Hydrogen mapping and mitigation using modelling and experimental studies to inform safety case scenarios in nuclear decommissioning operations.
Submitting InstitutionLondon South Bank University
Unit of AssessmentGeneral Engineering
Summary Impact TypeTechnological
Research Subject Area(s)
Chemical Sciences: Physical Chemistry (incl. Structural)
Engineering: Interdisciplinary Engineering
Summary of the impact
This Case Study illustrates how research has had a significant impact on
the awareness and management of hydrogen hazards across the UK's
pre-eminent nuclear decommissioning and reprocessing organisation,
Sellafield Ltd (formerly BNFL). It has enabled Sellafield to:
- Underpin safety cases for nuclear decommissioning operations where
mitigation of hydrogen explosions is essential; this is estimated to
have saved Sellafield over £550 million;
- Provide 100,000 man-hours of training/CPD on hydrogen hazards to
- Access the science relating to hydrogen-air ignition probabilities and
options for mitigating potential explosions;
- Expand and update the Hydrogen Technical Guide and associated road map
on hydrogen safety with the latest research findings;
- Obtain continued expert advice to the Sellafield Hydrogen Working
In addition, LSBU has, since 2008, benefited from contract research from
Sellafield valued at over £1 million.
Professor Philip Nolan (Professor of Chemical Engineering) together with
members of the Explosion and Fire Research Team (EFRT) at London South
Bank University (Dr P Holborn (Senior Research Fellow), Dr J Ingram
(Senior Research Fellow), Dr P Battersby (Research Fellow) and Dr A
Averill (Research Fellow)) have explored a wide range of phenomena
relating to hydrogen generation and explosion mitigation since 1999 .
Apart from Battersby (commenced employment at LSBU in 2000), all other
EFRT staff involved in this research were employed by LSBU before 1999 and
are still LSBU employees.
The research has been, and continues to be, funded by Sellafield Ltd
(~£2M since 2000) who now also sponsor (£100k pa) the Team, and have
awarded it Centre of Expertise status. The objective of the research was
to develop a comprehensive Hydrogen Technical Guide (HTG) and an
associated road map to identify and solve problems due to hydrogen
generation across Sellafield's nuclear decommissioning operations.
To characterise the probability of ignition of hydrogen-air gas mixtures
by mechanical stimuli, research employing a large number of experimental
tests has been carried out  using (i) a drop-weight impact apparatus
(2003-7), (ii) glancing angle impact tests using a double-pendulum
apparatus (2007-9) and (iii) a pure friction force apparatus using
hydraulic or pneumatic rams to push loads up to 1000 kg in mass across a
target plate (2009-12). Theoretical analyses for both impact and
frictional force ignition mechanisms have been successfully developed.
Experiments employing a dynamic thermocouple have also been performed to
characterise the surface temperature generated during a mechanical
stimulus event  (2012-13).
The results suggest that in any analysis of ignition probability the
maximum surface temperature should be determined and considered in
relation to the temperatures that would be required to initiate hot
surface reactions sufficient to cause sparking and ignition. The research
- Identified the crucial parameters that influence mechanical ignition;
- Predicted the surface temperatures that could be generated by
- Related surface temperatures to ignition probability;
- Demonstrated that ignition can occur at lower sliding velocities than
Early research into the mitigation of hydrogen explosions (2003-6)
focused on establishing proof-of-concept using water mist spray .
Subsequent experimental work has been performed (2007-12) using both a
laminar burning velocity apparatus and a small scale cylindrical explosion
vessel, to investigate the usage of very fine water fog, nitrogen dilution
and sodium hydroxide additives upon the mitigation of hydrogen
deflagrations. The experiments were used to characterise (i) the
inhibition of the flame burning velocity , (ii) the mitigation of the
explosion overpressure  and (iii) the narrowing of the flammability
limits of hydrogen-oxygen-mixtures, in terms of fog density, nitrogen
dilution level and presence of additive . Complementary research was
also performed (2007-12) to model the effect of these mitigation measures
upon the burning velocity, explosion overpressure and flammability limits
of hydrogen mixtures and to investigate the potential for mitigation of
explosion overpressure in a full scale plant silo ullage . The research
characterised the conditions required for effective mitigation of
hydrogen-air explosions using water mist and demonstrated that:
- water mist can significantly reduce the rate of pressure rise and peak
- addition of sodium hydroxide to water fog produces an abrupt reduction
in the burning velocity of hydrogen flames above a critical fog density;
- a combination of water mist, nitrogen dilution and sodium hydroxide
additive can significantly narrow the flammability limits of
References to the research
 Kempsell ID, Wakem, MJ, Fairclough MP and Ingram JM. Hydrogen
explosions — an example of hazard avoidance and control. In: Hazards XVI,
Symposium Series No. 148, IChemE; pp. 523-540, 2001.
