Understanding the
possible change in UO2surface 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 UO2reactivity after consecutive exposures
to O2or γ-radiation in aqueous solutions containing
10 mM HCO3–. The experiments show that
the reactivity of UO2toward O2decreases 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 UO2reactivity 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 UO2.3under 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.