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Separating uranium from plutonium
August 08, 2003Moscow researchers have made the supercritical carbon dioxide work. Saturated with special reagents, carbon dioxide first extracts uranium from the spent nuclear fuel waste, then extracts plutonium and then flies away into the atmosphere.
As a matter of fact, the spent nuclear fuel consists of multiple elements. First of all, this is uranium that did not burn out and plutonium obtained as a result of nuclear reaction and numerous fission fragments, both radio-active and non-radio-active. Uranium and plutonium are extracted first of all: on the one hand, they are very dangerous, and on the other hand, they can be used to produce new heat-generating elements for atomic power-plants.
The so-called Purex process is used worldwide to extract uranium and plutonium. Purex process is applied not because it is perfect, but because nothing better has been invented so far. The essence of the process is as follows: old fuel elements get dissolved in the nitric acid and a special reagent - tributylphosphate - catches what is required.
As a result, two solutions are obtained based on the poisonous acid. One of the solutions contains almost the entire uranium and plutonium. The other contains the residuum: some uranium and plutonium, fission elements and corrosion products. The volume of these liquids is measured by thousands of tons.
The method proposed by the researchers under the guidance of Academician Miasoyedov, Vernadsky Institute of Geochemistry and Analytical Chemistry (GEOKHI), Russian Academy of Sciences, is revolutionary as compared to the Purex process. The reason is that the solid residual (zirconium, caesium, strontium and other fission elements) and two aqueous solutions are received at the output. One solution contains uranium salts, the other - plutonium salts. On top of that, it contains carbonic acid gas that returns back to the atmosphere. When this gas is condensed and slightly heated (up to + 35 degrees C), it passes to the supercritical condition and becomes as powerful dissolvent as, for instance, acetone. Utilization of supercritical CO2 gas is considered one of the major directions in green chemistry, which has been already evolving for several decades at a rush pace.
This particular supercritical technology extracts uranium at the first phase. To this end, the spent nuclear fuel is placed into the sealed chamber and poured with supercritical carbon dioxide under pressure, a special reagent being dissolved in the carbon dioxide. Uranium forms a complex with it and dissolves, and plutonium and fission elements - do not. They settle on the bottom of the chamber.
Then the pressure is decreased and the carbonic acid gas is passed through water. Uranium remains in the water in the form of salts solution, and pure carbonic acid gas flies away. It is interesting to note that this method allows to extract uranium practically to full extent.
The second phase is separating the residuum, i.e. the plutonium/uranium fission products mixture. To extract plutonium, the researchers want to repeat the process but with a different substance. It will react only with plutonium and will disregard other components of the mixture.
"We are proactively handling this phase of the process now, says Academician Miasoyedov. The extragent has already been found. By the way, scientists all over the world are trying to address the problem, however, the first results on separating uranium and plutonium have been achieved in Russia. Another issue is how to use plutonium for peaceful purposes. The future of nuclear power lies in fast reactors. These reactors require mixed fuel - uranium and plutonium. So, in this case the element we extracted will be helpful."
Informnauka (Informscience) Agency
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