RD&D-Programme 2004 - SKB

The catalytic effect of uranium dioxide on hydrogen reduction is being studied experimentally.
Results have recently been obtained from tests with depleted uranium dioxide and hydrogen
/15-9/. Dissolved carbonate complexes of hexavalent uranium were used as oxidants. The results
showed that the U(VI) concentration in bicarbonate solutions with hydrogen only decreased
when uranium dioxide surfaces were introduced. The tests were complicated by sorption of
uranyl carbonate on uranium dioxide surfaces. This was investigated in a separate study /15-31/.
There is literature data showing that thorium dioxide surfaces dissociate hydrogen molecules to
atoms /15-32/. Another study using the PVT (Pressure-Volume-Temperature) method /15-33/
shows that plutonium dioxide catalyzes the reaction of hydrogen with oxygen to form water
at room temperature. The same method was used at Studsvik to investigate uranium dioxide,
which, unlike plutonium dioxide or thorium dioxide, is very sensitive to oxidation by oxygen.
Owing to the fact that the PVT method requires total pressures of around 0.1 atm, no conclusion
could be drawn regarding water formation at such low hydrogen concentrations /15-34/. At the
same time, all experimental data suggest a much stronger effect of hydrogen in the presence of
radiation than with uranium dioxide alone.
Metallic particles containing the metals of the platinum group plus molybdenum and technetium
were extracted selectively from spent fuel with non-oxidizing reagent (phosphoric acid instead
of nitric acid) in order to avoid dissolution of the most active metals and changes in composition.
Completely bright particles were isolated, in contrast to the black precipitate that collects
during fuel dissolution with nitric acid. The dissolution rates of various components from these
particles under different redox conditions (pure argon and argon with ten percent hydrogen)
have been determined and published /15-35/.
Figure 15-5 shows data from leaching of metallic particles in solution containing approximately
0.08 mM dissolved hydrogen. After the slow saturation of the solution with hydrogen during the
first two days, the concentrations of all redox-sensitive radionuclides, including molybdenum,
decline rapidly to sub-ppb levels (uranium, strontium and caesium come from a small, poorly
soluble fuel fragment). These data clearly show that surfaces of metallic particles in fuel have a
very strong effect on hydrogen and contribute to its activation.
Concentration, ppb
100
10
1
0.1
0.01
0.00 5.00 10.00 15.00 20.00
Leaching time, days
Tc-99
Mo-100
Ru-102
Ru-103
Ru-104
U
Cs-137
Sr-90
Figure 15-5. Results of leaching of 4d metal particles in argon atmosphere with ten percent
hydrogen /15-35/.
176 RD&D-Programme 2004