RD&D-Programme 2004 - SKB

Oxidation of uranium dioxide with hydrogen peroxide has been studied at KTH. The mechanism
of the reaction and the reaction rate have been determined /15-42/. A comparison of the
reaction rate with data for other oxidants, including oxidizing radicals determined in a separate
study /15-43/, showed that the logarithm of the reaction rate is linearly dependent on the reduction
potential. This is of great importance, since it shows that thermodynamic data can be used
to estimate kinetic data for oxidation of uranium dioxide with oxidizing radicals, something
which is often lacking. Kinetic data from these studies, along with literature data, were used
for improved radiolytic modelling of fuel dissolution data from mass balance studies /15-27/.
Radiolytic modelling has also been carried out within the EU project SFS. The work was done
by Studsvik with the support of SKB /15-44, 15-45/.
Programme
Research activities are planned during the coming period both to obtain data on fuel dissolution
under repository-like conditions and to shed light on the mechanisms of the different processes
that contribute to fuel dissolution. SKB is participating in the EU project NF-Pro, and some of
the studies will be conducted there.
In order to obtain fuel data for the first phases in a scenario with a damaged canister, when the
temperature is still high, fuel in the presence of water vapour and hydrogen will be studied. This
will be done within the framework of the EU project NF-Pro, in cooperation with ITU.
A new autoclave with soft metal gaskets and other improvements to avoid air contamination has
been purchased and will be used to study fuel leaching in different atmospheres of argon and
hydrogen. A parameter study of temperature and hydrogen pressure during leaching will be conducted
to determine the lowest hydrogen concentration that is required to avoid fuel oxidation.
Test with fuel pellets will be conducted in Studsvik’s hot cells in an argon atmosphere and in
the presence of siderite (iron(II) carbonate) to study the effect of dissolved divalent iron on
fuel leaching. The tests with fuel pellets in an atmosphere or argon with hydrogen and carbon
dioxide (ten percent hydrogen and 0.03 percent carbon dioxide) will be concluded and the
results reported.
There is some Mox fuel in Clab that must also be disposed of, and studies of this type of fuel are
planned. Pending a permit to conduct the experiments in Studsvik, a study of Mox fuel leaching
under a hydrogen atmosphere has been started in Germany in cooperation with ITU.
There are many discussions of the behaviour of the so-called rim zone in high-burnup fuel. The
alpha activity is much higher there, and there are many small fission gas bubbles (cauliflower
structure), which complicates the picture. We are therefore planning a study where the rim zone
can be separated from a high-burnup fuel pellet and studied to determine how it is leached under
a hydrogen atmosphere. The work will be done in cooperation with ITU.
Leaching of synthetic non-radioactive metallic particles in an argon atmosphere with ten percent
hydrogen will be carried out. The results will be compared with what is obtained with radioactive
particles under the same conditions. In this way it is hoped that conclusions can be drawn
regarding the importance of different contributions to hydrogen activation.
A mechanical study of the hydrogen effect will be conducted within the framework of the EU
project NF-Pro. In order to estimate and quantify the contribution of the fuel’s surfaces and
alpha radiation, these factors must be studied separately. Tests with either depleted uranium
dioxide, unused fuel or Simfuel in aqueous solutions with different concentrations of dissolved
hydrogen or argon will be conducted to study the effect of surfaces. The production of oxidants
will be studied as a function of time in solutions with different concentrations of dissolved
plutonium-238, which is a strong alpha emitter, and with different concentrations of dissolved
argon or hydrogen. The experiments will be performed at Chalmers University of Technology.
RD&D-Programme 2004 179