Burnup Dependent Monte Carlo Neutron Physics Calculations of IAEA MTR Benchmark

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Burnup Dependent Monte Carlo Neutron Physics Calculations of IAEA MTR Benchmark

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    Burnup dependent Monte Carlo neutron physics calculations of IAEA

    MTR benchmark

    Khurrum Saleem Chaudri a , *, Sikander M. Mirza b

    a Department of Nuclear Engineering, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Nilore, Islamabad 45650, Pakistanb Department of Physics & Applied Mathematics, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Nilore, Islamabad 45650, Pakistan

    a r t i c l e i n f o

    Article history:

    Received 2 November 2014

    Received in revised form

    23 December 2014

    Accepted 30 December 2014

    Available online 22 January 2015

    Keywords:

    OpenMC code

    Monte Carlo methods

    MTR benchmark

    Depletion studies

    a b s t r a c t

    In this work, OpenMC code has been used for reactor physics calculations of the IAEA 10 MW MTR

    benchmark. To perform the burnup dependent studies i.e. Beginning of Life (BOL) and End of Life (EOL)

    calculations are carried out using OpenMC. The isotopic number densities have been generated using

    WIMS code and comparison of global core parameters has been performed. Along with the prevalent 9-

    isotope vector methodology, a set of 16-isotopes, based on their relative importance, have been

    employed in the whole core calculations and the corresponding computed values of integral parameters

    been compared. Comparison of effective core multiplication factor is made with studies performed by

    various organizations, employing different codes based on diffusion theory and Monte Carlo method-

    ologies. The OpenMC predicted values of the multiplication factor and cell averaged thermal uxes in

    centralux trap for HEU and LEU cores are found in good agreement with the corresponding published

    data.

    2015 Elsevier Ltd. All rights reserved.

    1. Introduction

    Recent times have seen emergence of open source nuclear

    reactor physics computational tools as a norm. Various codes

    including DRAGON (Marleau et al., 2006), OpenMC (Romano and

    Forget, 2013), OpenMOC (Boyd et al., 2014), MocDown (Seifried

    et al., 2014), PyNE (Scopatz et al., 2012) are examples of codes be-

    ing developed as open source. Active participation of nuclear

    community in such projects helps in verication &validation and

    debugging of such codes, and helps these codes to rise to the level

    of well-established codes.

    Availability of advanced computational resources & tools has

    rekindled interest in high delity simulations for design and safetyof nuclear reactors. Use of Monte Carlo based codes, Computational

    Fluid Dynamics (CFD) approach and multi physics calculations are

    few examples of such high delity simulation efforts in reactor

    physics. International Atomic Energy Agency (IAEA) Coordinated

    Research Project CRP-1496 is indicative of the combined efforts of

    nuclear industry think-tank towards enhancement of the state-of-

    art analysis using computational tools and their validation against

    experimental data. The IAEA benchmark Material Test Reactor

    (MTR) (Haack, 1992; Winkler and Zeis, 1980) was set for validating

    and verication of tools and methodology needed to perform

    necessary calculations to convert High Enriched Uranium (HEU)

    reactors to Low Enriched Uranium (LEU) reactors, using computa-

    tional tools including MTR-PC (MTR-PC, 2001). With the availability

    of advanced computational resources and tools, such benchmarks

    are being revisited by the nuclear community to re-assess their

    results (Bousbia-Salah et al., 2008).

    Monte Carlo simulations offer a powerful method of simulating

    complex physical phenomenon including neutron transport in

    multiplying media such as nuclear reactors. With detailed three

    dimensional modeling of actual geometry, continuous energy crosssection representation is possible in various Monte Carlo based

    codes including MCNP (MCNPX, 2003), Serpent (Leppanen, 2012),

    OpenMC (Romano and Forget, 2013) etc. Typically large computa-

    tional times required by various Monte Carlo based codes are

    surpassed by their superior features including their ability to

    perform calculations for complex heterogeneous geometries. These

    codes have been used mostly for benchmarking the results ob-

    tained from other deterministic codes based on diffusion and

    transport theory. According to the Smith challenge (Smith, 2003),

    in 2030 we would be able to calculate pin by pin power prole of a

    full scale PWR with required accuracy (1% uncertainty) using a* Corresponding author. Tel.: 92 51 111174327; fax: 92 51 9248600.

