the highly efficient accident prevention measures adopted for the
current Generation II nuclear power plants (NPP) and the still more
demanding ones for the Generation III ones, some accident scenarios with
a low probability may result in severe accidents, as recently
emphasized by the Fukushima-Daiichi accidents in Japan. This can result
in core melting, plant damage and dispersal of radioactive materials
outside of the plant containment, thus threatening public health and the
The ERMSAR 2013
conference (European Review Meeting on Severe Accident Research) was
hosted by the French Institut de Radioprotection et de Sûreté Nucléaire
(IRSN) in the Palais des Papes in Avignon (France) from 2 to 4 October
2013. It was organized under the auspices of the SARNET (Severe Accident
Research NETwork of excellence) network that is coordinated by IRSN.
This conference concluded a cycle of life of twice 4 years of the
network in the frame of the 6th and 7th Framework Programmes (FP6 and
FP7 respectively) of Research and Development of the European Commission
(EC) that co-funded the network activities. ERMSAR 2013 gathered 137
participants (including about 30 from outside Europe) from 61
organizations and 25 countries, confirming its status as a leading
international event on major nuclear reactor accidents. Fifty-two papers
and eleven posters were presented and significant time was allocated
after each presentation for questions and open discussions.
organisations (research centres, universities, industries, utilities,
safety authorities and Technical Safety Organizations) – from 22
countries worldwide – have networked since 2004 their capacities of
research in SARNET to resolve the most important open severe accident
issues in existing and future Gen. II–III NPPs. The main objective was
to optimize the use of the available research budgets and to constitute a
sustainable consortium in which common research programmes would be
defined and performed. The network partners were, from 2008: Tractebel
(Belgium), INRNE and Technical University of Sofia (Bulgaria), UJV
(Czech Rep.), VTT (Finland), AREVA NP SAS, CEA, EDF, IRSN and Thermodata
(France), AREVA GmbH, University of Bochum, GRS, FZ Jülich, KIT and
University of Stuttgart (Germany), Demokritos (Greece), MTA-EK, NUBIKI
and University of Budapest (Hungary), ENEA, RSE and University of Pisa
(Italy), LEI (Lithuania), NRG (the Netherlands), INR (Romania), CIEMAT
(Spain), IVS, UJD SR and VUJE (Slovakia), JSI (Slovenia), Chalmers
University and University of Stockholm (or KTH) (Sweden), PSI
(Switzerland), NNL, University of Newcastle and Imperial College of
London (UK), JRC/IET and ITU (EC), AECL (Canada), USNRC (USA), KAERI and
KINS (Korea) and BARC (India). Contacts were taken with JAEA and JNES
from Japan in the last two years to start a further collaboration.
overall work carried out by the network members, involving more than
230 researchers and 30 PhD students, represents an equivalence of about
40 full-time persons per year. Most key European R&D actors in the
severe accident area are members and the network takes also benefit from
the knowledge and contribution of important non-European organizations
from USA, Canada, Korea and India.
The outcomes of the FP7 project are very positive:
- Good efficiency of networking and sharing of knowledge among the partners,
progress of knowledge on the high-priority severe accident issues
through collaborative work (analysis of experiments, benchmarks among
numerical simulation codes, state-of-the-art reports) on corium,
containment and source term phenomena,
of knowledge in the ASTEC IRSN-GRS integral code that simulates whole
severe accident scenarios and in the DATANET database for storage of
experimental data (using the JRC STRESA tool),
- Dissemination of knowledge1
through education courses for young researchers and students or
managers and senior scientists, edition of a textbook on severe
accident, mobility of researchers and ERMSAR periodic conferences.
