Environmental Chemistry of the Actinide Elements MarcoS. Caceci CEA-DRDD/SESD B.P.6, F-92265 Fontenay-aux-Roses CEDD( France abstract Two actinide elernents, 232Th and 235.2389, exj.st in nature in deposits of technological importance and - as traces - in all rocks, soils and wa- ters. others, such as 231Pa, are produced in the course of ¡adioactive de- cay and live long enough to establish thetr own chemical paths in the en- vironment. Heavier long-lived synthetic nuclides are produced in large amounts by sequential neutron activation/spalfation of 238U in nuclear reactors: the mOSt Signif iCant being "tNp, 238'23e'24o'2Á2'244p,r, 241'2t13¡¡¡. 2a2'244'248¿¡' They are all radioactive (mostly alpha emitters) and consequently constitute a potential threat to the biosphere. Actinide recovery (partitioning) from spent fuel is a possible, but probably too dangerous, strateglz to reduce Iong term alpha activity in nuclear waste: actinides could be recycled and burned in reactors. Most countries are presently committed to a policy of lirnited reprocessing or of no re- processing at all: actinides are to be buried either as part of unrepro- cessed spent fuel or as constituents of engineered waste in the form, for example, of glass or synthetic rock. I{aste management must identify criti- cal- nuclides and asses their migration paths and mechanisms in the diffe- rent planned and possible repository scenarios, as wel-l as in the general terrestrial environment. All migrration paths are thought to be mediated by water transport. The aqueous chemistry of the actinide e.l-ements is characterized by: - many possible oxidation states, most of which exibit characteris;ic nar- row absorption bands. - "hard. acid behavior: the cations have a strong affinity for hydroxide, carbonate, fluorlde, phosphate, carboxylates. Some polynuclear species with hydroxide and carbonate ions are r¡el1 established as weII as extremely sta- ble. - existence (except for Pa) of unigue, very stabl-e, dioxo "actinyl' spe- cieS in the formal V and VI oxidation states. Complexation strengrth increa- ses 1n the sequence Anoz+ < An3* a- Ano¿z* < Ana*. while it 1s in general true that correspondlng oxidatlon states have similar chemical behavlor, there are important exceptions: for example, Pu (VI) is much less soluble in bases than U (vI) , and does not appear to form as strong M3OH5 clusters; Pu(Iv) forms well characterized polymers, but Th(Iv) apparently does not. Speciatlon dlagrams as a function of pH, Eh, and co2 partlal Pres- sure have been derlved and can be used to assess the dominamt species in any solution at eguilibrlun. speciflc Ion Interaction theory has been ap- plled to account for the lnfluence of ionic strength. These data, conbi- ned wlth solubillty products, which for a few species (hydroxides, carbo- nates) are known, allow to estimate worst-case repository solubilities.