Nuclear Safety
NEA/CSNI/R(2018)14
March 2020www.oecd-nea.org

Riverine Flooding: Hazard

Assessment and Protectionof Nuclear Installations
Workshop Proceedings
21-23 March 2018
Paris, France

Nuclear Energy Agency

NEA/CSNI/R(2018)14
Unclassified

English text only
17 March 2020

NUCLEAR ENERGY AGENCYCOMMITTEE ON THE SAFETY OF NUCLEAR INSTALLATIONSRiverine Flooding: Hazard Assessment and Protection of Nuclear
Installations
Workshop Proceedings
Paris, France
21-23 March 2018
This document is available as PDF only.

JT03458844
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2 │ NEA/CSNI/R(2018)14
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RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14│3

COMMITTEE ON THE SAFETY OF NUCLEAR INSTALLATIONSThe Committee on the Safety of Nuclear Installations (CSNI) is responsible for the Nuclear Energy Agency(NEA) programmes and activities that support maintaining and advancing the scientific and technicalknowledge base of the safety of nuclear installations.
The Committee constitutes a forum for the exchange of technical information and forcollaboration between organisations, which can contribute, from their respective backgrounds in research,development and engineering, to its activities. It has regard to the exchange of information between
member countries and safety R&D programmes of various sizes in order to keep all member countries
involved in and abreast of developments in technical safety matters.
The Committee reviews the state of knowledge on important topics of nuclear safety science andtechniques and of safety assessments, and ensures that operating experience is appropriately accounted forin its activities. It initiates and conducts programmes identified by these reviews and assessments in order
to confirm safety, overcome discrepancies, develop improvements and reach consensus on technical issuesof common interest. It promotes the co-ordination of work in different member countries that serve tomaintain and enhance competence in nuclear safety matters, including the establishment of joint
undertakings (e.g. joint research and data projects), and assists in the feedback of the results to participatingorganisations. The Committee ensures that valuable end-products of the technical reviews and analyses areprovided to members in a timely manner, and made publicly available when appropriate, to support broader
nuclear safety.
The Committee focuses primarily on the safety aspects of existing power reactors, other nuclear
installations and new power reactors; it also considers the safety implications of scientific and technicaldevelopments of future reactor technologies and designs. Further, the scope for the Committee includes
human and organisational research activities and technical developments that affect nuclear safety.

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONS4 │ NEA/CSNI/R(2018)14

Acknowledgements
The members of the Nuclear Energy Agency (NEA) Working Group on External Events(WGEV) wish to acknowledge the significant contributions of those individuals who had a
key role in the preparation of the “Technical Note on Riverine Flooding: Hazard
Assessment and Protection of Nuclear Installations” (NEA, 2018 1), and those who had aleadership role in the conduct and success of the NEA International Workshop “Riverine
Flooding: Hazard Assessment and Protection of Nuclear Installations”. Additional thanks
are extended to Abir Maalouf, NEA Assistant, in support of the workshop and the
preparation of these proceedings.

Preparation of the WGEV technical note on riverine flooding
Gernot Thuma
Shi Zhong Lei
Roland Beutler
Vincent Rebour
Wouter van Lonkhuyzen
John A. Nakoski
Curtis Smith
Victor Neretin

Gesellschaft für Anlagen- und Reaktorsicherheit gGmbH(GRS), Germany
Canadian Nuclear Safety Commission (CNSC), Canada
Swiss Federal Nuclear Safety Inspectorate (ENSI),
Switzerland
Radiological Protection and Nuclear Safety Institute(IRSN), France
Authority for Nuclear Safety and Radiation Protection(ANVS), Netherlands
Nuclear Regulatory Commission (NRC), United States
Idaho National Laboratory (INL), United States
Nuclear Energy Agency (NEA)

Presenters and panel members at the workshop
Gernot Thuma
John A. Nakoski
Vincent Rebour
Dries Gryffroy
Joey Zhaoliang WangJohn Perdikaris
Zdenko Simic
Claire-Marie Duluc
Denis Jautzy
Sylvain Saviot
Christian Strack
Saskia van Vuren
Rainer Hausherr
1

GRS, Germany - Workshop and Closing Session Chair
NRC, United States – Session 1 Chair
IRSN, France – Session 2 Chair
Bel V (Belgian institute of nuclear expertise), Belgium –Session 3 Chair
CNSC, Canada
Ontario Power Generation, Canada
Joint Research Centre-Petten, European Commission
IRSN, FranceElectricité de France (EDF), France
EDF, France
GRS, Germany
Rijkswaterstaat, Netherlands
ENSI, Switzerland

Document available online at: www.oecd-nea.org/nsd/workshops/wgev2018/docs/wgev_technical_note.pdf.
RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14

Vinh Dang
Carlo Scapozza
Christopher Cook
Roger Kay
Michelle Bensi
Yoshimitsu Fukushima
Yuri SimonovPaul Scherrer Institute, Switzerland
Federal Office for the Environment, Switzerland
NRC, United States
US Army Corps of Engineers, United States
University of Maryland, United States
International Atomic Energy Agency
World Meteorological Organization

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONS│56 │ NEA/CSNI/R(2018)14

Table of contents
List of abbreviations and acronyms ..................................................................................................... 7
Executive summary ............................................................................................................................... 9
1. Introduction ..................................................................................................................................... 121.1. Background ................................................................................................................................. 121.2. Objectives of the workshop ........................................................................................................ 121.3. Organisation of the workshop ..................................................................................................... 13
1.4. Topics of the workshop............................................................................................................... 13
2. Summary of the Workshop on Riverine Flooding: Hazard Assessment and Protection of
Nuclear Installations ........................................................................................................................... 14
2.1. Opening session .......................................................................................................................... 14
2.2. Session 1 – Regulatory approaches and operating experience.................................................... 14
Challenges and gaps identified ....................................................................................................... 17
2.3. Session 2 – Technical methods for hazard assessment ............................................................... 17
Challenges and gaps identified ....................................................................................................... 20
2.4. Session 3 – Protection of nuclear installations against river floods ............................................ 21
Challenges and gaps identified ....................................................................................................... 24
2.5. Workshop closing session ........................................................................................................... 25
3. Conclusions and recommendations................................................................................................ 27

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List of abbreviations and acronyms
AEP
ANVS
ASN
AUM
BDBF
Bel V
CGB
CNSC
CSNI
DBF
DEC
EDF
EESS-EBP
ENSI
GRS
HEC
IAEA
IRS
IRSN
JRC
LIP
NEA
NI
NPP
NRC
NUREG
OECD
OPG
PFHA
PMF
PMP
PRA
PSA

Annual exceedance probability
Autoriteit Nucleaire Veiligheid en Stralingsbescherming (Authority for
Nuclear Safety and Radiation Protection, Netherlands)
Autorité de Sûreté Nucléaire (Nuclear Safety Authority, France)
Additional ultimate means
Beyond design basis flood
Institut belge d’expertise nucléaire (Belgian institute of nuclear expertise)
Crue de grand bassin versant (large watershed flooding)Canadian Nuclear Safety Commission
Committee on the Safety of Nuclear Installations (NEA)
Design basis flood
Design extension conditions
Électricité de France
External Events Safety Section-Extrabudgetary Programme (IAEA)
Swiss Federal Nuclear Safety Inspectorate
Gesellschaft für Anlagen- und Reaktorsicherheit gGmbH (Germany)
Hydrologic Engineering Center (US Army Corps of Engineers)
International Atomic Energy Agency
International Reporting System
Radiological Protection and Nuclear Safety Institute (France)
Joint Research Centre (European Commission)
Local intense precipitation
Nuclear Energy Agency
Nuclear installation
Nuclear power plant
Nuclear Regulation Commission (United States)
NRC Technical Report Designation
Organisation for Economic Co-operation and Development
Ontario Power Generation (Canada)
Probabilistic flood hazard assessment
Probable maximum flood
Probable maximum precipitation
Probabilistic risk assessments
Probabilistic safety assessment

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONS

8 │ NEA/CSNI/R(2018)14
PSR
RFS
ROR
RWS
SSC
TGNEV
WGEV
WGRISKWMO
XFPRA

Periodic safety review
Reference flooding situation
Rupture d'ouvrage de retenue (failure of water-retaining structure)
Rijkswaterstaat (Directorate-General for Public Works and Water
Management of the Netherlands)
Systems, structures and components
Task Group on Natural External Events (NEA/CSNI)
Working Group on External Events (NEA/CSNI)
Working Group on Risk Assessment (NEA/CSNI)World Meteorological Organization
External Flood Probabilistic Risk Assessment