ERMSAR-2013 conference was the opportunity to present the main outcomes
of R&D (i.e. mainly new knowledge and new physical models for
computer codes) from 2008 to 2013 on the five following high-priority
topics: corium and debris coolability, molten corium concrete
interaction (MCCI), hydrogen mixing and combustion in containment,
source term, and ASTEC integral code. All topics included both
experimental and modelling activities, and assessment of the
applicability of simulation codes to plant accident scenarios. This
special issue of Annals of Nuclear Energy includes a selection of 20
ERMSAR 2013 joint synthesis papers.
essential and continuous task of the network was the ranking of R&D
priorities in order to define common future severe accident research
programs. The Severe Accident Research Priority (SARP) group has updated
the R&D priorities on the basis of the SARNET outcomes and on the
recent international R&D (see the paper “Conclusions on severe accident research priorities”, by W. Klein-Heßling et al.).
In particular it took into account OECD/NEA/CSNI projects, the Level 2
Probabilistic Safety Analysis (PSA) studies and the ASAMPSA2 FP7 project
on PSA2 best-practice guidelines that ended in 2013. The group also
accounted for the preliminary understanding of the Fukushima-Daiichi
accidents, which resulted only in slight reorientations of priorities
because most involved phenomena were already addressed as being of
Topic "Corium and Debris Coolability"
R&D aims at reducing the remaining uncertainties on the possibility
of cooling the reactor core structures and materials during a severe
accident, either in the core region or in the vessel lower head or in
the reactor cavity after vessel failure, so as to limit the progression
of the accident. This could be achieved either by ensuring the retention
of corium within the reactor vessel by water injection or at least a
slow corium progression and small flow rates of corium release into the
cavity. These issues are linked to severe accident management (SAM) for
current reactors and also for design and safety evaluation of future
objectives were to enhance the database on the remelting of debris and
corium pool formation in the lower head (relying on the LIVE experiments
with simulant materials at KIT), to develop and validate the models and
computer codes for simulation of in-vessel melt pool behaviour, to
perform reactor-scale analysis for in-vessel corium coolability and to
assess the influence of SAM measures on in-vessel coolability. Work
comprised experimental and modelling activities, with strong
cross-coupling between the tasks.
knowledge exists now concerning cooling of fuel in large intact
rod-like geometries, but efforts had to be made to address modelling of
air ingress into an overheated core following an earlier partial
oxidation in steam (see the paper “SARNET2 Benchmark on Air Ingress Experiments QUENCH-10, -16” by L. Fernandez-Moguel et al.).
main experimental efforts concentrated on debris bed formation and
cooling by water to demonstrate effective modes and establish cooling
rates and limits (see the paper “Evaluation of an effective diameter to study quenching and dry-out of complex debris bed” by N. Chikhi et al.).
Several experiments were performed in the following facilities: DEBRIS
at the University of Stuttgart for top and bottom flooding of hot
debris, POMECO at the University of Stockholm for boil-off conditions
with emphasis on basic laws and specific 2D effect, and PRELUDE at IRSN
for the effect of power distribution inside a larger debris bed with
multi-dimensional effects (see the paper “Experimental investigation on reflooding of debris beds” by S. Leininger et al.). Other facilities were more adapted to ex-vessel debris beds such as COOLOCE in VTT and DEFOR in KTH (see the paper “Analyses on Ex-Vessel Debris Formation and Coolability in SARNET frame” by G. Pohlner et al.).
parallel, efforts have aimed at assessing and validating the models in
system-level and in detailed codes for core degradation, oxidation and
debris behaviour, before improving these models. Modelling of porous
media has indicated a strongly favoured coolability by inflow of water
from lateral water-filled regions of the core with higher porosities.
view of the vulnerability of nuclear fuels that are stored before their
evacuation and final disposal or possible reprocessing, as highlighted
by the Fukushima-Daiichi accidents, work also addressed the accidents in
spent fuel pools of different geometries (see the paper “Syntheses of spent fuel pool accident assessments using Severe Accident codes” by J. Fleurot et al.).
