Wir schaffen Wissen – heute für morgen A. Dehbi, D. Suckow, T. Lind, S. Guentay Paul Scherrer Institut, Switzerland Large Scale Experimental Program at PSI on Safety Issues in a PWR Steam Generator ERMSAR2012, Cologne, Germany, March 21-23, 2012

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 2  Project Overview – International collaboration project  Steam condensation / Reflux condensation Background Objectives of the Reflux Project Reflux Single Tube Test Facility Reflux Test Plan  Steam Generator Mixing and Recirculation under SBO initiated Severe Accident Conditions Background System code approach CFD Simulation Facility Scaling Test Facility and Experimental Program  Summary Presentation Outline

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 3 1) Steam / Reflux condensation, DBA and BDBA :  Amount of coolant available for core cooling  Timing and efficiency of EOP and SAM measures To Improve system codes and bench-mark CFD - In the presence of non-condensable gases - In the presence of aerosols - At high pressures ( bar) 2)Mixing and recirculation in the steam generator during PWR severe accidents :  Thermal challenge to primary pressure boundary  Failure of the components  Source term To scale-up of the methodology and bench-mark CFD 1/7 scale facility with improved scaling and instrumentation The effect of the steam generator design (geometry) Two Part Project Overview

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 4 Reflux condensation: Background (1) o Reflux condensation removes residual decay heat under conditions with: reduced primary side water inventory secondary side heat sink LOCA, mid-loop operation o Condensate flows counter-current to steam Steam generated due to decay heat Condensation in the steam generator Condensate flows back into core Different modes of operation: film-wise, oscillatory, carry-over

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 5 Pure steam condenses efficiently Efficiency decreased by: Non-condensable (NC) gases -N 2 from accumulator injection -Air migration during mid-loop operation -H 2 from fuel (severe accident) Aerosols (severe accident)  Heat transfer and steam condensation during counter / co-current flow at high pressures (5-100bar) for high content of non-condensable gases N 2 and He (H 2 ) under co-current and counter current flow conditions => CCFL in laminar to turbulent flow regimes in the presence of aerosols Reflux condensation: Objectives

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 6 TRACE Condensation Model: Status* *A. Manera, CAMP Meeting, Nov Film Condensation without NC Gases Very good agreement for condensation without NC for large pressure range Improvements needed for very low pressure and low liquid film Reynolds numbers Film Condensation with NC Gases Default model of TRACE code very approximate Significant improvements with TRACE advanced condensation model Further developments still needed for: High content of NC (20-90%) High pressures ( bar)

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 7 RFLX Single Tube Facility  Tube 1: 45 x 20 mm  Tube 2: x 16.8 mm  Length 4.5 m  Material SS 316L  Steam: up to 25 kg/h  NC: N w-% He: 0-45 w-%  Primary: bar 400°C  Secondary: bar 35 – 295°C

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 8 RFLX Test matrix  Test matrix comprised of 5 Phases and 50 Tests ConfigurationCharacteristics, Variations IReflux mode, pure steam condensation NC mass fraction: 0% Primary total pressure: bar Injection from bottom / top IIReflux mode, steam + noncondensable (NC) NC mass fraction: 1% - 91% Primary total pressure: bar IIIOnce-through counter-current mode (NC) NC mass fraction: 1% - 91% Primary total pressure: bar up to CCFL point IVOnce-through co-current mode NC mass fraction: 1% – 91% Primary total pressure: bar CCFL onwards to flooding, spill-over VOpen tests To be assigned

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 9 Steam Generator Mixing: Background o SBO severe accident sequence Hot leg voided by venting coolant through pressurizer Cold leg loop seal plugged with water Primary: high pressure; Secondary: dry, depressurized (“high, dry, low”) o Hotleg counter-current natural circulation Transfer heat to hot leg, surge line and SG tubes Hot flow counter-current to cold flow Mixing of hot and cold gas in inlet plenum Flow recirculation through SG U-tubes  Thermal challenge to surge line and SG tubes  Source term

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 10 Mixing: System Code Approach o MELCOR 1.8.5: Pairs of flow paths to simulate counter-current and inlet plenum mixing o Required inputs: Recirculation ratio : flow rate in tubes/flow rate in hotleg Fraction of tubes which receive hot fluid (flow upwards) → To predict the actual response and mixing parameters requires experimentally validated CFD o SOARCA by US NRC (NUREG-1935, 2012): Limited data available, only average behaviour Dependence on geometry, data available for only one geometry => validity for other geometries?

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 11 Facility Scaling: Dimensionless Numbers o Westinghouse (1990) tests to experimentally investigate natural circulation flows during severe accidents in PWRs; limitations on data application: scaling (not conservative), data proprietary  Improved scaling : increased height and number of tubes, reduced tube diameter  increased resistance  increased Ri tube  Improved instrumentation

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 12 Schematic of the SG Mixing Facility Program will produce CFD-grade data : high spatial / temporal resolution of flow field o PIV (2 components of velocity and RMS) o Two component LDA: Velocity profiles (mean & RMS) Reynolds stresses, thermal stresses o Thermocouples (tubes), and thermocouple meshes (hot leg, inlet / outlet plenum) ParameterValue Hot leg length (m)1.2 Hot leg diameter (mm)83.7 Number of U-tubes262 Inner Diameter of tubes (mm)5.0 Tube height (straight) (m)2.2 SG bundle radius (m)0.235

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 13 Experimental Program 1. Base case: circulation and mixing under “high-dry-low” scenario 2. Effect of light gas: He Concern: presence of light gas, under low driving ΔT, could lead to stratification and impairment/disruption of counter-current flow (no data available) 3. Leakage: effect of tube leakage on mixing Leakage will progressively disrupt counter-current flow  no data available on the dynamics of this effect 4. Geometry of inlet plenum CFD indicates large variations in recirculation ratio  no experimental data available with different inlet plenum geometries NPPMixing factor Fraction hot tubes West % CE % Boyd: Geometry effect, base case

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 14 Summary The Reflux tests will: Extend available data base to improve system codes and bench-mark CFD In the presence of non-condensable gases In the presence of aerosols At high pressures ( bar)  Amount of coolant available for core cooling  Better evaluation of accident prgress  Timing and efficiency of EOP and SAM measures The Mixing tests will: Provide critical mixing parameters (not constant ) Improved scaling Effect of different geometries Light gas Effect of tube breach  Help protect pressure boundary by better informed accident management  Better assessment of the source term

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 15 Thank you for your attention

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 16 RFLX Facility, Measurements Local heat flux → set of thermocouples → thin-foil heat flux sensors Condensate film → flush-mounted pin and wire-wire electrode conductance probes Condensate flow → Coriolis mass flow meters Pressure → differential pressure transducers

Nuclear Energy and Safety Laboratory for Thermal Hydraulics Severe Accident Research Group, SACRE ERMSAR2012, March 21-22, 2012 PSI, , 17 CFD Simulations of Mixing o At NRC, C. Boyd: 1/7 Westinghouse and full-scale “A challenge … the extension of a limited set of available test data at 1/7th scale to the full scale conditions” o At PSI, A. Dehbi: 1/7 Westinghouse Flow turbulent and erratic, hot plume wanders around  High resolution temporal and spatial data required to benchmark codes