Nuclear Safety
NEA/CSNI/R(2020)5
December 2021
www.oecd-nea.org

Workshop on Experimental

Validation and Application of CFD
and CMFD Codes to Nuclear Reactor
Safety Issues, CFD4NRS-7
Shanghai Jiao Tong University
4-6 September 2018

Nuclear Energy Agency

NEA/CSNI/R(2020)5
Unclassified

English text only
20 December 2021

NUCLEAR ENERGY AGENCYCOMMITTEE ON THE SAFETY OF NUCLEAR INSTALLATIONSWorkshop on Experimental Validation and Application of CFD and CMFD
Codes to Nuclear Reactor Safety Issues, CFD4NRS-7

Shanghai Jiao Tong University, 4-6 September 2018
Please note that this document is available in PDF format only.

JT03487712
OFDE

This document, as well as any data and map included herein, are without prejudice to the status of or sovereignty over any territory,
to the delimitation of international frontiers and boundaries and to the name of any territory, city or area.

2  NEA/CSNI/R(2020)5

Unclassified

NEA/CSNI/R(2020)5  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 technical knowledge base of the safety of nuclear installations.
The Committee is a forum for the exchange of technical information and for collaboration
between organisations, which can contribute to its activities from their respective
backgrounds in research, development and engineering. It has regard to the exchange of
information between member countries and safety R&D programmes of various sizes inorder to keep all member countries involved in and abreast of developments in technicalsafety matters.
The Committee reviews the state of knowledge on important topics of nuclear safety
science and techniques and of safety assessments, and ensures that operating experience isappropriately accounted for in 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 issues of common interest. It
promotes the co-ordination of work in different member countries that serves to maintainand 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 participating organisations. The Committee ensures that valuable end-products of thetechnical reviews and analyses are provided to members in a timely manner, and madepublicly available when appropriate, to support broader nuclear safety.
The Committee focuses primarily on the safety aspects of existing power reactors, othernuclear installations and new power reactors; it also considers the safety implications ofscientific and technical developments of future reactor technologies and designs. Further,
the scope for the Committee includes human and organisational research activities andtechnical developments that affect nuclear safety.

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

4  NEA/CSNI/R(2020)5

Acknowledgements
The Nuclear Energy Agency (NEA) wishes to express its sincere gratitude to the organisingcommittee, the scientific committee and the local organising committee.

Organising Committee
Jinbiao Xiong, Shanghai Jiao Tong University, China, General Chair
Dominique Bestion, Commissariat à l’Energie Atomique, France, Co-Chair
Yassin Hassan, Texas A&M University, United States, Co-Chair
Ghani Zigh, United States Nuclear Regulatory Commission, United States, Co-Chair
Xiang Chai, Shanghai Jiao Tong University, China
Nils Sandberg, Secretariat, Nuclear Energy Agency
Matthias Krause, Observer, International Atomic Energy Agency

Scientific Committee
Henryk Anglart, Royal Institute of Technology (KTH), Sweden
Arnoldo Badillo, Imperial College, United Kingdom
Emilio Baglietto, Massachusetts Institute of Technology (MIT), United States
Dominique Bestion, Commissariat à l’énergie atomique et aux énergies alternatives (CEA),FranceUlrich Bieder, CEA, France
Chris Boyd, United States Nuclear Regulatory Commission (US NRC), United States
Robert Brewster, Westinghouse, United States
Xu Cheng, Karlsruhe Institute of Technology (KIT), Germany
Abdelouahab Dehbi, Paul Scherrer Institute (PSI), Switzerland
Yassin Hassan, Texas A&M, United States
Thomas Hoehne, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany
Yanping Huang, Nuclear Power Institute of China (NPIC), China
Wang-Kee In, Korea Atomic Energy Research Institute (KAERI), Korea
Seiichi Koshizuka, University of Tokyo, Japan
Eckart Laurien, Stuttgart, Germany
Dirk Lucas, HZDR, Germany
Elia Merzari, Argonne National Laboratory (ANL), United States
Fabio Moretti, Nuclear and Industrial Engineering (NINE), Italy
Hideo Nakamura, Japan Atomic Energy Agency (JAEA), Japan

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  5

Horst-Michael Prasser, Eidgenössische Technische Hochschule Zürich (ETH Zürich),
Switzerland
Afaque Shams, NRG (nuclear research and consultancy group), Netherlands
Jan-Patrice Simoneau, Electricité de France, France
Chul-Hwa Song, KAERI, Korea
Wenxi Tian, Xi'an Jiaotong University (XJTU), China
Jinbiao Xiong, Shanghai Jiao Tong University (SJTU), China
Hongxing Yu, Nuclear Power Institute of China (NPIC), China
Donghui Zhang, China Institute of Atomic Energy (CIAE), China
Ghani Zigh, US NRC, United States

Local Organising Committee (Shanghai Jiao Tong University, China)
Xiaojing Liu
Wenhai Qu
Xiang Chai
Tengfei Zhang

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

6  NEA/CSNI/R(2020)5

Table of contents

Executive summary ............................................................................................................................... 7
List of abbreviations and acronyms ................................................................................................... 10
1. Background ...................................................................................................................................... 13
1.1. Scope........................................................................................................................................... 13
1.2. Organisational aspects ................................................................................................................ 13
2. Technical session summaries from the Co-Chairs........................................................................ 15
2.1. Session 1: Two-Phase Flow (I) ................................................................................................... 15
2.2. Session 2: Containment (I).......................................................................................................... 16
2.3. Session 3: Experiment technique ................................................................................................ 172.4. Session 4: Flow-Induced vibration ............................................................................................. 18
2.5. Session 5: Bundle flow ............................................................................................................... 19
2.6. Session 6: Mixing (I) .................................................................................................................. 19
2.7. Session 7: Liquid metal ............................................................................................................... 20
2.8. Session 8: Two-Phase flows II .................................................................................................... 21
2.9. Session 9: Containment (II) ........................................................................................................ 21
2.10. Session 10: Mixing (II) ............................................................................................................. 22
2.11. Session 11: Pressurised thermal shock ..................................................................................... 23
2.12. Session 12: Plant application (I) ............................................................................................... 24
2.13. Session 13: UQ and coupling.................................................................................................... 24
2.14. Session 14: Plant applications (II) ............................................................................................ 25
Annex A. Keynote lectures.................................................................................................................. 26
Annex B. Poster papers ....................................................................................................................... 27
Annex C. Technical programme (presented orally) ......................................................................... 28
Session 1: Two-Phase flow (I) ........................................................................................................... 28
Session 2: Containment (I)................................................................................................................. 28
Session 3: Experiment technique ....................................................................................................... 28
Session 4: Flow-Induced vibration .................................................................................................... 29
Session 5: Bundle flow ...................................................................................................................... 29
Session 6: Mixing (I) ......................................................................................................................... 29
Session 7: Liquid metal...................................................................................................................... 30
Session 8: Two-Phase flow (II).......................................................................................................... 30
Session 9: Containment (II) ............................................................................................................... 30
Session 10: Mixing (II) ...................................................................................................................... 31
Session 11: Pressurised thermal shock .............................................................................................. 31
Session 12: Plant application (I) ........................................................................................................ 32
Session 13: UQ and coupling............................................................................................................. 32
Session 14: Plant applications (II) ..................................................................................................... 32

