Bristol research helps extend life of nuclear power stations, with major financial and environmental benefits
Submitting InstitutionUniversity of Bristol
Unit of AssessmentPhysics
Summary Impact TypeTechnological
Research Subject Area(s)
Physical Sciences: Other Physical Sciences
Engineering: Biomedical Engineering, Materials Engineering
Summary of the impact
Researchers at the University of Bristol's Interface Analysis Centre
played a key role in making it possible to extend the life of two nuclear
power stations. Their insights into how the microstructure of reactor-core
graphite degrades during service and how the material fractures enabled
Magnox Ltd to construct a convincing safety case for Oldbury nuclear power
station to operate for an extra four years and Wylfa power station to run
for an additional four to six years. In terms of the value of the
electricity generated, these extensions are worth some £5 billion. In
addition, the longer lifespan of these low-carbon power sources means that
less energy has to be generated from other, high-carbon sources, with the
environmental benefit of an overall reduction in CO2 emissions.
Graphite bricks are used as a moderator for Magnox and advanced
gas-cooled nuclear reactors (AGRs). The graphite is polygranular, porous
and quasi-brittle (i.e., the material remains brittle but behaves in a
non-linear fashion prior to macroscopic failure).
During service, the graphite reactor core is subject to an environment of
neutron irradiation and hot CO2 gas which changes the
microstructure and the mechanical and physical properties of the graphite.
The mass-loss and associated decrease in strength is monitored by
periodically removing surveillance specimens installed at commencement of
operation and samples trepanned from fuel and interstitial channel bricks.
These provide material on which to undertake a range of measurements
including porosity (weight loss) and mechanical properties. However, it is
important to be able to understand the mechanisms that lead to these
changes in order to maintain safe operation for the whole of the reactor's
service life, which in the case of the Oldbury and Wylfa Magnox power
stations is more than 40 years rather than 25 years as originally
To provide improved understanding of the mechanisms leading to the
observed changes in the properties of pile grade A (PGA) graphite used for
Oldbury and Wylfa, Magnox Ltd placed a series of research contracts with
the University of Bristol between 2005 and 2011.
The aim was to investigate the mechanisms leading to changes in (i) the
microstructure and (ii) the deformation and fracture characteristics. A
range of novel, state-of-the-art experimental techniques were employed and
developed to undertake this research.
In 2002, a new materials analysis technique — a focused ion beam (FIB)
instrument — was introduced to the University of Bristol. This allowed the
machining and imaging of materials on a nanometre scale. The technique was
used here to provide high spatial resolution tomographic images of the
virgin and irradiated PGA graphite for the first time. When combined with
commercial software, it was possible to obtain 3-D representations of the
underlying pore structure to a nanometre-scale resolution [1,2]. The size,
shape and inter-connectivity of the pores was established, revealing that
the overall porosity developing over service life was both irregular in
shape and tortuous in morphology, and that the graphite assumed a skeletal
structure when highly porous.
Deformation and fracture behaviour was considered at both the macro- and
micro-scale. In general, at the macro-scale force-displacement traces
showed pre-peak non-linearity and a post-peak progressive decrease in
force [3,4]. Digital image correlation studies using flexural tests showed
macro-crack initiation involving the formation of three- to
five-millimetre-long process zones. Within such localised regions,
micro-cracks initiate and grow as a precursor to the formation of
macro-cracks. The latter propagate preferentially through the porous
matrix, but are impeded and deflected by the filler particles . Detail
of the fracture mode was investigated at the micro-scale using a
combination of FIB milling and imaging, which provided new insights into
the underlying fracture mechanisms [5,6]. The research team found that a
key stage in the overall cracking mechanism was the ability of the
material to store elastic strain energy. Combined with micro-cracking, the
elastic strain energy has to achieve a critical value to allow macro-crack
formation, leading to failure of a component .
Dr. P. J. Heard, University of Bristol Research Fellow; appointed October
Dr. G. M. Hughes, University of Bristol Research Assistant; appointed
December 2003; moved to Department of Materials, University of Oxford,
Dr. S. Nakhodchi, University of Bristol Research Associate; appointed
2007; moved to K.N. Toosi University of Technology, Iran, 2012.
Prof. P. E. J. Flewitt, School of Physics, University of Bristol;
References to the research
 Hughes, G.M. and Wootton, M.R. (2008), A preliminary examination of
the pore structure of graphite using focussed ion beam microscopy, Magnox
Electric Report, MEN/ESTD/GEN/REP/0060/07, Issue 1. Can be supplied upon
 Hughes, G.M., Heard, P.J. and Wootton, M.R. (2009), Examination of
the pore structure of PGA using FIB and Amira 3-D reconstruction software,
Magnox North Report, MEN/EWST/GEN/0001/09. Can be supplied upon request.
 *Nakhodchi, S., Smith. D.J. and Flewitt, P.E.J. (2013), The formation
of fracture process zones in polygranular graphite as a precursor to
fracture, Journal of Materials Science, 48, 720-732. DOI:
 Heard, P.J., Wootton, M.R., Moskovic, R. and Flewitt, P.E.J. (2010),
Crack initiation and propagation in pile grade A (PGA) reactor core
graphite under a range of loading conditions, Journal of Nuclear
Materials, 401, 71-77. DOI: 10.1016/j.jnucmat.2010.03.023.
 *Heard, P.J., Wootton, M.R., Moskovic, R. and Flewitt, P.E.J. (2011),
Deformation and fracture of irradiated polygranular pile grade A reactor
core graphite, Journal of Nuclear Materials, 418, 223-232.