    E-mail address:[email protected](K.S. Chaudri).

    Contents lists available atScienceDirect

    Progress in Nuclear Energy

    j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / p n u c e n e

    http://dx.doi.org/10.1016/j.pnucene.2014.12.018

    0149-1970/

    2015 Elsevier Ltd. All rights reserved.

    Progress in Nuclear Energy 81 (2015) 43e52

    mailto:[email protected]://www.sciencedirect.com/science/journal/01491970http://www.elsevier.com/locate/pnucenehttp://dx.doi.org/10.1016/j.pnucene.2014.12.018http://dx.doi.org/10.1016/j.pnucene.2014.12.018http://dx.doi.org/10.1016/j.pnucene.2014.12.018http://dx.doi.org/10.1016/j.pnucene.2014.12.018http://dx.doi.org/10.1016/j.pnucene.2014.12.018http://dx.doi.org/10.1016/j.pnucene.2014.12.018http://www.elsevier.com/locate/pnucenehttp://www.sciencedirect.com/science/journal/01491970http://crossmark.crossref.org/dialog/?doi=10.1016/j.pnucene.2014.12.018&domain=pdfmailto:[email protected]
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    desktop PC in one hour. Introduction of multi-core processors has

    now revised this timeline to 2019 (Martin, 2007). Recent Monte

    Carlo codes including MCNPX (Fensin et al., 2010) and Serpent

    (Leppanen and Pusa, 2009) are equipped with capability to

    perform burnup analysis. Clearly, this approach entails a corre-

    sponding huge computational effort.

    One method that can give accurate enough result for problems

    involving burnup dependent number densities when using Monte

    Carlo based codes is employing lattice codes like WIMS (Halsall

    and Tauman, 1997) in conjunction with Monte Carlo codes. Such

    lattice codes can generate burnup dependent number density

    values for an equivalent unit cell representative of full geometry.

    These burnup dependent number densities can then be used in

    neutron physics codes performing full three-dimensional analysis

    of core. This approach has been used and veried in several works.

    One such example of generating burnup dependent number den-

    sity in WIMS and then using them in detailed three dimensional

    modeling with MCNP was veried against experimental data of

    TRIGA reactor (Jeraj et al., 2002). This methodology used U-235, U-

    236, U-238, Pu-239, Pu-240, Pu-241, Pu-242, Tc-99, Rh-103, Xe-

    131, Cs-133, Xe-135, Nd-143, Pm-147, Sm-149 and Sm-151 (16

    isotopes in total). Similar methodology, with varying number of

    isotopes included in burned fuel, was used to perform MTRbenchmark static calculations with MCNP5 code (Bousbia-Salah

    et al., 2008). This methodology used only U-235, U-236, U-238,

    Pu-239, Pu-240, Pu-241, Pu-242, Xe-135 and Sm-149 (9 isotopes in

    total) on basis of number densities provided in original IAEA

    benchmark specication (Winkler and Zeis, 1980) for Monte Carlo

    methodology based codes.

    In present work both above stated methodologies, to obtain

    burnup dependent number densities, have been employed and the

    corresponding values of computed integral parameters have

    compared for the IAEA 10 MW benchmark reactor (Winkler and

    Zeis, 1980). The difference in values of reactivities obtained by us-

    ing different number of isotopes used is quantied. The OpenMC

    code has been used to perform three dimensional neutron physics

    analysis while WIMSD-4 is used for generation of number densities

    at various stages of fuel burnup. Various reactor physics parameters

    including effective multiplication factor and spatial ux proles in

    different energy groups have been calculated and compared.

    2. Computational framework

    IAEA 10 MW benchmark reactor, specication provided in Ap-

    pendix-F of IAEA-TECDOC-233 (Winkler and Zeis, 1980) has been

    used to perform the analysis presented in this study. Standard

    values of the reactor design parameters have been used as quoted

    in literature (Bousbia-Salah et al., 2008). Analyses are performedfor

    HEU core i.e. 93% enriched fuel and LEU core i.e. 20% enriched fuel.

