DISSOLUTON OF PLUTONIUM DIOXIDE 10 CFR 20 TABLE Of The previous discussion on the nature of solution-phase Pu species which could be in equilibrium with Pu02 indicated that if Pu(IV) species were the only soluble species in solution, the amount of monomeric Pu which would be found in solutions contacting crystalline PuQ, would be extremely small (provided no complexes were present). However, when Pu0z is placed in near-neutral solutions, Pu appears in the aqueous phase in concentrations for exceeding that predicted by simple chemical solubility. Furthermore, 238pu0> appears to "dissolve" —iog [Pu] IN FILTERED WATER faster than 239pu. (Patterson et al. 1974). Adams et al. (1975) reported "soluble" concentrations of 733pu0) microsphere dissolution studies which were near 107!! M4. Bondietti and Reynolds (1976) and Dahlman et al. (1976) reported soluble Pu species near 107!° to 10-!! M Pu in 239Pu0, dissolution experiments, with the oxidation states of Pu in solution determined. Smith et al. (1972) reported ultrafiltration data for Pu02 suspended in water. From their report, 238py concentrations of 1078 M can be estimated for the Pu species which passed an ultrafilter with 26 A pores. The "dissolution" of Pu02 can be summarized with two general statements: first, the observed concentrations of "soluble" Pu can exceed that expected if chemical solubility alone (i.e., the dissociation of Puf{IV) monomeric species from a solid) controls the solution phase concentrations; and secondly, the specific activity of the incorporated Pu isotope appears to affect the rate of dissolution. The abnormal solubility of PuQ) in water has led to several hypotheses. The higher radiation density of 23 Puy has been implicated as the reason for its faster apparent dissolution rate (Patterson et al., 1974; Fleischer, 1975). These authors, among others, suggested that fragmentation of the oxide lattice could release Pu: of colloidal dimensions; consequently, the solution phase would contain Fu in forms other than simple ions. This phenomenon (agpregate recoil) results from alpha emission recoils which have sufficient kinetic energy to easily break chemical bonds (Fleischer, 1975). Fleischer (1975) proposed a model which suggested that the differences in “solubility” between the 238 and 239 isotopes of*Pu when present as the oxide was solely due to this aggregate recoil effect. Recoils which occur at the surface of the oxide could account for + 10" ejected atoms of Pu per effective recoil. Fleischer compared hia model to experimental data on Pu solubilization and found good agreement between observed dissolution rates and theoretical emissions of aggregates of Puls. es 6 7 8 9 pH Fig. 5. Comparison of reported concentrations of plutonium in filtered water with the dominant specie in Fig. 4. See Table 2 for number key. Boxed numbers indicate speciation was studied; circles denote commercial radwaste burial ground. 462 Patterson et ai. (1974) also suggested that the radiolysis of water at the oxide surface could produce species which might reduce Pu(1V) to Pu({IIT), increasing the solubility of Pu. A major product of o-radiolysis of water is H20,, and in acid solutions reduction of higher oxidation states to Pu(III)} is well known. However, Kraus (1949) pointed out that hydrogen peroxide would oxidize Pu(III), but not reduce Pu(IV), at near-neutral pH's. The action of peroxide on Pu in acidic solutions is very complex, and the results of Pu oxidation state changes observed for acid solutions (the commonly reported situation) may not be relevant at environmental pH's. 46: