RD&D-Programme 2004 - SKB

oxidant consumption in the bulk solution. At the end of the test, the uranium dioxide surface
will be examined by spectroscopic methods to determine if oxidation has occurred. Published
independent experiments with alpha radiolysis in hydrogen-saturated solutions and uranium
dioxide surfaces show that no oxidation occurs /15-29/. Titanium surfaces will also be studied
with respect to uranium. Even though titanium surfaces are not expected to influence the results,
the tests will be repeated in autoclaves with quartz surfaces. Independent results from leaching
of similar uranium dioxide pellets in glass vessels with only six percent hydrogen gas in argon
do not show any increase in the uranium concentrations with time either /15-20/.
Influence of pH, salinity and temperature
The dissolution rate of spent fuel is dependent on more than just the redox potential, even
though this is the most important factor. Other factors that influence the dissolution rate are the
composition of the fuel and the groundwater. Experiments with leaching of fuel samples taken
from different segments in a fuel rod with burnup varying from 21 to 49 MWd/kgU /15-36/
show a weak and almost linear increase of leaching (cumulative released fraction) up to
40–43 MWd/kgU, followed by a decrease. The influence on the formation of secondary phases
in connection with fuel dissolution is investigated separately /15-37/. The water composition in
all other studies has been chosen to avoid the formation of secondary phases, since this makes it
more difficult to interpret the results /15-38/.
Fuel dissolution at different pHs from 3 to 9.2 and different carbonate concentrations from
0 to 10 mM has been studied by flow-through leaching /15-39/. The tests show that low pHs
increase the dissolution rate under oxidizing conditions. In hydrogen-saturated solutions, pH
had no influence. The importance of very high pHs typical of concrete leaching has also been
investigated /15-40/.
The influence of temperature has been studied in the range 20–70°C with the aid of autoclaves
filled with argon or hydrogen, and studies at higher temperature are planned, see under the
heading “Programme” below.
SKB has only performed a few of its own measurements of fuel dissolution at high salinities.
Studies of the dissolution of alpha-doped uranium dioxide employing the isotope dilution
method at low and high salinities and development of analysis methods are planned in
cooperation with Posiva within the framework of the EU project NF-Pro.
Radiolysis studies and radiolytic modelling
Mass balance tests have been conducted where the time dependence for formation of hydrogen
and oxygen has been studied in a closed system with approximately two grams of fuel fragments
and with initially oxygen-free solutions. In order to permit mass balance calculations, a new
leaching system has been used permitting simultaneous analysis of both gas phase and solution.
Oxygen and hydrogen in the gas phase were measured at different times by means of gas
phase electrode detectors. The aqueous phase was analyzed with respect to hydrogen peroxide
(luminescence measurements) and dissolved radionuclides (ICP-MS). The way in which the
composition of the solution influences radiolytic oxidant production and fuel dissolution has
been studied and modelled /15-41/. Carbonate and chloride concentrations in the solution were
varied between 0 and 10 mM.
Since analysis of the gas phase during the course of the test always entails a risk of leakage and
loss of radiolysis gases, particularly as their concentrations grow higher with time, long-term
tests with sealed glass vials have also been carried out. In the test series, the size of the fuel fragment
(surface-area-to-volume ratio) and the composition of the solution (chloride and carbonate
contents) were varied. The vials were opened and the gas phase and the solution were analyzed
after one, two and three years. The measured concentrations of radiolytic oxygen and hydrogen
peroxide that did not react with the fuel surface, as well as the concentrations of dissolved
uranium, were used to calculate a mass balance of radiolytic oxidants /15-21/.
178 RD&D-Programme 2004