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14

Executive summary
The March 2011 accident at the Fukushima Daiichi nuclear power plant (NPP) triggered
discussions about natural external events that are low frequency but high consequence. In
order to address these issues and determine which events would benefit most frominternational co-operative work, the Nuclear Energy Agency (NEA) established the Task
Group on Natural External Events (TGNEV) under the Committee on the Safety of Nuclear
Installations (CSNI). The CSNI then decided in 2014 to reorganise the TGNEV, whichbecame the Working Group on External Events (WGEV) and would have the objective ofimproving understanding and treatment of external hazards that would support thecontinued safety performance of nuclear installations, as well as of improving theeffectiveness of regulatory practices in NEA member countries. The WGEV is composedof a forum of experts who exchange information and experience on external events in theirrespective member countries, thereby promoting co-operation and maintenance of aneffective and efficient network of experts.
The WGEV “Riverine Flooding – Hazard Assessment and Protection of NPPs” task was
designed to share regulatory practices and technical approaches for riverine flood hazard
assessment (both deterministic and probabilistic) and for flood protection of NPPs. The
first activity involved in this task was to survey CSNI members so that they could share
information on such regulatory practices and technical approaches. The results of thissurvey are available in the “Technical Note on Riverine Flooding: Hazard Assessment andProtection of Nuclear Installations” (NEA, 2018). Following an initial assessment of thesurvey results, an international workshop was organised to involve relevant experts in
identifying the activities required to address knowledge gaps. In this context, theinternational workshop on “Riverine Flooding: Hazard Assessment and Protection of
Nuclear Installations” focused on current national regulatory approaches and operating
experience, technical methods for the assessment of riverine flooding events, protection of
nuclear installations against river floods and promotion of the effectiveness of preventative
measures.
The workshop was held at the Organisation for Economic Co-operation and Development(OECD) Conference Centre on 21-23 March 2018, with 37 participants representing
regulators and their technical support organisations, industry, universities and governmentorganisations from 9 countries, as well as representatives from international organisations.
A total of 20 technical presentations were given in 3 sessions. All workshop presentations
are available for download on the NEA website.
At the end of each technical session, a panel session was held allowing more detailed
discussions on the presentations in that session, with a general discussion session
concluding the workshop. During these discussions, participants expressed a strong interest
to continue such efforts after the NEA international workshop.

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Some of the issues that were highlighted by the participants include:•

Bridging the gap between hydrologists dealing with the science-based aspects offlooding and decision making in a regulatory framework is a challenge.

•

Correct and reliable weather forecasts in relation to flooding are needed.

•

While the use of historic information, paleodata and simulations can supplement
instrumental data, it is difficult to incorporate them into the existing hazard
assessment framework. Treating uncertainties can also be very challenging.

•

When assessing the impact on a facility, it may be necessary to consider otherfactors (associated affects) in addition to the maximum probable flood level.

•

A more rigorous understanding of the fragility of facilities is needed to strengthen
the technical basis for establishing appropriate protective measures.

•

Maintaining the public’s trust in protective measures for critical infrastructures will
require proactive approaches, and not only in terms of communication.

•

The key challenges associated with probabilistic safety analysis (PSA) for NPPs
and other nuclear installations are the nature of flooding hazards and plant impacts
from flooding hazards.

•

The main technical challenges of the probabilistic flood hazard assessment (PFHA)
methodology are time-consuming calculations, characterisation of probability
distributions for each input parameter and dealing with dependent input parameters.

•

A balance is needed between the appropriate consideration of the spectrum ofassociated effects from flooding and the information necessary to support decision
making for adequately protecting the facility from realistic external floods.

•

The concept of a “dry site” needs to be reconsidered. Even sites that were designedwith the “dry site” concept need to consider flooding effects.

Based on the results of the international workshop, the NEA proposes the following conclusions andrecommendations:•

It is important to share information between the nuclear industry and non-nuclear
organisations, as well as between neighbouring countries.

•

It is necessary to augment temporally and spatially sparse historical data withsimulations and other information. Further work needs to be done to understandhow metrics, such as a selected value for annual exceedance frequency, can be usedin regulatory decision making.

•Uncertainties with data and modelling need to be better understood and quantified
and decision makers should be aware of the uncertainties in the scientific results
and appropriately take them into account.

•

Co-ordination of work between NEA working groups is a key factor.

•

New approaches and models need to be developed to identify and address thechallenges introduced by climate change.

The workshop demonstrated that it is important for the nuclear and meteorological communities to work
together to take advantage of methods, models, data and experiences in the understanding of flooding
hazards and impacts to local, national and regional infrastructure. Additionally, co-operation among

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14│ 11

subject matter experts, including non-nuclear experts, will be key, as well as regional co-operation to shareexperience and data. The NEA will use insights gained from this workshop and the supporting survey,
along with insights from other activities, to identify potential future work that will address the areas
recommended for follow-up.

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1. Introduction
1.1.

Background
The Task Group on Natural External Events (TGNEV) was established by the Nuclear
Energy Agency (NEA) Committee on the Safety of Nuclear Installations (CSNI) in 2013
so as to address natural external events with low frequency but high consequence and to
determine which events would benefit from international co-operative work. In June 2014,
the CSNI decided to reorganise the TGNEV into a Working Group on External Events(WGEV) with the objective of improving understanding and treatment of external hazards
that would support the continued safety performance of nuclear installations, andimproving the effectiveness of regulatory practices in NEA member countries. The WGEV
constitutes a forum of experts who exchange information and experience on external events
in member countries, thereby promoting co-operation and maintaining an effective andefficient network of experts.
At its 58th meeting, the CSNI approved the recommended task on riverine flooding – hazard
assessment and protection of nuclear power plants (NPPs) to be pursued by the WGEV. It
was proposed that the first activity would be a survey of CSNI members to share regulatory
practices and technical approaches for riverine flood hazard assessment (both deterministic
and probabilistic) and flood protection of NPPs. The result of this survey is available as the“Technical Note on Riverine Flooding: Hazard Assessment and Protection of Nuclear
Installations” (NEA, 2018). Then, following an initial assessment of the survey results, aworkshop was planned to involve relevant experts in identifying activities required toaddress the knowledge gaps.
To achieve the aim of the working group, it was decided to convene an international
workshop on “Riverine Flooding: Hazard Assessment and Protection of Nuclear
Installations” at the OECD Conference Centre in Paris between 21 and 23 March 2018,
where specialists from across the world could gather to exchange information and share
their own country’s or company’s experiences on riverine flood hazard assessment andprotection of nuclear installations against flooding.

1.2.

Objectives of the workshop
The main objective of this international workshop was to provide a forum to review anddiscuss current national regulatory approaches and operating experience, technical methods
for the assessment of riverine flooding events, protection of nuclear installations (NIs)against river floods and to promote the effectiveness of preventative measures. Key focus
areas were:•

Regulatory requirements and operating experience with regard to riverine flooding.

•Technical methods for hazard assessment, identification of good practices andknowledge gaps.

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14•

Protection of NIs against river floods and the effectiveness of preventative
measures.

Information obtained as a result of this workshop should help to provide an understandingof key regulatory issues related to the characterisation and assessment of riverine flooding
events, flood protection and the determination of activities to address knowledge gaps.

1.3.

Organisation of the workshop
The workshop was organised into three technical sessions and an opening and closing
session as follows:•

Opening Session•

Session 1: Regulatory Approaches and Operating Experience•

Session 2: Technical Methods for Hazard Assessment•

Session 3: Protection of NIs Against River Floods•

Closing Session

In the closing session, discussions during technical sessions of the workshop weresummarised and conclusions and recommendations were developed for possible further
CSNI actions. The participation was open to experts from regulatory authorities and theirtechnical support organisations, research organisations, universities, operatingorganisations, industry associations and observers from NEA member countries as well asa small number of experts from non-NEA member countries. Thirty-seven representatives
from nine countries as well as the International Atomic Energy Agency (IAEA), theEuropean Commission (EC) and the World Meteorological Organization representatives
attended the workshop. The list of participants is provided in Appendix 1. A total of 20
technical presentations were given in the 3 sessions. The detailed workshop agenda isprovided in Appendix 2. Full copies of all workshop presentations are available fordownload on the NEA website.