    Three type of conguration with both HEU and LEU have beenstudied: (i) Fresh, (ii) Beginning of Life (BOL) and (iii) End of Life

    (EOL).

    In this work, tools used for the simulation include OpenMC

    (Romano and Forget, 2013) and WIMSD-4 (Halsall and Tauman,

    Fig. 1. Radial cross sectional view of the modeled core at core mid plane indicated in Fig. 2.

    K.S. Chaudri, S.M. Mirza / Progress in Nuclear Energy 81 (2015) 43e5244

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    Fig. 2. Axial cross sectional view of the benchmark core with AA marks indicating core mid plane.

    Fig. 3. Standard Fuel Element (SFE) cross sectional view as modeled in OpenMC.

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    1997). The OpenMC is modern structured code which can perform

    Monte Carlo based calculations for nuclear reactors. OpenMC is

    being developed as an open source code under the banner of

    Computational Reactor Physics Group (CRPG) of Massachusetts

    Institute of Technology (MIT). This code is written in Fortran 2008.

    Like MCNP, OpenMC uses ACE format cross sections. Constructive

    Solid Geometry (CSG) is used to model geometry in this code. For

    the time being, rst and second order surfaces can be modeled in

    OpenMC. Input of the code is provided in XML format. Code version

    0.5.4 was used to perform the analysis presented in this work. 3-

    dimensional geometry was modeled in OpenMC to perform the

    presented calculations. ACE formatted ENDF/B-VII.1 cross sections,

    prepared for 293.6 K, obtained from the National Nuclear Data

    Center (NNDC) Brookhaven National Laboratory (BNL), were used

    in the simulations presented in this work. OpenMC team has a plan

    of implementing burnup capability in the code but for the time

    being OpenMC cannot perform burnup calculations.

    WIMSD-4 (Halsall and Tauman, 1997) is a well tried and tested

    code for group constant generation and fuel burnup studies.

    WIMSD performs one dimensional transport theory based calcu-

    lations for the modeled geometry and then provides homogenized

    or region-wise cross section as requested by the user. One dimen-

    sional unit cell model based on Standard Fuel Element (SFE), sameas used in IAEA-TECDOC-233 Appendix-F, was used in the WIMSD

    code to obtain burnup dependent number densities.

    3. Computational methodology

    The IAEA benchmark 10 MW MTR core was modeled with exact

    specication as provided in the IAEA documentation (Winkler and

    Zeis, 1980). Radial and axial cross sectional view of modeled core

    are shown in Figs. 1 and 2 respectively. Figs. 3 and 4 show the

    exploded view of Standard Fuel Element (SFE) and Control Fuel

    Element (CFE). Various colors inFigs. 1e4represent different ma-

    terials including fuel, aluminum, water etc.

    The OpenMC based Monte Carlo simulation was carried out for

    20,000 particles per cycle and total of 850 cycles were employed.

    First 50 cycles from these 850 cycles were discarded and then score

    accumulation was started. Keeping number of histories (particles

    per cycle multiplied by no. of cycles) xed, better source conver-

    gence is achieved if we use relatively large number of particles per

    cycle (Brown, 2006; Macdonald, 2012). A ssion source uniformly

    distributed in reactor was used as input. To assess the convergence

    of source, Fig. 5 shows the results obtained for Shannon entropy

    and effective multiplication factor against cycle number. Fig. 5

    provides quantitative measure that results being presented in this

    study are accurate.

    To accumulate scores including uxes and ssion, OpenMC

    provides a very handy feature of dening a mesh and then accu-

    mulate the scores over that mesh. Various radial and axial meshes

    were dened to get ux proles in respective directions. To get the

    ux at central axial plane of core in xy-direction, a 100 100 mesh

    has been dened. Flux inx-direction is obtained for 100 points forcentral radial plane. Axialux (z-direction) is obtained for30 points

    at the central plane ofux trap. For ssion scores, a 6 5 mesh is

    dened with one mesh covering one full fuel assembly.

    Burnup dependent number densities, for use in the OpenMC

    calculations, were generated by the WIMS code. Unit cell based on

    SFE, used in IAEA-TECDOC-233 (Winkler and Zeis, 1980) and else-

    where (Bousbia-Salah et al., 2008) was modeled in WIMS. This

    Fig. 4. Control Fuel Element (CFE) cross sectional view simulated in OpenMC.

