Technetium-99

Technetium-99 (⁹⁹Tc) is an isotope of technetium which decays with a half-life of 211,000 years to stable ruthenium-99, emitting beta particles, but no gamma rays. It is the most significant long-lived fission product of uranium fission, producing the largest fraction of the total long-lived radiation emissions of nuclear waste. Technetium-99 has a fission product yield of 6.0507% for thermal neutron fission of uranium-235.

Technetium-99m (^99mTc) 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.

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.^[2] The next shortest-lived fission product is samarium-151 with a half-life of 90 years, though a number of actinides produced by neutron capture have half-lives in the intermediate range.

Releases

An estimated 160 TBq (about 250 kg) of technetium-99 was released into the environment up to 1994 by atmospheric nuclear tests.^[2] 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.^[3]

In the environment

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, 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. In 2012 the crystalline compound Notre Dame Thorium Borate-1 (NDTB-1) was presented by researchers at the University of Notre Dame. It can be tailored to safely absorb radioactive ions from nuclear waste streams. Once captured, the radioactive ions can then be exchanged for higher-charged species of a similar size, recycling the material for re-use. Lab results using the NDTB-1 crystals removed approximately 96 percent of technetium-99.^[4]

Transmutation

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 short-lived ¹⁰⁰Tc (half-life = 16 seconds) which decays by beta decay to stable ruthenium (¹⁰⁰Ru).

See also

• Isotopes of technetium
• Technetium (99mTc) sestamibi

References