CFD for Containment Analyses Hydrogen Management in LWRs

CFD for Containment AnalysesHydrogen Management in LWRs

JAHRESTAGUNG KERNTECHNIKTopical Session, May 19, 2011

Berliner Congress Center

Dirk C. [email protected]

Jahrestagung Kerntechnik, May 19, 2011 - Berlin 2

Contents

� Introduction:• NRG• Hydrogen Risk in LWRs

� CFD for Hydrogen Management:• NRG Hydrogen Distribution Analyses

� CFD Requirements & Guidelines

• NRG Hydrogen Combustion Analyses

� Summary


Established 1998

Staff ~340

Locations Petten & Arnhem

� NRG offers consultancy services in:

• in-core fuel management• in-service inspections• project services • safety assessment and licensing

� NRG is one of largest suppliers of medical isotopes worldwide

� more info on http://www.nrg.eu

Introduction – NRG

310 km to Hannover

120 km to Essen


Introduction - Hydrogen Risk in LWRs

Hydrogen is a key safety issue for water cooled rea ctors

- Large amounts of hydrogen may be generated and released in the reactor containment during a severe accident

- Hydrogen and oxygen can form a flammable or explosive gas mixture, depending on the H2-Air-Steam composition

- Hydrogen combustion/explosion may damage (safety) relevant equipment and challenge the integrity of the containment

- The potential risk of hydrogen depends on the H 2-Air-Steam composition in the containment


Introduction - Hydrogen Risk in LWRs

For assessment of the Hydrogen Risk it is important to:

1. predict the local gas composition and local process conditions (p, T, turbulence) in the containment during a severe accident transient � H2 distribution analyses

2. predict the combustion process � H2 combustion analyses

3D

complex

For both, 3D CFD modeling is required


CFD for Hydrogen Management

CFD model should be able to capture• distribution and mixing of H2, Air and Steam

• complex 3D phenomena : e.g. condensation, stratification, jets/plumes,

buoyancy effects, turbulent mixing, H2 combustion / explosion

• mitigation measures : e.g. recombiners, sprays, coolers

Status of CFD for Containment Analyses one decade a go

• extensive user modeling

• long computation times• reliability not demonstrated

• no quality guidelines

�� CFD modeling made a big step forward in last 10 yea rs


Containment Analyses at NRG

� FLUENT CFD code selected for its:- excellent parallel performance

- efficient solver

- flexible meshing capabilities

� Parallel computer cluster (>400 CPUs)

� NRG developed, implemented, verified and validated specific ‘user-defined’ sub-models for:

- condensation/ evaporation on walls and in bulk- rain-out of mist

- heat transfer with walls

- diffusion and turbulent mixing- recombiners and sprays

- combustion

� Optimization of numerical schemes for speedup of calculations

CFD for Hydrogen Management


Verification & Validation of the H 2 distribution model

Separate effect tests:- SARNET condensation benchmark

- SARNET recombiner benchmark

- SARNET-2 spray benchmark (single falling droplet)

Validation against experiments in large-scale tests facilities:- TOSQAN (7m3) � IRSN- THAI (60m3) � Becker Technologies / GRS

- MISTRA (100m3) � CEA

- PANDA (200m3) � PSI

NRG H2 Distribution Analyses



PHASE 2:4300 – 6800 s hot steam injection

Initially a N2-rich

atmosphere at

ambient condition

H2O

H2

N2PHASE 1:0 – 4300 sH2 injection

THAI-HM2 benchmark: formation and dissolution of a stratified H2-rich layer by a buoyant plume


THAI-HM2 benchmark: formation and dissolution of a stratified H2-rich layer by a buoyant plume

CFD analysis NRG

Experiment

4760 s4300 sTHAI vessel


� Good prediction of build-up and break-up of stratification� Quality Guidelines developed for mesh resolution, time step size and turbulence modeling� Solver settings are optimised to reduce calculation time