 Jones S, Averill AF, Ingram JM, Holborn PG, Battersby P, Nolan PF, et
al. Impact ignition of hydrogen-air mixtures. In: Hazards XIX, Symposium
Series No. 151, IChemE; pp. 401-409, 2006.
 Averill AF, Ingram JM, Battersby P, Holborn PG, Nolan PF. A
fundamental study of the generation of interfacial temperatures with metal
surfaces and coatings under conditions of sliding friction and mechanical
impact. Part 1: thermal analysis and theoretical evaluation of surface
temperature. Transactions of the Institute of Metal Finishing, 2013, 91,
pp. 269-274. Doi: http://dx.doi.org/10.1179/0020296713Z.000000000114
 Jones S, Averil AF, Ingram JM, Holborn PG, Battersby P, Nolan PF, et
al. Mitigation of hydrogen-air explosions using fine water mist sprays.
In: Hazards XIX, Symposium Series No. 151, IChemE; pp. 440-447, 2006.
 Ingram JM, Averill AF, Battersby P, Holborn PG, Nolan PF. Suppression
of hydrogen oxygen-nitrogen explosions by fine water mist: Part 1. Burning
velocity. Int J Hydrogen Energy 2012, 37, 19250-57. Doi:
 Battersby P, Averill AF, Ingram JM, Holborn PG, Nolan PF. Suppression
of hydrogen-oxygen-nitrogen explosions by fine water mist: Part 2.
Mitigation of vented deflagrations. Int J Hydrogen Energy 2012, 37,
19258-67. Doi: 10.1016/j.ijhydene.2012.10.029
 Ingram JM, Averill AF, Battersby P, Holborn PG, Nolan PF. Suppression
of hydrogen oxygen-nitrogen explosions by fine water mist containing
sodium hydroxide additive. International Journal of Hydrogen Energy, 2013,
38, 8002-8010. Doi: 10.1016/j.ijhydene.2013.04.048
 Holborn PG, Battersby P, Ingram JM, Averill AF, Nolan PF. Modelling
the mitigation of a hydrogen deflagration in nuclear waste silo ullage
with water fog. Process Safety and Environmental Protection, 2013, 91,
476-482. Doi: 10.1016/j.psep.2012.11.001
Details of the impact
This case study illustrates how the research of the Explosion and Fire
Research Team (EFRT) has benefitted the UK's leading nuclear
decommissioning and reprocessing company, Sellafield Ltd, through the
provision of technical expertise and knowledge that has enabled the
Company to manage effectively aspects of its decommissioning operations
where potential hydrogen explosion hazards may exist.
Nuclear plants pose many challenges in relation to hydrogen safety. An
uncontrolled release of hydrogen (generated through radiolysis and
corrosion) in a plant enclosure with a suitable ignition source could
result in an explosion with potential loss of human life, significant
infrastructural damage and environmental impact due to radioactive
contamination, for example, as occurred at Fukushima in 2012.
In this context, Sellafield Ltd (formerly British Nuclear Fuels Ltd), the
UK's pre-eminent nuclear decommissioning and reprocessing company,
approached the ERFT (1999) because of their modelling and experimental
expertise in the area of Flammable Gas explosions.
In recognition of the potential safety risks associated with hydrogen in
its decommissioning and reprocessing operations, Sellafield established a
Hydrogen Working Party in 1995 with overall responsibility for the
technical guidance and advice on hydrogen safety matters to all Sellafield
sites and its accountability to government regulatory agencies. The
research of the EFRT has, and continues to make, a major contribution to
the scientific understanding, direction and activities of the Hydrogen
Working Party (HWP). The HWP has met over 100 times in total, including 34
times since 1 January 2008. It is made up of Sellafield plant safety and
design managers and engineers with Professor Nolan and Dr Steve Graham
(National Nuclear Laboratory) as the only two external members of the HWP.
Nolan remains the only formal academic representative on the HWP, and
other members of the EFRT attend on a regular basis to present on specific
In this context, EFRT staff were the primary authors of Sellafield's
Hydrogen Technical Guide (HTG), first issued in 2002, which continues to
be expanded and updated on the basis of the latest research findings by
the EFRT, for example, with a guide to the assessment of ignition
probability carried out since 2008 [1-3]. The HTG remains the Company's
standard for the management of hydrogen safety and is applicable to all
existing and new facilities. The HTG also provides the national regulator
(Office for Nuclear Regulation) with the necessary assurances that
Sellafield can manage its hydrogen safety affairs effectively and with
confidence; even a minor incident can result in a plant shutdown with
significant cost implications.
