Annals of Nuclear Energy 55 (2013) 225–229

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Annals of Nuclear Energy

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Technical Note

Optimum burnup of BAEC TRIGA research reactor

0306-4549/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.anucene.2012.12.019

⇑ Corresponding author. Tel.: +880 27790683.E-mail address: sayemabs@yahoo.com (M.S. Mahmood).

Zoairia Idris Lyric, Mohammad Sayem Mahmood ⇑, Mohammad Abdul Motalab, Jahirul Haque KhanInstitute of Nuclear Science & Technology, Atomic Energy Research Establishment, Ganakbari, Savar, Dhaka 1349, Bangladesh

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 September 2012Received in revised form 17 December 2012Accepted 18 December 2012Available online 19 January 2013

Keywords:MVPMVP-BURNTRIGALEU fuelOptimum burnup

The TRIGA Mark II research reactor of BAEC (Bangladesh Atomic Energy Commission) has been operatingsince 1986 without any reshuffling or reloading yet. Optimum fuel burnup strategy has been investigatedfor the present BAEC TRIGA core, where three out-to-in loading schemes have been inspected in terms ofcore life extension, burnup economy and safety. In considering different schemes of fuel loading, optimi-zation has been searched by only varying the number of fuels discharged and loaded. A cost function hasbeen defined and evaluated based on the calculated core life and fuel load and discharge. The optimumloading scheme has been identified for the TRIGA core, the outside-to-inside fuel loading with ten fuelsfor each cycle starting with five fuels for the first reload. The discharge burnup has been found rangingfrom 17% to 24% of U235 per fuel element and optimum extension of core operating life is 100 MWDfor each loading cycle. This study will contribute to the in-core fuel management of TRIGA reactor.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The TRIGA Mark II research reactor of BAEC (Bangladesh AtomicEnergy Commission) has been operating since 1986 and no reshuf-fling or reloading has been done yet. The reactor is a light watercooled, graphite-reflected one, designed for continuous operationat a steady state power level of 3 MW (thermal). The fuels are ofsingle type i.e. only LEU fuel (20 wt.% and 19.7% enriched uraniumand 0.47 wt.% erbium). It was commissioned to implement variousfields of basic nuclear research like neutron activation analysis,neutron radiography, and neutron scattering experiment, also formanpower training and production of radio-isotopes for its usesin agriculture, industry and medicine. This work is the outcomeof the current endeavor to form a plan for core life extension andin-core fuel management of the TRIGA MARK II (Bangladesh) reac-tor. Since its commissioning in 1986, the reactor has been burnt forsome 700 MWD until early 2012. Our research result shows that,after 1200 MWD of burning, the core excess reactivity will fall be-low the necessary reactivity to keep the reactor running. Hence, itcan be said, this TRIGA reactor is now passing post mid-life of itsfirst burnup cycle. Careful planning is necessary for its future lifecycles. Previous study regarding in-core fuel management investi-gated only the fuel element reshuffling at different operation inter-vals to obtain the highest core life span (Huda et al., 2008); butfrom the work of (Ravnik, 1987) it was revealed that such regularreshuffling will not ensure maximum utilization of the fuels insuch small core. Ravnik showed that the fuel is best utilized when

the burn-up interval is as long as possible. This verifies the exacti-tude of our present strategy of running a cycle until the reactivitygoes down to the transition point where reactor stops operation. Inthis article optimum fuel burnup strategy has been investigated. Inthe effort of finding the optimum fuel loading scheme for the coreat hand, several loading strategies have been modeled andinspected. Those arrangements were assayed by balancing theirmerits and demerits from the perspective of core life, burnupeconomy and safety.

We have studied three fuel reloading schemes in successive cy-cles after first cycle operation of the reactor and compared the corelife extension along with design envisaged maximum fuel dis-charge burnup as well as the radial power peaking factor. In suchsmall core research reactor the power distribution cannot be flat-tened (Ravnik and Mele, 1988) and hence we just cared aboutkeeping the power peaking factor within the safety limit. Also inconsidering different schemes of fuel loading, optimization hasbeen searched only varying number of fuels discharged and loaded.This study will contribute to decide economic fuel loading strategyselection for TRIGA research reactor.

2. Calculation tools and techniques

The calculations here are performed by the coupled Monte Carlo(MC) burnup code MVP-BURN (Okumura et al., 2000) using thecross-section library based on JENDL 3.3 (Shibata et al., 2002).The sole feature of continuous energy MC method is that it doesnot seek reliance on space and energy approximation. Apart fromits continuous energy treatment, it can make an exact replica ofany kind of three dimensional complex systems and thus can

Irradiation channel Control Rod dorlortnoctneisnarTtnemeleleuF

Graphite dummy Pneumatic transfer tube noitacolecruoS

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G26G27G28G29

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Fig. 2. BAEC TRIGA reactor core configuration.

