Application of CFD in Hydrogen Safety Management

Transcription

1 8 TH CONFERENCE ON SEVERE ACCIDENT RESEARCH ERMSAR 2017 EU DuC = N Application of CFD in Hydrogen Safety Management D.C. Visser, E.M.J. Komen, N.B. Siccama and F. Roelofs Presented by Afaque Shams J.G.T. Te Lintelo May16-18, 2017 Warsaw, Poland

2 Contents Introduction Traditional Approach Need for Complementary CFD Full Scale Application Summary 2

3 Introduction Large quantities of flammable hydrogen gas can be produced and released into the containment during a severe accident in a LWR. Combustion of it may damage relevant safety systems and the containment walls. Fukushima confirmed key safety issue of LWRs with respect to hydrogen: Release Distribution Mitigation Combustion Notwithstanding mitigation measures, temporary existence of flammable gas clouds can not be excluded Reliable simulations are needed to assess the effectivity of the mitigation measures and the residual risk of hydrogen deflagrations! 3

4 Traditional Approach Basic Steps: 1. Definition of accident sequences 2. Evaluation of hydrogen and steam sources (LP Code) 3. Determination of hydrogen distribution and effects of mitigation systems on system scale (LP Code) 4. Evaluation of flammability limits using Shapiro diagram Shapiro diagram for hydrogen-air-steam mixtures 4

5 Traditional Approach Typical LP model of containment Comparison LP - CFD (table is taken from IAEA-TECDOC-1661, 2011) Subject LP Code CFD Code Computational time short long Nodalization ~100 volumes ~ cells Prediction of flammable mixtures average concentrations local concentration Flow paths adequate good Stratification limited good Combustion no yes Validation good limited Typical CFD model of containment ~1million nowadays 5

6 Need for Complementary CFD H2 Combustion Traditional Approach (LP codes + Shapiro) is; OK for slow deflagrations characterized by slow pressure increase and no effect of turbulence on flame speed Inadequate for fast deflagrations characterized by dynamic pressure peaks with strong effect of turbulence (not captured by LP codes) 6

7 Need for Complementary CFD H2 Combustion The need for complementary CFD is demonstrated by comparing the two experimental deflagration tests THAI-HD10 and ENACCEF 153. Both tests have: similar initial H2 concentration, and similar location in Shapiro, but ENACCEF (Including baffles, e.g. SG room) THAI (empty vessel, e.g. NPP containment) 7

8 Need for Complementary CFD H2 Combustion a totally different pressure evolution! THAI-HD10: limited turbulence generation at flame front limited flame speed ~ 5 m/s slow deflagration LP + Shapiro = OK ENACCEF 153: large turbulence generation because of baffles high flame speed ~ 300 m/s fast deflagration intermediate peak pressure resulting from pressure waves much larger than mean 8

9 Need for Complementary CFD H2 Combustion In fast deflagration, intermediate pressure peaks occur because of pressure waves Dynamic pressure loads (turbulent flame speed) depend on: turbulence generated at flame front (dominant effect) hydrogen concentration Detailed simulation of turbulence and geometry is important for fast deflagrations LP codes do not model turbulence and do not capture local/small-scale geometry Shapiro diagram does not include turbulence and geometry effect 9

10 Need for Complementary CFD H2 Distribution & Mitigation Distribution & Mitigation 3-dimensional configuration of the containment (compartments and flow paths) Local details/effects: Effectiveness of mitigation systems depending on local composition of the gas mixture (recombiners) Condensation/evaporation affecting local hydrogen concentration Geometrical details influencing hydrogen distribution/mixing Turbulent mixing and complex flow phenomena (jets, buoyancy, stratification) 10

11 Full Scale Applications Complementary CFD analyses with respect to hydrogen risk for Borssele NPP in the Netherlands: MELCOR LP Code to evaluate various severe accident scenarios LB LOCA and Loss-of-Feedwater complementary CFD analyses CFD Model: 5 million computational volumes Computational Effort: 2 weeks on 32-core computer CFD model and mesh of Borssele containment 11

12 Full Scale Applications Hydrogen distribution during LB-LOCA Streamlines coloured by temperature showing effect of recombiners Validate effectiveness of mitigation measures/procedures Optimization of recombiner positions 12

13 Summary Added value of complementary CFD analyses: 3-Dimensional configuration of the containment Local details/effects influencing the hydrogen concentration Complex flow phenomena and turbulence effects Modeling mitigation systems Determination of dynamic pressure load (intermediate peak pressures) in fast deflagration CFD application to full scale containment of the Borssele NPP 13

14 Questions? 14