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  7

Executive summary
The Workshop on Experimental Validation and Application of CFD and CMFD Codes to
Nuclear Reactor Safety Issues, CFD4NRS-7 was the seventh in a series of workshops
focusing on Computational Fluid Dynamics for Nuclear-Reactor Safety (CFD4NRS). It
was held in Shanghai Jiao Tong University on 4-6 September 2018 and featured 5 keynote
lectures, 45 oral presentations and 6 poster presentations. One hundred and twenty
participants attended the workshop and the emphasis was, in a congenial atmosphere, on
offering exposure to state-of-the-art (single-phase and multiphase) CFD applications
reflecting topical issues in nuclear power plant design and safety, but in particular on
promoting the release of high-resolution experimental data to continue the CFD validation
process in this application area.
The reason for the increased use of multi-dimensional CFD methods is that a number ofimportant thermal-hydraulic phenomena occurring in nuclear power plants cannot beadequately predicted using traditional one-dimensional system hydraulics codes with therequired accuracy and spatial resolution when strong three-dimensional motions prevail.
Established CFD codes already contain empirical models for simulating turbulence, heattransfer, multiphase interaction and chemical reactions. Nonetheless, such models must be
validated against test data before they can be used with confidence.
The necessary validation procedure is performed by comparing model predictions against
trustworthy experimental data. However, reliable model assessment requires CFD
simulations to be undertaken with full control over numerical errors and input uncertainties.
The writing groups originally set up by the Nuclear Energy Agency (NEA) have been
consistently promoting the use of best practice guidelines (BPGs) in the application of CFD
for this precise purpose, and BPGs remain a central pillar of the simulation material
accepted at this workshop, as it was during the previous ones. In order to assess the maturityof CFD codes for use in reactor safety and design, it is necessary to establish a database of
CFD-grade experimental material; this remains the second pillar of the CFD4NRS series
of workshops.
The third pillar is the evolving use of CFD modelling in multiphase applications, these days
known commonly as computational multi-fluid dynamics (CMFD). Here, the challenges
are considerable. Not only are the governing equations an order of magnitude more
complex than for single-phase applications, but a validation database for which there isgenuine three-dimensional involvement remains quite sparse. Of course, multiphase CFD
is not used only in the area of nuclear power plant applications, and importantdevelopments are taking place in other industrial arenas, such as in the chemical andprocessing industries, and in environmental studies. Prudence dictates that the CFD4NRS
series of workshops should not provide reporting space for such non-nuclear CMFD
applications, but should recognise that links with diverse application areas need to be
maintained. It is important to ensure that nuclear applications learn from developments inother areas and not repeat mistakes.
Emphasis in this workshop was placed on single-phase and multiphase CFD simulations
with a focus on validation in areas such as heat transfer, free-surface flows, direct contact
condensation and turbulent mixing. Many papers were related to nuclear power plantrelevant safety issues, such as pressurised thermal shock, critical heat flux, boron dilution,hydrogen distribution in containments, thermal striping and fatigue, and/or advanced
design concepts, such as tight-lattice fuel configurations and passive safety options. A few

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

8  NEA/CSNI/R(2020)5
papers discussed Uncertainty Quantification (UQ) in CFD, which was seen as a promising
step, and something that should be further explored.
Although some papers discussed the challenges in obtaining new experimental data, it was
felt that this area needs to be emphasised even more, in particular related to two-phase flow,
for which sophisticated measurement techniques are required, and for which informationis scarce. It is also very important to deepen understanding of the physics before starting a
numerical analysis.

Conclusions and recommendations
The session topics, as expected, were wide and various, including such issues as advanced
reactor modelling, flow mixing issues, boiling and condensation modelling, multiphase andmultiphysics problems, plant application, hydrogen transport in containments, advanced
measuring techniques, and single and multiphase flow in reactor cores and sub-channels.
As is customary at the panel session, which in this case was led by D. Bestion (CEA), therespective session chairpersons of the presentations given during the oral sessions provided
summaries, and general comments were invited from the audience. These session
summaries are included in the present document.

Specific recommendations
The nuclear CFD community should be encouraged to apply and further develop the use ofUncertainty Quantification (UQ) methods in regard to their simulations, including
uncertainties arising from the numerical solution procedure, the physical models employed,
and in the application of initial and boundary conditions.
CMFD has been more extensively applied to investigate the two-phase phenomena related
to nuclear reactor safety. The closure model development or improvement should be basedon CFD-grade experiment data with complete sets of local information. Joint efforts in thecommunity are demanded.
More validation efforts are demanded for CFD simulation on flow-induced vibration.
Currently, the vibration frequency can be predicted fairly well. However, the damping ofvibration is not so well reproduced.
More emphasis should be given on the experiment measurement error.

General conclusions and recommendations
Delegates were satisfied that the subject areas covered by the workshop werecomprehensive within the nuclear CFD community, and that leading experts in the field
adequately covered the current state of the art and projected future trends. The message
received was that “small is good”, and that the workshop should remain strictly focused on
CFD issues, and should not broaden its boundaries beyond this area.
The current format, length and interval between CFD4NRS workshops were generally
considered still to be appropriate, as was the rotation of venues worldwide. Hence, nochanges are proposed in this regard.
The competition of poster presentation should be organised to encourage students to present
their work without submitting full papers.
Since previous CFD4NRS workshops had rarely been attended by colleagues from China,
great efforts were devoted to let domestic colleagues know about the current workshop.
WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  9Over half of the participants were from domestic universities and institutes and, in addition,
the workshop attracted participants from many European countries and the United States.
In the panel session at the close of CFD4NRS-7, delegates confirmed their interest in
attending the planned follow-up workshop, if possible, and considered the two-year interval
between workshops to be appropriate.
It was also recommended that selected papers from the workshop could constitute a special
issue of the Nuclear Engineering and Design (NED) journal.

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

10  NEA/CSNI/R(2020)5

List of abbreviations and acronyms
ADS

Automatic Depressurization System

AIAD

Algebraic interfacial area density

ANL

Argonne National Laboratory (United States)

ASME

American Society of Mechanical Engineers

AULA

Abwaschmodell für unlösliche Aerosole (wash-off model foraerosols)

BIT

Bubble-induced turbulence

BORA

RPV mock-up at 1/5 scale (EDF)

BPGs

Best practice guidelines

BWR

Boiling water reactor

CEA

Commissariat à l’énergie atomique et aux énergies alternatives(French Alternative Energies and Atomic Energy Commission)

CFD

Computational fluid dynamics

CFX

Computational fluid dynamix (commercial software)

CHF

Critical heat flux

CIAE

China Institute of Atomic Energy

CMFD

Computational multi-fluid dynamics

CNRA

Committee on Nuclear Regulatory Activities (NEA)

CSNI

Committee on the Safety of Nuclear Installations (NEA)

DNB

Departure from nucleate boiling

DNS

Direct numerical simulation

EDF

Electricité de France

ETH Zürich

Eidgenössische Technische Hochschule Zürich (Swiss Federal
Institute of Technology in Zürich)

FCVS

Filtered Containment Venting System

FIV

Flow-induced vibration

FluentCommercial multi-purpose CFD package by ANSYS Inc.

FoM

Figure of merit

FSIFOAM

Finite volume fluid structure interaction solver based onpartitioned approach

GEMIX

Mixing experimental facility (PSI)

GENTOP

GENeralized TwO Phase flow

HFT

Hot functional testing
WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  11

HTC

Heat transfer coefficient

HZDR

Helmholtz-Zentrum Dresden-Rossendorf (Germany)

IAEA

International Atomic Energy Agency

ICC

Ice condenser containment

IRWST

In-Containment Refueling Water Storage Tank

JAEA

Japan Atomic Energy Agency

KAERI

Korea Atomic Energy Research Institute

KIT

Karlsruhe Institute of Technology (Germany)

LBE

Lead-bismuth eutectic

LDV

Laser-Doppler velocimetry

LES

Large eddy simulation

LIF

Laser-induced fluorescence

LIM

Large interface model

LOCA

Loss-of-coolant accident

LOFT

Loss-of-flow transient

LWR

Light-water reactor

MICAS

Mock-up of the hot plenum ASTRID reactor project at CEA

MIT

Massachusetts Institute of Technology (United States)

NEA

Nuclear Energy Agency

NED

Nuclear Engineering and Design (Elsevier journal)

NEPTUNE-CFD

French multiphase CFD code

NINE

Nuclear and Industrial Engineering (Italy)

NPIC

Nuclear Power Institute of China

NRS

Nuclear reactor safety

NSC

Nuclear and Radiation Safety Center (China)

OpenFOAMOpen source Field Operation and Manipulation (open source
software for computational fluid dynamics)

PANS

Partially Averaged Navier-Stokes

PAR

Passive Autocatalytic Recombiner

PFM

Pressure Fluctuation Model

PGSFR

Prototype Gen-IV sodium-cooled fast reactor

PIV

Particle image velocimetry

PSI

Paul Scherrer Institute (Switzerland)

PTS

Pressurised thermal shock

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

12  NEA/CSNI/R(2020)5
PWR

Pressurised water reactor

RANS

Reynolds-averaged Navier-Stokes

RPV

Reactor pressure vesselSA

Severe accident

SBLOCA

Small break loss-of-coolant accidents

SETCOM

Separate Effect Test for Condensation Modelling

SJTU

Shanghai Jiao Tong University (China)

TAMU

Texas A&M University

THAI

Thermal-hydraulics, Hydrogen, Aerosols and Iodine

TIV

Turbulent Induced Vibration

TUNN

Technical University of Nishny Novgorod (Russia)

UPTF

Upper Plenum Test Facility

UQ

Uncertainty Quantification

URANS

Unsteady Reynolds-averaged Navier-Stokes

US NRC

United States Nuclear Regulatory Commission

V&V

Verification and validation

VOF

Volume-of-fluid

XJTU

Xi'an Jiaotong University (China)

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  13

1. Background

This workshop, part of a biennial series of the Nuclear Energy Agency (NEA) andInternational Atomic Energy Agency (IAEA) sponsored events that began in Garching in
2006, follows the format and objectives of its predecessors in creating a forum whereby
numerical analysts and experimentalists can exchange information on the application ofcomputational fluid dynamics (CFD) to nuclear power plant safety and future design issues.
The emphasis was on offering exposure to state-of-the-art (single-phase and multiphase)
CFD applications reflecting topical issues arising in nuclear power plant design and safety,
but in particular to promote the release of high-resolution experimental data to continue the
CFD validation process in this application area.