Key areas of impact that EFRT staff have contributed to through their
involvement with the HWP and the HTG since 2008, are:
- Ignition and flammability research applied to Sellafield's Magnox
Swarf Storage Silo (MSSS)  retrieval project (2010): The EFRT
research provided the HWP with greater understanding of (i) the effects
of partial inerting/flammable limits on ignition; (ii) interpreting and
applying international inerting standards to develop an operable
ventilation design; (iii) mechanical ignition mechanisms and developing
probability assessments; and (iv) hydrogen release and dispersion
mechanisms from the waste in MSSS. The savings involved in installation
and operating costs over the proposed fully inerted system and through
an earlier start/quicker retrieval programme is estimated by Sellafield
to be greater than £500 million .
- Ignition probability applied to Sellafield's Silo Direct Encapsulation
Plant  (2010): The EFRT's work provided a detailed understanding of
all ignition mechanisms and developing probability assessment associated
with this plant. The saving to the Company is estimated to be greater
than £50 million through reduced engineering requirements .
- Benefits to other projects since 2008, including: (i) the development
of a safety case to allow the floc retrieval process to continue
following submersible pump blockage; (ii) development of a safe method
for sludge header closed pipe flange removal; (iii) a flask passive
ventilation design; (iv) a hydrogen instruments engineering standard;
(v) a strategy for sealed ion exchange cartridge hydrogen management,
and (vi) allowing the feasibility of employing water mist explosion
mitigation strategies on the MSSS plant to be evaluated .
Collectively, these projects are estimated to have saved Sellafield in
excess of £100K through allowing alternative less costly options to be
developed and justified .
- The HTG is widely used internally within Sellafield Ltd, with
approximately 60% of the 10,000 staff regularly referring to the Guide
to successfully deliver work, including safety assessments, new
engineering projects and project planning. The HTG has been used as the
core of over 100,000 man-hours training and CPD on hydrogen hazard since
- A senior member of the Flammable Gas Centre of Expertise at Sellafield
Ltd referred to the HTG as "The Bible", and has confirmed that the HTG
has attracted interest from the Department of Energy in the US  where
no equivalent exists .
The investment in contract research made by Sellafield in the above
research carried out by the EFRT since 2008 has been just over £1M. In
2011, Sellafield selected and now sponsor the EFRT as its Centre for
Expertise in Flammable Gases under a 5 year contract worth £500k. The EFRT
also continue to receive contract research commissions from individual
plants at Sellafield for specific issues.
Sources to corroborate the impact
 Wakem, M., Fairclough, M.P., Kempsell, I.D., Ingram, J.M., Hydrogen
Technical Guide, E1.30 Issue 3, Sellafield Ltd, Nuclear Decommissioning
Authority, September 2008 — (available on request from LSBU).
- The Sellafield Ltd. Hydrogen Technical Guide. Note that this is
designed to be used in conjunction with the "Hydrogen Hazards Handbook"
which is comprised of 15 supporting documents (426 pages) primarily
written by EFRT staff.
 Averill A.F. Nolan P., Ingram J.M., Battersby P., Holborn P., A
compendium of mechanical impact ignition results with an illustrative
Bayesian Belief Network, Report to Sellafield Ltd. HWP(09)P107, February
2009 — (available on request from LSBU)
- A compendium of EFRT mechanical ignition research data used to
provide the basis for an ignition probability assessment method.
 Ingram J.M., Thomas P.M., Assessment of ignition probability,
S&ERM Technical Manual, E1.30 SD 10, July 2009 — (available on request
- The Hydrogen Technical Guide Supporting Document describing the
assessment of ignition probability and developed using EFRT research
 Averill A.F., Mechanical ignition during waste retrieval in MSSS,
Report to Sellafield, HWP(10) P128, March 2010 — (available on request
- The application of the LSBU mechanical ignition assessment
methodology to the Sellafield MSSS plant.
 Report of independent consultants (The Innovation Partnership, 2013).
Contact: Managing Director, The Innovation Partnership (email@example.com)
— Summarises and quantifies Sellafield's views on the contribution and
value of the EFRTs research to its operations and the management of
 Averill A.F., Ingram J.M., Initial assessment of ignition event
probability during lidding of SDP boxes and storage in the buffer
location. Report to Sellafield Ltd. HWP(10)P136, December 2010 —
(available on request from LSBU)
- The application of the LSBU mechanical ignition assessment
methodology to the Sellafield SDP plant.
 Fairclough, M.P., Hydrogen explosion overpressure trials strategy,
Report RP/B38PLN- 3755/SAFE/00048, Sellafield Ltd. January 2009 —
(available on request from LSBU).
- Report outlining different hydrogen explosion mitigation strategies
for use in MSSS.
 Hydrogen Safety and Management Working Group — US Department of
Energy (EFCOG SAWG) — www.efcog.org/wg/sa_hsig/docs/Hydrogen%20safety%20initiative.pdf