Department of Nuclear Engineering & Radiation Health Physics Modeling the Oregon State University...

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


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



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


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


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


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


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


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


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%


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


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


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


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


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


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


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


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


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


Thermal 9.60E12 1.04E13 +8

Total 1.79E13 1.85E13 +3

Lazy Susan

Fast 4.40E11 3.53E11 -20


Thermal 3.00E12 3.65E12 +22

Total 5.24E12 5.55E12 +6

Purpose

The OSTR

Attila

Phase I

Phase II

Phase III

Conclusions

Acknowledgements


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


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)


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


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


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


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