BEHAVIOR OF ACTINIDES (TH, U, NP AND PU) AND RARE-EARTHS (LA, CE AND ND) DURING AQUEOUS LEACHING OF A NUCLEAR GLASS UNDER GEOLOGICAL DISPOSAL CONDITIONS

Leaching experiments in aqueous media were carried out on non-radioact ive and radioactive nuclear waste glasses to investigate the release b ehaviour of 4 actinides (Th, U, Np and Pu) and 3 lanthanides (La, Ce a nd Nd) from the solid into solution. The experiments were conducted un der Rowing conditions using synthetic aqueous solutions. The lanthanid e and actinide concentrations were measured in the leachates before an d after filtration to 0.45 mu m and 1.8 nm. Over 98.5% of the lanthani des and Th released from the glass were retained in the alteration pro ducts on the glass surface, probably coprecipitated with a siliceous g el. The retention was enhanced by the presence in the gel of 20 wt% P oxides from the initial 5 x 10(-3) M PO4 solution. In the leachates, m ore than 90% of the lanthanides and Th were associated with colloidal particles. Uranium and Np retention varied from 40% in a 5 x 10(-3) M CO, medium to 95% in a 5 x 10(-3) PO4 medium. In the CO3 medium, U and Np formed stable CO3 complexes that diminished the formation of preci pitates: in the PO4 medium, precipitation of PO4 or more complex phase s may have occurred. Pu exhibited atypical behaviour to the extent tha t it was strongly bonded to colloidal particles in solution, concentra ted in the fraction exceeding 0.45 mu m, and modifyed the calculated P u leach rates. These particles may be the result of limited disaggrega tion of the alteration film. As most of the Pu was found at valence IV , it may be strongly retained in the alteration layer by precipitation or coprecipitation. Under oxidizing conditions, Np and U are found at high valences as Np(V) and U(VI), yielding charged forms in solution (NpO2+ and UO2+). The ianthanides, Th and Pu were present at lower val ences as Ln(III), Th(IV) and Pu(IV), yielding Ln(3+), Th4+ and Pu4+ in oxidizing media. The different behaviour of U and Np compared with th e lanthanides, Th or Pu, observed during these experiments shows that the valence is the determining factor, and must be taken into account in assessing the evolution of radionuclides during the alteration of n uclear waste glass. (C) 1998 Elsevier Science. All rights reserved.