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
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
•
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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
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Purpose of this Research
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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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.
The OSU TRIGA Reactor
•
Purpose
•
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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•
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.
Attila
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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• 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
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
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36104 cells
720 Cells
336 Cells
Attila – Cross Sections
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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• 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
Phase I: Analysis of a Benchmark
Reactor Using Attila and MCNP
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
• 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
Acknowledgements
Water Universe
Fuel
Reflector
Water
Fuel Followed
Control Rod
Air Followed
Control Rod
T
X
Air
Aluminum
T - Center Thimble
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Spacer
Graphite
X - Experiment Holder
Lead
Fuel Rod
Reflector,
Experiment
or Thimble
Control Rod
Top View
Side View
Water
Phase I – Results
ICIT Thermal, Epithermal and Fast Flux Distribution
1.4E+13
Attila
Phase I
Phase II
Phase III
Flux (#/sec-cm^2)
Purpose
The OSTR
MCNP-Thermal
Attila-Thermal
MCNP-Epithermal
Attila-Epithermal
MCNP-Fast
Attila-Fast
1.2E+13
1E+13
8E+12
6E+12
3.00E+13
2.50E+13
Flux (#/sec-cm^2)
1.6E+13
FFCR Total Flux Distribution
MCNP
1.50E+13
Attila
1.00E+13
4E+12
5.00E+12
2E+12
0
Conclusions
2.00E+13
-30
-20
-10
0
10
20
-30
30
0.00E+00
-10
0
10
20
30
Elevation (cm)
Elevation (cm)
Acknowledgements
-20
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%
Attila/WIMS over-predicts
Benchmark Reactor k-effective
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MCNP
Attila/WIMS
Attila/SCALE
All Rods inserted
1.038
1.065
1.045
Attila/SCALE over-predicts
Rods Withdrawn
1.089
1.116
1.096
Reactivity by $1.08
Configuration
Reactivity by $4.15
Phase II – Depletion Studies
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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• Since 1976, the core has operated almost 1200 MWdays.
• 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?
Phase II – The ‘Quad Cell’
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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•
Experiment Holder location can be configured as ICIT, CLICIT or
another fuel rod
•
Control Rod can be moved vertically
•
All materials homogeneous
Phase II – Results
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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• 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 MWdays 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.
Phase II – Results
40.00
Attila
Phase I
Phase II
30.00
Elevation (cm)
The OSTR
Elevation (cm)
Purpose
40.00
Coarse Step
20.00
Medium Step
Fine Step
10.00
0.00
-25.00
-20.00
Phase III
-15.00
-10.00
-5.00
30.00
Coarse Step
20.00
Fine Step
10.00
0.00
-1.40
0.00
Percent Depletion
-1.20
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
Percent Depletion
U-235 Depletion in the Quad Cell
Conclusions
Medium Step
U-238 Depletion in the Quad Cell
Acknowledgements
80.0
Coarse Step
60.0
80.0
Medium Step
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20.0
0.0
-20.0
-40.0
0.0E+00
Coarse Step
60.0
40.0
Elevation (cm)
Elevation(cm)
Fine Step
Medium Step
40.0
Fine Step
20.0
0.0
-20.0
5.0E+12
1.0E+13
1.5E+13
Flux (#/sec-cm^2)
2.0E+13
Fast Flux in the Central Fuel Element
-40.0
0.0E+00
4.0E+12
8.0E+12
1.2E+13
Flux (#/sec-cm^2)
Thermal Flux in the ICIT Location
Phase III – Modeling the Current Core
State: Depletion vs. Snapshots
Purpose
The OSTR
• Limitations preclude using Attila to perform accurate full core
depletion calculations.
 Library (High temperature / no ZrH)
 Component movement
 Number of time-steps
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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Health Physics
•
•
•
•
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.
Phase III – Snapshot Calculations
(continued)
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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• 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.
Phase III – Measured parameters
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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Health Physics
• 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.
Phase III – Results: Φ(E)
Maxwellian Adjusted MCNP Spectrum
STAY'SL Spectrum w/o Co Bare reaction
Attila Spectrum
STAY'SL Energy Weighted Average Spectrum
1.E+20
Purpose
The OSTR
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
1.E+18
phi(E)/E (neutrons/cm^2-sec-MeV)
Attila
1.E+16
1.E+14
1.E+12
1.E+10
ICIT Facility
1.E+08
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1.E+06
1.E-11
1.E-09
1.E-07
1.E-05
1.E-03
1.E-01
ENERGY (MeV)
5.23, Neutron Spectrum in the ICIT Facility
1.E+01
1.E+03
Phase III – Results: Φ(E)
Energy Group
Measured
(STAY’SL/MCNP)
Predicted
(Attila)
Percent
Deviation
Fast
1.00E13
9.75E12
-3
Epithermal
2.80E13
2.63E13
-6
Phase I
Thermal
9.00E12
1.21E13
+34
Phase II
Total
4.70E13
4.82E13
+2
Phase III
Fast
8.80E12
8.45E12
-4
Epithermal
2.20E13
2.39E13
+9
Thermal
3.10E11
1.34E11
-57
Total
3.11E13
3.25E13
+4
Fast
1.90E12
1.76E12
-7
Epithermal
6.40E12
6.30E12
-2
Thermal
9.60E12
1.04E13
+8
Total
1.79E13
1.85E13
+3
Fast
4.40E11
3.53E11
-20
Epithermal
1.80E12
1.54E12
-14
Thermal
3.00E12
3.65E12
+22
Total
5.24E12
5.55E12
+6
Facility
Purpose
The OSTR
Attila
Conclusions
Acknowledgements
ICIT
CLICIT
Rabbit
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Lazy Susan
Phase III – Results: Φ(z)
1.6E+13
Purpose
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
Thermal Flux (neutrons/cm^2-sec)
The OSTR
1.4E+13
1.2E+13
1.0E+13
8.0E+12
6.0E+12
Measured Thermal Flux
Predicted (Attila) Thermal Flux
4.0E+12
Measured Thermal Flux
adjusted for self shielding
2.0E+12
0.0E+00
0.00
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10.00
20.00
30.00
Elevation (cm)
GRICIT Thermal Flux distribution
40.00
Phase III – Results: reactivity
Component
Measured Worth
Predicted Worth
Purpose
Transient Control Rod
$4.08
$2.69
The OSTR
GRICIT
-$0.10
-$0.08
Attila
ICIT
-$0.38
-$0.24
Phase I
Component predicted and measured reactivity worth
Phase II
Phase III
Conclusions
Acknowledgements
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Core 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)
Near-Critical core state in the ICIT, CLICIT and Normal cores
Phase III – Results: Φ(r)
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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Radial thermal flux distribution in the CLICIT core at 5 cm above the fuel midplane
Conclusions
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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• 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.
Further work
Purpose
The OSTR
Attila
Phase I
Phase II
Phase III
Conclusions
Acknowledgements
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• 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.
Questions?
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