that this is a metastable nuclear isomer, i.e., that its half-life of 6 hours is considerably longer than most nuclear isomers that undergo gamma decay...
is a short-lived (half-life about 6 hours) metastable nuclear isomer
Nuclear isomer
A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
used in nuclear medicine
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
, produced from molybdenum-99. It decays by isomeric transition
Isomeric transition
An isomeric transition is a radioactive decay process that involves emission of a gamma ray from an atom where the nucleus is in an excited metastable state, referred to in its excited state, as a nuclear isomer....
to technetium-99, a desirable characteristic, since the very long half-life and type of decay of technetium-99 imposes little further radiation burden on the body.
Radiation
The weak beta emission is stopped by the walls of laboratory glassware. Soft X-rays are emitted when the beta particles are stopped, but as long as the body is kept more than 30 cm away these should pose no problem. The primary hazard when working with technetium is inhalation of dust; such radioactive contamination in the lungs can pose a significant cancer risk.Role in nuclear waste
Due to its high fission yield, relatively long half-life, and mobility in the environment, technetium-99 is one of the more significant components of nuclear waste. Measured in becquerels per amount of spent fuel, it is the dominant producer of radiation in the period from about 104 to 106 years after the creation of the nuclear waste. The next shortest-lived fission product is samarium-151 with a halflife of 90 years, though a number of actinides produced by neutron captureNeutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
have halflives in the intermediate range.
Releases
An estimated 160 TBqBecquerel
The becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...
(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 the primary release of technetium-99 into the environment is by the Sellafield
Sellafield
Sellafield is a nuclear reprocessing site, close to the village of Seascale on the coast of the Irish Sea in Cumbria, England. The site is served by Sellafield railway station. Sellafield is an off-shoot from the original nuclear reactor site at Windscale which is currently undergoing...
plant, which released an estimated 550 TBq (about 900 kg) from 1995-1999 into the Irish Sea
Irish Sea
The Irish Sea separates the islands of Ireland and Great Britain. It is connected to the Celtic Sea in the south by St George's Channel, and to the Atlantic Ocean in the north by the North Channel. Anglesey is the largest island within the Irish Sea, followed by the Isle of Man...
. From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year.
In the environment
The long half-life of technetium-99 and its ability to form an anionic species makes it (along with 129IIodine-129
Iodine-129 is long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission decay products, where it serves as both tracer and potential radiological contaminant....
) 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
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
(e.g., 137Cs, 134Cs) and strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
(e.g., 90Sr
Strontium-90
Strontium-90 is a radioactive isotope of strontium, with a half-life of 28.8 years.-Radioactivity:Natural strontium is nonradioactive and nontoxic, but 90Sr is a radioactivity hazard...
). 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
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
are less able to adsorb onto the surfaces of minerals, so they are likely to be more mobile. By comparison plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
, uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
, and caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
are much more able to bind to soil particles. For this reason, the environmental chemistry of technetium is an active area of research.
Transmutation
An alternative disposal method, transmutationNuclear transmutation
Nuclear transmutation is the conversion of one chemical element or isotope into another. In other words, atoms of one element can be changed into atoms of other element by 'transmutation'...
, has been demonstrated at CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
for technetium-99. This transmutation process is one in which the technetium (99Tc as a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
target) is bombarded with neutrons to form the short-lived 100Tc (half-life = 16 seconds) which decays by beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
to ruthenium
Ruthenium
Ruthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element...
(100Ru).