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    methodology of using unit cell to generate burnup dependent

    number densities has its roots in the methodology which is being

    used in lattice cell calculation codes (usually transport theory

    based) coupled with 3D whole core codes, based on diffusion or

    transport theory. Such combinations of codes extensively used in

    nuclear community are WIMS/CITATION, DRAGON/DONJON,

    CASMO/SIMULATE etc. Lattice cell codes calculate the group aver-

    aged cross-sections for few-group to be used in whole core calcu-

    lations. Group constants for each different fuel assembly (different

    enrichment, different control rodor burnable poison conguration)

    are calculated separately based on equivalent unit cell made for

    each fuel assembly. Calculations reported in Appendix F of IAEA-TECDOC-233 by different organizations have used this methodol-

    ogy, i.e. lattice cell cross section generation in conjunction with 3D

    calculations, extensively. Main approximations used in generating

    burnup dependent number densities using WIMS are that unit cell

    was based on Standard Fuel Element (SFE) and equal power dis-

    tribution forall fuel assemblies, and hence fuel plates, is assumed to

    provide input for POWERC card in WIMS code.

    At this point, two different approaches were encountered in

    reported literature. For Monte Carlo based code calculations, only 9

    isotopes (U-235, U-236, U-238, Pu-239, Pu-240, Pu-241, Pu-242,

    Xe-135 and Sm-49) are used in IAEA-TECDOC-233 (Winkler and

    Zeis, 1980). Lately, Jeraj (Jeraj et al., 2002) has used WIMSD-5 in

    combination with MCNP4B and validated his result against

    experimental values to determine a set of 16 isotopes to getcomputational result close to experimental values. These 16 iso-

    topes include above mentioned 9 isotopes plus Tc-99, Rh-103, Xe-

    131, Cs-133, Nd-143, Pm-147 and Sm-151. In this study, both ap-

    proaches (9 and 16 isotope burnup vector) are used so as toquantify the difference between the two mentioned approaches.

    4. Results

    Monte Carlo based static reactor physics calculations have been

    performed for the IAEA benchmark MTR core. Table 1 contains

    results of multiplication factor comparison calculated by using 9

    and 16 isotopes burnup vector with the help of OpenMC code. For

    comparison, the range of corresponding values reported in IAEA-

    TECDOC-233, Appendix-F (Winkler and Zeis, 1980) are also

    included. It is evident fromTable 1that results obtained by using

    16-isotopes burned number densities from WIMS code are giving

    better results as compared to 9-isotopes approach. Figs. 6 and 7

    provide the comparison ofssion product inventory obtained forHEU and LEU cores. Both these gures clearly show that the

    number densities of 7 isotopes included in addition to 9-isotope

    approach are quite signicant. All the isotopes are present in

    greater quantity as compared to Xe-135 in both HEU and LEU cores.

    The rest of results presented in this work are calculated by using 16

    isotopes burnup vector in OpenMC code. Slightly different number

    densities of Xe-135 in HEU and LEU cores are obtained. This is due

    to the fact that abscissae ofFigs. 6 and 7are showing percentage of

    U-235 burnt which is achieved at different times (days of opera-

    tion) for both HEU and LEU core so a direct comparison cannot be

    made.

    A study was conducted to see the difference in multiplication

    factor while using different set of cross section libraries. ENDF/B-

    VII.1 and JEFF-3.2 libraries (293 K) were used in this study, basedon their availability.Table 2summarizes the results obtained. It can

    be seen that the difference in multiplication factor by using

    Fig. 5. Convergence assessment by observing Shannon entropy and effective multi-plication factor against number of cycles.

    Table 1

    Comparison of multiplication factor results using 9-isotope and 16-isotope methodologies.

    Core enrichment, stage keff Difference (pcm), (9e16 isotopes)

    This work Reported (Winkler and Zeis, 1980)

    9-Isotopes 16-Isotopes

    93%, BOL 1.06094 0.00024 1.04190 0.00023 1.02333e1.04199 1904 47

    93%, EOL 1.04305 0.00024 1.02006 0.00024 1.0090e1.05337 2299 48

    20%, BOL 1.05916 0.00025 1.03750 0.00023 1.02127e1.05782 2166 48

    20%, EOL 1.04452 0.00024 1.01925 0.00023 1.0120e1.05468 2527 47

    Fig. 6. Fission fragment inventory for the case of HEU core.