1.1.

Scope
The emphasis in this workshop was placed on single-phase and multiphase CFD
simulations with a focus on validation in areas such as heat transfer, free-surface flows,
direct contact condensation and turbulent mixing. And indeed, many papers were devoted
to these issues. The uses of systematic error quantification and the application of bestpractice guidelines (BPGs) were as strongly encouraged as in previous workshops in thisseries, leading to the rejection of some papers which did not address these issues
adequately. Papers submitted related principally to nuclear power plant-relevant safetyissues, such as pressurised thermal shock, critical heat flux, boron dilution, hydrogen
distribution in containments, thermal striping and fatigue, and/or advanced designconcepts, such as tight-lattice fuel configurations, and passive safety options. For the firsttime at these workshops, it was considered that the technology was sufficiently advanced
to discuss Uncertainty Quantification (UQ) in CFD.
Nonetheless, emphasis should always be placed on the presentation of new experimental
data, especially those relating to two-phase flow, for which sophisticated measurement
techniques are required, and for which information is scarce. It is also very important todeepen understanding of the physics before starting a numerical analysis. Experiments
providing data suitable for CFD or computational multi-fluid dynamics (CMFD) validation
were also welcomed, though these should include local measurements using multi-sensor
probes, laser-based techniques (Laser-Doppler velocimetry [LDV], particle image
velocimetry [PIV] or laser-induced fluorescence [LIF]), hot-film/wire anemometry,
imaging or other advanced measuring techniques for local measurements. It is rapidly
becoming an obligation for papers describing experiments to include a discussion on
measurement uncertainties.

1.2. Organisational aspects
There were around 120 registered attendees at the CFD4NRS-7 workshop, which was
almost the same level of participation as for the previous workshops. The number ofExtended Abstracts received for evaluation following the initial announcements was 76.
This is similar to previous workshops in the series. All the abstracts were evaluated forsuitability by at least two reviewers, and invitations to write a full paper sent out at threehierarchical levels:


unconditional (favourable reviews had been received from all the reviewers);

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES14  NEA/CSNI/R(2020)5


conditional (at least one reviewer was unsure of the final acceptance of the paper);

guarded (it was anticipated that major revision of the paper would be necessary).

The number of technical papers finally received for evaluation was 56. All technical papers
received were evaluated by two to three reviewers, each according to journal standards. Of
these, 45 were accepted for oral presentation, and 6 to be presented in poster form. The
remaining five papers were withdrawn by their author(s) for various reasons. Five keynote
lectures were given, each to introduce the morning/afternoon sessions, as appropriate. In
addition, two posters were displayed relating to the OECD-TAMU cold leg mixing
benchmark exercise (for which no accompanying paper was requested).
The series of CFD4NRS workshops had rarely been attended by colleagues from China.
For this workshop, great efforts were devoted to let domestic colleagues know about theevent. Over half of the participants were from domestic universities and institutes. The
workshop attracted participants from European countries (15 participants from Germany
and 13 from France), which was more than expected. However, the number of participantsfrom Asian countries other than China decreased. The reason for this decrease could havebeen the conflict with the International Topical Meeting on Nuclear Reactor Thermal
Hydraulics, Operation and Safety NUTHOS-12 conference held in October 2018, at
Qingdao, China.
A competition had been planned for the poster session to encourage the students to present
their work. However, the competition could not be organised due to limited participants inthe poster session.
The next workshop in the series, in 2020, will take place at Saclay, France, and Electricitéde France has already agreed to organise the event.
In the panel session at the close of CFD4NRS-7, delegates confirmed their interest in
attending the follow-up workshop, if possible, and considered the two-year interval
between workshops to be appropriate. The two-and-a-half-day duration of formal
presentations, with a fieldtrip during the afternoon of the third day, was also accepted as anadequate format.
It was also decided that selected papers from the workshop should constitute a special issueof the Nuclear Engineering and Design (NED) journal, as for previous workshops in thisseries, and indeed Professor Y. Hassan, General Editor, confirmed this intention. As aconsequence, at the conclusion of the workshop, the individual session chairs wereapproached to declare which of the papers presented in their sessions should go forward
for archival recognition in this regard. The evaluation process was subsequently set inmotion and the special issue was published1.

1.

Xiong, J., D. Bestion, H. Yassin, N. Sandberg and X. Chai (Eds.) (2020) “Special Section on the‘7th Workshop on the CFD for Nuclear Reactor Safety (CFD4NRS-7)’”, Nuclear Engineering and
Design,Volume358,
www.sciencedirect.com/journal/nuclear-engineering-anddesign/vol/358/suppl/C#article-15.

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  15

2. Technical session summaries from the Co-Chairs

2.1. Session 1: Two-Phase Flow (I)The session included four papers. The first presented the development and validation
methodology of the baseline model for poly-disperse bubbly flow based on multiphase
computational fluid dynamics (CFD) in the Euler-Euler framework. The limitations ofclosure model development based on the integral experimental data was pointed out.
Comparison of simulation results with integral experimental data was believed only suitedfor the validation of the whole model setup in most cases. Two examples were given fordevelopment or improvement of closure models based on better understanding andreflection of local phenomena. The first example is updating the bubble-induced turbulence(BIT) model based on the direct numerical simulation (DNS) data. The other was the
dedicated experiment to investigate the lateral lift force model. The GENeralized TwO
Phase flow (GENTOP) concept was also briefly discussed. The GENTOP concept
combines poly-disperse and segregated flows and allows for transitions between the
different morphologies. Obtaining CFD-grade data with complete local information andhigh resolution in time and space demands joint efforts in the community.
The second presentation reported on the development of the wall boiling model for twophase natural circulation flow in an inclined channel with downward-facing heated wall
based on the experiments. The presence of elongated bubbles had been identified andquantified. The long sliding bubbles near the heated wall challenge the classical wallboiling model. Based on the hypothesis that there exists a liquid film between the slug
bubble and wall, heat transfer is dominated by liquid film evaporation in the portion
occupied by the slug bubbles. A reduction factor is introduced to account for the presenceof the slug bubble, while the remaining part is still calculated with the RPI model. The
model is implemented into ANSYS CFX and validated with lab- and large-scale data. In
the current model the correlation of reduction factor is still very empirical and does notinclude enough local information. More validation work is required.
The third paper was about CMFD-based departure from nucleate boiling (DNB) prediction
in Westinghouse pressurised water reactor (PWR) fuel with four-vane and folded-vane grid
spacer designs. A total of eight cases had been analysed using a selected baseline set ofSTAR-CCM+ boiling/DNB closures. The validation confirms that two-phase CFD canalready predict qualitative geometrical effects, but the quantitative accuracy of thepredicted DNB heat flux is not sufficient. Selected boiling closures are expected to be mostaccurate over the range of conditions characterised by bubbly flow. From the current
achievement it can be predicted that two-phase CFD can be used to help screen spacer griddesign and reduce the need of very costly critical heat flux (CHF) tests.
The fourth paper presented the three-field model to model annular-mist flow in boilingwater reactors (BWR), where the liquid film, liquid droplets and the steam are treated as
three different fields. The model was validated against experimental data for heated pipes.
Even though preliminary good prediction was achieved, more models based on local
quantities need to be developed in order to have a predictive tool that can be used forcomplex geometries. The main obstacle to surpass is the entrainment modelling which ismodelled using empirical correlations and large dependency on the droplet diameter. The
effects of the droplet diameter on the film mass flow rate is due to the droplet deposition.

