1. INSTANT/PHISICS – RELAP5 coupling A. Epiney, C. Rabiti, Y. Wang, J. Cogliati, T. Grimmett, P. Palmiotti

2. Overview Adding the PHISICS tool suite to RELAP5-3D –A new transport/diffusion solver: INSTANT –A New cross section model: XS-MIXER Examples –Typical PWR –Takeda 4 benchmark –NGNP MHTGR Summary and outlook

3. Vision Increase of neutronics modeling accuracy Modeling flexibility Uncertainty assessment PHISICS RELAP5- 3D Lean software inter-dependency Low impact for user

4. RELAP5-INSTANT vs RELAP5-NESTLE FeaturesRELAP5 - NESTLERELAP5 - PHISICS Energy group2-4Not bounded DiffusionYes TransportNoYes Triangular MeshNoYes Unstructured MeshNoYes AdjointNoYes Multi-Dimensional Cross Section TablesNoYes SpeedWinLose (Future ?) Discontinuity FactorsYesFuture ? Cylindrical GeometryNoFuture Perturbation TheoryNoFuture DepletionNoFuture Localized refinementNoFuture

5. New transport core solver: INSTANT Accessible through new keyword “INSTANT” $------------------------------------------------------------ $ REACTOR KINETICS INPUT $------------------------------------------------------------ 30000000 instant gen 30000001 no-gamma 3600.0e+6 0.0076 6 1.0 0.48 … Compatible with RELAP5: cross section models control rod model Existing RELAP5 inputs will run just by changing “nodal” to “instant” INSTANT control parameters (if different from default) could be provided through separate input file

6. New cross section model PHISICS PHISICS gives additional access to: –Unlimited number of energy groups (memory limit) –Transport XS vs. Diffusion Coeffs. –Square root vs. linear structure temp. FB –Simple HTML input –Different FB tabulations for different materials Table allows to account for cross terms Multiple points address non linearity Functional representation in the future

7. New cross section model PHISICS Accessible through new keyword “PHISICS” $------------------------------------------------------------ $ REACTOR KINETICS INPUT $------------------------------------------------------------ 30000000 instant phisics 30000001 no-gamma 3600.0e+6 0.0076 6 1.0 0.48 … –Compatible with RELAP5 CR model –Kinetic nodes to TH mapping: as in “Gen” FB Zones and Regions for: –Structure temperature –Fluid temperature –Fluid density –Poison concentration

8. The “Gen” feedback structure Every kinetic node is assigned to a FB Zone –(Zone figures are assigned to axial meshes like for compositions) Every FB Zone has: –a number of heat structure FB regions Every HSFB region feeds back one (weighted) structure temperature –a number of volume FB regions Every VFB region feeds back: (weighted) density, temperature and poison concentration Total feedback variables: HSFB + 3*VFB

9. Implementation (using RELAP5 XS models) RELAP 5: Plant and TH INSTANT XS, Geometry Power Compatibility with existing RELAP5 XS and control rod models

10. Implementation (PHISICS XS) RELAP 5: Plant and TH XS-MIXER INSTANT T f, T c, ρ c,… CR positions XS Power Steady state search using several energy group (>4) has been already implemented Geometry Controls …

11. Software Structure RELAP Input Reader Feed input Branch to a special input file RELAP /PHISICS driver Data Type PHISICS input file reader Construction- destruction Data Type INSTANT DRIVER (pointing local data type interface) Data Type Feeding RELAP5INSTANT Neutronics TH coupling

12. Cartesian Geometry: Typical PWR Rod in/out cases Full core model 17x17 nodes 13 axial levels 11 Materials 36 Feedback zones 2 Energy group CR out CR in

13. PWR Rod Out Easy visualization with INSTANT(VTK file) *surface order 1, volume 4 Convergence evolution Keff InitialConverged INSTANT*1.016391.00348 RELAP1.017311.00483 Delta0.000920.00135

14. PWR Rod Out: K eff Evolution

15. PWR without control rods Power distribution difference (%) NESTLE/INSTANT First iteration Converged Feedbacks tend to reduce the difference in power distribution

16. PWR Rod Out Comparison Comments –Difference in k eff reasonable –Difference in assembly power higher than expected. Possible reasons: Higher spatial resolution in INSTANT (NESTLE mesh refinement study could confirm this) Different implementation of vacuum BC

17. PWR Rod Out Spatial convergence and computational time –INSTANT P0 Initial conditions Converged Computational time [%] Surface orderVolume order 3456 01.017361.01754 1 1.016391.01652 2 1.016451.01653 3 1.01652 S. orderVolume order 3456 01.004071.00440 11.003481.00368 21.003611.00372 31.00371 S. orderVolume order 3456 0100146 1182328 2422696 3902 INSTANT converges spatially Best computational cost – accuracy ratio for Surface order 1, Volume order 4

