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Department of Nuclear Engineering & Radiation Health Physics
Modeling the Oregon State University TRIGA Reactor Using the Attila Three-Dimensional
Deterministic Transport Code
2007 TRTR ConferenceSeptember 17 – 20, 2007
By S. Todd Keller
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
Department of Nuclear Engineering & Radiation Health Physics
Purpose of this ResearchPurpose: 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
Department of Nuclear Engineering & Radiation Health Physics
The OSU TRIGA Reactor
• TRIGA Mark II, 1100 KWt steady state, 3000 MWt 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
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 keff 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
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 cells 720 Cells 336 Cells
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
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
Fuel
Reflector
Water
T - Center Thimble
X - Experiment Holder
T
X
Fuel FollowedControl RodAir FollowedControl Rod
Water Universe
Lead
Air
Water
Aluminum Spacer
Graphite
Fuel Rod Control RodReflector,Experimentor Thimble
Top ViewSide View
Department of Nuclear Engineering & Radiation Health Physics
Phase I – Results
Benchmark Reactor k-effective
ConfigurationMCNP Attila/WIMS Attila/SCALE
All Rods inserted 1.038 1.065 1.045
Rods Withdrawn 1.089 1.116 1.096
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
0
2E+12
4E+12
6E+12
8E+12
1E+13
1.2E+13
1.4E+13
1.6E+13
-30 -20 -10 0 10 20 30
Elevation (cm)
Flu
x (#
/sec
-cm
^2)
MCNP-Thermal
Attila-Thermal
MCNP-Epithermal
Attila-Epithermal
MCNP-Fast
Attila-Fast
ICIT Thermal, Epithermal and Fast Flux Distribution
0.00E+00
5.00E+12
1.00E+13
1.50E+13
2.00E+13
2.50E+13
3.00E+13
-30 -20 -10 0 10 20 30
Elevation (cm)
Flux
(#/s
ec-c
m^2
)
MCNP
Attila
FFCR 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%
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
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
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
Department of Nuclear Engineering & Radiation Health Physics
Phase II – Results
0.00
10.00
20.00
30.00
40.00
-25.00 -20.00 -15.00 -10.00 -5.00 0.00
Percent Depletion
Ele
vati
on
(cm
)
Coarse Step
Medium Step
Fine Step
0.00
10.00
20.00
30.00
40.00
-1.40 -1.20 -1.00 -0.80 -0.60 -0.40 -0.20 0.00
Percent Depletion
Ele
vati
on
(cm
)
Coarse Step
Medium Step
Fine Step
-40.0
-20.0
0.0
20.0
40.0
60.0
80.0
0.0E+00 5.0E+12 1.0E+13 1.5E+13 2.0E+13Flux (#/sec-cm^2)
Ele
vati
on
(cm
)
Coarse Step
Medium Step
Fine Step
U-235 Depletion in the Quad Cell
U-238 Depletion in the Quad Cell
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
-40.0
-20.0
0.0
20.0
40.0
60.0
80.0
0.0E+00 4.0E+12 8.0E+12 1.2E+13
Flux (#/sec-cm^2)
Ele
vati
on
(cm
)
Coarse Step
Medium Step
Fine Step
Fast Flux in the Central Fuel Element
Thermal Flux in the ICIT Location
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
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
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
Department of Nuclear Engineering & Radiation Health Physics
Phase III – Results: Φ(E)
1.E+06
1.E+08
1.E+10
1.E+12
1.E+14
1.E+16
1.E+18
1.E+20
1.E-11 1.E-09 1.E-07 1.E-05 1.E-03 1.E-01 1.E+01 1.E+03
ENERGY (MeV)
phi(E
)/E (n
eutro
ns/c
m^2
-sec
-MeV
)
Maxwellian Adjusted MCNP SpectrumSTAY'SL Spectrum w/o Co Bare reactionAttila SpectrumSTAY'SL Energy Weighted Average Spectrum
ICIT Facility
5.23, Neutron Spectrum in the ICIT Facility
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
Department of Nuclear Engineering & Radiation Health Physics
Phase III – Results: Φ(E)
Facility Energy GroupMeasured
(STAY’SL/MCNP)Predicted(Attila)
PercentDeviation
ICIT
Fast 1.00E13 9.75E12 -3
Epithermal 2.80E13 2.63E13 -6
Thermal 9.00E12 1.21E13 +34
Total 4.70E13 4.82E13 +2
CLICIT
Fast 8.80E12 8.45E12 -4
Epithermal 2.20E13 2.39E13 +9
Thermal 3.10E11 1.34E11 -57
Total 3.11E13 3.25E13 +4
Rabbit
Fast 1.90E12 1.76E12 -7
Epithermal 6.40E12 6.30E12 -2
Thermal 9.60E12 1.04E13 +8
Total 1.79E13 1.85E13 +3
Lazy Susan
Fast 4.40E11 3.53E11 -20
Epithermal 1.80E12 1.54E12 -14
Thermal 3.00E12 3.65E12 +22
Total 5.24E12 5.55E12 +6
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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
0.0E+00
2.0E+12
4.0E+12
6.0E+12
8.0E+12
1.0E+13
1.2E+13
1.4E+13
1.6E+13
0.00 10.00 20.00 30.00 40.00
Elevation (cm)
Th
erm
al F
lux
(neu
tro
ns/
cm^
2-se
c)
Measured Thermal Flux
Predicted (Attila) Thermal Flux
Measured Thermal Fluxadjusted for self shielding
Department of Nuclear Engineering & Radiation Health Physics
Phase III – Results: reactivity
Component Measured Worth Predicted 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
AcknowledgementsCore Configuration Measured keff Predicted keff (Attila)
Normal Core(all rods at 50%)
0.9962 0.9972 (+$0.15)
ICIT Core(all rods at 50%)
0.9965 0.9958 (-$0.11)
CLICIT Core(transient rod = 50%
all other rods = 70%) 0.9969 0.9932 (-$0.57)
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
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 keff 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
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