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

16  NEA/CSNI/R(2020)52.2. Session 2: Containment (I)The session included four papers. With the further development of a wall-function
approach by the experimental data from the Separate Effect Test for Condensation
Modelling (SETCOM) facility, the first paper focused on the turbulent heat and masstransfer in the boundary layer during wall condensation and a detailed validation of aresolved boundary layer (low-Re) CFD approach. Combined with the classical scaling
approach based on the friction or shear velocity uτ (i.e. u+-y+ or T+-y+) and thedimensionless gravity force and the dimensionless heat flux, the wall boundary conditions
can be modelled by 3D surfaces instead of a 1D analytical correlation. The 3D models are
implemented numerically efficient for u+, T+, YS+ and turbulence by means of a nonequilibrium factor and an approximation by radial basis functions into an existing URANS
model. The paper was concluded with exemplary discussion of the current model
assessment results. It was considered that the models would need to be assessed by further
tests.
The second paper presented a three-dimensional numerical simulation of water running
down inclined surfaces coupled with a particle wash-off and particle transport model using
the open source software OpenFOAM. The water flow on inclined plates is simulated witha two-stage wash-off model for insoluble aerosols based on the Abwaschmodell für
unlösliche Aerosole (AULA) wash-off model for aerosols. The simulation results arecompared with the experimental data. Resuspension takes place when the wall shear
velocity exceeds a critical threshold (Shields criterion). The particle mass that leaves
through the outlet is evaluated and compared to the Thermal-hydraulics, Hydrogen,Aerosols and Iodine (THAI) laboratory experiments conducted by Becker Technologies
and to calculations done with AULA. Several simulations had been performed withdifferent plate inclinations, initial loading and varying particle properties, such as particle
density, particle diameter and wash-off rate. This work will contribute towards thedevelopment of a semi-empirical model to predict input parameters for AULA and thecreation of new, high-fidelity wash-off models.
In the third paper, a CFD model based on a Eulerian-Lagrangian approach was developed
to simulate the nuclear aerosol transport to consider the influence of nuclear aerosol withdecay power on the buoyant flows, water-steam balance in the containment and the
distribution and flammability of the hydrogen-air-steam mixture. For the Eulerian phase,
an Unsteady Reynolds-averaged Navier-Stokes (URANS) approach closed by buoyant kω SST model was used. In the Lagrangian phase, drag, gravitational, lift, thermophoresis
and turbulent dispersion forces were considered. Additionally, for particles below 1 μm,
the effect of Brownian diffusion was also taken into account. The ISP40 STORM
experiment SD 11 was used to assess the CFD model. It was found that the default models
in OpenFOAM for turbulent dispersion over-predicted the deposition. To allow a firstassessment of the effect of decay heat associated with the particles in buoyant flows, abuoyancy driven cavity with Rayleigh number (Ra) of 109 was considered. SnO2 particles
with a typical concentration of 1 g/m3 were considered as core melt aerosols. The decay
heat was modelled as a volumetric heat source on the particles. It was found that additional
heat transfer to the fluid induces local temperature and density changes and this visibly
affects also the turbulent transport in stratified flows.
The fourth paper presented considerations on the special case of the Loviisa Nuclear Power
Plant which combines the Russian VVER-440 reactor and the western ice condenser
containment (ICC). The VICTORIA experimental facility, linearly scaled from Loviisa
Nuclear Power Plant containment, was constructed by Fortum to provide the necessary
WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  17

experimental information of the Loviisa ICC thermal hydraulics during small break lossof-coolant accidents (SBLOCA) and during severe accidents (SA). NRG Petten had
previously developed and validated models for containment modelling making use of the
ANSYS Fluent CFD code. In a joint research project, Fortum and NRG developed an ice
condenser model that was integrated in the CFD containment model of NRG. The modelof the VICTORIA facility was constructed for the CFD simulations to validate the complete
containment model.

2.3. Session 3: Experiment techniqueThis session was the only one related to experiments and included three presentations. The
first paper dealt with pool scrubbing experiments dedicated to the phenomena in the wet
well of a BWR during a severe accident. They were conducted at the THAI facility, which
is a versatile medium scale containment test facility. Downcomers – circular, downwards
oriented pipes – inject the aerosol laden gas-vapour mixture into the water volume of thepressure suppression chamber. Aerosol scrubbing is relevant in a late phase of the accident
when flow rates through these downcomers are low and bubble formation is characterisedby little dynamics. The measured decontamination factors for insoluble aerosols werecorrespondingly low, in the order of magnitude of 10, which is much less than what can beachieved in a Filtered Containment Venting System (FCVS) with optimised injection
nozzles. The bubble sizes were measured by high-speed camera observation followed byimage processing. The work makes a very important experimental input to severe accident
mitigation and modelling applied to BWRs.
The second paper described experiments conducted at an adiabatic 5x5 rod bundle tocharacterise the flow field downstream of a spacer grid. The paper showed that refractive
index matching techniques for the application of optical techniques, like PIV, increasingly
become the state of the art in the area of velocity field measurements in bundles. In case of
the present paper, PIV was applied to measure a single-phase flow of water at ambientconditions. The paper includes the development of a high-fidelity measurement approachfor PIV measurement for cross flow downstream of a spacer grid in a rod bundle. Also, theuncertainty of PIV measurement is quantified, which is very important for CFD validation.
In general, the results are very useful for CFD code validation regarding coolant cross flow
and cross mixing. Valuable results can be expected from the work at Shanghai Jiao Tong
University in this field.
The topic of the third paper was coolant mixing experiments in a vessel with a topologicalsimilarity to the pressure vessel of a PWR, although not representing it in a geometricalsimilarity, using conductance probes and wire-mesh sensors. The authors challenged thescaling rules for mixing experiments regarding deboration and subcooling transients at
PWRs. By performing generic experiments in different scales, they planned to obtain CFDgrade measurement data suitable for answering the question of the Reynolds number
influence. The first results on time-resolved transport scalar measurements were presented.
More was planned to come. The Technical University of Nishny Novgorod (TUNN) has
obviously more than one test facility in different scales, which might provide interesting
insight into scaling issues in the future. During the discussion there was a debate about thenecessity of such a study, since coolant mixing in the RPV was already widely studied in a
number of test facilities ranging up to a scale of 1:1 in case of the Upper Plenum Test
Facility (UPTF) in Germany. Furthermore, there was experimental evidence from real plantcommissioning tests. Still, careful studies are always welcome, and the authors wereadvised to increase the resolution of their sensors and to better reflect available literatureon the issue.
WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

18  NEA/CSNI/R(2020)52.4. Session 4: Flow-Induced vibration
There were three excellent presentations about flow-induced vibration (FIV). Thisphenomenon is analysed by coupling fluid and solid dynamics. It was considered that theusual approach of fluid dynamics is large eddy simulation (LES) because this phenomenon
is strongly unsteady. One of the three presentations used LES. However, two of threepresentations used Reynolds-averaged Navier-Stokes (RANS) (URANS) and the provided
results were very good. If RANS is available, it is more efficient. The general impression
was that the vibration frequency was well simulated even though the frequency was shiftedfrom the natural frequency of the solid. However, the damping of vibration was not so well
reproduced. This will be studied further.
In the first paper, an advanced numerical framework called NRG-FSIFOAM (finite volume
fluid structure interaction solver based on partitioned approach) was presented with theobjective to study the dynamics of Turbulent Induced Vibration (TIV) and to use if for theapplication of nuclear reactor safety (NRS). The NRG-FSIFOAM is based on the use ofpartitioned algorithms for the fluid and solid problems coupled by means of the state-ofthe-art approach for the simulation of strongly coupled FSI problems. To overcome thedefault defect of URANS models which are not capable of modeling the fluctuations at thesmaller scales, an innovative Pressure Fluctuation Model (PFM) was implemented in NRGFSIFOAM with the objective of complementing the average field computed with the
URANS approach with a stochastic model that is able to reproduce the chaotic fluctuationsof the turbulent field. This numerical framework was first validated in good agreement withstrongly coupled FSI benchmark cases. The capabilities of the PFM were then assessed fora nuclear application case for which experimental measures are available. It was found that
the PFM is able to reproduce the dynamics of the vibrations observed in the experiments
while the classical URANS model fails to predict physical oscillations.
The second paper described how the codes ANSYS CFX-MOR and ANSYS CFX–ANSYS
Mechanical are validated against the Vattenfall Rod Vibration Experiment data. The
experimental setup consisted of a Plexiglas test section with a slender stainless steel rod in
the middle, which was pulled and then released. The calculated time dependent vibration
amplitude was compared with measured data for the water and air test cases. The analyses
showed that the nature of the vibrations for the cases with flowing fluid was well predicted,
while underestimations of the vibration amplitude and phase shift were observed in thecases with stagnant flow.
The third paper presented results of numerical and experimental activities to validate thecalculations of flow-induced vibrations of equipment flexible structures. A geometricallysimple model of a bluff body which represents two flexible cylinders arranged in-line in
the working medium cross flow was taken as a subject of research. Modes with the
Reynolds numbers Re of 104 were studied. Hydrodynamics was modelled using an LES
model for the turbulent flow. The validation showed that the hydrodynamic calculation andcalculation of the cylinder vibration levels at the vortex shedding frequency are insatisfactory agreement with the available experimental data. However, the coupledcalculation of vibration levels provides overassessment for the natural frequency. The
obtained results testify to the conservative nature of the applied approach to calculate
vibration amplitudes of flexible bodies.