1.4.

Topics of the workshop
Items addressed in the workshop include:•

Regulatory considerations and guidance for flood hazard assessment and flood
protection of NIs;•

Operating experience with flood protection measures;•Riverine flood hazard assessment in member countries;•

Riverine flood hazard assessment based on probable maximum precipitation andprobable maximum flood;•

Probabilistic flood hazard assessment for rivers and probabilistic risk assessments(PRA) standards;•

Data collection, and advances in modelling river floods and risk management;•

Protection of NIs against river floods in member countries.

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2. Summary of the Workshop on Riverine Flooding: Hazard Assessment andProtection of Nuclear Installations
The workshop consisted of an opening session, three technical sessions with participant
presentations followed by short discussions and a closing session summarising thediscussions and developing conclusions and recommendations for possible further Nuclear
Energy Agency (NEA) Committee on the Safety of Nuclear Installations (CSNI) actions.
The contributions presented were devoted to discussions of international guidance for flood
hazard assessment and flood protection at nuclear installation (NI) sites, national regulatory
framework in assessing flood hazard and flood protection of NIs, case studies on flooding
and hazard assessment, countries’ approaches in riverine flood hazard assessment,
statistical and probabilistic methods for riverine flood hazard assessment, data collection
and advances in modelling river floods, advances in flood risk management, operating
experience with flood protection measures.

2.1.

Opening session
The workshop was opened by the Acting Head of the Nuclear Safety Division of the NEA,Mr Andrew White, who welcomed the participants, gave a brief overview of the recent
Agency activities and wished a very productive and fruitful workshop. The workshop
Chair, Dr Gernot Thuma (Gesellschaft für Anlagen- und Reaktorsicherheit gGmbH[GRS]), briefly discussed the background and objectives of the workshop. He stated thatthe objectives are to review and discuss current national regulatory approaches andoperating experience, technical methods for the assessment of riverine flooding events,
protection of NIs against river floods and to promote the effectiveness of preventative
measures. He also mentioned the main priorities and challenges of this workshop such as
understanding of key regulatory issues related to the characterisation and assessment ofriverine flooding events, identification of knowledge gaps and research needs to address
knowledge gaps by establishing a community and relationships to enhance the assessmentof riverine flooding hazard and protection of NIs against river floods.

2.2.

Session 1 – Regulatory approaches and operating experience
This session was devoted to the regulatory approaches and operating experience that many
countries have implemented at the national level to strengthen the assessment of riverine
flooding hazard and protection of NIs.
The following presentations were made:•

IAEA guidance for flood hazard assessment and flood protection at NI sites,
Yoshimitsu Fukushima (IAEA);•

Regulatory considerations for flood hazard assessment and flood protection of NIs
in the United States, Christopher Cook (NRC, United States);

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14•

Regulatory requirements for flood hazard assessment and flood protection of NIs
in France, Vincent Rebour (IRSN, France);•

Undertaking vulnerability assessments of critical water resources infrastructure,
John Perdikaris (Ontario Power Generation, Canada);•

Operating experience at Fort Calhoun, Roger Kay (US Army Corps of Engineers,
United States);•

Riverine flood hazard assessment in Germany, Christian Strack (GRS, Germany).

National regulatory requirements and approaches as well as international guidance
regarding flood hazard assessment and protection of NIs against river floods were discussed
in this session. Through the presentations, information on regulations, hazards assessment
and protective measures against river floods were shared among the member states. In
many member states, the design basis level is based on annual exceedance frequency(e.g. less than or equal to 10-4 per year) for natural events. In other member states designbasis level is derived from a deterministic approach (i.e. probable maximum precipitation(PMP) and probable maximum flood (PMF)) and not associated with a probability.
Mr Fukushima (IAEA) presented the IAEA guidance for flood hazard assessment and flood
protection at NI sites. He informed that IAEA Safety Guide SSG-18 provides guidance toestablish design basis parameters for meteorological and hydrological phenomena,
measures for site protection and strategies for monitoring nuclear installations located on acoastal or a river site or an estuary site. It was noted that the results of probabilistic methods
should be checked for consistency with the results of a simplified deterministic analysis
and, in all cases (deterministic, statistical and probabilistic approach), quantitative estimateof the uncertainties should be determined. He stated that climate change adds further
uncertainty to the meteorological and hydrological analyses and should be considered. Suchapproaches as paleoflood analysis of the site area should also be considered. He underlinedthat in deriving the design basis flood for an NI site, combined events should be considered
as well as single events and the annual exceedance frequency for each combination shouldbe estimated. The measures for site protection such as dry site concept and permanent
external barriers were discussed. He said the EESS-EBP drafted the “Safety Report:
Assessment of External Flooding (Excluding Tsunami) and High Wind Hazards in Site
Evaluation for Nuclear Installations”. An important lesson learnt from the Fukushima
Daiichi NPP accident was that the plant design bases need to be revised based on the resultsof the latest scientific models that employ both conservative analysis methods andassumptions and also assess and characterise uncertainties.
The US NRC representatives discussed the regulatory considerations for flood hazard
assessment and flood protection of NIs in the United States. Dr Cook informed on the postFukushima flood walkdowns undertaken to verify that procedures or activities can be
executed as specified within the allocated time frame, and to understand cliff-edge effects.
As a result, many licensees have enhanced procedures, practices and training programmes,
and identified better locations to store flood protection equipment. It was noted that postFukushima hazard reevaluations showed that the majority of the exceedances was
associated with the local intense precipitation hazard (LIP>85%), which was not part of theoriginal design basis. He presented the update of NUREG-0800 Standard Review Plan
related to hydrology and flooding hazards assessment. He also mentioned that floodinundation maps are the US government standard for communication of flood hazard
information (warning time, inundation depth/duration, etc.), which should be shared with

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16 │ NEA/CSNI/R(2018)14
NPPs, and he spoke of how the flood hazard analysis results could feed into theprobabilistic risk assessment (PRA) models to derive the safety goals.
The representatives from IRSN discussed the regulatory requirements for flood hazard
assessment and flood protection of NIs in France. Dr Rebour mentioned that a guide onprotection of nuclear installations against external flooding was published in 2013 taking
into account the «Le Blayais» flooding event (December 1999) and the «Fukushima»
flooding accident. The definition of “Reference Flooding Situation" (RFS) was introducedin this guide as well as methods for the characterisation of the RFSs and principles forprotection of NIs. According to this guide, it is needed to consider a set of 11 RFSs todefine design basis floods (DBF). It recommends taking into account all situations likely
to be encountered on the basis of relevant past experience (e.g. historical evidence) byapplying statistical and deterministic methods. The general aim is to derive RFSs withfrequency of exceedance of 10-4 per year and to cover associated uncertainties. The
evolution of the hazard with time should be also considered. Regarding flood protection,
he noted that measures requiring neither human intervention nor energy supplies should be
preferred. It is also recommended to consider a beyond design basis flood/design extension
conditions (BDBF/DEC) scenario as DBF scenario plus margin, which is specific to eachflooding scenario. Lastly, he said the follow-up activities in the domain should include thedevelopment of probabilistic flood hazard assessment to support the current
deterministic/statistical flood risk assessment.
Dr Perdikaris (Ontario Power Generation) discussed the vulnerability assessment of criticalwater resources infrastructure. He said that the assessment and ranking of the vulnerabilityof affected groups should serve as the starting point in defining priorities and means ofremedial interventions He presented how OPG evaluates flood risk by looking at thewatershed itself and then following a systematic assessment approach. He also spoke oflooking at the population exposure to the flooding event and taking life safety into the floodrisk assessment. He stated that climate change needs to be incorporated into the overallvulnerability index quantification method. In addition, given the level of uncertainty
associated with climate change, several future climate scenarios need to be examined inorder to quantify their risk and impact. The knowledge gaps and future research needs on
this matter were also discussed.
The representatives from the US Army Corps of Engineers presented the operating
experience related to 2011 Missouri river flood at Fort Calhoun NPP. Dr Kay noted that
reservoir regulation during extreme events requires flexibility to be built into mainstream
reservoir systems, as well as consideration of changing conditions on the ground andchanging climate. He stated that unprecedented runoff occurred in the Missouri River Basinabove Sioux City (Iowa) during May-July 2011, which was higher than the total annual
runoff in 102 of 113 years in the period of record. It was mentioned that some local levees
raised by farmers made the situation even worse at the NPP site. The nuclear reactor had
been shut down and defueled in April 2011 for scheduled refuelling. The Fort Calhoun
plant had taken a number of protective measures (e.g. makeshift barriers), which helped tokeep the water away from safety related buildings.
The representative of the GRS discussed the regulatory requirements and methodology forriverine flood hazard assessment in Germany. Mr Strack informed that according to nuclear
safety standard KTA 2207 at river sites the design basis water level is based on the flood
runoff from the catchment basin of the river with an exceedance frequency of 10-4 per year.
For the derivation of this design basis runoff, all relevant parameters (e.g. condition andcharacteristics of the drainage area and overflow/failure of dikes/embankments) have to be

