Life extensions of nuclear power plant
Submitting InstitutionOpen University
Unit of AssessmentElectrical and Electronic Engineering, Metallurgy and Materials
Summary Impact TypeTechnological
Research Subject Area(s)
Chemical Sciences: Physical Chemistry (incl. Structural)
Engineering: Manufacturing Engineering, Materials Engineering
Summary of the impact
The lifetimes of Hartlepool and Heysham I nuclear power stations have
been extended from 2011 to 2019 as a direct result of our research into
the development and application of new measurement techniques for the
accurate determination of residual stresses. These life extensions are
contributing to the health of the UK economy, maintaining jobs, ensuring
security of electricity supply, and deferring the need for decommissioning
and replacement of two nuclear power stations at a cost of several billion
pounds each. The electricity generated during the life extension period
has a market value of over £8 billion. New numerical modelling methods,
underpinned by our measurements, are now used by the nuclear industry in
life assessment procedures.
Since the 1990s The Open University (OU) has led research into the
determination of spatially varying residual stresses in welded components.
Over the past 15 years it has developed and applied novel techniques to
study welded structures for the nuclear power generation industry.
1998-2001: Edwards (Senior Lecturer/Reader) worked closely with
Nuclear Electric, within the EU-funded thematic network TRAINSS (contract
BRRT-CT97-5043), on measuring residual stresses in weld repairs
[3.1] using neutron diffraction.
1999-2001: Edwards (Reader) led the £3.5m consortium to design and
construct ENGIN-X [3.2], the dedicated materials engineering beamline at
the UK's ISIS neutron source.
2001-present: Edwards (Reader/Professor), Fitzpatrick
(Senior Lecturer/Reader/Professor), and Bouchard (Professor,
joined the OU 2008) initiate and participate in the NeT European
consortium for the standardisation and validation of residual stress
measurement and modelling in power plant welds [3.3, 3.4].
2004-08: Edwards (Professor, left OU 2011) and James
(Research Fellow, appointed to permanent position 2006) delivered the
British Energy Boiler Spines Research Programme contract (£0.5m). The work
characterised spatially resolved material properties and studied residual
stresses in ten welded benchmark and nuclear component mock-ups [3.4].
These measurements were used to validate improved numerical methods for
simulating welding residual stresses [3.5] and revealed, for
the first time, the nature of residual stress concentrations in multi-pass
weld repairs and the dominant effect of weld capping passes.
2006-08: Fitzpatrick (Reader) researched residual stress
generation in welds for light water reactor applications for the Japanese
Nuclear Energy Safety Organization.
2010-13: James (Research Fellow) extended this research to the
characterisation of residual stresses in dissimilar-metal welds using the
ISIS and SNS neutron facilities. The joints were made by AREVA for the
European pressurised water reactor using new welding procedures designed
to mitigate the risk of stress corrosion cracking.
2009-12: James (Research Fellow) undertook measurements on the
Australian OPAL research reactor where delayed hydride cracking had been
identified as the cause of a leaking reactor reflector vessel [3.6].
References to the research
3.1 Journal article. Bouchard, P. J., George, D., Santisteban, J.
R., Bruno, G., Dutta, A., Edwards, L., Kingston, E. and Smith, D. J.
(2005) `Measurement of the residual stresses in a stainless steel pipe
girth weld containing long and short repairs', International Journal
of Pressure Vessels and Piping, vol. 82, no. 4, pp. 299-310, DOI:
3.2 Journal article. Santisteban, J. R., Daymond, M. R., James,
J. A. and Edwards, L. (2006) `ENGIN-X: A third generation neutron strain
scanner', J. Appl. Cryst., vol. 39, pp. 812-825, DOI:
3.3 Journal article. Turski, M. and Edwards, L. (2009) `Residual
stress measurement of a 316L stainless steel bead-on-plate specimen
utilising the contour method', Int. J. Pres. Ves. & Piping,
vol. 86, pp. 126-131, DOI: 10.1016/j.ijpvp.2008.11.020
3.4 Journal article. Pratihar, S., Turski, M., Edwards, L. and
Bouchard, P.J. (2009) `Neutron diffraction residual stress measurements in
a 316L stainless steel bead-on-plate weld specimen', International
Journal of Pressure Vessels and Piping, Vol. 86, pp.13-19. DOI:
3.5 Journal article. Smith, M. C., Bouchard, P. J., Turski, M.,
Edwards, L. and Dennis, R. J. (2012) `Accurate prediction of residual
stress in stainless steel welds', Computational Materials Science,
vol. 54, pp. 312-328, DOI: 10.1016/j.commatsci.2011.10.024
3.6 Journal article. Muránsky, O., Holden, T. M., Kirstein, O.,
James, J. A., Paradowska, A. M. and Edwards, L. (2013) `Evaluation of
residual stresses in electron-beam welded Zr2.5Nb0.9Hf Zircadyne flange
mock-up of a reflector vessel beam tube flange', Journal of Nuclear
Materials, vol. 438, nos 1-3, pp. 154-162, DOI:
10.1016/j.jnucmat.2013.02.045 Listed in REF2.
