CFD simulation of fibre material transport in a PWR core under loss of coolant conditions

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1 CFD simulation of fibre material transport in a PWR core under loss of coolant conditions T. Höhne, A. Grahn, S. Kliem Forschungszentrum Dresden- Rossendorf (FZD) Institut für Sicherheitsforschung Postfach , D Dresden Text optional: Institutsname Prof. Dr. Hans Mustermann Mitglied der Leibniz-Gemeinschaft

Kliem Forschungszentrum Dresden- Rossendorf (FZD) Institut für

$differential pressure measurements, modifications of the insulation, the strainer size and the mesh size at the strainers - But: Still possible, that a small amount of smaller fractions of the fibre$

2 Transport of fibre material Leckstörfall mit Freisetzung von Mineralwolle Transport of fibre material in a German PWR Status in German NPPs : - Back flushing procedures, implementation of differential pressure measurements, modifications of the insulation, the strainer size and the mesh size at the strainers - But: Still possible, that a small amount of smaller fractions of the fibre material can be transported into the RPV - Core coolability must be guaranteed all the time! Szenario, Assumptions : - LOCA, SCRAM, ECCS hot leg injection, after 1600s switch to sump cooling mode with 150 kg/s per loop, decay heat 80 MW, leak in cold leg, no stable natural circulation Seite 2

$small amount of smaller fractions of the fibre material can be transported into the RPV - Core coolability must be guaranteed all the time!$

3 Szenario During hot leg ECC injection, the fibres enter the upper plenum and can accumulate at the fuel element spacer grids, preferably at the uppermost grid level (1) upper support plate, (2) control rod guide tubes, (3) fuel element, (4) RPV, (5) core wall, (6) lower support plate, (7) perforated drum, (8) hot leg ECC injection of colder fluid Source. AREVA NP Measurements at UPTF: Establishment of downwards directed break-trough channels at the core General Aim: calculation of break-through channels and of the distribution of mineral wool fibres across the grid spacers (local mass load, pressure) Seite 3

(8) hot leg ECC injection of colder fluid Source.

4 Previous CFD-Calculations: Calculations: Seite 4

phase, incompressibel - Water & Fibre material - Turbulence model: SST - Automatic wall funtions - uppermost spacer grid plane

5 Boundary Conditions, Model Assumptions Boundary conditions: - RPV pressure at cold leg 2.5 bar, averaged coolant temperature 380 K, - ECC water 150 kg/s per loop, temperature 330 K, Model assumptions: - Fluid: two phase, incompressibel - Water & Fibre material - Turbulence model: SST - Automatic wall funtions - uppermost spacer grid plane collects all the fibres that arrive there: 3 D subdomain for strainer model implementation - Initial state: inner circulation uppermost spacer grid plane fuel element Seite 5

6 Strainer Model - Accumulation of fibre material Seite 6

Strainer Model implementation of strainer model for the spacer grid, which completely retains the insulation material carried by the coolant accumulation of the insulation material rise to the

7 Strainer Model implementation of strainer model for the spacer grid, which completely retains the insulation material carried by the coolant accumulation of the insulation material rise to the formation of a compressible fibrous cake permeability to the coolant flow is calculated in terms of the local amount of deposited material and the local value of the superficial liquid velocity. porosity distribution due to streamwise increase of compacting pressure pressure drop in fibrous layers according to Davis and Ergun self compaction under p k (experiments, TH Zittau) strainer model for the spacer grid Seite 7

amount of deposited material and the local value of the superficial liquid velocity.

8 Results 4 Loops 5kg Fibre Material Injection Seite 8

9 4 Loops 5kg Fibre Material Injection Mass load fibres [kg/m²] 40 s after start Mass load fibres [kg/m²] 68 s after start 3 akkumulierte Accumulated fibres Fasern [kg] [kg] Zeit Time / s Seite 9

after start 3 akkumulierte Accumulated fibres Fasern

10 Further improvement of the modelling VGB Project: "Qualifizierung von CFD-Programmen für Fragestellungen der Reaktorsicherheit (FZD/SA"AT" 41/09 B) Seite 10