226 Z.I. Lyric et al. / Annals of Nuclear Energy 55 (2013) 225–229

better simulate physical phenomena. The Monte Carlo code MVP(Mori et al., 1992) and an associated code BURN, joined together,execute an entire burnup calculations by taking turns for differentburnup steps. MVP computes the microscopic reaction rates, andthen these reaction rates serve as the input for the depletion calcu-lation performed by the BURN module. The depletion calculationmakes a normalization of the reaction rates using the total thermalpower of the system. As mentioned in the prelude of this docu-ment, the first burnup cycle will end after 1200 MWD burnupwhen the excess reactivity is insufficient to make up for theCold-to-Hot swing. Core excess reactivity loss curve with theburnup (Fig. 1) shows that, after 1200 MWD of burning, the coreexcess reactivity will fall below $4.5. As prescribed by the GeneralAtomics (GA) safety report (General Atomics, 1981), after the equi-librium xenon condition is reached, $3.43–$5.14 is necessary tokeep the reactor running overriding the negative reactivity changedue to the rise of temperature.

The present core of the reactor consists of a total of 100 fuelelements (including five fueled follower control rods), six con-trol rods, 18 graphite dummy elements, one Central Thimbleand one pneumatic transfer system irradiation terminus. Allthese elements are placed and supported in-between the topand bottom grid plates and arranged in seven concentric hexag-onal rings (A, B, C, D, E, F, and G) of a hexagonal lattice asshown in Fig. 2. Detail description of the MVP model of BAECTRIGA reactor can be found in Ref. Rahman et al. (2007). Tomake the first move, the 1200 MWD burnt core was shuffled in-side out. In this format, the highest burnt fuel was interchangedwith the lowest burnt one, the second highest with the secondlowest et seq. The reactivity gain in this reshuffling is $0.6. Atthe end of this shuffled cycle the core being reshuffled withtwo-cycle burnt fuels, cannot lead to anywhere satisfactory.Hence, the repeated shuffling method was discarded right awayto avoid shorter cycle length. To get a realistic reactivity yield ineach series of cycles continually, some fresh fuel has to beincorporated in the core; next we studied three different reloadschemes to get a sustainable layout as follows,

i. First the fuels in the core have been rearranged in an out-to-in scheme as shown in Fig. 3. The loading pattern for thisstep is to discharge five mostly burnt fuels at the beginningof each cycle and refurnish them with the consecutive max-imum burnt fuels and so on and finally loading five newmembers at the peripheral locations. This is scheme 1.

ii. Secondly, the outside to inner loading scheme has beenrepeated as in the first case, but this time with ten fresh fuels(Fig. 4). We call it scheme 2.

0123456789101112

0 100 200 300 400 500 600 700 800 900 100011001200130014001500Burnup (MWD)

Cor

e Ex

cess

Rea

ctiv

ity ($

)

Fig. 1. Change of reactivity with burnup calculated by MVP-BURN.

iii. Concurrently, a transient core was on trial as scheme 3. Inthis exploitation, out-to-in loading method has been appliedas the former cases but with five fresh fuels for the first load-ing cycle and ten for the following.

Our investigation is limited to no greater than 10 dischargedfuels at a time to reduce wastage of fuel and no reshuffling hasbeen made prior reload to avoid the fuel accounting complexity.Also the choice of five or multiple of five fuels is optimum to makea symmetrical loading distribution in reference to the present coreconfiguration. All the schemes have been tested with two furthercycles after the first cycle (i.e. the fresh core cycle) burnup. Thenumbers of reloading cycles are limited to only two keeping inmind that the research reactor core configuration is prone tochange after few cycles due to utilization purpose and withinfew successive cycles the characteristics i.e. discharge burnupand operating cycle length are assumed to be same. For each ofthe schemes, same number of fuel loading and same end-of-cyclecore excess reactivity has been considered in all cycles. The choiceof fuel replacement in each ring has been made based on ring wiseequilibrium burnup consideration. And clockwise next fuel ele-ment location in a ring has been chosen to operate out or shiftfor subsequent cycles. Finally, to compare the fuel loading schemesthe cost function has been defined as follows,

f ¼ F � DL

unit=MWD

whereF = unit fuel loaded in a cycle; 100 unit is assigned per freshfuel,D = unit fuel consumed in a cycle; total of percent burnup valueof each discharged fuel at the beginning-of-cycle is assigned asconsumed unit,L = reactor operating life in a cycle (MWD).

It is to be noted that no in-to-out fuel loading case has beenconsidered, because in such case during continual loading, verynaturally situation would arise that the outer and inner fuels will

Fig. 3. Fuel loading scheme 1 with arrows showing the flow of fuels (out-to-in with five fuels).

Z.I. Lyric et al. / Annals of Nuclear Energy 55 (2013) 225–229 227

almost reach to the same burnup value that will hinder to sustainthe loading scheme economically.