Topic "Molten Corium Concrete Interaction"
addressed situation is, after a postulated vessel lower head failure,
the corium presence in a reactor pit which is initially dry but with the
possibility of water injection later during MCCI. A major question was
why an anisotropic ablation of basemat was observed with siliceous
concretes and an isotropic one with carbonaceous concretes. The work was
based on the analysis of recent 2D real material MCCI-OECD (in Argonne
National Labs. in USA) and VULCANO (in CEA) experiments in an oxidic
pool and of analytical experiments with simulant materials on 2D heat
convection in a bubbling pool (see the paper “Towards an European
consensus on possible causes of MCCI ablation anisotropy in an oxidic
pool” by M. Cranga et al.). The scaling issue was also investigated to support reliable plant calculations (see the paper “Transposition of 2D Molten Corium Concrete Interactions from experiment to reactor” by C. Spengler et al.).
potential factor that might strongly affect MCCI is the possible
metal/oxide stratification and the spatial distribution of metallic and
oxide phases. Experiments provided new insights at the large-scale MOCKA
facility (in KIT) (∼1 ton of corium) with simulant materials and at the
VULCANO (in CEA) and the SICOPS (in AREVA GmbH) facilities with
prototypic materials (see the paper “Experiments on MCCI with oxide and steel” by J.J. Foit et al.).
for corium coolability, recent data suggest that early water flooding
(i.e. at a time when the concrete molten fraction is low) has a good
coolability potential but this must be confirmed by further experiments.
The efficiency of late top flooding seems more doubtful, especially for
siliceous concretes. Bottom flooding is being studied as an efficient
alternative to top flooding.
benchmarks were performed among simulation codes, either on VULCANO and
COMET experiments (the latter in KIT) or on VVER-1000 reactor MCCI
scenarios, in order to evaluate the code capabilities.
corium composition and properties determine the interactions both with
the reactor vessel and in the later phases with the concrete basemat.
Analytical and larger scale tests supplied new data on melt-liquidus
composition for the NUCLEA thermodynamic database (see the paper
“Quality Improvements of Thermodynamic Data applied to Corium
Interactions for Severe Accident Modelling in SARNET2” by P.D.W.
Bottomley et al.).
Topic "Containment Issues"
focus is on the threat to containment integrity caused by highly
energetic phenomena, particularly steam explosion and hydrogen
may be caused by fuel–coolant-interaction after corium pouring into the
flooded reactor cavity. The work, aiming at completing the SERENA2
OECD/NEA/CSNI project, was based on experiments in the facilities KROTOS
(CEA), TROI (KAERI), MISTEE and DEFOR/PULiMS (KTH), and DISCO-FCI (KIT)
and on the simulation codes MC3D (IRSN) and JEMI-IDEMO (IKE) (see the paper “Status of steam explosion understanding and modelling” by R. Meignen et al.).
combustion (deflagration and detonation) may be caused by ignition of a
gas mixture with high local hydrogen concentrations, which may be due
to the imperfect mixing of the containment atmosphere. Benchmarks have
been performed to assess the effect of specific phenomena and/or safety
systems with the potential to affect the containment atmosphere
behaviour and to evaluate the code capabilities and improvements:
- On containment sprays (see the paper “Achievements of spray activities in nuclear reactor containments during the last decade” by J. Malet et al.),
- On hydrogen combustion (see the paper “SARNET hydrogen deflagration benchmarks – main outcomes and conclusions” by A. Bentaib et al.),
- On steam condensation on walls (see the paper “Lesson learned from the SARNET Wall Condensation Benchmarks” by W. Ambrosini et al.),
- On passive autocatalytic recombiners.
a theoretical benchmark based on a generic NPP containment allowed an
in-depth comparison of thermal–hydraulic models in Lumped-Parameter
codes (see the paper “Generic containment: Detailed comparison of containment simulations performed on plant scale” by S. Kelm et al.).
This led to recommend that results of a single simulation of a
transient in a NPP containment with a lumped-parameter code cannot be
trusted before at least an additional independent simulation (preferably
with a different code) of the same transient.
Topic "Source Term"
activities focused on reducing the uncertainties associated to
estimates of radioactive releases to the NPP external environment. Two
major issues were investigated: the impact of oxidising conditions on
source term and the iodine chemistry within the reactor coolant system
(RCS) and containment.
main emphasis was placed on iodine and ruthenium, given their high
radio-toxicity and volatility. The transport and chemical transformation
of these two elements through the RCS and inside the containment were
investigated (see the paper “Transport of ruthenium in primary circuit conditions during a severe NPP accident” by T. Kärkelä et al.).