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    different libraries is quite small, with maximum difference of

    255 pcm. The results provided hereafter are calculated using ENDF/B-VII.1 library (293 K).

    Table 3 compares the results of multiplication factorfor HEUand

    LEU cores for fresh, BOL and EOL congurations with reported re-

    sults. Name of the organization along with the code used to pro-

    duce the reported value are provided in the table. It is evident that

    results match well and lie within the spectrum of reported results.

    The differences are due to different computational tools (using

    different methodology) and different cross sections libraries used

    to perform these simulations.

    Table 4provides the comparison of thermal neutron ux aver-

    aged over the whole water box i.e. 8.1 7.7 60 cm3. The com-

    parison of calculated values in this work matches very well with

    other organization values. Modeling of full details of heterogeneous

    geometry in OpenMC code is providing slightly higher ux as

    compared to diffusion theory based codes where homogenous

    geometry is modeled.

    Figs. 8e10 show thermal, epithermal and fast ux spatial pro-

    les obtained for 93% fresh core. Consistent with the expected

    behavior, thermal ux has a large peak in ux trap present at the

    center of the core while fast ux shows a minimum in this region.

    Sharpness of thermal ux peak in ux trap matches well with the

    corresponding reported results presented in IAEA-TECDOC-233,Appendix-F.

    Figs. 11e13show thermal, epithermal and fast ux radial prole

    obtained for all 6 core compositions i.e. 93% and 20% Fresh, BOL,

    EOL. Figs.14e16give the corresponding axial ux plots for thermal,

    epithermal and fast ux respectively for the 6 studied core com-

    positions. All 6 cases for thermal, epithermal and fast ux are

    plotted on the same graph in order to provide a comparison ofux

    magnitude obtained when altering core composition. The trend

    obtained fromFigs. 11e16reveal the fact that 93% BOL and EOL

    cores display maximum ux values among the six studied core

    compositions. These values occur in sets of two i.e. 93% BOL& EOL,

    20% BOL & EOL and nally 93% & 20% Fresh cores (in order of

    maximum to minimum values ofux). The kinks in axial thermal

    ux prole near the top and bottom of fuel are due to thermali-

    zation of neutrons from axial reector.

    Figs. 17 and 18 provide the comparison of power fractions (%)

    produced in each assembly for HEU and LEU cores respectively.

    Anticipated pattern of larger power fraction being produced near

    Fig. 7. Inventory ofssion fragment present in the case of LEU core.

    Table 2

    Comparison of multiplication factor results using different cross section libraries with OpenMC code.

    Cross section library HEU (93%) LEU (20%)

    Fresh BOL EOL Fresh BOL EOL

    ENDF/B-VII.1 1.19382 0.00025 1.04190 0.00023 1.02006 0.00024 1.15494 0.00025 1.03750 0.00023 1.01925 0.00023

    JEFF-3.2 1.19229 0.00025 1.03935 0.00024 1.01823 0.00024 1.15296 0.00026 1.03539 0.00026 1.01709 0.00025

    Absolute difference (pcm) 153 50 255 47 183 48 198 51 211 49 216 48

    Table 3

    Computed Eigen-values for the IAEA 10 MW MTR benchmark core.

    Organization (code) HEU (93%) LEU (20%)

    Fresh BOL EOL Fresh BOL EOL

    T hi s wor k ( OpenMC) 1 .19 382 0.00025 1.04190 0.00023 1.02006 0.00024 1.15494 0.00025 1.03750 0.00023 1.01925 0.00023

    Bousbia et al., (MCNP5) 1.18962 0.00034 1.05768 0.00032 1.03959 0.00031 1.17238 0.00033 1.05617 0.00032 1.04111 0.00032

