RD&D-Programme 2004 - SKB

simulating the repository environment in the laboratory. Oxygen is present in the air around
us, but in the anoxic experiments the partial pressure of oxygen must be virtually zero. This is
difficult to achieve in practice. Traces of oxygen tend to diffuse in due to the big difference in
partial pressure outside and inside the experiment. Despite flushing with inert gas or the use of
glove boxes filled with inert gas, the conditions in the experiments tend to be more oxidizing
than in the repository.
In order to be able to distinguish the effect of radiolytic oxidation caused by radiation in the
experiments with spent fuel, it is necessary to reduce the influence of oxygen contamination.
Only then is it possible to observe the influence of different reducing components. The results
of experiments involving dissolution of a fuel rod in an argon atmosphere are reported in /15-6/.
The use of metal tubes, glass-metal welds and better seals in the measurement system reduced
the inward diffusion of contaminating oxygen. It could then be observed how radiolysis alone
contributes to an oxidative dissolution of the fuel. After 280 days, two small metal foils of iron
and copper were added to simulate the influence of the canister’s different materials. The fission
products caesium, molybdenum and strontium continued to leach out (Figure 15-2), while the
concentrations of uranium, neptunium and technetium fell. This shows that despite the strong
radiation, the contact with iron creates reducing conditions.
Two other ways of avoiding contamination with atmospheric oxygen have also been tried:
use of an oxygen trap and inert gas under high pressure /15-7/. In the first case, an oxygen
trap with saturated iron(II) carbonate solution was placed in the same closed inert-gas-flushed
space as the test vessel /15-8/. In the second case, a sturdy and well-sealed pressure vessel was
used, where any leakage can easily be checked via changes in the pressure. No leakage may be
tolerated, since tests have shown that even if inert gas flows out of the vessel it is possible for
oxygen to diffuse in upstream /15-9/.
The term “reducing conditions” should not be used indiscriminately for all experiments with
hydrogen-saturated solutions. For even though the hydrogen is potentially highly reducing, it
is also kinetically inert at normal temperatures. This, along with the importance of the negative
redox potentials measured in hydrogen solutions, is discussed in /15-9/.
30
6x10 —6
Sr-90
Tc-99
25
Cs-137
Mo-100
Concentration, ppb
20
15
10
U-238
U-238
4x10 —6
(U)
mol/L
2x10 —6
5
Addition of iron
and copper foil
0
0 100 200 300 400 500
Leaching time, days
0
Figure 15-2. Measured concentrations of 238 U and the fission products 137 Cs, 90 Sr, 99 Tc and 100 Mo
as a function of leaching time. The concentrations of 238 U are shown on the right-hand axis in
mol/l. The vertical dashed line shows the time for introduction of iron and copper electrodes
(each with a surface area of about 30 mm 2 ).
172 RD&D-Programme 2004