STUDIES ON THE PARTITIONING OF ACTINIDES FROM HIGH LEVEL WASTE AND OTHER WASTE STREAMS: RECENT DEVELOPMENTS IN INDIA

摘要

TRUEX process uses Octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (O(Φ)D(iB)CMPO or simply CMPO) for the partitioning of actinides from acidic waste streams. CMPO is one of the most effective bifunctional organophosphorus compounds that can extract even trivalent actinides from high level waste (HLW). In spite of its excellent extracting power capabilities, CMPO has certain drawbacks like third phase formation in presence of high concentration of the extracted metal ions. TRUEX process uses tri-n-butyl phosphate (TBP) as a phase modifier along with CMPO to suppress the formation of third phase. Despite this, the tendency cannot be completely eliminated. Moreover, high degree of loading of CMPO by uranium lowers the extraction of trivalent actinides drastically. The acid, extracted in the organic phase, also interferes with the stripping of americium using dilute nitric acid in the conventional TRUEX process. A two-step process is developed using TBP and CMPO as extractants with certain modifications incorporated in the conventional TRUEX process. The first step of the proposed process is a 'uranium depletion step' in which uranium in HLW is removed using 30% TBP in n-dodecane as extractant. Neptunium and plutonium are also quantitatively extracted into TBP along with uranium after oxidising them to their hexavalent state using potassium dichromate. Neptunium and plutonium, extracted in TBP, are recovered using a mixture of hydrogen peroxide (0.25 M) and ascorbic acid (0.05 M) in 2.0 M nitric acid medium as strippant. Neptunium and plutonium are reduced to inextractable Np(V) and Pu(III), respectively, with this strippant. Uranium left in the TBP phase can be stripped with dilute nitric acid. Americium and curium as well as traces of uranium, neptunium and plutonium left in the uranium-lean HLW raffinate are partitioned in the second step, using an extractant mixture containing 0.2 M CMPO and 1.2 M TBP in n-dodecane (TRUEX solvent). A conventional process uses dilute nitric acid and oxalic acid for the stripping of americium and plutonium, respectively. Since major fraction of plutonium is removed during the uranium depletion step in the present process, it is advantageous to strip plutonium along with americium in a single step. This is achieved with the use of citric acid solution (0.1 M) in a buffer medium consisting of formic acid and hydrazine hydrate as strippant. The interference of the CMPO-extracted nitric acid during the stripping of americium is avoided with the use of this strippant.