Development and Validation of CFD Methods forNuclear Reactor Safety AssessmentSAFIR2018 Interim Seminar24 March 2017, Innopoli, Espoo, Finland

Timo Pättikangas, Juho Peltola, Risto Huhtanen, Ville Hovi,Joona Leskinen and Ismo KarppinenVTT Technical Research Centre of Finland

Juhaveikko Ala-Juusela and Timo SiikonenAalto University

Giteshkumar Patel, Vesa Tanskanen and Elina HujalaLUT

Tommi Rämä and Timo ToppilaFortum Power and Heat Oy

202/05/2017 2

Development and Validation of CFD Methods forNuclear Reactor Safety Assessment

WP 1: CFD benchmarks on mixing and stratification- Participation in OECD/NEA benchmarks

WP 2: CFD modeling of PPOOLEX experimentsStratification of pressure suppression pool of BWR

WP 3: OpenFOAM solver for nuclear reactor safety assessmentDevelopment and validation of open-source CFD codeCo-operation of VTT, Aalto University, LUT, Fortum, KTH and otherinternational partners

WP 4: Coupled CFD-Apros simulations of NPP components

Two-way coupled calculation of steam generator

WP 1: CFD benchmarks onmixing and stratification:OECD/NEA HYMERESPanda HP1_6_2 benchmark

Risto HuhtanenVTT Technical Research Centre of Finland

402/05/2017 4

OECD/NEA–PSI HYMERES BenchmarkPanda Test HP1_6_2

Motivation: Stratification of hydrogen may be important insevere accidents.

Erosion of density stratification in Panda-vesselExperimental arrangement by PSI in SwitzerlandInternational benchmark, blind simulation of the experimentExtensive post simulations in several steps

502/05/2017 5

MS_1

TC_1, MS_2

TC_2, MS_3TC_3, MS_4

MS_5, TC_4

MS_6, TC_5

MS_7

MS_8TC_15

TC_14

24

3 m

4.97 m

6.27 m

6.7 m

8 m

7.5 m

5.62 m

TC_7 TC_8

TC_12

TC_11

TC_16

TC_13

TC_10TC_9

TC_6

Five cell levels:Basic grid: 10 cmTop of Vessel-1 andconnection pipe sector of Vessel-2: 5 cmPlume and connection pipe: 2.5 cmPlume area: 1.25 cmInjection pipe and plate: 0.625 cm

Number of cells: 2 872 435 (Blind)

Computational grid and measuring points

602/05/2017 6

Initial and boundary conditions

Helium [vol] Density [kg/m3]Constant flow58.4…61.2 g/sSteam 100 %

Injected mixture is lighterthan in the lower part, butheavier than in the top partof the vessel

Initial temperature 107 °CSteam injection temperature 107°C -> 151 °C

Constant pressure outletabout 1.3 bar

702/05/2017 7

Used models and methods (Post simulation)

The commercial code ANSYS Fluent version 16.2 was usedTurbulence was modelled with SST k- model, where turbulentbuoyancy terms were addedComputational grid improvements, more cells (3.4 million).Radiation heat transfer was added.Injection pipe was included in the calculation.

802/05/2017 8

Temperature

902/05/2017 9

Conclusionsk- SST turbulence model with added buoyancy terms seem toperform well in stratification modelling.The post-simulation shows essential improvement when radiation istaken into account.Increasing number of cells does not improve the results unless allrelevant physical models are included (radiation in this case).Calculated temperature is too high already when the jet hits the plate(same result for all simulations).The velocity and temperature profiles in pipe exit was boundarycondition in the blind simulation.In post simulation, the inflow pipe is included in the simulation domain.

WP 2: Simulation of PPOOLEXsparger test SPA-T1

Risto Huhtanen and Timo Pättikangas

VTT Technical Research Centre of Finland

1102/05/2017 11

PPOOLEX test facility at LUT

Sparger submerged in the water pool.

Model of a BWR pressure suppression pool.Study of formation and breaking of thermal stratification.Experiment by Markku Puustinen, Jani Laine and Antti Räsänen (LUT).

1202/05/2017 12

Description of the PPOOLEX experiment SPA-T1

Motivation: Thermal stratification reduces the capacity of the pressuresuppression pool when it is acting as a heat sink.

In the experiment, steam is injected through a pipe into a poolwith cold water (13°C)Injection is done through 8 orifices (ø8 mm) around the pipeIn the stratification phase (13 650 s) steam flow is 30 g/sIn the mixing phase (1 240 s) steam flow is 123 g/sIn the calculation, it is assumed that steam is condensed when enteringthe pool, direct-contact condensation is not simulatedThe mass, momentum and enthalpy of steam are added as a sourceterm to the cells in front of the orifices

1302/05/2017 13

Temperature and velocity at the end of thestratification phase, t = 13 650 s

m/s°C

VelocityTemperature

1402/05/2017 14

Development of temperature profileStratification 260 – 13 900 s, mixing 13 900 – 15 150 s

1502/05/2017 15

Temperature comparison, vertical Line 1

Point Height [m]

T4109 2.022

T4101 0.522

Stratification near the bottomis not strong enough

z = 0.522 m

z = 2.022 m

1602/05/2017 16

Conclusions

Stratification of cold bottom part is mainly maintained in the firstphase. However, there is too strong mixing in simulation during thestratification phase compared to experiments.This could be improved by refining the grid in the density gradientlayer.The higher steam mass flow in mixing phase mixed the fluid in thesimulation properly.Due to too effective mixing in the stratification phase, the thermaltransient in mixing phase is mild.Heat loss to the environment has been taken into account. Theinfluence is not large.

WP 3: OpenFOAM solver for nuclearreactor safety assessmentJuho Peltola and Timo PättikangasVTT Technical Research Centre of Finland

Juhaveikko Ala-Juusela and Timo SiikonenAalto University

Giteshkumar Patel, Vesa Tanskanen and Elina HujalaLUT

Tommi Rämä and Timo ToppilaFortum Power and Heat Oy

1802/05/2017 18

WP3: OpenFOAM solver for NRS assessmentDevelopment and validation of open-source CFD solver in co-operation with national and international partnersModelling of Departure from Nucleate Boiling (VTT)Direct-contact condensation (LUT)Heat transfer in fuel rod bundles (Aalto, Fortum)

DNB

Available in the OpenFOAM Foundation development repository:https://github.com/OpenFOAM/OpenFOAM-dev

WP 4: Coupled CFD-Apros simulationsof NPP components

Ville Hovi, Timo Pättikangas, Joona Leskinenand Ismo KarppinenVTT Technical Research Centre of Finland

2002/05/2017 20

Steam collector

Feedwater Feedwater

Emergency Feedwater

Hot Leg

Cold Leg

Water level control

Pressuremeasurements

Break Location

WP 4: CFD-Apros coupling

CFD

Heattransfer

Steam

Feedwater

VVER-440 steam generator