Details of the impact
The Head of Research and Development, EDF Energy, writes: `We have a
fifteen-year long relationship with the OU and consider its expertise in
combining small scale and large scale residual stress measurement
techniques, together with analytical modelling techniques, to be unique.'
The OU and Nuclear Electric measured residual stresses in a weld repair
[5.2], using neutron diffraction, following a steam leak at Hunterston
power station in 1997. This led to an OU/British Energy research training
partnership (within the EU-funded thematic network TRAINSS, 1998-2001)
involving neutron diffraction measurements of residual stress in long and
short weld repairs [3.1]. These were the first measurements of their kind
and revealed the surprisingly severe nature of residual stress fields
associated with repairs which, most importantly, demonstrated that the
simplified 2-D finite element predictions of stresses at weld repairs
routinely used by British Energy to support safety cases for repairs were
not conservative. `This resulted in a multi-million pound program of
weld modelling development and validation at British Energy (2001—10) to
underwrite safety cases for repaired AGR [Advanced Gas-Cooled
Reactor] weldments prone to reheat cracking' [5.1]. The UK AGRs
operate at coolant temperatures significantly higher (over 600°C) than the
water-cooled reactors used elsewhere in the world where temperatures are
below 350°C: this is a demanding operating regime for ageing plant
susceptible to creep degradation (for example reheat cracking and creep
crack growth) in the presence of residual stresses.
In 2004 British Energy funded our `Boiler Spines Residual Stress
Measurement Programme', a five-year research project to quantify residual
stresses using neutron diffraction and the contour method, and measure
spatially resolved weld properties using digital image correlation, on
carefully designed benchmark welds and mock-ups of complex nuclear plant
welds. This work has had a direct impact on life-extensions of nuclear
plant in the UK [5.3]. Our results were used by British Energy to develop
and validate the use of more realistic heat source and material hardening
models in weld residual stress simulations [3.5], to characterise the
nature of residual stress concentrations at weld repairs [5.4] and to
validate predicted creep relaxation of residual stresses. The outcomes of
this research have mitigated uncertainty associated with the development
of reheat cracking: `The new understanding of weld repairs and refined
life analysis methods, underpinned by the OU measurements, has allowed
the lives of Hartlepool and Heysham 1 AGR power stations to be extended
to 300,000 hours, rather than the previous restriction of 175,000 hours
caused by uncertainty about creep damage to weld repairs in the main
boiler support structures. The previous restriction would have required
the reactors to close early. With each reactor generating around £700k
of electricity per day, these life extensions represent a major
contribution to the UK economy, on jobs and on security of electricity
supply. It also deferred the need for decommissioning and replacement of
two nuclear power stations at a cost of several billion pounds each'
[5.1]. In aggregate the economic impact of lifetime extensions for the
Hartlepool and Heysham 1 twin-reactor power stations, from 2011 to 2019,
amounts to electricity generation worth over £8 billion.
The application and impact of our research has continued through
characterising residual stresses in dissimilar metal welds for pressurized
water reactors in Japan and France, and on a critical component in a
reactor in Australia that was rendered barely usable due to a water leak,
until a repair was justified. `Our collaborative measurements with the
Open University team to validate the FEA [finite element analysis] model
of the reflector leak was a critical input to the process that
identified the successful reactor repair method. The OPAL reactor
provides neutrons to both undertake neutron beam science and the
production of radio pharmaceuticals for Australia. If the reactor had
not been repaired then the economic impact would run to many millions of
[Australian] dollars per annum.' [5.5].
Finally, new numerical modelling methods, underpinned by our
measurements, are now widely used by the regulated nuclear industry in
life assessment procedures for pressurised water reactor plant. The weld
modelling technology validated by our measurements has been captured in
the R6 Failure Assessment Procedure [5.6], transferred by EDF Energy to
the Australian Nuclear Science and Technology Organisation, and applied to
the pressuriser surge nozzle of Sizewell B Power Station to justify
relaxation of the inspection interval, `saving the station £1m per
annum in lost electricity generation' [5.1].
Sources to corroborate the impact
5.1 `Impact of OU research on nuclear power plant operations', Letter
from Head of Research and Development, EDF Energy, dated 2 July 2013.
5.2 Edwards, L., Bouchard, P. J., Dutta, M., Wang, D. Q., Santisteban, J.
R., Hiller, S., Fitzpatrick, M. E. (2005) `Direct measurement of the
residual stresses near a 'boat-shaped' repair in a 20 mm thick stainless
steel tube butt weld', International Journal of Pressure Vessels and
Piping, vol. 82, no. 4, pp. 288-298, DOI:
5.3 Science and Technology Facilities Council (2012) Neutron
Scattering: Materials research for modern life, STFC Media Services
5.4 Bouchard, P.J. (2009) Hartlepool and Heysham 1 Power Stations —
Current understanding of boiler spine weld repairs residual stress
concentration effects, Report E/REP/BBGB/0023/AGR/07, British Energy
5.5 Head, Institute of Materials Engineering, Australian Nuclear Science
and Technology Organisation, Australia.
5.6 EDF Energy Ltd (2012) R6 Revision 4: Assessment of the integrity
of structures containing defects, Sections III.15 and V.5, 2012, EDF
Energy Ltd: Gloucester, UK [online] http://www.r-desk.co.uk/r6-procedure