RD&D-Programme 2004 - SKB

Influence of hydrogen on dissolution of alpha-doped uranium dioxide
Since radical-rich gamma and beta radiation have a favourable effect on hydrogen activation
accompanied by consumption of radiolytic oxidants, the relevance of the results for the scenario
with spent fuel in a repository can rightfully be questioned. Since virtually only alpha radiation
is expected to persist for a long time, the favourable effect of gamma and beta radiation would
diminish progressively. A material that resembles old fuel is alpha-doped uranium dioxide,
where different quantities of an alpha-emitting isotope are homogeneously distributed in the
uranium dioxide matrix. Alpha-doped uranium oxide has been synthesized in cooperation
with ITU in Karlsruhe. As a part of SKB’s work in the EU project SFS, leaching of a uranium
dioxide pellet doped with ten percent uranium-233 was studied in a titanium autoclave under
three different hydrogen pressures. Leaching was started with a 10 mM sodium chloride solution
under a hydrogen pressure of 16 bar. Samples of the solution were taken at different times
and analyzed by alpha spectrometry. After four months, the estimated uranium concentration
in the solution was still less than 3·10 –10 M. To dissolve all oxidized uranium from the pellet,
a bicarbonate solution was added to the autoclave, but no changes were noted in the uranium
concentration. No measurable dissolution of uranium could be observed, not even when the
hydrogen pressure was reduced to 1.6 bar during a 178-day period. The hydrogen pressure was
further reduced by a factor of ten to 0.16 bar (1.6 bar of argon with a hydrogen concentration of
6.08 percent by volume) during a 141-day period. Preliminary results from alpha radiometric
analyses of uranium-233 are shown in Figure 15-6.
The results show that no measurable increase in the very low uranium concentrations in the
solution could be observed during the entire test period (more than 15 months). If, in this case,
uranyl carbonate species were produced in a layer several tens of microns thick near the surface
(range of alpha particles), some of them would diffuse away and contribute to an increase of the
uranium concentration in the solution. These results are confirmed by the measured levels of
radiolytic oxygen, which lie below the detection limit (10 –8 M) for the oxygen sensor that was
in the autoclave solution during the entire test. The same applies to any radiolytic oxygen or
hydrogen peroxide: some of this would diffuse away from the surface and cause an increase in
the oxygen levels, which would eventually be recorded by the oxygen electrode, since the test
lasts such a long time. There is almost no gamma or beta radiation, and published data /15-23/
with 5 MeV helium ions (alpha radiation) indicate that there is no influence of hydrogen on
100
10 mM NaCl 10 mM NaCl + 2mM HCO 3
—
10 —7
10
10 —8
P H2 (bar)
1
0.1
10 —9
10 —10
10 —11
233 U (mol/l)
0.01
10 —12
0.001
10 —13
0 200 400 600 800 1,000
Time (days)
Figure 15-6. Measured hydrogen pressure and 233 U concentrations in the solution as a function
of time.
RD&D-Programme 2004 177