1. Department of Nuclear Engineering & Radiation Health Physics Modeling the Oregon State University TRIGA Reactor Using the Attila Three-Dimensional Deterministic Transport Code 2007 TRTR Conference September 17 – 20, 2007 By S. Todd Keller

2. Department of Nuclear Engineering & Radiation Health Physics Outline Purpose The OSU TRIGA Reactor The Attila Code  The Method  Geometry  Cross Section Libraries Phase I: Benchmark studies (Attila vs. MCNP)  The Benchmark Reactor  Results - Φ(r), reactivity Phase II: Depletion Studies  Reactor Operating History  The unit cell  Results - Flux and number density vs. time step duration Phase III: Current Core State (Attila vs. OSTR)  Model/Code limitations  Core ‘snapshot’ calculations  Results - Φ(E), Φ(r), reactivity, power Conclusions and Future Work

3. Department of Nuclear Engineering & Radiation Health Physics

4. Purpose of this Research Purpose: To create a computer model of the OSU TRIGA reactor which is efficient, accurate and easy to use, and to validate the model by comparison with an industry standard code and measured reactor parameters. Why create another computer model? TRIGA reactors have previously been modeled using MCNP. They have also been modeled using Bold Venture, Burnup, CAN, Citation, DIF, DTF-IV, Exterminator II, FEVER M1, ITU, KENO, LEOPARD, MCRAC, ORIGEN, OZGUR, PARET, RELAP, STAR, TORT, TRICOM, TRIGAP, TRIGLAV, Twenty Grand, WIGL, WIMS… No previous modeling techniques met all three criteria. Stochastic models have inherent limitations. TRIGA reactors incorporate unique materials/geometries. Once Attila is validated for the OSTR, it will be a useful tool for future safety analyses. Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

5. Department of Nuclear Engineering & Radiation Health Physics The OSU TRIGA Reactor TRIGA Mark II, 1100 KW t steady state, 3000 MW t pulse, peak thermal flux ~1.5E13. Sample locations: Lazy Susan, ICIT, CLICIT, GRICIT, Thermal Column, Pneumatic Rabbit and Beam Ports. FLIP core loaded in 1976. Approximately 28,000 MW-Hr operation since BOL. Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

6. Department of Nuclear Engineering & Radiation Health Physics Attila An accurate, efficient, three-dimensional transport code operated via GUI.  Geometry input via CAD (Solidworks)  Material property input via XS data file Linear discontinuous finite element method.  Source Iteration  Diffusion Synthetic Acceleration  Preconditioning Solution to a k-eigenvalue criticality problem is k eff and flux moments at every point in the problem. Solution post–processing  Flux, current, number densities, reaction rates Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

7. Department of Nuclear Engineering & Radiation Health Physics Attila – Geometry Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements Accepts many formats As much detail as needed Use surfaces/facets to control mesh Advanced meshing controls included with Attila  Adjust mesh size by region  Azimuthal segmentation  Axial segmentation 36104 cells720 Cells336 Cells

8. Department of Nuclear Engineering & Radiation Health Physics Attila – Cross Sections Accepts many formats Memory and Time ~ (tets) x (groups) Three Principal Library types utilized:  WIMS-ANL based Cross sections  SCALE5 based cross sections Create fine group library Create 3-D model Extract 2-D slice Run 2-D slice with fine group library Obtain desired spectra Collapse fine group library  Transpire depletion library Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

9. Department of Nuclear Engineering & Radiation Health Physics Phase I: Analysis of a Benchmark Reactor Using Attila and MCNP Simplified benchmark model created. Incorporates most materials and structures found in the OSTR. Attila ↔ MCNP differences  Clad structures (fuel, control rod absorbers) homogenized in Attila model, discrete in MCNP model.  Core components ‘faceted’ in Attila model.  SCALE: ENDF/B-V, WIMS & MCNP: ENDF/B-VI Benchmark ↔ OSTR differences Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements Top View Side View

10. Department of Nuclear Engineering & Radiation Health Physics Phase I – Results Benchmark Reactor k-effective Configuration MCNPAttila/WIMSAttila/SCALE All Rods inserted1.0381.0651.045 Rods Withdrawn1.0891.1161.096 Attila/WIMS over-predicts Reactivity by $4.15 Attila/SCALE over-predicts Reactivity by $1.08 Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements ICIT Thermal, Epithermal and Fast Flux DistributionFFCR Total Flux Distribution Deviation of Attila flux from MCNP flux (per component): -9.7% to +2.8% Deviation of Attila flux From MCNP flux (all components): -2.4%

11. Department of Nuclear Engineering & Radiation Health Physics Phase II – Depletion Studies Since 1976, the core has operated almost 1200 MW- days. Eleven major core re-configurations. Three principal operating modes. Regulating control rod always moving. Equilibrium Xenon is never reached. Extremely complex operational history! How best to model such a history?  Can many short operating periods be lumped together?  How long a time step is too long?  How do isotope number densities vary with time? Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

12. Department of Nuclear Engineering & Radiation Health Physics Phase II – The ‘Quad Cell’ Experiment Holder location can be configured as ICIT, CLICIT or another fuel rod Control Rod can be moved vertically All materials homogeneous Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

