RD&D-Programme 2004 - SKB

of divalent iron shows that trivalent plutonium is the dominant soluble species. Studies have
been conducted to identify solubility-limiting phases of trivalent plutonium and determine
their solubility (Chalmers University of Technology). Experimental methods for preparing and
analyzing plutonium in different oxidation states were developed during the period. A study of
Pu(OH) 3 (s) solubility under a hydrogen pressure of 50 bar /15-47/ showed that the plutonium
concentrations in solution after one year were between 10 –8 M at a pH of 10 and 10 –5 M at a
pH of 3. All attempts to prepare PuOHCO 3 (s), which is expected to be formed under repository
conditions, resulted in X-ray amorphous phases. The results of measurements of the solubility
of solid phases of plutonium carbonate were recently published /15-48/.
In the scenario with a damaged canister, the redox conditions in the near-field are of very great
importance. For example, the actinides that are present in the fuel have a much lower solubility
in the tetravalent state than in the penta- or hexavalent states. Radiolysis can influence the redox
conditions, and the radiation is intensive at the surface of the fuel. This can cause the uranium
oxide in the fuel to be oxidized to a higher valence state and go into solution as uranyl ions. The
actinides neptunium and plutonium, as well as redox-sensitive fission products such as technetium
and selenium, can also be affected in a similar manner and be oxidized to a higher valence
state, resulting in increased solubility and mobility. Colloids can also be formed in a damaged
canister, especially in conjunction with the reduction of radionuclides released from the fuel to
lower and less soluble valence states, or if dissolved divalent iron is oxidized and precipitates.
A research programme for studying the redox processes that are expected to occur in a damaged
canister, especially their kinetics, has been under way for several years at SKB. The influence of
uranium dioxide surfaces on the concentrations of oxidizing radionuclides has been investigated
under anoxic conditions. Other important components in the near-field are cast iron and its
corrosion products, as well as dissolved divalent iron.
In the EU project In Can Processes (InCan), two closely-related areas were studied /15-49,
15-50/:
1. Reduction of hexavalent uranium and pentavalent neptunium in the presence of corroding
iron (the environment inside a waste canister).
2. Theoretical ab initio calculations of the possibility of reducing U(VI) solution with divalent
iron.
Reduction of hexavalent uranium and pentavalent neptunium in solutions containing corroding
iron showed rapid decreases in the concentrations in solution. Examination of the iron surfaces
by Rixs (Resonant inelastic X-ray scattering) showed that tetravalent uranium and tetravalent
neptunium had been precipitated on the iron surface. Electrochemical studies of reduction of
hexavalent uranium in solution with subsequent determination of the valence state showed that
after one day half of the uranium had been reduced to E h –400 mV due to actively corroding
iron in the system, which resulted in elevated concentrations of Fe(II) ions in solution. Tests in
0.1 M sodium chloride solution also showed rapid decreases in the uranium concentration. From
the original concentration of 500 ppb, the concentration fell to less than 10 ppb in three days; in
the solution, however, 86 percent of the uranium in solution was still hexavalent.
Ab initio calculations showed that Fe(II) ions in solution can reduce hexavalent uranium to
pentavalent uranium, which is in turn disproportioned to tetravalent and hexavalent uranium.
These results improve our understanding of the electrochemical experiments, which showed
reduction of uranium both in solution and on corroding iron.
A study of the interaction between hexavalent uranium and magnetite (synthetic or formed on
a corroded iron surface) in the presence of different hydrogen pressures showed a reduction
in the concentrations of uranium in the solution /15-51/. Tests with carbon steel coupons,
where magnetite had been identified by X-ray diffraction and XPS (X-ray Photoelectron
Spectroscopy), show a considerably greater ability to reduce dissolved hexavalent uranium.
Examination of the magnetite surfaces by Xanes (X-ray absorption near edge structure) and
XPS showed the presence of tetravalent uranium on magnetite surfaces.
RD&D-Programme 2004 181