Annual Report

Atomistic Simulation of Dislocation Creep in
UO2 Nuclear Fuel
Researcher: Dr Paul Fossati
Supervisor: Prof Robin Grimes
Sponsors: EPSRC
Understanding the behaviour of fission gas in
nuclear fuel is critical to improving the simulation
of fuel performance. Indeed, fission gases
can be detrimental in different ways whether
they accumulate in bubbles in the fuel or between
the fuel pellets and the clad. This projects
aims at simulating two aspects of the interaction
between dislocations and fission gases,
particularly xenon. The first one is the change
in the mechanical behaviour of UO2 caused by
individual xenon atoms or small bubbles slowing
and pinning dislocations. The other facet
is the enhanced mobility of xenon caused by
the strain fields surrounding the dislocations
cores, and its effect on long-range xenon diffusion.
Both aspects are also of significant interest
for the description of the formation of the
high-burnup structure, which involves both dislocations
piling up and gas bubbles but is not
yet understood at the atomistic scale. This project
is part of the Consortium for Advanced Simulation
of Light Water Reactors (CASL), which is
focused on fuel performance simulation.
Fundamental Properties of Thoria Based Mixed
Oxides
Researcher: Dr Navaratnarajah Kuganathan
Supervisor: Prof Robin Grimes
Sponsors: EPSRC
Thoria (ThO2) has attracted a great deal of interest
as the next generation nuclear reactor fuel
due to its higher abundance, higher thermal
conductivity, higher chemical stability, higher
melting temperature, higher corrosion resistance
and lower thermal expansion compared
to uranium based fuels. During operation in reactor,
uranium, thorium and plutonium atoms
undergo fission producing a variety of fission
products such as Xe, Kr, Cs, Ba, Br and I. Fission
product inert gases (Xe and Kr), produced during
fission in nuclear fuels, are estimated to be
15% of the total fission yield but are insoluble
in the fuel matrix. These inert gases are initially
accommodated at defect sites in the fuel lattice
and are known to have a deleterious effect on
fuel performance, particularly at high levels of
burn-up.
Using first-principles density functional theory
(DFT), we calculate perfect lattice properties
of ThO2, the energetics of defects and the interaction
of Xe and Kr atoms and clusters with
the defects. This project aims to calculate the
structures of atoms and clusters of noble gases
with defective ThO2 to provide soundly based
models for the experimentalist to use in the
interpretation of experimental data. Quantum
mechanical calculations, in addition to giving
structural information, have the added bonus
of elucidating electronic structure and properties.
Our simulation results are validated with
experiments carried out by our collaborators at
Bhabha Atomic Research Centre (BARC) in India.
This projects is funded by EPSRC as part of
the INDO-UK project.
ZrC and MAX Phases for Future Fission
Environments
Researcher: Dr Eugenio Zapata-Solvas
Supervisor: Prof Bill Lee
Sponsors: EPSRC CAFFE Consortium
The aim of this project is to develop new Zrbased
carbides, including Zr-based MAX phases,
for coating Zr-alloys cladding, providing
accident tolerance in fission reactors of future
nuclear power plants. Powders are synthesised
and densified by hot press and spark plasma
33 http://www.imperial.ac.uk/nuclear-engineering