11 Improvements - Next Steps CAD Geometry - real Konvoi structures in the upper plenum - ECC injection nozzle ( Hutze ), SG bottom - Core geometry as porous body, horizontal flow components are possible Decay Heat Distribution - 3D extraction of node-wise heat source from DYN3D - import into CFX and interpolation Steam Production - three phase flow, injection of steam via volumetric source - re-suspension of the insulation material with upwards flow Seite 11

extraction of node-wise heat source from DYN3D - import into CFX and interpolation Steam Production - three phase

12 New Grid - Konvoi Geometry PWR Konvoi Modular Grid 19 Mio. Cells Seite 12

13 New Grid - Konvoi Geometry Hot leg incl. Hutze, SG Bottom Hot leg Upper plenum incl. structures, SSFE Core: spacer grid levels, FE head, bottom SG Bottom PWR Konvoi Modular Grid 19 Mio. Cells Cold leg incl. ECCS pipe Seite 13

14 New Grid Core: Porous Media PWR Konvoi Modular Grid 19 Mio. Cells Seite 14

15 Core Modelling and Decay Heat Distribution Seite 15

16 New: Full Core: Porous Media - Core Permeability β= Flow Resistance: Directional Loss Model STREAMWISE LOSS: Option = Permeability and Loss Coefficient Resistance Loss Coefficient = 3.38 [m^-1] TRANSVERSE LOSS: Option = Streamwise Coefficient Multiplier Streamwise Coefficient Multiplier = Core Support Plate Permeability = [m 2 ] - Resistance Loss Coefficient = 9.8 [m^-1] Seite 16

Coefficient Resistance Loss Coefficient = 3.

17 New: Full Core: Porous Media Nodewise calculation of decay heat with DYN3D (10x193 Nodes): - Begin of Cycle of a generic Konvoi reactor core, - coolant mass flow rate 400 kg/s from bottom to top, - no crosswise mixing, - decay power 80MW (approx. 2% of the nominal power, 1600s after SCRAM), - no boiling - Volumetric Heatsource: - Extraction of 1930 Volumetric Heatsource Points of Decay Heat calculation in DYN3D - Transformation and use of interpolation algorithm in ANSYS CFX - Calbration algorithm with overall Decay Heat Value (spacer planes) DYN3D CFX coupling CFX (Heatsources) Seite 17

2% of the nominal power, 1600s after SCRAM), - no boiling - Volumetric Heatsource: - Extraction of 1930 Volumetric Heatsource Points of Decay Heat

18 Results of Inner Circulation with ECC injection Seite 18

19 Results of Inner Circulation RPV, upper spacer grid indicated, ECC water injection over 120 s Seite 19

20 Preliminary Results: ECC water injection in 4 loops - 5kg Fibre Material Injection Seite 20

21 4 Loops 5kg Fibre Material Injection RPV, inlet nozzle plane, ECC water injection with fibre material (5kg ), 25 s, isosurface at 750 ppm Seite 21

22 Steam production Seite 22

23 Steam Production Simplified Model - Simplified RPV model (5 Mio. nodes, 14 Mio. Elements) consists of Hot leg, Upper Plenum, Core, spacer grids - 3 phase flow (solid, gas, liquid), multiphase flow models, strainer model - Steam injection via volumetric sources into subdomain core (0.95 [kg m -3 s -1 ], 410 [K]) - Start of ECC injection with 1.25 kg isolation material (150 kg/s) Grid model nozzle plane 0-9s after ECC injection Seite 23

24 Preliminary Results - Fibre Material Injection RPV, upper spacer grid, ECC water injection with fibre material (1.25 kg ), 9 s, isosurface at 750 ppm Seite 24

25 Summary Major modeling improvements were done for: Geometry (use of original Konvoi geometry) Decay heat simulation Steam production Preliminary Results: - the fiber material at the uppermost spacer grid plane is not evenly distributed - first, it is accumulated at the positions of the breakthrough channels - steam production makes the flow in the upper plenum situation more complex Seite 25

26 Acknowledgments The project was funded by the Nuclear Special Committee Plant engineering of VGB PowerTech (Germany). Thank you! Seite 26

CFD Topics at the US Nuclear Regulatory Commission. Christopher Boyd, Ghani Zigh Office of Nuclear Regulatory Research June 2008

CFD Topics at the US Nuclear Regulatory Commission Christopher Boyd, Ghani Zigh Office of Nuclear Regulatory Research June 2008 Overview Computational Fluid Dynamics (CFD) is playing an ever increasing