Technetium

Radioactive wastes are extremely damaging since they travel up the food chain and cause cancer in humans. Hence, radioactive wastes storage and their disposal have become an ever growing problem throughout the world. The Savannah River Site in South Carolina houses some 160 million gallons (600 million liters) of such wastes still waiting for a permanent disposal solution.

The radioactive isotope Tc-99 of the 43rd element in the periodic table Technetium is found abundantly in the nuclear waste. It has a long half-life of about 200,000 years and can be carcinogenic when released into the environment. Scientists at the Berkeley National Laboratory, California, have devised methods to examine the in-situ chemical properties of Technetium and other chemical species and use it for the evaluation of safe methods to isolate them from the environment.

Setting Technetium into a cement waste form is a possible method advised by the Berkeley Lab’s Chemical Sciences Division. However, the problem encountered with this is that Technetium takes on a water-soluble form in cement. Technetium commonly exists in its very soluble state as the pertechnetate ion (TeO­­­4-). And for the method of setting it into cement waste form to be successful, it is essential to hold Technetium in its reduced less soluble state like TeO2.

The cement formulation for immobilizing waste involving blast furnace slag and fly ash mixtures started as a project at the Savannah River Technology Centre. The method used the principle of X-ray absorption spectroscopy to examine the element in cement-blast furnace slag mix where the slag acted as the reducing agent. An element with a positive or a negative charge is called an ion and oxidation state is determined by the number of electrons added to or subtracted from the elemental atom. The oxidation state gives us information on the mobility and water soluble state of Technetium.

To study using this technique, a sample is irradiated with X-rays which ejects an inner shell electron of the ion making the other electrons fall back to the inner shell. This emits the characteristic radiation of the particular element that the ion is based upon which helps to determine the oxidation state of the chemical.

X-ray absorption fine structure (XAFS) spectroscopy has been beneficial since it helps to determine the oxidation state of Technetium in cement without chemically separating it from cement and also gives structural information about the nearby atoms. XAFS spectroscopy also helps to study the chemistry of technetium in radioactive waste forms. The technique can be used to study other radionuclides in cement as well. This technique has proven successful and useful in the choice of waste form effective at keeping a species isolated.

Other methods of long-term radioactive waste storage under research include mixing waste with ceramic materials and glass (vitrification). The research papers would be published in Radiochimica Acta sometime later this year.