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  19

2.5. Session 5: Bundle flow
Three papers were presented in session five for fluid flow and heat transfer of single phasein rod bundle. Two papers used the commercial CFD code, STAR-CCM+ and the third
paper developed in-house code using the OpenFOAM library. The turbulence models usedin these papers are the realisable k-ε model, PANS (Partially Averaged Navier-Stokes) kw model, k-w SST model. The CFD simulations are verified in all three papers but thevalidation process was performed only in one paper. It is impressive to make an effort for
multiphysics coupling and application of high-end turbulence model.
The first paper described research on the particle behaviour and cladding oxidation in a
PWR fuel bundle by CFD methodology to improve conclusions for design optimisation.
The movement of discrete particles in the moderator can already be simulated by a
Lagrangian model with the particle tracking approach. A reference 3D PWR corrosion
model was started to be developed, which can calculate local parameters on a fuel rod
surface 2D model to predict oxide thickness and component deposition taking into accountthe mixing effects during the moderator transport. The calculated 2D surface coolant
temperatures were coupled to a dedicated fuel rod corrosion model, covering the full fuelrod in azimuthal and axial direction.
In the second paper, a PANS model was used. This is an energy based, filteredrepresentation of the Navier-Stokes equations. As part of the bridging models paradigm
family, it is possible with such a model to resolve the turbulent space and time scales ofinterest. The most important large-scale unsteadiness can be resolved at a minimal
computational cost. The model and its solution verification had been tested for a turbulent
flow inside a 5x5 fuel bundle, with a single spacer grid and split-type mixing vanes. The
results will be compared with particle image velocimetry data available in literature.
The third paper described how the local heat transfer behaviour of wire-wrapped rod bundle
in liquid metal cooled reactors, can be studied with OpenFOAM to calculate the detailed
temperature and heat flux distribution on the surface of rod and wire. In the study, two CFD
models were created for a bare rod bundle and a wire-wrapped 19-rod bundle. For the bare
rod model, a periodical boundary condition was used to simulate the heat transfer ofdeveloped flow. For the wire-wrapped 19-rod bundle case, the normal inlet outlet boundary
was imposed to the fluid region with a length of two wire pitches. In the CFD studies, themodels were divided into solid and fluid regions. The k-omega SST model was used forthe turbulent simulation. The simulation result showed a cosine like local heat transfer
distribution in the circumferential direction of the rod.

2.6. Session 6: Mixing (I)There were three presentations in the session. The first addressed the verification andvalidation in LES of a triple parallel jet flow. In the verification part, the numerical error
was first estimated with two LES on different grids where no SGS model was set. It was
shown that the kinetic energy was mostly overestimated in the mixing region of the flow,
leading to a negative effective SGS kinetic energy. Then, the contribution of the SGS model
was investigated. The order of convergence of the SGS kinetic energy with the
Smagorinsky model was found to be in good agreement with other results. Moreover, theWALE model was found to converge more rapidly towards the total kinetic energy thanthe Smagorinsky model. The LES on the coarsest mesh had a lower discrepancy withexperiment than those on the finest mesh; the origin of this unexpected results needs to beinvestigated.
WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

20  NEA/CSNI/R(2020)5
The second presentation was about water mixing during In-Containment Refueling Water
Storage Tank (IRWST) heatup for the AP1000® Plant. The CFD results show that ANSYS
CFX can simulate the flow and heat-transfer characteristics of natural convection in the
IRWST, and the error of the simulation was acceptable. With thermal phase change
included in the model, the results matched the test data better than that without thermal
phase change, especially later in the test when the IRWST was at elevated temperatures,
when subcooled boiling is more likely to occur. Finally, both the CFD results and hot
functional testing (HFT) data show that the temperature distributions in the IRWST are
horizontally stratified.
In the last presentation, the turbulent natural convection formed in a rectangular enclosure
with a curved surface heated from below was shown with experimental and numericalmethods. The dependency of the Nusselt number on the Rayleigh number is quantitatively
investigated. It was found that the Nusselt number is proportional to the Rayleigh number
and the exponent is larger than the values found in the literature. The distribution of fluid
velocity and turbulent kinetic energy in the region near the curved surface was also obtainedwith the PIV method. Comparing against the experimental data, it was found that thepresented numerical data underestimate the wall temperature profile over the heating
section and the v2-f turbulence model with three different turbulent flux models fails to
reproduce the wall temperature. It was also found that the predicted velocity distribution
near the heating section is much larger than the experimental data. The discrepancy of thewall temperature between the numerical and experimental results can be attributed to theoverestimation of fluid velocity and the components of turbulent heat fluxes.
The session was well attended and showed that turbulent mixing processes are still valuable
research-objectives in the nuclear community.

2.7. Session 7: Liquid metal
There were four presentations in the session. The first paper was about CFD analysis of theflow field in the MICAS experiments (mock-up of the hot plenum ASTRID reactor project)
at CEA which represents a water model of the hot pool of the Astrid reactor. Trio_CFD
was used to perform the RANS computation and the obtained results, even with the use ofa linear k-epsilon model, were found to be qualitatively in good agreement with theexperiments.
The second paper evaluated the pressure drop in a fuel bundle and reflector of the fuelassembly for a prototype Gen-IV sodium-cooled fast reactor (PGSFR) using computational
fluid dynamics (CFD) and analytical methods. The commercial CFD codes, ANSYS CFX
and STAR-CCM+, were used to predict the pressure drop in a fuel bundle and reflector.
The RANS turbulence models used in this CFD analysis are the standard k-e model,
Realizable k-epsilon model and SST k-omega model. The CFD calculation and correlations
predicted the bundle friction factor, which agrees well with the experimental data.
The third paper investigated the influence of turbulence modelling on the heat transfer,especially for fluids with low Prandtl numbers like sodium and lead-bismuth eutectic(LBE). A detailed CFD model was prepared and presented. Special attention was given to
optimise the mesh for accurate geometry representation and fast convergence. Simulations
were run with three different turbulent Prandtl numbers (0.9, 1.5 and variable values
generated by a look-up table method). It was found that the influence of the choice of Prt is
low for fluids with very small Pr numbers like sodium, while it is of much higher

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  21

importance for fluids with a lower thermal conductivity. The conjugate heat transfer has asmall impact at the touching zone between rod and wire.
In the fourth presentation, a CFD calculation was performed to validate the design of areactor flow distribution test facility for the PGSFR. The porous media approach was
adopted to simulate the core region, which has complex geometries of 313 fuel assemblies
for computational efficiency. The full velocity and pressure distribution in the test vessel
were acquired from a CFD analysis. The flow rate and pressure drop of the fuel assemblies
and the IHX were compared with the target values suggested as the reference parameters,
and the vessel-wise velocity and pressure distribution were observed for surveying any
defective flow in a vessel. From the results of the CFD analysis, it was confirmed that the
global flow behaviour in the test vessel was hydrodynamically acceptable.

2.8. Session 8: Two-Phase flows II
This session consisted of four papers. The first three were loosely related, dealing withdifferent aspects of gas-liquid flows, while the fourth paper dealt with solid-liquid phasechange.
The first paper addressed improved turbulence modelling for a rising Taylor bubble.
Specifically, a damping source term was added to RANS k-Epsilon and k-Omega models,
which resulted in significantly improved agreement with experimental data and highresolution LES simulations.
The second paper discussed improved interphase heat transfer coefficient (HTC)
correlations for Euler-Euler closures. A review of existing interphase HTC correlations waspresented, an improved HTC correlation was proposed, and results using this newcorrelation were shown for steam condensation in subcooled water, subcooled wall boilingand flashing nozzle flow. The results were very encouraging and some suggestions forfurther development were discussed.
The third paper described sensitivity studies of different boiling interfacial momentum
closures, including drag, lift and wall lubrication force models. There was a good
discussion with the audience related to the experimental data used for comparison andassumptions made about the initial bubble size distribution used in the simulations.
Finally, the fourth paper was on numerical modelling of frozen walls in a molten salt
reactor. The CFD simulations were coupled to neutronics predictions for the heat sources,
and the frozen salt in the CFD simulations was captured by a temperature-dependent
porosity. Preliminary results show that maintaining a frozen salt wall layer may be achallenging design problem.
The papers in this session demonstrated the range of models and applications of two-phase
CFD simulations, as well as their associated challenges.

