RD&D-Programme 2004 - SKB

238 U
237 Np
239 Pu
Concentration, mol/l
10 —7
10 —8
10 —9
10 —10
10 —11
10 —6 0.5 MPa (H 2 +0.03%CO 2 )
0.5 MPa Ar
144 Nd
153 Eu
10 —12
0 100 200 300 400 500
Leaching time, days
Figure 15-3. Measured concentrations of actinides (U, Pu, Np) and lanthanides (Eu, Nd) as a
function of the time for leaching of spent fuel powder in a 10 mM NaCl, 2 mM NaHCO 3 solution
and either 0.5 MPa hydrogen mixed with 0.03 percent carbon dioxide or only 0.5 MPa argon.
concentrations of actinides and lanthanides in solution samples taken at different points in time
(Figure 15-3) show similar values as in the stainless autoclave. Investigation of the behaviour
of redox-sensitive nuclides such as uranium, technetium and neptunium during the first days
(which were missed before) showed that their already low starting concentrations declined with
time. This suggests that they are reduced, which is also found in other studies /15-18, 15-19/.
The fact that only low concentrations of radionuclides were measured in autoclave solutions
/15-19/ indicates that this reduction takes place at the surface of the fuel.
Another goal of the study was to follow and understand what happens with oxygen that is
generated by radiolysis of water in hydrogen-saturated solutions. A gas sample taken from
the autoclave after 356 days and analyzed by mass spectrometry showed oxygen concentrations
below the detection limit (approx. 50 ppm or 5·10 –8 mol/l). The same results were found
in another, independent study /15-20/. When these data are compared with the high oxygen
concentrations measured in similar tests and during similar time periods /15-21/ but under an
argon atmosphere (Figure 15-7), plus the extremely low concentrations of uranium and other
experimental observations, it can be concluded that radiolytic oxidants must be consumed by
hydrogen under such conditions and that no measurable oxidative dissolution of fuel occurs
/15-17/. This is confirmed by an analysis of the released fraction of fission products, such as
strontium-90 (Figure 15-4). This was not noticed in stainless autoclaves, and one explanation
may be that despite all efforts to avoid contamination with air, the gas analysis showed that there
was nitrogen in the autoclave /15-17/. But since corresponding oxygen quantities would have
caused a nearly tenfold higher increase of the concentrations of strontium-90 if the oxygen had
only been consumed to oxidize the fuel surface, this indicates that dissolved hydrogen consumes
not only radiolytic oxidants, but also the oxygen from air contamination.
These and other experimental data suggest that hydrogen must be activated under experimental
conditions in order to be able to consume oxidants from radiolysis and oxygen from air
contamination, and cause reduction of radionuclides.
There are two possible ways to activate hydrogen under experimental conditions: either by
reactions with radicals from radiation-induced radiolysis, or by reaction with fuel surfaces
(uranium dioxide surfaces and/or metallic particles in the fuel).
174 RD&D-Programme 2004