    ANL (VIM) 1.189 0.0033 e* 1.045 0.0036 1.168 0.0033 e* 1.048 0.0034

    ANL (DIF2D) 1.18343 1.02333 1.03366 1.16830 1.02127 1.03934

    EIR (CODIFF) 1.19394 e* 1.07230 1.15937 e* 1.04981

    OESGAE (EXTERMINATOR) 1.1966 1.0320 1.0090 1.1813 1.0320 1.0120

    CEA (NEPTUNE) 1.202 1.04041 1.05337 1.187 1.0394 1.05468

    JAERI (ADC) 1.18104 1.04199 1.02195 1.18339 1.05782 1.04122

    CNEA (EXTERMINATOR) 1.20018 1.03620 1.01425 1.18150 1.03334 1.01300

    e

    *: Value not calculated.

    Table 4

    Computed average water box thermalux 1014 (n/cm2-s)for the IAEA 10MW MTR

    benchmark core.

    Organization (code) HEU (93%) LEU (20%)

    Fresh BOL EOL Fresh BOL EOL

    This work (OpenMC) 1.87004 2.17595 2.23519 1.82359 1.93065 1.97141

    Bousbia et al.,

    (MCNP5)

    e* e* e* e* e* e*

    ANL (VIM) e* e* e* e* e* e*

    ANL (DIF2D) e* 2.1345 2.1999 e* 1.9017 1.9498

    EIR (CODIFF) e* 2.220 2.285 e* 2.025 2.068

    OESGAE

    (EXTERMINATOR)

    e* e* e* e* e* e*

    CEA (NEPTUNE) e* e* e* e* e* e*

    JAERI (ADC) e* e* e* e* e* e*

    CNEA

    (EXTERMINATOR)

    e* 1.9831 2.0507 e* 1.7220 1.7691

    e*: Value not calculated.

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    Fig. 8. Thermal ux prole (a) contour and (b) 3D surface plot for HEU fresh core at core mid-plane.

    Fig. 9. Epithermal ux prole (a) contour and (b) 3D surface plot for HEU fresh core at core mid-plane.

    Fig. 10. Fast

    ux pro

    le (a) contour and (b) 3D surface plot for HEU fresh core at core mid-plane.

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    the center is followed. This pattern is predominant in fresh core. At

    BOL, when more burnt fuel assemblies are near the middle and lessburnt assemblies are present near the periphery, much atter

    prole is obtained as compared to fresh core.

    Fig. 11. Radial thermal ux at core mid-plane for studied core compositions.

    Fig. 12. Radial epithermal ux proles for studied cases at core mid-plane.

    Fig. 13. Studied core composition radial fast

    ux pro

    le at core mid-plane.

    Fig. 14. Axial thermal ux for the studied core compositions at core mid-plane.

    Fig. 15. Axial epithermal ux at core mid-plane for the studied core composition.

    Fig. 16. Axial fast

    ux for the studied core compositions at core mid-plane.

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    5. Conclusion

    Burnup dependent steady state neutron physics calculations are

    performed for the IAEA 10 MW MTR benchmark using Monte Carlo

    based code OpenMC. To perform the burnup dependent neutron

    physics calculations, burnt fuel number densities are generated

    using WIMS code. Two different approaches (9 isotopes vs. 16

    isotopes) adopted by different researchers were compared and 16

    isotope approach was found better. The latter approach was then

    used to perform analysis presented in this work. Multiplication

    factor, ux (thermal, epithermal and fast) and power fraction pro-

    duced in different fuel assemblies were calculated. The results were

    found to be in very good agreement with those presented in liter-

    ature. It can be concluded that 16 isotopes approach can be used to

    Fig. 17. Power fractions (%) distributions in each fuel assembly for different HEU core congurations.

    Fig. 18. Power fractions (%) distributions in each fuel assembly for different LEU core con

    gurations.

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    perform burnup dependent neutron physics calculations using

    Monte Carlo based codes. With the availability of computational

    resources and tools, such exercises can help validate both the

    methodology adopted to perform the calculations and the rela-

    tively newer tools used to perform these calculations.

    Acknowledgements

    Authors would sincerely like to thank Anis Bousbia-Salah for his

    cooperation and helpful discussions during this work. Authors

    would also like to thank the whole team of OpenMC in general and

    Paul K. Romano in particular for their discussions, support and help

    in using the code OpenMC.

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