13. Department of Nuclear Engineering & Radiation Health Physics Phase II – Results 2100 MW-day operating history simulated. 50% normal mode, 40% CLICIT, 10% ICIT. EOL state-point calculated using coarse, medium and fine time steps.  Coarse: Three steps (1050 MW-days Normal, 840 MW- days CLICIT, 210 MW-days ICIT)  Medium: 10 time steps  Fine: 30 time steps At EOL, fluxes and number densities compare well, regardless of step size used. Multiplication factor of unit cell compares well with manufacturer data. Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

14. Department of Nuclear Engineering & Radiation Health Physics Phase II – Results U-235 Depletion in the Quad CellU-238 Depletion in the Quad Cell Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements Fast Flux in the Central Fuel ElementThermal Flux in the ICIT Location

15. Department of Nuclear Engineering & Radiation Health Physics Phase III – Modeling the Current Core State: Depletion vs. Snapshots Limitations preclude using Attila to perform accurate full core depletion calculations.  Library (High temperature / no ZrH)  Component movement  Number of time-steps Simplified depletion calculation possible. How accurate? Alternative approach: core ‘snapshot’. Burnup history of each fuel element is tracked. Quad cell depletion calculation can be used to determine isotopic composition of fuel at any time. The only depletion library available was developed for analysis of power reactors. Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

16. Department of Nuclear Engineering & Radiation Health Physics Phase III – Snapshot Calculations (continued) Fuel grouped into three types Higher burnup fuel typically near core center, but exposure is more uniform than might be expected.  Reflector  Major core shuffle in 1989 Control rod fuel followers have lowest exposure. Three fuel types are radially zoned and then full core calculations are performed with the core in ICIT, CLICIT and NORMAL configuration. Calculated flux and reactivity are then compared with measured parameters. Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

17. Department of Nuclear Engineering & Radiation Health Physics Phase III – Measured parameters Flux spectra measured in all experiment locations in 2005 using STAY’SL/MCNP dosimetry unfolding code (Ashbaker).  MCNP used to predict Φ(E)  Flux foils used to measure Φ(E) at discrete energies and correct the spectrum predicted by MCNP. Thermal flux measured in new facility (GRICIT) Reactivity worth of ICIT, GRICIT and a control rod evaluated. Near critical core state evaluated. Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

18. Department of Nuclear Engineering & Radiation Health Physics Phase III – Results: Φ(E) 5.23, Neutron Spectrum in the ICIT Facility Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

19. Department of Nuclear Engineering & Radiation Health Physics Phase III – Results: Φ(E) FacilityEnergy Group Measured (STAY’SL/MCNP) Predicted (Attila) Percent Deviation ICIT Fast1.00E139.75E12-3 Epithermal2.80E132.63E13-6 Thermal9.00E121.21E13+34 Total4.70E134.82E13+2 CLICIT Fast8.80E128.45E12-4 Epithermal2.20E132.39E13+9 Thermal3.10E111.34E11-57 Total3.11E133.25E13+4 Rabbit Fast1.90E121.76E12-7 Epithermal6.40E126.30E12-2 Thermal9.60E121.04E13+8 Total1.79E131.85E13+3 Lazy Susan Fast4.40E113.53E11-20 Epithermal1.80E121.54E12-14 Thermal3.00E123.65E12+22 Total5.24E125.55E12+6 Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

20. Department of Nuclear Engineering & Radiation Health Physics Phase III – Results: Φ(z) GRICIT Thermal Flux distribution Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

21. Department of Nuclear Engineering & Radiation Health Physics Phase III – Results: reactivity ComponentMeasured WorthPredicted Worth Transient Control Rod$4.08$2.69 GRICIT-$0.10-$0.08 ICIT-$0.38-$0.24 Component predicted and measured reactivity worth Near-Critical core state in the ICIT, CLICIT and Normal cores Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements Core ConfigurationMeasured k eff Predicted k eff (Attila) Normal Core (all rods at 50%) 0.99620.9972 (+$0.15) ICIT Core (all rods at 50%) 0.99650.9958 (-$0.11) CLICIT Core (transient rod = 50% all other rods = 70%) 0.99690.9932 (-$0.57)

22. Department of Nuclear Engineering & Radiation Health Physics Phase III – Results: Φ(r) Radial thermal flux distribution in the CLICIT core at 5 cm above the fuel midplane Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

23. Department of Nuclear Engineering & Radiation Health Physics Conclusions SCALE based cross section libraries are easier to create than WIMS based libraries and give better results. Flux distributions predicted by Attila agree well with fluxes predicted by MCNP. Predicted values of k eff do not agree as well. Even for a thirty year old core, depletion time steps can be taken as large as desired without impacting model accuracy. Just because a code has a GUI doesn’t mean it is easy to use! With proper cross section data and fuel exposure history, flux and reactivity of a thirty year old core can be accurately predicted, even if the core model isn’t perfect. Attila is accurate and efficient. It is also frequently upgraded to improve/expand its capabilities. Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

24. Department of Nuclear Engineering & Radiation Health Physics Further work Improve spatial resolution near control rod tips – some negative fluxes remain in these regions. Benchmark TRIGA library. Develop TRIGA specific depletion library. Incorporating core axial zoning in addition to radial zoning. Develop the capability to model pulse behavior. Purpose The OSTR Attila Phase I Phase II Phase III Conclusions Acknowledgements

25. Department of Nuclear Engineering & Radiation Health Physics Questions?