{4
Bondietti and Reynolds (1976) reported that substantial amounts of an oxidized
Pu species (Pu(V) or Pu(V1}} occurred in neutral solutions contracting high-
fired Pu02.
The PuQ, used for those studies was in the microspheres, having a
diameter range of 149-174 um, a specific surface area of 0.012 m?/g, and made
by the "sol-gel" process,
The microspheres had been calcined at 1150
C.
For
the experiments described here, 2-3 mg amounts of the microspheres were used
(weighing accuracy was 0.01 mg).
Sodium bicarbonate (1 mM) and organic
solutes were equilibrated with the microspheres for varying time periods.
{2
When bicarbonate was used, two types of Pu were found in the aqueous phase.
The initial period of water contact resulted in the solution phase Pu being
“dissolution” rates which were very erratic; this observation also pervades
the titerature. Thus, Patterson et al., 1974, in their review and experiments,
reported that the initial dissolution rates of Pu02 (238 and 239 isotopes) are
higher and more erratic than the subsequent, longer-term rate which was relaThose investigators attributed the initial, rapid rate as
tively constant.
possibly due to the initial reactions of the oxide with the solution. We
would propose that an alternative but not unrelated mechanism could involve
recoil aggregates; that is, the decay events which occurred postcalcination
would cause a buildup of recoil-derived fragments at the microsphere surface.
When placed in water, these are released to the aqueous phase as colloidal
material.
In our case, we have observed that the Pu in this initial release
behaves as Pu. and also sediments in the ultracentrifuge, while the oxidized
Pu species do not.
239 p,, (0)
a
-(i
oy
The initial contribution of this refractory Pu resulted in initially higher
3
This Pu species displayed the proper-
ties of Pu polymer (Bondietti and Reynolds, 1976) because it failed to extract
as an fonic species into organic solvents and sedimented in the ultracentrifuge.
While this Pu02-like species dominated the initial equilibration period (several
weeks), the presence of an oxidized Pu species (V or V1) was also observed.
The rate of appearance of this oxidized Pu was nearly constant over periods of
up to 127 days. The net result of the appearance of these two Pu forms was to
have an initially high and erratic rate, which appeared to be due to the
refractory species, and a subsequent slower rate which was entirely contributed
by the oxidized species.
SOLUBLE Pu’, 10g
dominated by a "refractory" Pu species.
242
O
pu (3.7% 0F TOTA
The oxidation of Pu appears to be due to radiolysis.
Hydrogen peroxide will
oxidize tracer amounts of Pu(IV)} to Pu(¥) or (VI} in alkaline solutions (Connick,
1954).
We have observed, for example, the oxidation of Pu(IV) by peroxide
under the following initial conditions:
M NaHCO3.
107!! M Pu(IV), 107° M H209, and 1073
Figure 6 illustrates that in the case of the micro pheres) oxidation
apparently occurs near the oxide surface.
The appearance of 733py °7
as
shown in the figure, increased with time, starting at about 1.5 x 107!) g
dissolved 739Py at 1 week, and increasing to about 12.9 x 10-!! g dissolved
Pu at 10 weeks.
In addition to the Pu originating from the oxide, the aqueous
phase (20 ml) initially contained about 66 x 107!!
g of ?7%2pu. The 2"2Pu was
added initially in the tetravalent state.
Throughout the 10-week period,
only about 3% of the total added 242py appeared in the oxidized state, even
though the mass of 242py was comparable to the 233py mass appearing in solution.
o
2
4
6
8
TIME (weeks)
-3
M sodium biFig. 6. Appearance of oxidized plutonium in 10
carbonate solution contacting 1.77 mg of Pus. The
242
239p. originated from the Pu0,, while "Pu had been
added separately.
464
465
10