2.9. Session 9: Containment (II)The Containment II session contained three presentations related to analysis and validationof different modelling challenges related to three aspects of the hydrogen issue: mixing,
mitigation and combustion.
The first paper addressed the assessment of a Passive Autocatalytic Recombiner (PAR)model based on the manufacturers’ correlation and implemented in Fluent The main
challenges were represented by the flow conditions, i.e. the flow field around the PAR was
WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

22  NEA/CSNI/R(2020)5
opposing the buoyant chimney flow cheated by the PAR. Experimental results obtained
within the multi-compartment THAI+ facility during the NEA THAI-III project were used
as experimental reference. Both simulation and experiments revealed that the imposed
counter-current flow did not affect the PAR performance once the PAR was operating. The
level of agreement between measurement and simulation indicates that the relatively simple
correlation based model for the recombination rate can be applied without introducing
significant modelling errors, even under the given challenging boundary conditions.
The second paper addressed the systematic assessment of an OpenFOAM-based turbulent
mixing model against a complete experimental test series on the erosion of a helium
stratification by means of a vertical jet, conducted in the PANDA facility within the
NEA SETH-2 project. The nine different cases consider variation of the density gradients,
gas mixture and injection locations. Even though the results did not reveal a fundamental
disagreement with the experiments, several inconsistencies related to the mixing rate orheat transfer (gas temperature level) were identified in the thorough assessment of all
different experimental tests. Different variations, e.g. of turbulence model constants or
inclusion of thermal radiation modelling, suggest it would be good to take a closer look atthese issues and create specific guidelines or modelling standards for this type of problem.
The last paper investigated the effect of the initial turbulence level on the downward
propagation of a slow flame on basis of a deflagration experiment conducted within the
NEA THAI project. The GASFLOW-MPI code was used in the systematic parametricanalysis to assess the counteracting effects of thermo-diffusivity instability and higher
initial turbulence level which promote flame propagation and the Rayleigh-Taylor
instability and buoyancy, which tend to suppress the downward flame propagation. The
work encourages the inclusion of the instabilities and turbulence intensity into theformulation of the flame velocity. On the experimental side, the analysis encourages themeasurement of the initial turbulent condition in combustion experiments.
Containment type simulations can be characterised by large computational domains andlong transients, which usually prohibited repetitive analysis runs to address sensitivities. It
can be noted that all presented work was based on multiple simulations, conducted either
to quantify mesh sensitivity to different initial and boundary conditions (scenarios / test
cases) to base model assessment on a broader basis. Looking at the modelling approaches,
there was a harmonisation but still no complete consensus about the physics to be included
for containment typical analysis (e.g. gas radiation) about model coefficients(e.g. turbulence transport). This should be addressed in more specific guidelines on
containment flows to generalise the experience obtained for individual model assessments.

2.10. Session 10: Mixing (II)There were four presentations in this session. The first presented the analysis of thethermohydraulic behaviour in the cold legs and downcomer of a pressurised water reactor
primary circuit during an intermediate-break loss-of-coolant accident scenario located on
one of the cold legs. First, a physical analysis of the accidental scenario was proposed,
including a Phenomena Identification and Ranking Table analysis to comprehend themodelling complexity on an industrial geometry. Second, a presentation of NEPTUNECFD was given, including a discussion on the physical models that will be used to deal
with this accidental scenario.
In the second presentation, large eddy simulation (LES), using code NEK5000, of anexperimental cold leg benchmark investigating buoyant mixing among two fluids was

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  23

developed for verification and validation. It was concluded that instabilities due toresolving more of the interface using the molecular Schmidt number were essential for
accurately predicting this type of flow.
In the third presentation, a recently published pressure-based compressible multiphase flow
model was validated for applications relevant to nuclear safety, such as flashing of highpressure water through valves and nozzles and steam jets into suppression pools. Good
matches with the experimental results were observed, thus validating the compressible
multiphase flow model for critical scenarios in nuclear safety.
In the last presentation, through the comparison of the simulated results with the results
performed in the THAI facility, it was found that STAR-CCM+ 11.02 with the fluid filmmodel simulating the steam condensation predicted the steam concentration, gas
temperature, and vessel wall temperature with an uncertainty range of approximately 20%.
This difference between test and simulation might be caused by radiative heat transfer
between the steam-air gas mixture and the inner wall, which is not considered in the CFD
simulation. Therefore, it was proposed that the radiative heat transfer model be included in
the best practice guidelines (BPGs) to accurately simulate the natural convection flow
induced by the steam condensation.

2.11. Session 11: Pressurised thermal shock
In this session, three papers were presented. The composition of this session was optimal
to demonstrate a full line of efforts to solve a safety-relevant problem by means of CFD
simulations. The first paper gave an overview of calculations performed in the frame of the
PTS benchmark based on ROCOM data. The injection of a deborated plug with and without
density difference was discussed, both by showing the experimental results and bycomparing them to CFX calculations. The achieved good performance of the codedemonstrates in an impressive way that CFD is mature for this kind of single phase mixing
applications, of course under the conditions outlined by the Best Practice Guidelines.
The application of this achievement for the simulation of the bounding subcooling transientin a German type PWR was presented in the third paper. This paper showed it was possible
to demonstrate that simplified models based on experimental evidence obtained at the fullscale test facility UPTF provide conservatisms and the CFD simulations were able touncover additional safety margins regarding the subcooling of the RPV wall during thebounding loss-of-coolant-accident (LOCA) scenario.
The second paper dealt with DNS related to the PTS issue. The NRG in Petten, in the
Netherlands, conduct DNS simulations in a simplified downcomer geometry with the aimof generating high-quality data for the validation of turbulence models. The idea behind
this is the indirect transfer of knowledge from DNS calculations to the real reactor scale byvalidated RANS or LES models. DNS was successfully applied to a case without buoyancy
and the result of this numerical experiment is available for code validation. The next step
concerns a test case with density difference, which causes a considerable increase incomplexity. Some more simplifications of the geometry will be necessary. Still, an increaseof the size of the needed mesh – up to 1.5 billion cells – seems to be necessary and a
sufficiently powerful computer hardware has to be found.
With only three presentations, the session was able to give a comprehensive picture of theefforts needed to solve a safety relevant problem using CFD. From fundamental DNS
analyses allocated at the front end of the CFD validation chain (paper #2) via the alreadyquite applied research at ROCOM and the related code validation exercises (paper #1) up
WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

24  NEA/CSNI/R(2020)5
to the full-scale experiment (UPTF) final application to real plant conditions (paper #3), a
practically complete picture was given. This demonstrates the success of the CFD4NRS
movement.

