Annual Report

ding; and, the formation of, and structure of,
fuel pel let cracking and the role that this plays
in pellet clad mechanical interaction.
Heterogeneous Deformation in Zirconium
Alloys
Researcher: Vivian Tong
Supervisor: Dr Ben Britton
Sponsors: Rolls-Royce
Zirconium is used in the nuclear industry in
thin walled tubes and is therefore of practical
importance for engineering applications. It has
anisotropic mechanical properties leading to
strong crystallographic textures, twinning, and
heterogeneous plastic strains during forming
operations such as sheet rolling and tube reducing.
Micromechanical modelling cannot at present
accurately predict how plastic strain concentrations
and cracks develop, or which deformation
modes are favourable in HCP materials. Therefore,
high quality experimental characterisation
is needed to determine how different types
of deformation and heat treatments affect the
grain growth, texture, and mechanical properties,
and how these are related to the deformation
mechanisms (slip, twinning, recrystallisation,
etc.). In particular, this project aims to
characterise twinning behaviour of zirconium,
focussing on its dependence on strain rate,
grain size, geometrically necessary dislocation
density, and local texture.
Electron backscatter diffraction (EBSD) is being
used to characterise the micro and macro
texture, grain size, and twinning fraction. Digital
image correlation will be used to measure
macroscopic strain and strain rate. High resolution
EBSD (HR-EBSD) will be used to characterise
microscopic elastic and plastic strain during
deformation. The accuracy and limitations
of HR-EBSD are also explored to validate the
technique.
Inverse Numerical Method for Calculating the
Temperature Dependent Thermal Conductivity
of Nuclear Materials
Researcher: Tsveti Pavlov
Supervisors: Prof Robin Grimes, Dr Mark Wenman,
and Dr Paul Van Uffelen (Institute for Transuranium
Elements)
Sponsors: European Commission and Imperial College London
In the general context of nuclear fuel safety and
after the accident in Fukushima, investigating
the behaviour of nuclear materials under
extreme conditions is of prime importance for
the analysis of the reactor operational limits.
Relevant experiments in an experimental reactor
are time consuming, expensive and their
analysis is challenging because of limited instrumentation
possibilities.
Thus, this project will focus on the development
of a method for the calculation of thermophysical
properties such as thermal conductivity.
The technique will be validated and
applied to commercial and novel fuel materials
at high temperatures. The proposed method
uses experimental thermograms obtained via
laser- flash heating of a disc-shaped sample in
combination with finite element analysis and
parameter optimization. The experimental part
involves heating samples to a steady state temperature
via two lasers (on the back and front
sides) and subsequently subjecting the front
sample surface to a short laser pulse, resulting
in a temperature transient (thermogram). A thermal
camera records the temperature transients
at 30 points along the radius on the rear surface
of the sample. An optimization technique
known as the Levenberg-Marquardt method
is applied, whereby 5 parameters (emissivity,
Centre for Nuclear Engineering Annual Report 2014-2016 36