 *Moskovic, R., Heard, P.J., Flewitt, P.E.J. and Wootton, M.R. (2013),
Overview of strength, crack propagation and fracture of nuclear reactor
moderator graphite, Nuclear Engineering and Design, accepted
manuscript in press. DOI: 10.1016/j.nucengdes.2013.05.011.
 Flewitt, P.E.J., Smith, D. and Heard, P.J. (2012-2015) "QUasi-Brittle
fracture: a 3D Experimentally-validated approach, `QUBE'", EPSRC
 Humphries, P.N., Bond, G., Scott, T.B., Heard, P.J., Cooke, D.J. and
Eccles, H. (2011-2015). "The Post-Disposal Behaviour of C-14 and
Irradiated Graphite. `C14-BIG'", EPSRC EP/I036354/1, £728,414.
 Fox, N., Flewitt, P.E.J., Heard P.J., Scott, T.B., Allan, N. and
Cryan M.J. (2012-2016). "Beta-enhanced thermionic energy converters
and nuclear batteries employing nanostructured diamond electrodes",
EPSRC EP/K030302/01, £951,947.
Details of the impact
As part of the research described above, the University of Bristol was
asked in 2007 to undertake a study of the micro- and nano-scale structure
of the nuclear graphite used in Magnox power stations. At that time, the
Oldbury station was due to cease generation in 2008, and Wylfa in 2010. As
detailed in the letter of support from Magnox [a], there was a concern
that the graphite bricks within the core of the reactor could fracture,
which might lead to an alteration in the coolant gas flow path within the
bricks, local hotspots and possibly melting of the fuel. It was
acknowledged that the reactor core was a life-limiting factor for both the
Oldbury and the Wylfa reactors. Further studies were commissioned from
Bristol by Magnox between 2007 and 2011. Confidential reports were
returned to Magnox, and some of the work was published in the open
literature to give the work an independently reviewed pedigree.
Bristol's research improved the understanding of the degradation and
fracture mechanisms of the graphite. It showed that the material is
unlikely to undergo a sudden catastrophic failure, and that instead it
behaves in a "quasi-brittle" way, in which failure is more graceful and
preceded by a more gradual deformation. The novel use of FIB combined with
small-scale mechanical testing to investigate this material at Bristol was
listed as a highlight in the Health and Safety Commission Nuclear Safety
Advisory Committee Review Group, dated 2 October 2007 [b].
As a result of this and other work, a safety case was constructed by
Magnox and a decision taken to extend the closure point of the Oldbury
power station from 2008 to 2012. The lifespan of the Wylfa power station,
which had been scheduled to cease operation in 2010, was also extended,
with one reactor closed in 2012 and the other operating until at least
The safety cases for the operation of nuclear power stations are
incremental, in that the lifetime of each station is continually
reassessed. End-of-life is delayed as long as it is deemed safe to do so.
At each stage, the nuclear authorities require evidence to support the
case for lifetime extension. The safety cases are reviewed by the Office
for Nuclear Regulation (ONR), an agency of the Health and Safety Executive
that oversees the process and grants consent for continued operation. In
addition, Periodic Safety Reviews (PSRs) must be carried out by the
licensee of a nuclear power plant. These provide the opportunity to
undertake a comprehensive review of plant safety, including operational
history, ageing factors and the advancement of safety standards.
A recent ONR Project Assessment Report [c] referred to the lifetime
extension for the Oldbury nuclear power station, stating: "In support of
the graphite safety case and continued operation the licensee also carried
out a programme of work associated with the material properties of the
graphite moderator ....". The work referred to includes that carried out
at Bristol, and as stated in the Magnox letter of support [a], "The
research undertaken by the University of Bristol contributed to the
necessary improved understanding of how the microstructure of reactor core
graphite degrades during service and how such changes modify the
associated deformation and fracture... This information, when combined
with our other research programmes and detailed assessments, was used to
support the arguments in the Safety Cases and provided confidence to the
Regulator that Magnox was making every effort and indeed was successful in
understanding holistically how this complex material (PGA graphite)
performs in service. Indeed it allowed Oldbury to achieve 45 years'
operation and one reactor at Wylfa continues to operate."
The value of this life extension can be estimated by considering that
operation of a 500-megawatt power station turbine/generator unit (roughly
the size of Oldbury) for four years generates revenue of approximately
£1.7 billion for the electricity produced. Hence life extension of four
years for Oldbury and Wylfa corresponds to approximately £5 billion.
Additionally, since nuclear is a low- carbon energy source, considerable
benefits to the environment are realised by life extension of the nuclear
fleet. For example, a coal-fired power station of the same output as the
combined Oldbury and Wylfa power stations operated for four years would
emit approximately 60,000,000 tonnes of CO2. The Bristol
research has therefore played a significant role in helping the UK meet
its CO2 reduction targets.
Current work at Bristol is directed at understanding the behaviour of the
graphite used in AGR reactor cores, as well as possible routes for
Sources to corroborate the impact
[a] Technical Manager, Engineering Function, Magnox Ltd. Provided a
letter of support to corroborate the assertion that the research at
Bristol was significant and important in furthering the understanding of
PGA graphite, and that this work was used to support the safety case that
allowed a lifetime extension.
[b] Health & Safety Executive, http://www.hse.gov.uk/aboutus/meetings/iacs/nusac/021007/p04.pdf.
Highlighted Bristol's FIB work to map pore interconnectivity in graphite
and measure pore sizes, as part of the Magnox safety-related research
[c] ONR-D1-OBA-PAR-11-025, http://www.hse.gov.uk/nuclear/pars/2011/oldbury-6.pdf.
Gives details of the extension of the Oldbury power station from 31
December 2008 to the end of December 2012 (Introduction) and details of
supporting work on the graphite safety case (section 3.3).