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taken into account. He noted the assessment of flood runoffs is based on the statisticalevaluation and extrapolation of gauge measurements. The peak runoff of the 10-2 per year
flood event is derived from representative gauge measurements using standard statisticalprocedures. The peak runoff of the 10-4 per year flood event is extrapolated from the peakrunoff of a flood event with an exceedance frequency of 10-2 per year by using the Pearson
Type III probability distribution with a conservative skewness coefficient. Finally, the10-4 per year flood runoff is converted to a design basis water level using a suitablehydraulic model (applying the information on the slope of water surface). It was underlinedthat the conservative approach described in KTA 2207 is not binding and other approaches
may also be used.
The presentations and discussions highlighted a number of commendable practices. It wasnoted as a commendable practice that the design basis is based on an annual exceedance
frequency less than or equal to 10-4 per year for riverine flooding. Associated effects, suchas debris, hydrostatic pressure, which happen at lower than design basis flood levels, could
impact the plant safety and they should be taken into account. The resilience of the facility
against external flooding hazards needs to be demonstrated in a systematic way for thesafety case. Reliable forecasting is far more important for plants that tend to use temporary
measures for flood protection versus those using permanently installed measures. The
operators of NIs should establish a strong relationship with forecasting organisations sothat they can effectively implement their strategies to protect their facility from flood
events.

Challenges and gaps identified:

2.3.

•

There seems to be a gap between hydrologists dealing with the science-based
phenomenon of flooding and decision making in a risk-informed regulatory
framework. Bridging this gap is a challenge.

•

Data extrapolation for low probability flooding events is a commendable practice,
but it is challenging to do.

•

Weather forecast data related to flooding (LIP, etc.) should be correct and reliable,
which is a challenge.

Session 2 – Technical methods for hazard assessment
This session was devoted to the technical methods in modelling river floods and hazard
assessment.
The following presentations were made:•Riverine flood hazard assessment based on Probable Maximum Precipitation and
Probable Maximum Flood, Yuri Simonov (WMO expert, Hydro-meteorologicalResearch Centre of Russia);•

PMP and PMF under a changing climate, John Perdikaris (Ontario Power
Generation, Canada);•

Probabilistic flood hazard assessment for rivers and updates on the PRA standard,Michelle Bensi (University of Maryland, United States);•

Global sensitivity analyses applied to hydraulic modelling, Claire-Marie Duluc(IRSN, France);

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18 │ NEA/CSNI/R(2018)14•

Illustrative example - nuclear plant along the Rhône River, Sylvain Saviot (EDF,
France);•Riverine flood hazard assessment in Switzerland, Carlo Scapozza (Federal Officefor the Environment) and Vinh Dang (Paul Scherrer Institute, Switzerland);•Data collection, and advances in modelling river floods, Roger Kay (US Army
Corps of Engineers, United States).

Dr Simonov, the expert of the World Meteorological Organization (WMO), provided thepresentation on riverine flood hazard assessment based on probable maximum precipitation(PMP) and probable maximum flood (PMF). He noted that there are two groups of methods
for deriving design flood parameters: probabilistic approach (flood frequency analysis) anddeterministic approach (estimation of the so-called PMP and PMF). He stated that methodsof statistical extreme analysis are widely used in hydrology and considered appropriate for
return periods of up to 103 or 104 years. He said that PMP/PMF events are considered to beat least two orders of magnitude greater (104 to 106 years) and they cannot be considered ashaving a specific return period. He gave the definitions of PMP and PMF, and discussed
the considerations, methods/models currently used for PMP/PMF estimation, datarequirements and some other issues (maximisation, transposition, enveloping, accuracy,
plausibility etc.). It was underlined that close co-operation of meteorological andhydrological specialist is needed in this area. He also mentioned that factors of climatechange need to be considered in PMP/PMF estimation. Lastly, he said that some examplesof PMP/PMF estimates for different climate/orographic conditions are given in the“Manual on estimation of Probable Maximum Precipitation” (WMO-No. 1045), 2009.
Dr Perdikaris (Ontario Power Generation) discussed the PMP and PMF under a changing
climate. He presented the history of PMP development, approaches to PMP estimation andnoted that there is no universally accepted methodology for deriving the annual exceedance
probability (AEP) for the PMP. He mentioned the statistical approach to evaluating extreme
precipitation events as well as its advantages and disadvantages. Methods for estimating
PMP and steps of the most commonly used PMP estimate such as moisture maximisation
have been discussed. He noted the accuracy of PMP/PMF estimation rests on the quantityand quality of data on extraordinary storms and floods and the depth of analysis and study.
There are no methods to quantitatively assess the accuracy of PMP and PMF, and it is mostimportant to analyse, compare and harmonise results of PMP/PMF from multiple
perspectives. He also discussed the development of the PMF hydrograph and incorporationof climate change for determining PMF, which was illustrated on case study of Canadianbasins. It was noted there is no big change (±5%) in PMP/PMF in Ontario region withclimate change between two different time periods (1971 to 2000 and 2041 to 2070). Lastly,
he shared his opinion on existing knowledge gaps and future research needs in the areas of
uncertainty in climate change projection, a systematic approach to risk management, andproactively maintaining public trust.
Dr Bensi (University of Maryland) discussed the probabilistic flood hazard assessment forrivers and provided updates on the US PRA standard. She informed that ASME/ANS RAS standard (Part 8 addresses external flooding) describes the technical elements requiredfor performing a PRA (Level I, At-power) in support of nuclear plant safety and riskinformed applications. She said that significant effort being undertaken to revise the current
version of Part 8 (XFPRA) of the ASME/ANS RA-S standard with the aim to reflect recent
lessons learnt and identified flood-specific challenges. It was noted that key challenges
associated with XFPRA for NPPs are the nature of flooding hazards (diverse flooding
hazards and diverse characteristics of flooding events) and plant impacts from flooding
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hazards (diverse ways in which they may affect a plant, and diverse strategies to protect
against or mitigate). She stated that the revision of Part 8 of the standard is performed using
many non-prescriptive requirements, providing significant flexibility in addressing
requirements and developing extensive non-mandatory appendices with commentary,clarifications, and insights. She discussed the key technical elements of revising Part 8 of
ASME/ANS RA-S such as hazard analysis, fragility and plant response.
Mr Saviot from EDF presented an illustrative example of applying the ASN guide n°13 forriverine flood hazard assessment of NPP site along the Rhône River. He noted that
according to the guide n°13, two «Reference Flood Situation» (RFS) should in particular
be considered in flood hazard assessment for sites at the Rhône River: large watershed
flooding (CGB) and failure of a water-retaining structure (ROR). He discussed theexamples of flood hazard assessment for failure of the Vouglans dam with a height morethan 100 m (ROR case) and a 1 000-year return period flood (taking the upper bound of the
70% confidence interval) plus 15% increase (CGB case). It was mentioned that
TELEMAC-MASCARET system was used for numerical modelling of a hydraulicstructure (hydraulic dam, waterways, hydroelectric power station). The numerical resultsof the base case and the most unfavourable case were presented.
Ms Duluc form IRSN presented the global sensitivity analyses applied to hydraulicmodelling. She noted the objective of this activity was to develop a rigorous and objective
methodology to define the most influencing parameters for hydraulic modelling with theassociated uncertainties. She discussed the main steps of an uncertainty propagation study
and the role of IRSN's computational environment PROMETHEE in a global sensitivity
analysis. The application of this methodology to hydraulic studies were presented for a 1D
hydraulic model of the Rhône River; a 2D hydraulic model for flooding and levee breachstudies on La Garonne river; and 2D hydraulic model for the study of tsunamigenic
potential of the Azores-Gibraltar Fracture Zone (AGFZ). It was mentioned that global
sensitivity analysis allows identifying the most influencing parameters in a rigorous way
as well as some rare combinations of critical flooding situation, which would not have been
identified solely based on expert opinion. She also noted the main challenges of thismethodology such as time-consuming calculations, characterisation of probability
distributions for each input parameter and dealing with dependent input parameters.
Mr Scapozza from the Federal Office for the Environment provided an overview of flood
protection measures used in Switzerland as well as lessons learnt from extreme events andclimate changes. He informed that average damages from flood amount to CHF 350 million
per year and investments in flood protection is approximately 1/3 of all public investments.
He mentioned the climate in Switzerland is warming and therefore extreme events will
occur more often. He discussed the project EXAR whose goals are to establish a commonbase for the risk and damage potential of extreme flooding events along the Aare and Rhine
River, based on existing studies, and to derive hazard scenarios for events of different
probability levels, especially for extremely rare events (down to annual exceedance
frequency of 10-7 per year). He gave an example of modelling the catchment area of the
Aare River and expected results of peak runoff on observation-area. Mr Dang (PaulScherrer Institute) provided more details on the project EXAR. He briefly described the
PFHA methodology, decomposition into subsystems, meteorological and weather
scenarios, hydraulics, structures and natural processes as well as flood scenario event treemodelling and synthesis. Lastly, he noted the challenges of this project including selectionof representative hydrographs, verification and plausibility of hydrographs and theirfrequencies, identification of key sites (whose behaviour have an impact on total system),and uncertainty and sensitivity analysis in multi-level analysis.
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Dr Kay (US Army Corps of Engineers) discussed the data collection and advances inmodelling river floods. He noted that data by itself is facts and statistics collected together
for reference or analysis, but when put in certain context becomes information useful for
decision making. He said the long-term monitoring of hydrologic systems (precipitation,
streamflow etc.) is essential for understanding system behaviour and is a basis forpredictive modelling and thus risk management. Data gathering need to be pooled, qualitycontrolled and archived, and archives then need to be maintained, updated and should
ideally be made accessible to everyone. It may well be one of the greatest future challenges
to decide who pays for the costs of ensuring longevity and continuity of data gathering,monitoring and archiving. He mentioned that different methods can be used for datacollection such as aerial photogrammetry, laser scanning, multipurpose remote survey boat
etc. He also informed that Hydrologic Engineering Center (HEC) has been developing
computer software for hydrologic engineering and planning analysis procedures since its
inception in 1964. He provided a list of software developed by HEC, and discussed the areaof application of some codes in modelling river floods. It was noted that some of HEC
software are available for download on their website and may be used by individuals
outside of the Corps of Engineers.
The presentations and discussions led to a number of commendable practices. Assessment
methods used for evaluating the hazards from riverine flooding vary widely. It was notedthat the results of the models developed using these various methods should not be taken
as the definitive answer, but should be cross checked when feasible using other models or
methods. This approach will help to understand better the uncertainty and the effects ofassumptions on the results of the analysis so that the results can be put into context insupport of decision making. The development of probabilistic flood hazard assessment
methodologies for combining hazards (dependent and independent) and building acomplete probabilistic approach, including uncertainty propagation was noted as acommendable practice.
It was underlined that close co-operation of meteorological and hydrological specialist isneeded in this area. It was also mentioned that factors of climate change need to beconsidered in PMP/PMF estimation.

