RD&D-Programme 2004 - SKB

A study has been conducted in cooperation with Posiva of the mechanism for dissolution of
pre-oxidized gadolinium-doped uranium dioxide /15-10/. The results show that release of
hexavalent uranium in the solution causes an increase in the lattice parameter for uranium
dioxide, which suggests that the whole matrix is enriched in tetravalent uranium.
In the EU project In Can Processes (InCan), the dissolution of uranium dioxide was studied
with the aid of isotope dilution. This method can be used to measure the dissolution rate even
if precipitation occurs /15-11/.
Under oxidizing conditions, unirradiated uranium dioxide from different suppliers was found to
have different dissolution rates and give rise to different concentrations of uranium in solution.
We believe that this may be a result of differences in the sintering process. The concentrations
after extended exposures were between 9 and 13 ppm and showed clear signs of simultaneous
dissolution and precipitation in a quasi-steady state. Corrosion experiments under actively
reducing conditions showed an initial pulse followed by precipitation and low levels of the
uranium concentration after a few days. The concentration of uranium in solution was then
less than 4·10 –11 mol/l. This is lower than predicted by available data on uranium solubility.
Experiments were also conducted with uranium dioxide that contained 5 and 10 percent
uranium-233. These materials have an alpha activity equivalent to that of spent fuel 10,000 and
3,000 years after deposition in a deep repository. It has been assumed that fuel corrosion under
reducing conditions can be affected by radiolysis of the water due to alpha radiation. Radiolysis
would create local oxidizing conditions at the fuel surface and thereby increase the dissolution
rate. No sign of this was seen in the experiments that were conducted within the framework of
the InCan project. All samples had uranium concentrations of less than 0.02 ppb, i.e. the same as
undoped samples. By adding a solution of enriched uranium-235, it was possible to see that the
uranium concentration in an experiment that lasted a month was around 0.005 ppb.
Experiments were also conducted with fuel samples from previous studies. The samples were
equilibrated for six months in groundwater with low ionic strength in a 10 bar hydrogen atmosphere
to create anaerobic and preferably reducing conditions. The solution was then exchanged
for fresh oxygen-free groundwater and a solution enriched in uranium-235 was added. More
was added than desirable (the total concentration of uranium after addition was 40 to 125 ppb)
and precipitation started immediately. After a week the uranium concentrations were 20 to
50 ppb, and after 18 days they were 6 to 15 ppb. Despite ongoing precipitation, it could be seen
from the changes in the isotope ratios that the spent fuel continued to dissolve at a rate of 0.2 to
0.3 ppb per day. This may be due to the effects of beta and gamma radiation.
Influence of hydrogen on dissolution of fuel
Hydrogen is expected to be present in large quantities during long periods of time in a damaged
canister with a hole of limited size. This is because hydrogen gas is produced by anoxic iron
corrosion at a higher rate than the rate of mass transport by diffusion outside the canister
/15-12, 15-13/. The first results from fuel dissolution in stainless autoclaves under a hydrogen
pressure of 50 bar showed that hydrogen has a great influence on fuel dissolution /15-14/. The
concentrations of actinides and fission products during the entire test period were very low
and constant within analytical margins of error, which suggests very low fuel dissolution rates.
Even though the concentrations of dissolved hydrogen in the autoclave (approximately 40 mM)
were many orders of magnitude higher than the concentrations of redox-sensitive radionuclides
and radiolytic oxidants, hydrogen gas is expected to be chemically inert at low temperatures.
There are, for example, figures in the literature that indicate that hexavalent uranium /15-9/,
heptavalent technetium /15-15/ and pentavalent neptunium /15-16/ are not reduced in the
presence of dissolved hydrogen at room temperature.
In order to prevent all contact between the leachant and metal surfaces and thus avoid
discussing their influence, a new test was performed in autoclaves with internal quartz
surfaces under a hydrogen pressure of five bar /15-17/. The results of analyses of the
RD&D-Programme 2004 173