Development of a new extractant and a new extraction process for minor actinide separation

Abstract

Many kinds of new extractants have been studied for the separation of minor actinides, Am and Cm in particular, from high-level liquid waste (HLLW) which is generated from spent fuel reprocessing for the recovery of U and Pu. The authors have developed a new type of extractant for the extraction of Am and Cm, N,N,N',N'-tetraoctyldiglycolamide (TODGA) which is one of the diamides and shows the tridentate feature. To apply such a new extractant to the separation process for Am and Cm from HLLW, many criteria should be investigated and satisfied; e.g., high separation factor between An(III) and fission products, chemical and radiolytic stability, high extraction capacity, fast kinetics, compatibility with hydrocarbon diluents, controllability for the behaviour of U, Pu and Np which exist as residue of the former step, and so on. TODGA is fully soluble in n-dodecane, but has a relatively low extraction capacity. Therefore, TODGA was modified and a new DGA extractant, N,N,N',N'-tetradodecyldiglycolamide, was developed, which can extract Ln with one-third of the extractant concentration. Since both TODGA and TDdDGA extract Zr and Pd, effective masking agents for them were examined and selected. To extract Np quantitatively, the method to reduce Np(V) to Np(IV) was studied. With those achievements, a counter-current extraction test with 0.1M TDdDGA in n-dodecane and simulated solution of HLLW was carried out using a small-scale mixer-settler. As results of the counter-current extraction test, very clear phase separation was observed without any crud formation during the operation and quantitative recovery of Nd which is a substitute for Am was obtained. 1. Introduction Many research activities are being conducted in the world to develop partitioning and transmutation (P&T) system, in which long-lived minor actinides, MA (Np, Am and Cm), are partitioned and transmuted to short-lived or stable nuclides [1]. Heat generating elements, Sr-Cs, long-lived fission products (LLFP) such as Tc, platinum group metals (PGM) and rare metals in spent nuclear fuel are also target elements to be separated in P&T system. In most of the partitioning processes, trivalent actinides, Am and Cm (An(III)), are separated by solvent extraction from high-level liquid waste (HLLW) generated from spent fuel reprocessing for the recovery of U and Pu. Neptunium, one of MA, can also be separated with U and Pu. Many kinds of extractants have been developed and studied for the separation of Am and Cm from HLLW. Most of the extractants which can be applied to the separation of Am and Cm from HLLW also extract rare earth elements (RE). Therefore, Am and Cm are separated from RE at the following second step.