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.
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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.
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