Challenges and gaps identified:•

The lack of data to go to very low-frequency events creates a significant challenge.
While the use of historic information and paleodata can supplement instrumental
data, this is not sufficient and other sources of information should be used. It wassuggested that supplemental data could be generated using simulations based on
phenomenological modelling of riverine systems. However, it was also recognisedthat there are many dimensions to data that makes it difficult to incorporate datafrom other sources into the existing hazard assessment framework. It was further
noted that treating the uncertainty in data and modelling can be very challenging.

•

It was concluded that it may not be sufficient to just identify the flood level, butthere are other factors (associated affects) that should be considered when assessing
the impact on a facility. For example, hydrodynamic loads, sediment transport,
duration of the flood, debris in the floodwater, erosion, etc. should be considered.
The challenge is to determine how to integrate their impacts in support of decision
making on appropriate protective measures.

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2.4.

•

A more rigorous understanding of the fragility of facilities for integrated flooding
impacts would strengthen the technical basis for establishing appropriate protective
measures.

•

Maintaining the public’s trust in protective measures for critical infrastructures will
require proactive approaches not only in communication, but should include further
research into areas such as: current and emerging risks; development of better DBF
estimates; and implementing a credible and diverse portfolio of initiatives tomitigate risk.

•

It was noted the key challenges associated with PSA for NPPs and other NIs are
the nature of flooding hazards (diverse flooding hazards and diverse characteristicsof flooding events) and plant impacts from flooding hazards (diverse effects anddiverse strategies to protect against or mitigate).

•

The main technical challenges of the PFHA methodology were identified as timeconsuming calculations, characterisation of probability distributions for each input
parameter and dealing with dependent input parameters.

Session 3 – Protection of nuclear installations against river floods
This session was devoted to the technical means and methods for protection of NPPs andother NIs against river floods.
The following presentations were made:•Protection of NIs against river floods in Belgium, Dries Gryffroy (BelV, Belgium);•Protection of NIs against river floods in France, Denis Jautzy (EDF, France);•

Protection of NIs against river floods in Germany, Gernot Thuma (GRS, Germany);•

Protection of NIs against river floods in Switzerland, Rainer Hausherr (ENSI,
Switzerland);•Operating experience with flood protection measures, Zdenko Simic (JRC, EC);•

Advances in flood risk management in The Netherlands - dike failure & solutions,
Saskia van Vuren (RWS, the Netherlands).

Dr Gryffroy (BelV) provided the example of flood protection measures used at Tihange
NPP site in the Meuse river valley. He mentioned that the original design basis flood (DBF)
was taken as the historical flood of 1926 plus 20% with a water level at 69.80 m and thesite platform at 71.50 m (based on “dry site” concept). He informed that reassessment of
DBF was performed during PSR based on combination of potential flooding phenomena(high river flow rates + wind waves + impact of mobile dam (Ampsin) downstream of
Tihange NPP) and using a 2D-hydrodynamic model of the Meuse river floods. The revised
DBF is a 1 000-year flood (confidence interval 70%) increased by 15% with resulting waterlevels between 73.20 m (west) and 72.65 m (east), which leads to complete flooding of thesite platform. He said that the flooding protection concept for the Tihange NPP includes
two layers of protection: peripheral protection for the whole site (DBF + 40 cm margin)
and additional ultimate means (AUM) for protection of each unit against BDBF or DEC(DBF + 1 m). He discussed the new site protection against the revised DBF including
peripheral site protection, upstream dike, isolation structure at entrance of water intake
channel, isolation structure in discharge channels, and pumping stations to evacuate sewage