Ho

t w

all

Co

ldw

all

T (K)

inle

t

steam~1.2g/s

steam/air ~4g/s

steam~12g/s

steam~1.2g/s

steam~1g/s

steam/air ~4g/s

steam/He ~2g/s

TOSQAN test ISP-47: Flow and condensation under different representative conditions

EXP vs. CFD


� Correct prediction of condensation rate during different transients


PANDA test ST1_7_2 (SETH-2):break-up/erosion of a stable helium layer by a low momentum air plume


� Good a priori prediction of stratification break-up

0.0

0.1

0.2

0.3

0.4

0.5

0 5000 10000 15000 20000 25000

time (s)

volu

me

fra

ctio

n h

eliu

m

.

(-)

cfd - h=7.5m

cfd - h=6.9m

cfd - h=6.3m

cfd - h=5.6m

cfd - h=0.5m

exp - h=7.5m

exp - h=6.9m

exp - h=6.3m

exp - h=5.6m

exp - h=0.5m

EXP vs. CFD

helium vol%

time


TOSQAN spray test T113 (SARNET):Mixing of a stable helium stratification by spray injection from a single nozzle

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250 300

time (s)

heliu

m v

olum

e fr

actio

n (-

)

cfd - 0.9mcfd - 1.9mcfd - 2.8mexp - 0.9mexp - 1.9mexp - 2.8m

EXP vs. CFDCFDTOSQAN


� Successful application of NRG spray model


CFD Quality Assurance

� Essential Quality Requirements for Containment Anal yses:• Conservation of mass

• Mesh independent

• Time step independent

� In-house Quality Guidelines developed for:• Mesh resolution• Turbulence model/parameters

• Transient settings (time step, # iterations per time step, etc)



Interim Summary

� In-house quality guidelines and specific sub-models are developed for Containment Analyses

� NRG’s Containment modeling in FLUENT for H2 distribution/mitigation analyses is validated against the available large-scale experiments in TOSQAN (7m3), THAI (60m3), MISTRA (100m3) and PANDA (200m3)

� Reliable predictions are obtained with CFD for H2 distribution / mitigation, using the quality guidelines and sub-models developed by NRG

� Practical computing times are feasible using parallel computing and optimised numerical schemes


�� 5~10 million cells are required for a typical full scale LWRcontainment (transient scenarios will take about 2 weeks on 40 CPUs)


Combustion model implemented in FLUENT by NRG is validated against H2deflagration experiments in ENACEFF (F), THAI (D), and FLAME (USA) facility:

� Combustion process and resulting pressure loads on containment walls show a strong dependence on:

• Mesh resolution• Time step • Initial turbulence

� Experiments by Bradly, Groff and Cammarota (methane / propane –air deflagrations) confirm the strong influence of initial turbulence on deflagrations

NRG H2 Combustion Analyses

0 0.005 0.01 0.015 0.02 0.025 0.030

5

10

15

20

25

30

35

40

Time [s]

Flam

e Pr

opag

atio

n Ve

loci

ty [m

/s]

rmax 1mm

123 �� a higher initial

turbulence level results in higher flame speed


This results in the following requirements- Experimentally: the initial turbulence has to be measured- Numerically: Successive mesh size and time step refinement has

to be applied

At the moment, adequate validation of the H 2 combustion analyses is not possible , because no H2 combustion experiments are currently available where the initial turbulence is measured

NRG H2 Combustion Analyses

�� CFD analyses for H 2 management in LWRs should be focused on distribution and mitigation


Summary

� Hydrogen management is a key severe accident issue for existing and new LWRs

� CFD analyses are an essential tool for adequate Hydrogen Management

� CFD Containment Analyses give reliable results provided that thequality guidelines are followed and valid sub-models are used

� CFD reached a level at which it can be applied in full-size plant analyses by making use of parallel computing and optimised numerical schemes


The End

“Thanks for your attention”

CFD for Containment Analyses Hydrogen Management in LWRs

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