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Advancements in Predictive Models for PWR Lower Head Failure: A Comprehensive Benchmark Study

This presentation outlines significant progress in predictive modeling for lower head failures (LHF) in pressurized water reactors (PWRs). The study encompasses various experimental and numerical investigations, presenting benchmark calculations and comparative analyses of different models developed by IRSN, FZD, CEA, and EDF. Key findings include insights into mechanical behavior, crucial failure modes, and the efficacy of different modeling approaches. Understanding these factors contributes to enhancing safety assessments and developing effective accident mitigation strategies for nuclear reactors.

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Advancements in Predictive Models for PWR Lower Head Failure: A Comprehensive Benchmark Study

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  1. Progress on PWR Lower Head Failure Predictive Models _________________ V. Koundy1, F. Fichot1, H.-G. Willschuetz2, E. Altstadt2, L. Nicolas3, JS. Lamy4, L. Flandi4 1. IRSN – 2. FZD – 3. CEA – 4. EDF (SARNET WP10-2 WORKING GROUP)

  2. Background (exp. and num. studies on LHF) Benchmark calculations Presentation of the different models used Comparison of the different results Conclusions Presentationoutline

  3. Background SA assessments Late phase of a postulated SA Definition of appropriate accident mitigation strategies Loads A good understanding of the mechanical behaviour of the RPV lower head • Experimental programmes : LHF/OLHF(SNL), FOREVER(RIT),… • Modelling investigations (2D simplified, 2D FE and 3D FE)- 2D calculations (Benchmark) - 3D calculations by Cast3m (complementary results)

  4. Calculation benchmark“OLHF-1 experiment” - 1/5 scale LH experiment- 1/2 scale wall thickness - Thermocouples along 4 meridians: ● 3D calculations ● 2D calculations (0°longitude) - Uniform heating (corium gradually relocates to the LH) - Experimental findings (failure time/location…) - Severe necking (from 76mm to 1mm) - T° as a function of time at measurement points - Transient internal pressure

  5. Pressure, dead weight T° on the I/O surfaces (given by the thermocouples) T° in the wall (thermal solution) Temperature loading Presentation of the models used1. The FZD FE model (in ANSYS) Axisymmetric FE model432 four-node elements ● Plasticity ● Creep/Damage modelling(Fortran routines) 1. 2. a creep DB (D : J. Lemaitre) ● Failure criterion : D=1 Material DB REVISA & OLHF Programmes

  6. ● Creep/Damage modelling • : Norton-Bailey (Sandia) • 2. D : Kachanov – by post-evaluation (properties : RUPTHER) • ● Failure criterion : eq limor D=1 Presentation of the models used2. The CEA FE model (in CAST3M) Axisymm. FE model576 eight-node elts. Pressure, dead weight T° on the I/O surfaces (given by the thermocouples) T° profile in the wall (linear) Temperature loading

  7. ● Creep/Damage modelling 1. (Norton-Bailey) 2. D : Kachanov formula ● Failure criterion : D=1 (Similar to that used by CEA) Presentation of the models used3. The EDF FE model (in Code_Aster) Axisymm. FE model1152 eight-node elts -Very fine mesh-Elt. n° doubled OLHF & Rupther Programmes Pressure, Temperature T° Thermocouple readingsT° in the wall (thermal model)

  8.  Ro r ● Theory of shells of revolution under symmetric loading ● Large displacements, large strains : 0    /2  100 intervals Thickness 11 calculation points ● Creep/Damage modelling 1. : Norton-Bailey (Sandia) 2. D : Kachanov formula (Rupther) ● Failure criterion : D=1 (Like in the CEA model) Presentation of the models used4. The IRSN simplified model (in Astec) • The ruptured vessels remain axisymmetric (like in LHF and OLHF) • Hemispherical LH  Ovoid (egg-shape) Pressure, Temperature(T° in the wall : thermal model)

  9. OLHF-1 Benchmark Comparisonof different 2D calculation results

  10. Longitude 180° 0° Nodal temperatures given by SNL 0° Uz=0 12.3 MPa 90° Ur=0 OLHF-1 Benchmark Boundary and loading conditions Hemispherical part –Meridian section at a longitude of 0° – Uz=0 andUr=0 Constant pressure – Nodal temperatures (thermocouples)

  11. OLHF-1 Benchmark Calculated – experimentally determined results () All the calculations : damage criterion (except Cast3m) Cast3m stopped prematurely “necking criterion” ( the damage was negligible at the predicted time ) Certain results of Cast3m will not be compared to the other results

  12. 17.7cm 16.9cm  14.6cm (TEST) 70-75° (failure locations) 9.5cm OLHF-1 Benchmark Comparison the elongation as a function of time • Similar fail. locations (latitudes 70-75°) Test: latitude of 75° • trup texp(discrepancy value =3%) • Uz : discrepancy value =35% (fast Uz) • Code_Aster, Ansys~experimental curve

  13. OLHF-1 Benchmark Wall thickness profile along a meridian line (at t=trup) 0° • CEA, FZD-initial wall thickness profile (thicker at the top)-“curves”similar to “exp. curve”- min. thicknesses over-estimated • Aster, Astec- « curves » ~ exp. Curve (polar angles up to 70°)-not parabolic behaviour 90°

  14. 0° 90° 160 OLHF-1 Benchmark Comparison of different calculated Vm (int. & ext. surfaces at the pole) as a function of time • FE codes (good agreement) • Simplified model- stress values underestimated - quite good results after 160 min

  15. Benchmark conclusion Calculation Benchmark 2D models : - able to correctly predict the exp. failure time and failure location. - the global mechanical behaviour of the OLHF-1 experiment was well represented (by all the models). - not able to give information relating to crack propagation and breach size ( 3D models are necessary for these issues). An IRSN/CEA joint programme Complementary study of crack propagation and crack size using a 3D model of Cast3M

  16. Crack opening and propagation3D calculations using Cast3m ●Constitutive law Unified coupled damage viscoplastic model (Lemaitre-Chaboche) ● Failure criterion : D=1 or Eps=100% 11488 CU20 (20-node elements) ●Crack propagation modelling -Depressurisation (perfect gas)-Damaged elements removed from the mesh Variation of initial wall thickness Thermocouples : 4 meridian lines Top ~ 20°C and Linear axial T° gradient in the cylinder Small cylindrical extension Pressure, dead weight, T° loading

  17. 17.6 cm2 14 cm2 Uz=15.7 cm Uz=14.6 cm 3D calculations Model : Interpretation of the OLHF/LHF experiments Yes OLHF-1 correctly interpreted No NB: LHF material (more sulpher) behaved in a brittle manner / OLHF material Exp. failure section > Calculated failure section Unsatisfactory interpretation of the LHF experiments The 3D model validated by the OLHF experiments is unsuitable for the LHF experiments  Need of a failure criterion which can take into account the variability in material behaviour

  18. Conclusions • 2D Calculations are able to interpret the global mechanical behaviour of lower head failure experiments. Failure time and failure locationcan be correctly estimated. • For crack opening and propagation, 3D calculations are necessary, but the failure criterion must take into account the variability in material behaviour. The development of such a criterion is one of the objectives of the current joint research programme between IRSN, CEA and INSA Lyon.

  19. 0° 40° 90° OLHF1 Benchmark Comparison of different results

  20. OLHF1 Benchmark Comparison of different results

  21. OLHF1 Benchmark Comparison of different results Maximum local measurement of the exp. Value : ~ 140%

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