The two data lines for ***Pu represent experiments with and without Pu09. The 3% oxidized Pu probably formed during addition to the bicarbonate solution. In any event, 242pu in the bulk solution was not being oxidized, but Pu oripinating from the microspheres was. This suggests that impurities in the solution or atmospheric oxygen are not involved but that the oxidation occurred prior to or during transfer from the solid to solution. We might expect that the intense radiation field at the surface of the oxide is involved. ORGANIC SOLUTES TABLE 3- DISSOLUTION RATES OF PuO2 MICROSPHERES IN INORGANIC AND It appears that for large particles of Pus, where the radtation field near the oxide surface can be substantial (about 10° rad/hour at 20 um for the microspheres used in the above work), oxidation of Pu to the more soluble penta- or hexavalent state results in concentrations of Pu which thus exceed solubility limits of the tetravalent state. For submicron particles of Pu0>, the importance of this radiolysis effect would be reduced since the number of emergent alpha particles from the interior of the oxide would be less, and consequently, radiolysis would be less intense. Apart from radiolysis-induced oxidation, however, the actual dissociation of Pu from the oxide matrix may be physical. Fleischer's model (1975) explained the appearance of Pu solely as a physical effect. Thus, aggregates of Pu could be displaced from the oxide surface by recoil phenomenon; oxidation may or may not be related to this effect in that these fragments enter a zone of intense radiolysis. Thus, the actual "dissolution" may not be related to oxidation-only the subsequent solubility of the oxidized Pu resulta in it being observed in solution. Table 3 summarizes observations on the long-term dissolution of Pu0) microspheres in inorganic and organic solutes. Bicarbonate, acetate, citrate, and ethylenediaminetetracetic acid (EDTA) were employed as solutes. The rates of Pu appearance in the aqueous phase represent only nonrefractory Pu. The refractory Pu observed during the initial phases of oxide-solution contact are not included; thus, the table reflects only the long-term, rather constant rate. One of the interesting results is that EDTA and citrate slightly accelerate the dissolution rate over acetate or inorganic solutes, and plutonium CIV) is the oxidation state of the solution phase species. This is due either to a failure of oxidation to occur in the presence of complexers of Pu(IV) or Overall, the similar to reduction by the organics (Bondietti et al,, 1976), rates suggest that the oxide is extremely insoluble and that complexers like EDTA do not substantially change the dissolution rates, This observation has been made by Rabbe et al. (1973). Solute NaHCOs Days 33-91 Dissolution Rate py) 0.27** Q5+ 10°)° g/cm?-day Acetic Acid pH 2 760 1.9 32 pu 4 760 0.23 20 pH 6 680 0.05 92 Citrate (pH 6) 715 2.7 3 EDTA (pH 6) 715 5.5 1 Pu (V) or (VI) state. *Fraction of non-refractory Pu which was in the **Average of 3 separate samples in NaHCQ;. The basic evidence suggests that the dissolution of Puls is very slow, on the order of hundreds of years for submicron particles (Fleischer, 1975). Due to radiolysis effects, oxidized species can occur in some cases but their environmental stability is probably low (Bondietri et al., 1976; Dahlman et al., 1976). The radiolysis-induced oxidation may be important in waste managementrelated problems, but probably not for submicron, dispersed, environmentaltype Pu0y. However, the effect of oxidation may be important in laboratory atudies of the type described here. Obviously, understanding the speciation of Pu in experimental solutions is essential for interpretation of the results. 466 % 467