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water. It was noted that dedicated flooding alert procedure (onset of “pre-alert phase”,“alert phase” and shutdown of all units) and flood protection procedures (at site level or
per unit) were developed. The second layer of protection (AUM) includes for each unit 1
ultimate diesel generator (6 kV) in a dedicated bunkered building, redundant pumps and(mostly) fixed piping for water makeup and associated flooding procedures. Long-term
flooding management strategy and combinations of hazards/events (causally or noncausally linked) were identified as challenges.
Mr Jautzy from EDF informed about the flood protection measures and warning systemused in France. He discussed the external flood risk management including its purpose,risks taken into account (site isolation, platform flooding, loss of off-site power, loss ofultimate heat sink) and three types of sites considered (river, sea shore and estuary). He
said that organisational measures for each site include definition of equipment protections
and operational actions to be implemented based on design basis assessment for each risk,
as well as equipment monitoring and maintenance programmes, and operations particular
rules (actions required and justifications). The protection measures – both internal(permanent and temporary protections) and external (to ensure that local infrastructure does
not increase risks) – were presented. It was noted that the warning system is common to alltypes of external flooding events and includes four levels of monitoring and actions,i.e. watch (regular operations), vigilance (strengthened monitoring), pre-alert (increase
surveillance and checks) and alert (anticipate NPP shutdown). Lastly, he provided a coupleof examples of application of the event management (warning system) for Belleville NPP(dam break warning) and for Chinon NPP (flooding).
Dr Thuma from GRS informed on the flood protection measures against river floods usedin Germany. He presented the German safety requirements for NPPs according to which
all safety systems have to be functional during and after design basis natural events
including design basis floods. He noted that DBF is defined in the pertinent nuclear safety
standard KTA 2207 as a flood event with an exceedance frequency of 10-4 per year. It was
mentioned that loads resulting from DBF have to be combined with classical civilengineering loads (dead loads, live loads, etc.), loads due to potential consequential eventsof the flood and seismic loads. KTA 2207 defines different types of sites. The type of site
has consequences for the parameters that need to be taken into account in the hazard
assessment. The protection measures against flooding can be subdivided into two
categories: structural measures and administrative measures. KTA 2207 requires thecompilation of a comprehensive plant-specific concept demonstrating the appropriatenessof the combination of the individual protection measures. He mentioned that preferred
protection measures are permanent structural measures such as adequate site elevation,dikes and waterproofing of buildings etc. It was stated that temporary flood protection
measures may be applied only for water level between the 10-2 per year flood event andthe 10-4 per year flood event. He noted that flood protection measures shall be subject toregular in-service inspections or, alternatively, be designed such that the required
protection is ensured during the planned service life. Structural flood protection measures
shall be supplemented by organisational and administrative measures, such as ensuring theaccessibility of the plant, provision of necessary equipment, integration in flood warning
systems, and specification of a water level for a precautionary shutdown.
Mr Hausherr from ENSI discussed the protection of NPPs and other NIs against river floods
in Switzerland. He said that older plants were designed against a break of upstream dams
or against a flood with exceedance frequency of 10-3 per year. ENSI required the existing
NPPs to reassess flood protection against the new flood hazards that were finished in 2010
(before the Fukushima Daiichi accident) and to assess the margins. It was mentioned that
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safety cases regarding flooding are usually done by detailed 2D hydraulic calculations
using up to date software. In the course of this reassessment, one «dry site» became «wet»
and additional short-term, medium-term and long-term protective measures had been
implemented. After the Fukushima Daiichi accident, ENSI requested operators to develop
new safety cases related to flooding, including blockage of river water intake structures,and its harmonisation with sediment transport modelled using fractioned grain size. One
safety case showed that clogging of the water intake structure of the bunkered system could
not be ruled out. This led to additional measures to prevent clogging of water intake
structures of the bunkered system (periscope pipes) and for component cooling of the
bunkered system. Another safety case showed possibilities to increase the safety margins
by minor modifications to the groundwater wells (stronger manhole lid, backfitting of acheck valve). For mobile accident management equipment either local storage or external
storage was foreseen.
Dr Simic from JRC (European Commission) discussed the results of a statistical andengineering analysis of flooding and flooding protection related events in the nuclear power
plants from 4 databases (French, German, US and the IAEA/NEA International Reporting
System (IRS)) during the last 20 years. He noted this topical study has been conducted toreview the recent worldwide operating experience related to flooding events and flooding
protection, in order to formulate lessons learnt. In total, 263 event reports were analysedwith real and potential flooding events (i.e. when flooding protection deficiency was
discovered without flooding). He said that the events were analysed and classified
considering 13 categories in order to create statistical analysis and assess its possible
insights. It was mentioned that most of the events are occurring during power operation,but events frequency is higher during outage for all databases. The events were analysedbased on categorisation related to three groups: event conditions (i.e. root causes and causal
factors), event consequences and corrective actions. He stated that valves and passive
components are the main reasons for floods in all databases, while seals and drainage arealso important cause for the IRS and US databases. Training, qualification and procedureswith design configuration and analysis are the most important root causes and causal factors
in all databases, where maintenance activity is also important for the IRS and the equipment
performance for the French database. He noted that it is important to have a holistic viewon maintenance, inspection and analysis of flood hazard and protection measures(especially for non-safety systems interaction, drainage system, fire protection systems,
major cooling systems and all infrastructural barriers preventing water propagation).
Configuration management, flood protection engineering programme, communication andtraining are all important elements for reducing flood risk.
Ms Van Vuren from RWS gave a presentation on advances in flood risk management in
the Netherlands - dyke failure and solutions. The presentation focused on dike and coastal
protection systems, rather than nuclear installations. It was mentioned that the flood
protection scheme in the Netherlands has a length of 3 760 km and includes closuring ofestuaries by building storm surge barriers. She noted that the Netherlands recently adapted
new flood safety standards and performed assessments of all protection systems. She said
the study strived to set standards that balance risks across geographic regions. The goal of
the new flood safety standards is to provide protection for everyone at the level of risk lessthan 10-5 per year by 2050. Both the probability of flooding and the consequences offlooding are considered in the risk calculations. Several mechanisms of dikes and structures
failure were taken into account such as overflow/overtopping, erosion of outer/inner slope,
piping, dune retreat, non-closure and instability of construction. Dominant failure
mechanisms are uplifting and piping (for dikes) and non-closure of constructions (forRIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONS│ 23

24 │ NEA/CSNI/R(2018)14
structures). She stated that the maximum acceptable failure probabilities of dikes andstructures were established in the new safety standards for Petten NPP as 3×10-4 per year,
for Delft NPP as 1×10-5 per year and for Borssele NPP as 1×10-6 per year. It was notedthat the study is currently available on a public website along with related data.
The presentations and discussions led to a number of commendable practices. It was notedthat applying the concepts of defence-in-depth (in the sense of multiple barriers against
flooding) is important to establish appropriate protective measures for flood prevention andmitigation. This practice can be used to account for uncertainties in the analyses and for theeffectiveness of barriers (engineered features) and supports the development of emergency
measures in the event of a barrier breach. A commendable practice noted was also the useof both prevention and mitigation when developing protective measures against flooding.
It was suggested that a third line of defence against flooding would be to harden
components against the effects of flooding. It was noted as a commendable practice to also
assess the impact of the protective measures on other hazard protective features and event
prevention or mitigation features when assessing the effectiveness of protective measures.
Consideration should be given to all potential hazards when developing protective
measures. When establishing the criteria for protective measures to assure theireffectiveness, it is important to also consider how the equipment or feature could be
credited in other situations.
A commendable practice identified was to verify on a periodic basis that the protective
measures against flooding remain in good working order during the life of the plant. Thisshould include periodic walkdowns to make sure the protective features have not degraded
and remain consistent with the assumptions made in the hazard assessment. The use of
precursor events and other operating experience to inform flood hazard assessments andassure the effectiveness of protective measures was identified as a commendable practice.
Another commendable practice identified was to demonstrate the resilience of the facility
against flooding considering the protective measure that will be used during a flooding
event. It was suggested that analysis and evaluation alone were not sufficient, and that tothe extent practical the effectiveness of the protective measures should be demonstrated.
There was acknowledgement that forecasting of conditions that contribute to riverine
flooding is important to flooding protection. With this recognition, it was also noted there
should be stronger relationships between those organisations that forecast these conditions
and those organisations that need to act to protect vital infrastructure, including nuclear
installations. This relationship is particularly important for those facilities that rely on
temporary protective measures (i.e. sand bags). Also, it was noted that nuclear installations
should have strong relationships with those organisations that manage the water control
structures that could affect the nuclear installation. This is important to provide
co-ordination and warnings in the event the water control structure is used to mitigate or
prevent impacts on other critical infrastructure or to the public. It was noted that awarenessof flood impacts on infrastructure, including nuclear installations, was high with substantial
working being conducted both within the nuclear field and in other industries.

Challenges and gaps identified:•A balance is needed between the appropriate consideration of the spectrum ofassociated effects from flooding and the information necessary to support decision
making for adequately protecting the facility from realistic external floods.

•

The concept of a "dry site" needs to be reconsidered. It was the general view fromthe discussions at the workshop that due to the evolving understanding of flooding
RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14

hazards, even sites that were designed with the "dry site" concept need to consider
flooding effects. This is due, in part, to the potential for local intense precipitation
to create localised flooding on the site.

2.5.

