Annual Report

adiation damage, leaching and corrosion can
be studied without the danger of a high radiation
field. These features would be extremely
difficult and expensive to investigate using real
spent nuclear fuel.
Atomistic Simulation of UK Spent Nuclear Fuel
Researcher: Michael Cooper
Supervisor: Prof Robin Grimes
Sponsor: NDA and EPSRC
The UK has had an active nuclear industry since
the early 60s. As such it has accumulated a significant
amount of nuclear waste primarily from
Advanced Gas-cooled Reactors (AGRs). The
planned closure of the Thermal Oxide Reprocessing
Plant (THORP) at Sellafield means that
the National Decommissioning Agency (NDA) is
now interested in the option of deep geological
disposal of unprocessed Spent Nuclear Fuel
(SNF). This project aims to use an atomistic
modelling approach to investigate the suitability
of AGR-SNF for this form of disposal. In particular,
the transferability of data from the well
characterised Pressurised Water Reactor (PWR)
SNF is of interest.
As part of the UK Spent Fuel Research Group,
with members from Imperial College, Cambridge
University and Lancaster University, this
project aims to support their experimental work
on UO2 based simulant (SIMFuel) samples of
AGR-SNF. The atomistic modelling techniques
implemented include both static and dynamic
simulation using empirically fitted potentials
in addition to electronic structure calculations
utilising Density Functional Theory (DFT). Problems
that are suitable for investigation using
such techniques include: He bubble resolution,
fission product (FP) solution in UO 2 and
secondary SNF phases, FP migration and the
behaviour of dislocations. To date the partition
energies for the transfer of FPs between UO2
and the secondary phases, (Ba/Sr)ZrO3 has
been investigated by static energy minimisation
using empirical pair potentials. The results
of these calculations predict the segregation of
large trivalent fission products (Sm 3+ , Nd 3+ , Pr 3+ ,
and La 3+ ) into BaZrO3 from stoichiometric UO2.
Furthermore, excess Cr 3+ in the fuel results in
the partition of Y 3+ , Dy 3+ , and Gd 3+ as well as the
above FPs. This work has been submitted to the
Journal of Nuclear Materials. A similar approach
is now being applied to CrUO4. In parallel to the
work on (Ba/Sr)ZrO3 an investigation into the
migration of uranium vacancies as a function
of UO2 stoichiometry is being undertaken. It is
thought that this may underpin the method of
Xe and Kr transport though SNF.
Finally, a new method of empirically modelling
UO 2 that better reproduces the temperature
dependence of various mechanical and thermodynamic
properties is being studied. Understanding
the effect of temperature on UO 2 is
expected to be one of key differences between
PWR-SNF and AGR-SNF.
Low Melting Glass Wasteforms for Fukushima
Absorbing Membranes
Researcher: Dimitri Pletser
Supervisor: Prof Bill Lee
Sponsor: Hitachi-GE
The accident at Fukushima in Japan has led to
the generation of enormous volumes of High
Dose Spent Adsorbents (HDSA) which are currently
stored on site but need to be immobilised
in a solid wasteform and then permanently disposed
of. The majority of the radionuclides in
the HDSA are Cs and Sr with relatively short
half-lives so that the level of performance of
the wasteform is not as demanding as for vitrified
High Level Waste. The process used to im-
Centre for Nuclear Engineering Annual Report 2014-2016 52