Technetium-99

Technetium-99 (⁹⁹Tc) is an isotope of technetium which decays with a half-life of 211,000 years, emitting soft beta rays but no gamma rays, and has a fission product yield of 6.0507% for thermal neutron fission of uranium-235. It is the most significant long-lived fission product, with halflife over 2000 times as long as the next longest-lived fission product.

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.

Availability

As of 2005, technetium-99 is available to holders of an ORNL permit for US$83/g plus packing charges. [The CRC Handbook of Chemistry and Physics, 85th edition, The Elements]

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.*K. Yoshihara, "Technetium in the Environment" in "Topics in Current Chemistry: Technetium and Rhenium", vol. 176, K. Yoshihara and T. Omori (eds.), Springer-Verlag, Berlin Heidelberg, 1996.] The next shortest-lived fission product is samarium-151 with a halflife of 90 years, though a number of actinides produced by neutron capture have halflives 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. 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. [Technetium-99 behaviour in the terrestrial environment]

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 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.

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 shortlived ¹⁰⁰Tc (half life = 16 seconds) which decays by beta decay to ruthenium (¹⁰⁰Ru).

Nuclear isomer

Technetium-99m is a short-lived metastable nuclear isomer used in nuclear medicine.

ee also

* Isotopes of technetium

References