Workshop closing session
At the beginning of the closing session the rapporteurs summarised the content of thepresentations and discussions of the three technical sessions. Stimulated by thesesummaries, the participants raised, among others, the following points that deserve future
consideration:•

There are various methods for flooding hazard assessment, from very simple
approaches to highly complex and sophisticated models. Each method has its
specific advantages, drawbacks and sources of uncertainties. These factors should
be considered carefully when choosing the most suitable hazard assessment method
for a given purpose.

•

For any hazard assessment availability of data is of paramount importance.
Unfortunately, the database with respect to flooding is limited. Therefore, ways
have to be found to increase the relevant database and to supplement the data byadditional information, e.g. from physics-based models/simulations.

•

Flooding can have many different causes (e.g. rising water levels in adjacentwaterbodies or heavy precipitation) and may affect an installation in many ways.
Therefore, the hazard assessment should not focus on a particular flooding scenario
but should try to cover all reasonable scenarios.

•

Both flooding hazard assessment and flooding protection measure reliability
involve uncertainties. To account for the uncertainties and to increase therobustness of nuclear installations with respect to flooding risks, the flooding
protection concept should comprise multiple layers, e.g. selection of an elevatedsite, augmented by a dyke and waterproofing of safety related buildings as a thirdline of defence.

•

The demonstration of the effectiveness of flooding protection measures should
consider ageing effects and should not be limited to the consideration of the waterlevel (i.e. hydrostatic pressure) but should also include associated effects such asdebris and dynamic loads.

•

As the protection against one hazard may also affect the protection measuresagainst other hazards, the potential adverse effects of increasing the robustness withrespect to a specific hazard should be considered carefully.

•

Sharing of information regarding regulations, hazard assessment approaches andprotective measures is an important element to successfully understanding theimpact of external hazards on nuclear facilities. The sharing of information needs
to cross the boundary between the nuclear industry and other organisations thatcollect and analyse data on external hazards for use in assessing the impact ofsevere weather on society and its supporting infrastructure. Sharing includes
reaching out to organisations responsible for modelling and predicting weather(i.e. national weather services, universities, etc.) and international organisations,such as the World Meteorological Organization to facilitate a more integrated
national and international response to external hazards.

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26 │ NEA/CSNI/R(2018)14•

Probabilistic safety assessment provides an important input for a traceable andreproducible risk-informed regulatory decision-making process, but as highlighted
in recent work on PSA for natural hazards done by the Working Group on Risk
Assessment (WGRISK) (see, e.g. Workshop Proceedings NEA/CSNI/R(2014)9),
challenges remain for areas such as fragility analysis for non-seismic hazards,
including external flooding as discussed during this workshop. Within this context,
research results related to external flooding, including PFHA, being conducted byuniversities, regulators, the nuclear power industry, and other organisations should
be shared to the extent practical to develop a common data set for use in assessing
the impact to NPPs and other NIs.

The insights gained during the workshop, as noted above and based on engaging
discussions among participants, have a direct connection to the objectives of regulators that
include review of nuclear facility siting, SSC design, licensing decisions and oversightactivities. In several countries, deterministic methods such as the stylised, probablemaximum flooding approach, are currently used in nuclear facility siting, licensing andregulatory oversight decisions. In other countries, statistical methods are usually used toassess events with annual exceedance frequencies less than or equal to 10-4 per year. To
support the application of risk-informed decision making, research needs to continue tounderstand the impact of external hazards on nuclear facilities, by, for example:
establishing guidelines for PFHAs; leveraging data collected and maintained on riverine
flooding from organisations outside the nuclear industry; and establishing criteria related
to riverine flooding that are based on sound scientific principles with supporting data andanalytical techniques for use in risk-informed regulatory decisions. Co-ordination of thework of the WGEV with other NEA working groups, primarily with WGRISK, is importantfor understanding how better data and a better understanding of the phenomena associatedwith external events can be used to better understand the risk to public health and safety
from nuclear facilities. Building on the results of this research will support the balanceneeded in risk-informed decision making between the use of deterministic data andmethods and the application of probabilistic methods to extend the applicability of the datafor use in PSAs.

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3. Conclusions and recommendations
The following conclusions and recommendations are being made by the Nuclear Energy
Agency based on workshop presentations and discussions during particular technicalsessions and other, facilitated discussions:•Sharing of information between the nuclear industry and non-nuclear organisations
regarding regulations, hazard assessment approaches and protective measures is an
important element to successfully understanding the impact of external hazards on
nuclear power plants (NPPs) and other nuclear installations. Data received fromneighbouring countries can also be used to broaden information on flooding
phenomena.

•

Given the limited amount of available historical data, further work needs to be done
to understand how metrics, such as a selected value for annual exceedance
frequency, can be used in regulatory decision making and be clearly communicatedto the public. It is necessary to augment temporally and spatially sparse historical
data with simulations and other information in order to be able to make decisions
associated with rare events. Simulation and modelling should be validated withconsideration of available empirical data.

•

In the treatment of uncertainty, there are two important aspects: from a scientificperspective, there are uncertainties with data and modelling that need to be better
understood and quantified; from a regulatory perspective, decision makers should
be aware of the uncertainties in the scientific results and take them appropriately
into account in their decision-making process.

•Co-ordination of the work of the Nuclear Energy Agency Working Group on
External Events (WGEV) with other Nuclear Energy Agency (NEA) working
groups, primarily with the Working Group on Risk Assessment (WGRISK), is
important to understand how better data and a better understanding of thephenomena associated with external events can be used to better assess the risk topublic health and safety from NPPs and other nuclear installations. Building on the
results of this research will support the balance needed in risk-informed decision
making between the use of deterministic and probabilistic methods.

•

Climate change is introducing new challenges into the decision-making process thatwill need to be considered. New approaches and models will need to be developed
to identify and address these challenges.

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28 │ NEA/CSNI/R(2018)14

4. References
NEA (2018) “Technical Note on Riverine Flooding – Hazard Assessment and Protection of Nuclear
Installations”, OECD, Paris, NEA/SEN/SIN/WGEV(2018)1.

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14│ 29

Appendix 1: List of participants
BELGIUM
Mr Dries GRYFFROY

Bel V

CANADADr John PERDIKARIS

Ontario Power Generation

Mr Joey WANG

Canadian Nuclear Safety Commission

FINLANDMr Marko MARJAMÄKI

Säteilyturvakeskus

FRANCEMr Riadh ATA

Électricité de France

Ms Karine BEKADDOUR

Institut de Radioprotection et deSûreté NucléaireMr Hervé CORDIER

Électricité de France

Mr Vincent COULON

Électricité de France

Ms Claire-Marie DULUC

Institut de Radioprotection et deSûreté Nucléaire

Mr Yasser HAMDI

Institut de Radioprotection et deSûreté NucléaireMr Denis JAUTZY

Électricité de France

Mr Erik LECLERC

Institut de Radioprotection et deSûreté Nucléaire

Mr Giovanni MATTAROLO

Électricité de France

Ms Céline PICOT

Institut de Radioprotection et deSûreté Nucléaire

Dr Vincent REBOUR

Institut de Radioprotection et deSûreté Nucléaire

Mr Sylvain SAVIOT

Électricité de France

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONS30 │ NEA/CSNI/R(2018)14
GERMANYDr Gernot THUMA

Gesellschaft für Anlagen- undReaktorsicherheit gGmbH

Mr Christian STRACK

Gesellschaft für Anlagen- undReaktorsicherheit gGmbH

Mr Timo SCHMITTTÜV SÜD Industrie Service GmbHJAPANMr Taichi SATO

Nuclear Regulation Authority

Mr Takuya TORIYAMA

Nuclear Regulation Authority

SWITZERLAND
Mr Rainer HAUSHERR

Swiss
FederalInspectorate

Nuclear

SafetyMr Carlo SCAPOZZA

Federal Office for the Environment

Mr Vinh DANGPaul Scherrer Institute

THE NETHERLANDS
Ms Linda JANSSEN

Autoriteit Nucleaire Veiligheid en
Stralingsbescherming

Ms Saskia VAN VUREN

Rijkswaterstaat

UNITED STATESMr John NAKOSKI

United States Nuclear Regulatory
Commission

Dr Joseph KANNEY

United States Nuclear Regulatory
Commission

Dr Christopher COOK

United States Nuclear Regulatory
Commission

Dr Roger KAY

US Army Corps of Engineers

Dr Michelle BENSI

University of Maryland

Dr Curtis L. SMITH

Idaho National Laboratory

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14│ 31

INTERNATIONAL ORGANISATIONSEuropean Commission
Dr Zdenko SIMIC

European Commission,
Joint Research Centre

International Atomic Energy Agency
Mr Yoshi FUKUSHIMAInternational Atomic Energy Agency

