The two uranyl peroxides meta-studtite and studtite exist in nature and can form as alteration phases on the surface of spent nuclear fuel upon water intrusion in a geological repository....
Hydrogen peroxide
is produced upon radiolysis of water and has
been shown to be the main oxidant driving oxidative dissolution of
UO
2
-based nuclear fuel under geological repository conditions.
While the overall mechanism and speciation are well known for granitic
groundwaters, considerably less is known for saline waters of relevance
in rock salt or during emergency cooling of reactors using seawater.
In this work, the ternary uranyl–peroxo–chloro and uranyl–peroxo–bromo
complexes were identified using IR, Raman, and nuclear magnetic resonance
(NMR) spectroscopy. Based on Raman spectra, the estimated stability
constants for the identified uranyl–peroxo–chloro ((UO
2
)(O
2
)(Cl)(H
2
O)
2
–
) and uranyl–peroxo–bromo ((UO
2
)(O
2
)(Br)(H
2
O)
2
–
) complexes are
0.17 and 0.04, respectively, at ionic strength ≈5 mol/L. It
was found that the uranyl–peroxo–chloro complex is more
stable than the uranyl–peroxo–bromo complex, which transforms
into studtite at high uranyl and H
2
O
2
concentrations.
Studtite is also found to be dissolved at a high ionic strength, implying
that this may not be a stable solid phase under very saline conditions.
The uranyl–peroxo–bromo complex was shown to facilitate
H
2
O
2
decomposition via a mechanism involving
reactive intermediates.
Understanding the
possible change in UO2 surface reactivity
after exposure to oxidants is of key importance when assessing the
impact of spent nuclear fuel dissolution on the safety of a repository
for spent nuclear fuel. In this work, we have experimentally studied
the change in UO2 reactivity after consecutive exposures
to O2 or γ-radiation in aqueous solutions containing
10 mM HCO3
–. The experiments show that
the reactivity of UO2 toward O2 decreases significantly
with time in a single exposure. In consecutive exposures, the reactivity
also decreases from exposure to exposure. In γ-radiation exposures,
the system reaches a steady state and the rate of uranium dissolution
becomes governed by the radiolytic production of oxidants. Changes
in surface reactivity can therefore not be observed in the irradiated
system. The potential surface modification responsible for the change
in UO2 reactivity was studied by XPS and UPS after consecutive
exposures to either O2, H2O2, or
γ-radiation in 10 mM HCO3
– solution.
The results show that the surfaces were significantly oxidized to
a stoichiometric ratio of O/U of UO2.3 under all the three
exposure conditions. XPS results also show that the surfaces were
dominated by U(V) with no observed U(VI). The experiments also show
that U(V) is slowly removed from the surface when exposed to anoxic
aqueous solutions containing 10 mM HCO3
–. The UPS results show that the outer ultrathin layer of the surfaces
most probably contains a significant amount of U(VI). U(VI) may form
upon exposure to air during the rinsing process with water prior to
XPS and UPS measurements.
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