RD&D-Programme 2004 - SKB

The influence of hydrogen peroxide on uranium dioxide surfaces will be studied in an argon and
hydrogen atmosphere. The experiments will be conducted at KTH. The results will be used to
estimate the catalytic effect of uranium dioxide surfaces on consumption of radiolytic hydrogen
peroxide with dissolved hydrogen.
In cooperation with Posiva, dissolution of alpha-doped uranium dioxide will be studied in
solutions with different ionic strengths and redox conditions to determine the matrix dissolution
rate by means of the isotope exchange method. This is a part of the EU project NF-Pro.
Reactions with water vapour on the surface of uranium dioxide will be studied by means of
experimental microscopy in combination with quantum mechanics and molecular mechanical
modelling. The work will be done at the University of Michigan in the USA. Many studies in
recent years have shown that molecules of oxygen, hydrogen and water vapour dissociate when
they react with the uranium dioxide surface. This is probably a part of the catalytic mechanism.
The planned tests of fuel dissolution under repository conditions in the Chemlab probe in the
Äspö HRL are still in a planning phase due to delays of ongoing actinide tests. The experiments
require careful radiological monitoring during preparations, execution and associated transport.
The exact time required cannot be calculated, but the goal is to be able to start pilot studies in
the next few years.
15.2.13 Dissolution of gap inventory
Conclusions in RD&D 2001 and its review
It has always been assumed in the safety assessment that the radionuclide inventory in the gap
between fuel and cladding tube and in the grain boundaries is released immediately when it
comes into contact with water. In the review of RD&D 2001, SKI emphasized the importance
of obtaining better experimental data for this, which is usually described as instant release when
fuel dissolution is modelled in the safety assessment.
Newfound knowledge since RD&D 2001
Leaching of metallic particles extracted from spent fuel by selective non-oxidizing dissolution
of the fuel matrix in an argon atmosphere gave similar leaching rates for technetium and
molybdenum. These were in turn three orders of magnitude higher than the leaching rates for
ruthenium, rhodium and palladium. The results from leaching in an argon atmosphere with ten
percent hydrogen are dealt with in section 15.2.12.
Programme
Planned leaching tests to determine the fraction of readily soluble radionuclides that are released
in the fuel-clad gap are described in section 15.1.8.
15.2.14 Speciation of radionuclides, colloid formation
Conclusions in RD&D 2001 and its review
SKI comments that uncertainty and sensitivity analyses of solubilities based on thermodynamic
calculations must be presented and that quality issues relating to database management must be
taken into account. Kasam believes that SKB should devote greater attention both theoretically
and practically to plutonium chemistry.
Newfound knowledge since RD&D 2001
Solubility calculations show that plutonium can exist in certain groundwaters as trivalent
plutonium. A new literature study of the solubility of plutonium dioxide /15-46/ in the presence
180 RD&D-Programme 2004