Nuclear Energy Agency
Dr Victor NERETIN

Nuclear Energy Agency

Mr Andrew WHITE

Nuclear Energy Agency

World Meteorological Organisation
Dr Yuri SIMONOVWorld Meteorological Organization

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONS32 │ NEA/CSNI/R(2018)14

Appendix 2: Workshop programme
WEDNESDAY, 21 MARCH 2018__________________________________________________________________________________
8:30-9:00

Registration of the participants

9:00-9:15

OPENING SESSION
Session chaired by Dr Gernot THUMA – Workshop Chair (GRS, Germany)

09:00

NEA WELCOME AND OPENING REMARKS
Andrew White, Deputy Head of Nuclear Safety Technology and Regulation Division
(NEA)

09:05

WELCOME AND WORKSHOP INTRODUCTION, BACKGROUND, AND
OBJECTIVES, PRIORITIES AND CHALLENGES
Gernot Thuma, Chief, Reactor Safety Analyses Division (GRS)

09:10

WORKSHOP ORGANISATION & LOGISTICS
Victor Neretin (NEA)

Session 1

REGULATORY APPROACHES AND OPERATING EXPERIENCE

9:15 – 17:30

Session chaired by Mr John NAKOSKI (NRC, USA)

9:15 – 10:15

IAEA GUIDANCE FOR FLOOD HAZARD ASSESSMENT AND
FLOOD PROTECTION AT NI SITES
Yoshimitsu Fukushima (IAEA)

10:15 – 10:45

REGULATORY CONSIDERATIONS FOR FLOOD HAZARDASSESSMENT AND FLOOD PROTECTION OF NIS IN THE USA
Christopher Cook (NRC, USA)10:45 – 11:15
11:15 – 11:45

Coffee BreakREGULATORY REQUIREMENTS FOR FLOOD HAZARDASSESSMENT AND FLOOD PROTECTION OF NIS IN FRANCE
Vincent Rebour (IRSN, France)

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14

11:15 – 12:45

12:45 – 13:45│ 33

UNDERTAKING VULNERABILITY ASSESSMENTS OF CRITICAL
WATER RESOURCES INFRASTRUCTURE
John Perdikaris (Ontario Power Generation, Canada)
Lunch BreakSession 1

REGULATORY APPROACHES AND OPERATING EXPERIENCE (cont.)

13:45 – 14:15

OPERATING EXPERIENCE AT FORT CALHOUN
Roger Kay (US Army Corps of Engineers, USA)

14:15 – 14:45

RIVERINE FLOOD HAZARD ASSESSMENT IN GERMANY
Christian Strack (GRS, Germany)

14:45 – 15:15

Questions and answers on regulatory approaches and operating
experience

15:15 – 15:45

Coffee Break

15:45 – 17:15

PANEL SESSION 1
Open discussion from the floor with all the presenters in the Session 1
Panellists – John Perdikaris (Ontario Power Generation); Christopher Cook (US NRC);
Rainer Hausherr (ENSI); Vincent Rebour (IRSN); Rapporteur – Joey Zhaoliang Wang
(CNSC)

17:15 – 18:00

WGEV members internal discussion

THURSDAY, 22 MARCH 2018__________________________________________________________________________________
Session 2

TECHNICAL METHODS FOR HAZARD ASSESSMENT

9:00 – 9:10

Recap of Day, Overview of Session 2 Objectives

9:10-17:30

Session chaired by Dr Vincent REBOUR (IRSN, France)

9:10 – 9:35

RIVERINE FLOOD HAZARD ASSESSMENT BASED ON
PROBABLE MAXIMUM PRECIPITATION AND PROBABLE
MAXIMUM FLOOD
Yuri Simonov (Expert WMO, Hydro-meteorological Research Centre, Russia)

9:35 – 10:30

PMP AND PMF UNDER A CHANGING CLIMATEJohn Perdikaris (Ontario Power Generation, Canada)

10:30 – 10:55

PROBABILISTIC FLOOD HAZARD ASSESSMENT FOR RIVERS
AND UPDATES ON THE PRA STANDARD
Michelle Bensi (University of Maryland, USA)

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONS34 │ NEA/CSNI/R(2018)14
10:55 – 11:20
11:20 – 12:00

Coffee BreakRIVERINE FLOOD HAZARD ASSESSMENT AND CURRENT
DEVELOPMENTS IN FRANCEGLOBAL SENSITIVITY ANALYSES APPLIED TO HYDRAULIC MODELLING
Claire-Marie Duluc (IRSN, France)
ILLUSTRATIVE EXAMPLE - NUCLEAR PLANT ALONG THE RHÔNE RIVER
Sylvain Saviot (EDF, France)

12:00 – 12:30

12:30 – 13:30

RIVERINE FLOOD HAZARD ASSESSMENT IN SWITZERLAND
Carlo Scapozza (Federal Office for the Environment) and Vinh Dang (PaulScherrer Institute)
Lunch Break

13:30 – 14:00

DATA COLLECTION, AND ADVANCES IN MODELLING RIVER
FLOODS
Roger Kay (US Army Corps of Engineers, USA)

14:00 – 14:30

Questions and answers on technical methods for hazard assessment

14:30 – 15:30

PANEL SESSION 2Open discussion from the floor with all the presenters in the Session 2
Panellists – Michelle Bensi (University of Maryland); Carlo Scapozza (Federal Officefor the Environment); John Perdikaris (Ontario Power Generation); Gernot Thuma(GRS); Rapporteur – Christopher Cook (US NRC)

15:30 – 16:00

Coffee Break

16:00 – 16:10

Recap of Day, Overview of Session 3 Objectives

Session 3

PROTECTION OF NIS AGAINST RIVER FLOODS

16:10-18:15

Chaired by Mr Dries GRYFFROY (Bel V, Belgium)

16:10 – 16:45

PROTECTION OF NIS AGAINST RIVER FLOODS IN BELGIUM
Dries Gryffroy (Bel V, Belgium)

16:45 – 17:15

PROTECTION OF NIS AGAINST RIVER FLOODS IN FRANCE
Denis Jautzy (EDF, France)

17:15 – 17:45

PROTECTION OF NIS AGAINST RIVER FLOODS IN GERMANY
Gernot Thuma (GRS, Germany)

17:45 – 18:15

PROTECTION OF NIS AGAINST RIVER FLOODS INSWITZERLAND
Rainer Hausherr (ENSI, Switzerland)
RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2018)14│ 35

FRIDAY, 23 MARCH 2018__________________________________________________________________________________
Session 3

PROTECTION OF NIS AGAINST RIVER FLOODS (cont.)

9:00 – 9:30

OPERATING EXPERIENCEMEASURES
Zdenko Simic (JRC, EC)

9:30 – 10:00

ADVANCES IN FLOOD RISK MANAGEMENTNETHERLANDS - DYKE FAILURE & SOLUTIONS
Saskia van Vuren (RWS, The Netherlands)

10:00 – 10:30

Questions and answers on flood protection measures

10:30 – 11:00
11:00 – 12:30

12:30 – 13:30
13:30-15:00

WITH

FLOOD

PROTECTION

IN

THE

Coffee BreakPANEL SESSION 3
Open discussion from the floor with all the presenters in the Session 3
Panellists – Saskia van Vuren (RWS); Roger Kay (US Army Corps of Engineers); Denis
Jautzy (EDF); Dries Gryffroy (Bel V); Rapporteur – Michelle Bensi (University of
Maryland)
Lunch Break
CLOSING SESSION
Session chaired by Dr Gernot THUMA – Workshop Chair (GRS, Germany) andpanel members Mr John Nakoski (US NRC), Mr Dries Gryffroy (Bel V, Belgium)and Dr Vincent REBOUR (IRSN, France)
FINDINGS AND CONCLUSIONSRECOMMENDATIONS FOR FURTHER WORK

15:00

CLOSING REMARKS – Dr Gernot THUMA, Workshop Chair (GRS, Germany)

15:15

WGEV members internal discussion

RIVERINE FLOODING: HAZARD ASSESSMENT AND PROTECTION OF NUCLEAR INSTALLATIONS