18. PWR Rod Out Computational times INSTANT vs. NESTLE Spatial approximation used 4 th order  37 degree of freedom by node, by energy group  Flexibility comes at less computational efficiency We started at*: NESTLE 175sINSTANT 13760s (S1,V4)  INSTANT factor ~80 slower Ways of reducing the computational time Cross section threshold  gained factor of 10 Parallelization of INSTANT  how many cores you have? *Processor time in kinetics subroutines 1500 iterations

19. PWR rod Out Neutronics inactive if  XS<tolerance  Gained a factor of 10

20. PWR Rod Out Using parallelization for INSTANT  Scaling is almost perfect on shared memory otherwise dependent on node to node communication speed  factor of 10 or more possible by average users  Using both, TH skipping and parallelization the initial factor of 80 can be compensated The real conclusion is… Now you can choose your trade off between accuracy and computational cost

21. PWR Rod Out: Multi Group Test Cross section model test with PHISICS –2 groups (Diffusion coefficient / Total cross sect.) –2 group cross sections expanded to 8 and 20 groups Energy group i is expanded into j groups One can show that k eff does not change for X i,j =  i,j X i X=D,  fis,  abs,   s gi,j->gi’j’ =  i,j  i’,j’  gi->gi’ With some constraints:  = 1,  not  i,j  abs +sum  i,j sum  i’,j’  gi->gi’ < 1/ (3D i,j )

22. PWR Rod Out (PHISCS XS) Test confirms functioning of PHISICS XS model and multi-dimensional interpolation

23. PWR Rod In 1 control rod inserted XS by RELAP CR model Steady state calculation  Convergence evolution 2 nd group INSTANTNESTLE Keff converged 1.038381.03919

24. Takeda 4 benchmark 3D Hexagonal test 4 energy groups No TH feedback Small fast sodium cooled reactor NESTLE vs. INSTANT CR In CR Out

25. Takeda 4 benchmark (CR Out) Volume Order Surface Order 0123 51.07995 6 1.07350 71.079951.073511.07343 81.073441.07342 INSTANT Diffusion NESTLE coarse mesh diffusion methodnodal expansion solution technique 1.106931.07427 INSTANT: PN1 Solution converges towards PN2ND Solution Vol 6/ Surf 1 gives best computational time/accuracy ratio RELAP/NESTLE: coarse mesh method not appropriate, nodal expansion within 90pcm (between INSTANT Surf 0 and 1) UNIC Diffusion k eff = 1.07335

26. Takeda 4 benchmark (CR) Volume Order Surface Order 0123 50.85866 60.858760.85278 70.858760.852780.85241 80.852420.85238 INSTANT PN1 NESTLE coarse mesh diffusion methodnodal expansion solution technique 0.947930.85984 INSTANT: PN1 Solution converges towards PN2ND Solution Vol. 6/ Surf 0 gives best computational time/accuracy ratio RELAP/NESTLE: Coarse mesh diffusion not appropriate; nodal expansion still off by ~800 pcm UNIC Diffusion k eff = 0.85161

27. MHTGR (NGNP) for the MHTGR benchmark Features needed by the benchmark not supported by RELAP3D-NESTLE –Linear independent feedbacks not good enough –26 energy groups –Need of multiple feedback regions (T fuel, T graph ) in each zone –Triangular mesh for CR location NGNP is supporting the RELAP/INSTANT coupling

28. NGNP MHTGR Reactor vessel Core barrel Coolant channels Central reflector Fuel blocks Side reflector Control rod channels Hexagonal geometry Inner and outer reflector 3 Fuel rings RELAP5 representation for feedback zones

29. NGNP MHTGR INSTANTNESTLE Keff converged 1.053001.03080 XS by RELAP (2G) Steady state calculation  Convergence evolution 2 nd group INSTANT: Surf. 1/Vol. 6 NESTLE: coarse mesh diffusion method

30. Conclusion RELAP5-3D – PHISICS coupling add the following feature –Spatial/angular mesh refinement –Unlimited number of energy group –Cross section tabulation –In the future: depletion, time dependent, decay heat, adjoint sensitivity analysis We can match computational time with higher accuracy We preserve compatibility with past input deck

31. Extra Slides

32. Simple HTML input Different tabulations for different materials 540 600 650 700 750 800 540 555 570 585 600 Different tabulations with Different number of dimensions and points One dimension for each FB variable considered

33. Simple HTML input 2.62E-2 2.44E-2 6.811648 0.933646 … … Dimension 1 Dimension 2 => Full table input allows consideration of cross terms