Of particular importance is the prediction of volatile iodine and
ruthenium species in the containment atmosphere since they would be only
partly removed by containment sprays or by filtration when the venting
of the containment system is actuated. Full advantage was taken of
cooperation with international programmes such as Phébus FP, ISTP
(International Source Term Programme) and OECD/NEA/CSNI projects (i.e.
STEM, BIP, BIP2 and THAI). Benchmarks were organised on the basis of the
Phébus FPT3 (organised by IRSN) and THAI experiments (organized by
iodine and containment paint (e.g. adsorption) have been known to likely
happen in case of a severe accident. However, recent insights
concerning chemical mechanisms seem to be the key for a comprehensive
understanding of organic iodides formation and, as a consequence, for an
enhancement of predictability of gas phase iodine concentration in the
longer term (see the paper “Iodine-Paint Interactions during nuclear reactor severe accidents” by L. Bosland et al.).
there are other processes involved in the formation of gaseous iodine.
The experiments on iodine oxidation demonstrated also that gaseous
iodine species such as molecular iodine and volatile organic iodide can
reversibly react with the products of air radiolysis to form iodine
oxide particles (see the paper “Experimental and modelling studies
of iodine oxide formation and aerosol behaviour relevant to nuclear
reactor accidents” by S. Dickinson et al.).
revaporisation or physical resuspension of fission product deposits
from the primary circuit might be a major source term in the late phase
of severe fuel degradation. The work conducted on Phébus FP post-test
analysis at JRC/ITU, EXSI-PC experiments at VTT, and CHIP experiments at
IRSN support this statement (see the paper “Revaporisation of
fission product deposits in the primary circuit and its impact on
accident source term” by P.D.W. Bottomley et al.).
Topic "ASTEC code"
ASTEC integral simulation code, jointly developed by IRSN and GRS,
plays a key role in the network by capitalizing severe accident
knowledge through the implementation of the new physical models that are
being developed in each above Topic. The code assessment was done by
partners through comparison with more than 50 experiments and
applications to severe accident scenarios in diverse NPP types (PWR,
VVER, CANDU…), as well as benchmarks with other codes (see the paper
“Synthesis of the ASTEC integral code activities in SARNET. Focus on
ASTEC V2 plant applications” by P. Chatelard et al.). The current
development efforts for the next major version ASTEC V2.1, planned end
of 2014, address the reflooding of degraded cores and the adaptation of
core degradation models to BWR and CANDU.
Conclusions and SARNET perspectives
ERMSAR periodic conference organized by the network is becoming the
major worldwide conference on severe accident research. ERMSAR 2013 was a
cornerstone since the FP7 SARNET2 project ended in March 2013, and the
SARNET network is now fully integrated in the new international NUGENIA
on R&D on Gen. II and III NPPs. The NUGENIA Technical Area N°2,
entitled “Severe accidents”, covers the SARNET scope and extends to the
issues of emergency preparedness and response, as well to the impact of
severe accidents on the environment. In particular, the SARP research
priorities have been totally used to build the NUGENIA roadmap for
severe accident R&D in the next years. The next ERMSAR conference
will be hosted by CEA in Marseille (France) on 24–26 March 2015.
the participants to the network must be acknowledged, and in particular
the members of the Management Team (besides the guest editors below, A.
Auvinen, VTT, D. Beraha, GRS, P. Chatelard, IRSN, C. Journeau, CEA, I.
Kljenak, JSI, A. Miassoedov and W. Tromm, KIT, and R. Zeyen, JRC-IET).
The EC must also be acknowledged for the co-funding of the FP6 and FP7
projects, and particularly Michel Hugon, EC coordinating officer for
SARNET, for his very strong interest and support. Special thanks also to
the IRSN staff in Cadarache and the Avignon Tourism Office for the
practical organization of the conference. Last but not least, this
special issue would not have been possible without the help of the