The Chemical Behavior of Transuranium Elements and Barrier Functions in Natural Aquifer Systems
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THE CHEMICAL BEHAVIOR OF TRANSURANIUM ELEMENTS AND BARRIER FUNCTIONS IN NATURAL AQUIFER SYSTEMS JAE-IL KIM Kernforschungszentrum Karlsruhe, Institut fuir Nukleare Entsorgungstechnik, D-7500 Karlsruhe, and Institut ffir Radiochemie, Technische Universitit Milnchen, D-8046 Garching, GERMANY. ABSTRACT The chemical behavior of transuranium elements in natural aquifer systems is governed by a variety of geochemical reactions, such as dissolution reaction (solubility), hydrolysis, complexation with inorganics or organics, redox reaction, colloid formation, geochemical interaction with surfaces of various minerals, coprecipitation, mineralization, etc. This paper reviews the present state of knowledge on some of these particular reactions. The emphasis here is on how the individual reactions can be appraised for long-term prediction of the geochemical behavior of transuranium elements in the natural environment. Of the various possible reactions, the primary thermodynamic processes are discussed with notable examples: dissolution of transuranium compounds in aquatic solution; complexation with important anions present in groundwater; and colloid generation. Various laser spectroscopic methods in use for chemical speciation are mentioned briefly for their spectroscopic capability, as well as for their applicability. The present paper attempts to better understand the migration behavior of transuranium elements in natural aquifer systems. INTRODUCTION The worldwide increase in the use of nuclear energy generates, yearly, considerable amounts of transuranium elements. Based on what we know of the actual worldwide electric production of 213 GWe (total capacity: 320 GWe) presently by nuclear energy [1], the production of transuranium elements estimated by the KORIGEN [2] calculation corresponds to 77 tons of plutonium, 1.0 ton of americium, 4.3 tons of neptunium, and 0.3 tons of curium, after fuel discharge. Plutonium is being produced in a larger quantity than any other transuranium element. Four long-lived isotopes are important for the long-term behavior of this element in the natural 24 1 4 24 0 23 8 pu (14.4a). pu (6.4x10 3a); and pu (87.7a); 2 39 pu (2.41x10 a); environment: 23 7 Np (2.14x10 6 a) that has been considered, only Neptunium has one long-lived nuclide, recently, as a possible long-term hazard in ecosystems [3,4], because of its mobile nature, under aerobic conditions, due to the high chemical stability of the pentavalent state, NpO' [5]. 24 1 Am (433a) and 243 Am (7370a). From reactor Americum has two long-lived nuclides: production, Am becomes, some years after discharge, the second prevalent transuranium element, next to Pu, because its quantity grows through the beta decay of relatively short-lived 241 pu. Curium is produced from reactor operation in a smaller quantity than the other transuranium elements. However, with its most abundant but relatively short-lived nuclide 244Cm (18. Ia), Cm is the second most active element in spent fuels, after Pu, and it will persist in the environment for several human
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