2.12. Session 12: Plant application (I)This session had three papers for the CFD application to hot leg with Automatic
Depressurization System (ADS) in AP1000, AES-2006 reactor pressure vessel (RPV)
simulating VVER1200, and upper plenum/hot leg for AP1000. The three papers consideredsingle-phase fluid and two-equation models for turbulence. The STAR-CCM+ code wasused to simulate the fluid flow and heat transfer in hot leg with ADS and AES-2006 RPV.
The in-house code Code-Saturne was applied to predict flow and temperature distributionof coolant in upper plenum and hot leg of AP1000. The three papers performed theverification analysis but only one paper provided the validation result. The CFD predictions
are quite reasonable and showed a positive outlook for the application of CFD methods inthe optimal design of nuclear power plants.
The first paper used CFD to assess the susceptibility of vibration of an AP1000® reactor
ADS pipe where it intersects with the hot leg nozzle piping. These CFD results were used
in a qualitative manner to guide proposed design modifications and subsequent scale model
testing. Two types of CFD analyses were performed to fulfil different purposes: (i) steadyanalyses were performed to provide an indication of flow features such as separation points
and shear layer structure; (ii) unsteady simulations were then performed on the baseline
and final designs to further validate the final design selection.
In the second paper, a virtual model of the primary circuit at nuclear power plants withVVER reactors was developed by CFD methodology to prove basic technical solutions of
AES-2006 RPV design concerning questions of hydrodynamics and heat exchange. The
results of calculations using elaborated models are three-dimensional distribution oftemperature, pressure, velocity vector of the coolant and other values, characterising heatand mass transfer in the flow path of RPV primary circuit. The correspondence of thecalculation data to design data is shown by the results of the comparison within the rangeof technical tolerances and errors and that proves the correctness of the simulation.
The third paper conducted CFD analysis of PWR upper plenum thermal-hydraulicphenomena with CODE_SATURNE. To make a comparison between different geometries
for future research, a simplified AP1000 upper plenum geometry was adapted to a 1/5
scaled four-loop BORA (RPV mock-up at 1/5 scale) mock-up facility built by EDF R&D
to carry out a CFD simulation. The calculation domain includes the upper half core, upper
plenum and four hot legs. The geometry and mesh were generated by Salome and thecalculation was conducted using the Code_Saturne developed by EDF. A uniform flow rate
and a given temperature field distribution are imposed to the upper half core inlet and outlet,
respectively. Three-dimensional coolant temperature and flow fields at the core outlet level
and in the upper plenum and hot legs were obtained and discussed. The suction effects of
hot legs on the flow fields in the upper plenum were achieved. Also, the temperature fieldevolution in hot lets was analysed. This work is meaningful for the deep understanding of
the thermal-hydraulic phenomena in the PWR upper plenum.

2.13. Session 13: UQ and couplingThis session included two papers. The first provided the results of an Uncertainty
Quantification (UQ) method for CFD validation for turbulent mixing experiments fromWORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  25

GEMIX (mixing experimental facility [PSI]). NRG participated in this benchmark with a
UQ method based on the ASME Verification and Validation (V&V) standard for UQ in
CFD. The propagation of the uncertainty in the input parameters was sampled with theLatin Hypercube Sampling. The numerical uncertainty was determined by means ofRichardson extrapolation. A source of uncertainty which was not evaluated explicitly wasthat of the turbulence model input parameters, which depend heavily on the flow
conditions. In order to investigate this contribution, the turbulence model parameters havebeen considered as uncertain input variables. The results of this so called extended ASME
methodology were presented and discussed. It was found that the uncertainty in the
turbulence model input parameters has a significant contribution to the overall uncertainty
in the simulation results for the considered GEMIX mixing experiment.
The second paper described the development and validation of the multiscale multiphase,numerical platform NEPTUNE CFD/CATHARE. The coupling between the codes
CATHARE and NEPTUNE CFD was presented in the context of single-phase flows. A
verification test case was first provided to showcase the correct implementation of thecoupling. Finally, a validation test case with strong mixing effects based on a double Tjunction experimental facility was studied.

2.14. Session 14: Plant applications (II)This session included two presentations. The first concerned thermal-hydraulic analysis ofa reactor plant for an icebreaker ship. Coupled 1-D/3-D simulation was performed to
predict temperature at the core inlet, due to the concern that incomplete mixing could affectthermocouple readings used to control the plant. To accurately capture the mixing effects,
LES was used for the CFD part of the simulations. Verification results for the code couplingtechnology were also presented.
The second paper addressed the thermal control temperature predictions at the fuelassembly outlets for a VVER-1000 reactor. Inlet boundary conditions for the simulations
were provided by a sub-channel code. Results were compared to measurements, showing
good agreement. After further validation, it will be possible to predict thermocouple
readings in the hot legs as well.
These papers show the challenges of real-world plant applications of CFD, which include
complex geometries, very large models and complex physics, as well as some of thesolutions such as coupling to system and sub-channel codes.

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

26  NEA/CSNI/R(2020)5

Annex A. Keynote lectures
1. Guo-Han Chai; Nuclear and Radiation Safety Center (NSC), China
Application of CFD in Nuclear Safety License in China
2. Horst-Michael Prasser; ETH Zürich, Switzerland
High-Resolution Measurements in Two-Phase Flow Experiments in Adiabatic andHeated Rod Bundle and Sub-channel Models
3. Dominique Bestion; CEA, France
Requirements for CFD-grade Experiments for Nuclear Reactor ThermalHydraulics
4. Yassin Hassan; TAMU, United States
OECD-TAMU Cold Leg Mixing Benchmark Exercise
5. Seiichi Koshizuka, University of Tokyo, Japan
Guidelines for Verification and Validation for Engineering Simulation

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  27

Annex B. Poster papers

1. S. K. Bikezin, D. A. Oleksyuk
CFD Modelling of the 37 Rod Bundle Experiment with Different Spacing Grids
2. Chen Chong, Wang Mingjun, Tian Wenxi, Qiu Suizheng, Su Guanghui, Qi Yubo
Simulation of “Dead Legs” Phenomena for PWR Nuclear Power Plant
3. A. Graževičius, A. Kaliatka, E. Ušpuras
Numerical Study of the Natural Convection and Thermal Stratification Phenomena
in a Rectangular Enclosure with a Horizontal Heat Source
4. Y. Bouaichaoui, T. Höhne
CFD Benchmark Study of Pressurised Thermal Shock Experiment of the ROCOM
Test Facility
5. A. S. Noskov, A. A. Falkov, D. L. Shipov
STAR-CCM+ Verification and Application for a Reactor Core Fluid Dynamics
Analysis
6. Ji Wen-ying, Zong Wei-xin, Lv Yi-jun, Luo Han yan, Lu chang dong, Yang Jiang
CFD Analysis of the Inlet Blockage Accidents for Single Fuel Assembly of the LBECooled Fast Reactor
7. Yazhe Lu, Fan Yang, Zhiqiang Zou, Huanhuan Peng, Jian Deng
CFD Related Studies on Hydrogen Issue in Nuclear Power Plant
8. Luguo Liu, Zhongchuan Li, Sijia Du, Songwei Li, Xi Chen, Peiying Li, Yu Liu
CFD Application on Fuel Assembly Design in NPIC
9. Yingzi, Zhu, Jinbiao Xiong
Development of MPS-Based Eutectic Reaction Model for Severe Accident
Simulation
10. Mubashir Hassan, Jinbiao Xiong, Xu Cheng
CFD validation and sensitivity analysis of OECD-TAMU cold leg mixing
benchmark

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

28  NEA/CSNI/R(2020)5

Annex C. Technical programme (presented orally)

Session 1: Two-Phase flow (I)Co-Chairs: D. Bestion (CEA, France), J.-P. Simoneau (EDF, Germany)
1. D. Lucas, E. Krepper, Y. Liao, T. Höhne, R. Rzehak, F. Schlegel, T. Ziegenhein
Multi-Fluid Models for Gas-Liquid Flows
2. Hyoung-Tak Kim, Dae-Kyum Lee, Kwang-Hyun Bang
Wall Boiling Model for Flow Boiling in an Inclined Channel with Downward-Facing
Heated Wall
3. Robert A. Brewster, Emilio Baglietto, W. David PointerAssessment of Boiling Models for CFD-Based Prediction of DNB on Westinghouse PWR
Fuel Grids
4. Salvatore Raddino, Yann Le Moigne, Tobias Strömgren, Jean-Marie Le Corre
Modelling of Annular Flow with a Three-Field Approach

Session 2: Containment (I)Co-Chairs: Y. Hassan (TAMU, United States), J. Xiao (KIT, Germany)
1. S. Kelm, H. Müller, A. Hundhausen, C. Druska, A. Kuhr, H.-J. Allelein
Development and Validation of a Multi-Dimensional Wall-Function Approach for Wall
Condensation
2. K. Amend, M. Klein
Modelling and Simulation of Wash-Down of Fission Products by Water on Containment
Walls
3. M. Kampili, S. Kelm, H.-J. Allelein
Modelling of Aerosol Transport and Decay Heat Distribution in Containment Flows
4. T. Rämä, T. Toppila, D. Visser, A. Siccama
Validation of the Containment Analysis CFD Model for VVER-440 Type Containment with
Ice Condenser

Session 3: Experiment technique
Co-Chairs: H.-M. Prasser (ETH Zürich , Switzerland), J. B. Xiong (SJTU, China)
1. M. Freitag, B. von Laufenberg
Measurement and Evaluation of the Bubble Dynamics in the Vicinity of a Downcomer
under Conditions Typical for Pool Scrubbing in the Wet-Well of BWR

