A multidisciplinary team comprising the best European specialists in the field of "calixarene" synthesis has prepared new compounds capable of eliminating long-lived radioactive elements from liquid waste arising from the reprocessing plants of nuclear fuels.
Every year, reprocessing plants recycle
over 150000 tonnes of nuclear waste(1) from European facilities.
This process considerably reduces the final amount of waste arising.
The problem of storing this waste, which is characterised by the
presence of radioactive elements which are of variable longevity
and toxicity, nevertheless remains.
When the waste is of low activity and has a relatively short life span, storing it by means of shallow burial in supervised areas offers sufficient guarantees of safety and is not very costly. This type of storage is already a feasible and widespread option, particularly in France and Spain.
The problems are much more complex and worrying in the case of effluents consisting of variable quantities of radioelements which have a very high activity and long life, such as various isotopes of strontium, caesium and actinides (plutonium, americium, curium). No process for decontaminating such waste by the rapid nuclear transmutation of these elements into less harmful substances has been successfully carried out so far. The only solution currently envisaged - but not yet put into practice - is to bury such waste in deep geological formations (at more than 500m below ground) in order to render it "harmless" in total security for several thousands of years. The economic cost of such a storage system is naturally very high.
To attenuate the problem, one of the research paths actively pursued nowadays consists of sorting reprocessing waste by separating the highly radioactive components which are long-lived from other materials of clearly lower activity or with a shorter life. The volumes to be buried at depth are reduced accordingly.
A European research project has brought together specialists in molecular synthesis from Barcelona, Belfast, Mainz and Parma. The teams have succeeded in synthesising several molecular "cages", capable of trapping certain specific radioelements. The majority of these molecules are calixarenes (so called because of their structure in the form of a calyx), and capable of attracting the radioelements. The selectivity of these compounds is obtained by surrounding the molecule's cavity with active groups capable of "recognising" the radioelement to be eliminated and of keeping it in the cage, while excluding the other radioelements. Such compounds are the end-product of sophisticated chemical syntheses which, in some cases, require numerous steps.
For example, Professor Ungaro's team (University of Parma) has synthesised molecular traps in the form of a crown whose selectivity is much greater than that of the caesium extracts currently used. These exceptional properties have been jointly highlighted by the teams of Professors Schwing (ECPM/ Strasbourg) and Dozol (CEA/Cadarache), the project coordinator. Meanwhile, Professor Wipff (Queens University, Belfast) has interpreted the results obtained by the researchers thanks to molecular modelling involving long and complex calculations. Professor Boehmer's team (University of Mainz) prepared the calixarenes which, even when used at very low concentration, make it possible to extract all the actinides.
140 synthesised molecules
After numerous tests on reconstituted solutions, these calixarenes have demonstrated their ability to extract caesium and actinides from real highly radioactive solutions. "The major challenge was to find molecular traps capable of extracting radioactive elements and only those," stresses J.F. Dozol. "We could not have successfully carried out this operation," adds Professor Schwing, "without the collaboration of the best European specialists in calixarenes which this project has helped to bring together." More than 140 molecules were synthesised and tested during the project, permitting a significant advance in the extraction of caesium and actinides. These calixarenes, which have now been patented and marketed, have crossed the Atlantic and are currently being studied by American researchers.
Cooperation between the project participants has since continued under the coordination of the CEA. It takes the form of exchanges of young researchers in receipt of European research grants. These exchanges enable them to direct their syntheses and to select the molecules which will lead to significant reductions in the volume of nuclear waste.
(1) This figure does not include waste produced
in Central and Eastern European countries.