Performance of Ti/Pt and Nb/BDD anodes for dechlorination of nitric acid and regeneration of silver(II) in a tubular reactor for the treatment of solid wastes in nuclear industry

One of the problems frequently encountered in the processing of nuclear fuels is the recovery of plutonium contained in various solid wastes. The difficulty is to make soluble the plutonium present as the refractory oxide PuO2. The dissolution of this oxide in nitric acid solutions is easily performed by means of silver(II) a strong oxidizing agent which is usually electrochemically generated on a platinum anode. However, certain solid residues that must be treated to separate actinides contain important quantities of chloride ions that require after dissolution in nitric acid a preliminary electrochemical step to be removed before introducing Ag(I) for Ag(II) electrogeneration. Research is conducted to find electrocatalytic materials being able to replace massive platinum in view to limit capital costs. In the present work a set-up including a two-compartment tubular reactor with recirculation of electrolytes was tested with anodes made of boron doped diamond coated niobium (Nb/BDD) and platinum coated titanium (Ti/Pt) grids for the removal of chlorides (up to 0.1 M) and for silver(II) regeneration. The study showed that these two anodes are effective for the removal of chlorides contained in 6 M HNO3 solution as gaseous chlorine, without producing the unwanted oxyanions of chlorine. Furthermore, the regeneration rate of silver(II) on Nb/BDD anode is approximately equal to that obtained on Ti/Pt anode for the same hydrodynamic conditions in the tubular reactor. Accordingly, dechlorination as well as silver(II) regeneration can be performed in the same reactor equipped either with a Nb/BDD or a Ti/Pt anode. Besides, the service life of Nb/BDD anodes estimated by accelerated life tests conducted in 6 M HNO3 can be considered as very satisfactory compared to that observed with Ti/Pt anodes