Recent activities in the field of separation science and technology of radioactive elements

Chemical separation of radioactive elements has been extensively applied in nuclear science fields because there are long-term options for management of spent nuclear fuels: storage for nuclear wastes in a deep geological repository and separation and recycling of the actinide elements. Recovery of radioactive elements such as actinides and fission products from nuclear and industrial wastes either before or after release into the environment is another important and topical area of research due to their radioactivity which is often coupled with heavy metal toxicity [1]. The activities in separation science and technology of radioactive elements are introduced in this summary by surveying the recent publications in the Journal of Nuclear Science and Technology.

Target elements in the field of nuclear separation science are fission products such as cesium Cs and strontium Sr as well as uranium U and transuranic elements such as neptunium Np and plutonium Pu. Especially, chemical separation system of fission products at Fukushima Daiichi Nuclear Power Plant (NPP) plays an important role in reducing the volume of contaminated water. For the treatment of radioactive waste seawater, a coagulation–flocculation system using ferric hydroxide and polyacrylamide flocculant was applied to remove the radioactive cobalt Co, manganese Mn, antimony Sb, ruthenium Ru, and Sr remaining in the waste water after Cs removal [2].

Separation methods for U and Pu are reasonably well established and reviewed [3]. On the other hand, separation of the minor actinides from lanthanide Ln fission products is an ongoing challenge and an area of active research [4]. The adsorption properties of a porous silica-based adsorbent implementing 2,6-bis(5,6,7,8-tetrahydro-5,8,9,9-tetramethyl-5,8-methano-1,2,4-benzotriazin-3-yl)pyridine towards palladium Pd [5] in the presence of americium Am and Pu [6] and Ln [7] were investigated. The microstructure of the silicate particles impregnated with octyl(phenyl)-N, N di isobutyl carbamoyl methyl phosphine oxide as extractants for europium Eu and their change with the crosslinking degree of polymer were investigated [8]. The selectivity of the benzimidazole-type anion exchange resin for adsorption of the major elements present in fuel debris in HCl solution containing U, Ru, Pd, tin Sn, molybdenum Mo, zirconium Zr, nickel Ni, iron Fe, chromium Cr, Sr, barium Ba, Cs was analyzed, and shown that chromatographic separation of U was evaluated [9].

Isotope separation is also performed by chemical exchange processes based on the isotope fractionations between two isotopic compounds. To develop the calcium-48 ⁴⁸Ca enrichment process, a feasibility study on a band chromatography was made using 9 M HCl solution and crown ether resin synthesized in porous silica beads [10]. As another method for isotope separation, the feasibility of separating radioactive Cs isotopes using the effects of light-induced drift (LID), which is the massive flow of particles caused by the difference between the collision frequencies of the buffer particles in the optically connected ground and excited states were investigated [11].

Solvent extraction is one of the most important and popular methods for elemental separation using aqueous and organic solutions. The establishment of the plutonium uranium redox extraction (PUREX) process, some of its most recent developments and a general covering of all the new technological processes idealized to overcome the problems associated with the disposal of radiotoxic wastes, constitute a sample of the huge amount of investment that has been made in this area. A highly practical diamide-type extractant, which is an alkyl diamide amine with 2-ethylhexyl alkyl chains (ADAAM(EH)), was developed and the separation of Am and curium Cm was achieved in very high yield [12]. A liquid–liquid countercurrent centrifugal contactor with Taylor–Couette flow was applied to practical multi-species cases. Continuous separation of Mo, Zr [13] and zinc Zn [14] from a simulated high-level liquid waste (HLLW) with bis(2-ethylhexyl) phosphoric acid (HDEHP) as an extractant has been performed.

As another option for the separation technology, the pyrochemical processes at high temperature as well as room temperature molten salts [15,16], so-called ionic liquids, are increasing interest in the partitioning and transmutation scenario pursuing a significant reduction of the environmental burden of the long-lived nuclides contained in radioactive wastes. Both a selective U metal deposition on a solid cathode and a grouped recovery of actinides, U, Pu, Np, Am, and Cm, in a liquid cadmium Cd cathode were confirmed by electrorefining of irradiated metallic fuels in a LiCl-KCl melt at 773 K [17].

Current research activities in separation science and technology reported within the last few years were reviewed briefly. Minor actinide separation is still hot subject, and there are also a lot of options to separate elements in the extraction of actinides as well as fission products from nuclear wastes produced in reprocessing process as well as contaminated water in reactor core of Fukushima Daiichi NPP. For the future study, simple and rapid decontamination technology in environments would be expected.