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  29

2. W. Qu, S. Chen, J. Xiong, X. Cheng
Experimental Measurement of Turbulent Flow in a 5×5 Rod Bundle with Mixing Vane
Spacer Grids for CFD Validation
3. M. A. Bolshukhin, A. V. Budnikov, A. A. Barinov, D. N. Patrushev, S. M. Dmitriev
Experimental Studies on the Turbulent Flow Steady-State Mixing Processes in the LargeScale Test Facility

Session 4: Flow-Induced vibration
Co-Chairs: Y. H . Yang (SJTU, China), S. Koshizuka (University of Tokyo, Japan)
1. S. Sharma, D. De Santis, A. Shams
An Advanced URANS Solver to Predict Turbulence Induced Vibrations in Nuclear Reactor
Applications
2. A. Papukchiev
Numerical Analysis of Reactor Relevant Vibrations Using Advanced Multiphysics CFDCSM Methods
3. M. A. Bolshukhin, AV. Budnikov, E. I. Shmelev, D. A. Kulikov, A. I. Patrusheva
Approaches to Modelling of Hydrodynamic Forces to Calculate Flow-Induced Vibrationsof Equipment Flexible Structures

Session 5: Bundle flow
Co-Chairs: W. Tian (XJTU, China), W. K. In (KAERI, Korea)
1. D.-Y. Sheng, M. Seidl
Numerical Investigation of Particle Behavior and Cladding Oxidation in a PWR Fuel
Bundle by CFD Methodology
2. G. Busco, Y. A. Hassan
Solution Verification of PANS Modelling for a 5x5 PWR Fuel Assembly
3. X. Wang, X. Cheng
A CFD Study of Local Heat Transfer in Triangular Arrayed Rod Bundle

Session 6: Mixing (I)Co-Chairs: G.-H. Chai (NSC, China), T. Höhne (HZDR, Germany)
1. P.-E. Angeli
Verification and Validation in LES of Triple Parallel Jet Flow for a Thermal Striping
Investigation
2. H. Xu, R.F. Wright
Water Mixing During IRWST Heatup under Natural Circulation for the AP1000® Plant

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

30  NEA/CSNI/R(2020)5
3. X. Chai, B. Chen, J. Yao, X. Liu, J. Xiong, X. Cheng
Numerical Simulation of Heat Transfer Properties over a Curved Heated Surface in an
Enclosure

Session 7: Liquid metal
Co-Chairs: D. Bestion (CEA, France), A. Shams (NRG, Netherland)
1. U. Bieder, J. Maillard, Y. Gorsse, D. Guenadou
CFD Analysis of the Flow in the MICAS Experimental Facility, a Water Model of the Hot
Pool of a Sodium Cooled Fast Reactor
2. W. K. In, K.-G. Lee, S.-R. Choi, J.-S. Cheon
Evaluation of Pressure Drop in Fuel Bundle and Reflector for Sodium-Cooled Fast Reactor
3. M. Böttcher, R. Krüssmann
CFD Simulation of Liquid Metal Flow in a 19 Rod Wrapped Wire Assembly with Focus on
Turbulent and Conjugate Heat Transfer
4. W. S. Kim, S.-K. Chang, D.-J. Euh
CFD Calculation for Preliminary Analysis of SFR Reactor Flow Distribution Test

Session 8: Two-Phase flow (II)Co-Chairs: D. Lucas (HZDR, Germany), R. Brewster (Westinghouse, United States)
1. E. M. A. Frederix, A. S. Hamraz, J. G. M. Kuerten, E. M. J. Komen
Simulation of a Rising Taylor Bubble Using Improved Reynolds-Averaged Models
2. Y. Liao, E. Krepper, D. Lucas
A Baseline Closure Concept for Simulating Bubbly Flow with Phase Change: Interfacial
Heat Transfer Coefficient
3. D. Jin, J. Xiong, X. Cheng
Effects of Interphase Force Models and Turbulence Model on Two-Phase Flow in Verticaland Inclined Upward Pipe
4. G. Cartland-Glover, A. Skillen, D. Litskevich, S. Rolfo, D. R. Emerson, B. Merk, C. Moulinec
On the Numerical Modelling of Frozen Walls in a Molten Salt Fast Reactor

Session 9: Containment (II)Co-Chairs: S. Kelm (FZJ, Germany), X. Chai (SJTU, China)
1. Y. Halouane, A. Dehbi
CFD Simulations of Passive Autocatalytic Recombiner Operation inside the THAI+ TwoCompartment Facility

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODESNEA/CSNI/R(2020)5  31

2. R. Krpan, I. Kljenak
Simulation of PANDA SETH2 Experiments on Containment Atmosphere Mixing Caused by
Vertical Injection
3. J. Xiao, M. Kuznetsov
Effects of Initial Turbulence Intensity on Downward Hydrogen Flame Propagation in THAI
HD-23 Test

Session 10: Mixing (II)Co-Chairs: T. Höhne (HZDR, Germany), A. Shams (NRG, Netherland)
1. N. Mérigoux, J. LaviévilleApplication of NEPTUNE_CFD to Intermediate-Break Loss of Coolant Accident in the
Downcomer Side of a Pressurised Water Reactor
2. J. K. Lai, E. Merzari, Y. A. Hassan, P. Fischer, O. Marin
Verification and Validation of Large Eddy Simulation with Nek5000 for Cold Leg Mixing
Benchmark3. C. Narayanan, D. Lakehal, A. D. Carlson, L. Zhang, K. Fu
Application of Pressure-Based Compressible Multiphase Flow Approach to Flashing in
High-Pressure Valves and Under Expanded Gas Jets into Suppression Pools
4. H. S. Kang, R.-J. Park, D. H. Kimand M. Freitag
CFD Analysis for Dissolution of a Steam-Air Stratification by Natural Convection in the
THAI Facility

Session 11: Pressurised thermal shock
Co-Chairs: H.-M. Prasser (ETH Zürich, Switzerland), U. Bieder (CEA, France)
1. T. Höhne, S. Kliem
Numerical Simulation of the IAEA Benchmark Regarding ROCOM PTS Test Cases
2. S. Aggarwal, A. Shams, D. De Santis, E. M. J. Komen
Design of a Single-Phase PTS Numerical Experiment with Buoyancy Effects for a
Reference DNS
3. I. Cremer, R. Trewin, S. Grams, A. Mutz
Two-Phase Pressurised Thermal Shock Analysis with CFD Including the Effects of FreeSurface Condensation

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES

32  NEA/CSNI/R(2020)5

Session 12: Plant application (I)Co-Chairs: W. K. In (KAERI, Korea), W.X. Tian (XJTU, China)
1. R. A. Brewster, T. A. Bissett, G. A. Meyer, T. K. Meneely
CFD Analyses of theAP1000® ReactorHot Leg/Automatic Depressurization System (ADS)
Nozzle2. A. P. Skibin, V. Y. Volkov, L. A. Golibrodo, A. A. Krutikov, O. V. Kudryavtsev, Y. N. Nadinskiy
Development of the Virtual CFD model of VVER NPP Primary Circuit
3. L. Wang, M. Wang, T. XU, J. MIN, W. Du, W. Tian, S. Qiu, G. Su
CFD Analysis of PWR Upper Plenum Thermal Hydraulic Phenomena with
CODE_SATURNE

Session 13: UQ and coupling
Co-Chairs: S. Koshizuka (University of Tokyo, Japan), D. Bestion (CEA, France)
1. A. Cutrono Rakhimov, D. C. Visser, E. M. J. Komen
Uncertainty Quantification Method for CFD Validated for Turbulent Mixing Experimentsfrom GEMIX
2. C. Koren, C. Geffray
Progress in the Development and Validation of the Multiscale, Multiphase, Numerical
Platform NEPTUNE_CFD/CATHARE

Session 14: Plant applications (II)Co-Chairs: T. Höhne (HZDR, Germany), R. Brewster (Westinghouse, United States)
1. M. Bolshukhin, A. Budnikov, D. Sveshnikov, R. RomanovExperience with the Practical Application of the 1D-3D Coupled Thermal-Hydraulic
Analysis to Validate Future Operating Modes of the RITM-200 Reactor Plant for the NewGeneration Nuclear-Powered Icebreaker
2. D. A. Oleksyuk, V. A. Bugayeva, D. R. Kireeva
Simulation of Standard Temperature Control Indications at the Outlet of a Fuel Assembly of the
VVER 1000 Reactor of Rostov NPP Unit No. 2

WORKSHOP ON EXPERIMENTAL VALIDATION AND APPLICATION OF CFD AND CMFD CODES