Abstract
Diffusion coefficients for oxygen and hydrogen were determined from a series of natural uraninite–H2O experiments
between 50 and 700 C. Under hydrous conditions there are two diffusion mechanisms: (1) an initial extremely fast-path diffusion
mechanism that overprinted the oxygen isotopic composition of the entire crystals regardless of temperature and (2) a
slower volume-diffusive mechanism dominated by defect clusters that displace or eject nearest neighbor oxygen atoms to form
two interstitial sites and two partial vacancies, and by vacancy migration. Using the volume diffusion coefficients in the temperature
range of 400–600 C, diffusion coefficients for oxygen can be represented by D = 1.90e5 exp (123,382 J/RT) cm2/s
and for temperatures between 100 and 300 C the diffusion coefficients can be represented by D = 1.95e10 exp (62484 J/
RT) cm2/s, where the activation energies for uraninite are 123.4 and 62.5 kJ/mol, respectively. Hydrogen diffusion in uraninite
appears to be controlled by similar mechanisms as oxygen. Using the volume diffusion coefficients for temperatures between
50 and 700 C, diffusion coefficients for hydrogen can be represented by D = 9.28e6 exp (156,528 J/RT) cm2/s for temperatures
between 450 and 700 C and D = 1.39e14 exp (34518 J/RT) cm2/s for temperatures between 50 and 400 C, where
the activation energies for uraninite are 156.5 and 34.5 kJ/mol, respectively.
Results from these new experiments have implications for isotopic exchange during natural UO2–water interactions. The
exceptionally low d18O values of natural uraninites (i.e. 32& to 19.5&) from unconformity-type uranium deposits in
Saskatchewan, in conjunction with theoretical and experimental uraninite–water and UO3–water fractionation factors, suggest
that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low
d18O values have been interpreted as resulting from the low temperature overprinting of primary uranium mineralization
in the presence of relatively modern meteoric fluids having d18O values of ca. 18&, despite petrographic and U–Pb isotope
data that indicate limited alteration. Our data show that the anomalously low oxygen isotopic composition of the uraninite
from the Athabasca Basin can be due to meteoric water overprinting under reducing conditions, and meteoric water or
groundwater can significantly affect the oxygen isotopic composition of spent nuclear fuel in a geologic repository, with minimal
change to the chemical composition or texture. Moreover, the rather fast oxygen and hydrogen diffusion coefficients for
uraninite, especially at low temperatures, suggest that oxygen and hydrogen diffusion may impart characteristic isotopic signals
that can be used to track the route of fissile material.
