The Use of RELAP5-3D for Subchannel Analysis of SFR Fuel Assemblies

1. The Use of RELAP5-3D for Subchannel Analysis of SFR Fuel Assemblies Matthew J Memmott Supervisors: Jacopo Boungiorno, Pavel Hejzlar Massachusetts Institute of Technology RELAP User’s Group Meeting November 19, 2008

2. Outline • Background • Subchannel Model • Benchmarks • Application • Results/Conclusions • Future Work MIT Nuclear Science and Engineering

3. Background • SFR is currently the fast reactor of choice under GNEP (ABR1000) • MIT is investigating innovative fuel designs (annular fuel, bottle shaped, etc.) • Subchannel analyses required: fuel temps, clad temps, coolant velocities, etc. • Current subchannel codes cannot handle nonstandard geometries MIT Nuclear Science and Engineering

4. Subchannel Model • 4 Primary Considerations • Subchannel Geometry • Cross-Flow • Sodium Conduction • Turbulent Mixing MIT Nuclear Science and Engineering

5. Subchannel Geometry (I) MIT Nuclear Science and Engineering

6. Subchannel Geometry (II) • Composed of multiple 22 segment pipes • 2 segments for entrance/exit and shielding • 5 segments for plenum • 15 segments for core • Fuel rods divided into 6 segments (no peaking) • Allows for connection to each subchannel • Cannot evaluate azimuthal conduction in rods • Inlet conditions controlled by time dependent volume/junction MIT Nuclear Science and Engineering

7. Subchannels only (I) MIT Nuclear Science and Engineering

8. Subchannels Only (II) MIT Nuclear Science and Engineering

9. Cross-Flow (I) • Multiple junctions – Each volume of the pipe is connected to the adjacent subchannel volume of the same number • Transverse bundle flow resistances1 • Area equal to transverse area of the subchannel • Volume lengths in y and z input for each volume • VERY little mixing, almost no mixing due to tight pitch of the core/uniform velocity profile. 1. I. E. Idelchik, “Handbook of Hydraulic Resistance Second Edition”, pg. 608, Hemisphere Publishing Corporation, New York, USA, 1986. MIT Nuclear Science and Engineering

10. Cross-Flow (II) • Pressure-gradient driven • Modeled in RELAP5 using horizontal junctions • Creates an outlet temperature “profile” MIT Nuclear Science and Engineering

11. Sodium Conduction • Heat transfer due to conduction • Axial Conduction • Radial Conduction • Fourier’s Law • Control variables (CVs) in RELAP5-3D • Requires over 5,000 CV’s limit is 10,000 MIT Nuclear Science and Engineering

12. Axial Conduction (full power) MIT Nuclear Science and Engineering

13. Axial Conduction (4% power) MIT Nuclear Science and Engineering

14. Radial Conduction (full power) MIT Nuclear Science and Engineering

15. Radial Conduction (4% flow) MIT Nuclear Science and Engineering

16. Turbulent Mixing (I) • Mixing due to wire-wrap • Two “regions” of turbulent mixing • Inner channels • Edge and Corner Channels • Mixing modeled by two parameters: • ε* - Effective Eddy Diffusivity • C1L – Swirl Ratio MIT Nuclear Science and Engineering

17. Turbulent Mixing (II) ε* - Effective Eddy Diffusivity • Induced coolant “swirling” caused by wire wrap enhances mixing • Total mass flow in/out of each subchannel face is zero • Enhanced mixing flattens temperature profile in interior MIT Nuclear Science and Engineering

18. Turbulent Mixing (III) C1L – Swirl Ratio • Unidirectional orientation of wire-wrap causes a transverse flow in edge channels • This flow results in a circular flow inside duct wall • C1L is the ratio of transverse to axial flow in the edge channels • Effect of edge swirl velocity is to equalize edge/corner channel temperatures MIT Nuclear Science and Engineering

19. Effect of Mixing MIT Nuclear Science and Engineering

20. Constraints • Limiting feature of model is control variables: • ~6 control variables per volume • 22 volumes per subchannel • 51 subchannels for 9 ring, 1/12 assembly model • ~6732 Control variables out of 10,000 possible MIT Nuclear Science and Engineering

21. SUPERENERGY II • Developed by Chen and Todreas • Only valid for solid pin, hexagonal assemblies • Calculates sodium properties at single T • Can only evaluate 1-8 ring assemblies • Created a SUPERENERGY II model matching an 8 ring RELAP model • Outlet temperature profile comparison MIT Nuclear Science and Engineering

22. Benchmark I • Excellent Agreement ~3°C • Sodium properties are temperature dependent • The difference is due to the sodium properties calculated in the RELAP5 model • RELAP5 more accurate MIT Nuclear Science and Engineering

23. 206 204 220 202 224 Benchmark II: ORNL 19-Pin Test MIT Nuclear Science and Engineering

24. 41 41 41 41 32 32 32 32 18 18 18 18 17 17 17 17 4 4 4 4 1 1 1 1 9 9 9 9 38 38 38 38 ORNL 19-Pin Benchmark MIT Nuclear Science and Engineering

25. Example Application of RELAP as a Subchannel Analysis Code • “Cold” assembly dimensions • Rod-duct gap in edge channels is 1.21 mm • Wire-wrap diameter is 0.805 mm • Edge to interior subchannel flow area ratio ~2.0 • This results in a LARGE temperature distribution (~70°C) • Hot dimensions–still large T distribution (~35°C) • Flattened profile can be achieved by: • Decreasing edge channel area • Increasing flow resistance in edge channels MIT Nuclear Science and Engineering

26. Assembly Duct Ribs MIT Nuclear Science and Engineering

27. RELAP5-3D Advantages • Non-standard fuel geometries (annular, inverse, thermal expansion, etc.) • Various assembly designs (wire-wrap, grid spaced, duct ribs, etc.) • Temperature dependent coolant properties • Detailed fuel rod analysis • Steady State or Transient analyses MIT Nuclear Science and Engineering

28. RELAP5-3D Disadvantages • Huge input file (>23,000 lines of code) • Control Variable limited (~13 rings) • Long runtime (~3 to 26 hours, depending on size) • Can take significant time to modify input file due to size and complexity MIT Nuclear Science and Engineering

29. Results/Conclusions • RELAP5-3D subchannel model capable of modeling wide variety of fuel types, sodium cooled assembly geometries, and conditions • RELAP5-3D model within acceptable performance region dictated by benchmarking of traditional subchannel SFR codes with ORNL19 pin • Scope of RELAP5-3D would be greatly enhanced by the addition of a wire-wraped sodium subchannel mixing option MIT Nuclear Science and Engineering

30. Thank you! Questions? MIT Nuclear Science and Engineering

31. Extra Slides MIT Nuclear Science and Engineering

32. Annular Fuel • Decrease number of rods • Increase rod diameter • Create inner coolant channel, providing internal and external cooling of rod • Decrease clad surface temp • Decrease fuel max temp • Can safely increase power density MIT Nuclear Science and Engineering

33. Bottle Neck Fuel • Maintain constant plenum volume by decreasing plenum radius and increasing height • Decreases pressure drop in assemblies • Ideal plenum radius minimizes pressure drop at low ΔH • Potential for power uprate or decreased pump sizes MIT Nuclear Science and Engineering

34. MIT Nuclear Science and Engineering

35. Original Optimized MIT Nuclear Science and Engineering