3. Results and discussions

From Table 1 data, in all loading cases, the calculated peakingfactor values are less than the initial or fresh core value, which iswithin the design value 1.70 (General Atomics, 1981). This meansthe investigated loaded cores are suppose to satisfy the reactorcore safety criteria. After first cycle operation of 1200 MWD, 15%equilibrium burnup and 23.84% maximum burnup of U235 hasbeen reached. This burnup value is within the safety limited valuei.e. 50% for TRIGA fuel (U.S. Nuclear Regulatory Commission, 1987).The core excess reactivity as found in Table 2, at the beginning ofsecond cycle is $4.848 for scheme 1 and scheme 3 indicating a yieldof about $0.35, and for scheme 2 this gain is about $0.75. Now thispicture looks pretty inviting to carry on. Provisionally, the reactiv-ity cut-off is taken as $4.5, at gross which is much close to the peakvalue of GA reckoning. With this, the first reloaded cycle can last upto 100 MWD of burning for first and third schemes and 200 MWDfor scheme 2 whereat the reactivity drops to the threshold value.But the core life reduced in the subsequent cycle, 50 MWD for firstscheme and 100 MWD for second. Here the core being refilled withnew fuels in the least fuel worth locations and other locations withmostly two-cycle burnt fuels cannot take the end point of core life

as extended as of the first cycle. In scheme 3 the second cycle isjust as scheme 2 with loading of 10 fresh fuels. This cycle expandedup to 100 MWD. It can be expected for the upcoming cycles in thisscheme, discharge of fuels from the higher worth region and con-tribution of relatively younger fuels toward the inner rings willreach to equilibrium; due to the identical reloading architectureand eventuation in the consecutive cycles the following cyclescan be inferred to possess the same nature, fixing the cycle lengthsmore or less to 100 MWD with constant number (i.e. 10) of freshfuel loading in each cycle. The minimum discharge burnup ofU235 as shown in Table 3 has been found to be 18.75%, 17.05%,and 16.57% for first, second and third scheme respectively. The cal-culated core life and discharged fuel for each loading method aregiven in Tables 2 and 3 respectively, from where the cycle wise costfunction value for each method obtained are shown in Table 4. Thelowest value for the total cost is found for the first scheme. This va-lue is comparable to the third i.e. the transient core scheme whichalso posses the optimally longer equilibrium cycle period of100 MWD. Now this method sounds feasible indeed.

4. Conclusions

The cost for the first scheme is found to be least, but it requiresrelatively frequent reloading, whereas the cycle length is longer forthe second scheme in expense of higher cost. In each scheme, the

Fig. 4. Fuel loading scheme 2 with arrows showing the flow of fuels (out-to-in with 10 fuels).

Table 1Radial power peaking factor.

Cycle Scheme

1 2 3

1(fresh core) 1.69 – –2 1.61 1.63 1.613 1.61 1.63 1.62

228 Z.I. Lyric et al. / Annals of Nuclear Energy 55 (2013) 225–229

reload cycle immediate after the fresh core cycle is longer com-pared to the subsequent cycles, and hence retaining the first reloadcycle with small number of fresh fuel loading like first scheme,

Table 2Core excess reactivity ($) and core life extension (MWD).

Cycle Scheme

1 2

$ MWD $

1(fresh core) 10.718 1200 –2 4.848 100 53 4.856 50 4

addition of more fresh fuels in the following cycles like the secondscheme found to be economic as well as possessing longer cyclelength which makes the third scheme preferable. Hence for theTRIGA core, the outside-to-inside fuel loading, ten fuels for eachcycle starting with five fuels for the first reload can be an economicfuel utilization strategy. It is expected that the optimum burnupinformation in this research work will aid the in-core fuel manage-ment of TRIGA reactor.

Acknowledgments

The authors would like to thank the Reactor Physics group ofthe Nuclear Data and Engineering Division, Nuclear Science and

3

MWD $ MWD

– – –.252 200 4.848 100.962 100 5.173 100

Table 4Cost function value.

Cycle Scheme

1 2 3

2 3.851 3.979 3.8513 8.024 8.052 8.032Total 11.875 12.031 11.883

Table 3Percent discharge burnup of U235.

Cycle Scheme

1 2 3

S. No. Location % Burnup S. No. Location % Burnup S. No. Location % Burnup

1(fresh core) 1 C01 20.776 1 C01 20.776 1 C01 20.7762 C02 22.697 2 C02 22.697 2 C02 22.6973 C04 23.697 3 C04 23.697 3 C04 23.6974 C08 23.839 4 C08 23.839 4 C08 23.8395 C10 23.806 5 C10 23.806 5 C10 23.806

6 D02 17.0547 D05 17.5568 D08 19.4989 D11 17.65810 D14 17.624

Total burnup unit 114.815 Total burnup unit 204.205 Total burnup unit 114.815

2 1 C01 18.753 1 C01 17.841 1 C01 18.7532 C02 19.413 2 C02 17.937 2 C02 19.4133 C04 21.431 3 C04 18.103 3 C04 21.4314 C08 19.613 4 C08 18.300 4 C08 19.6135 C10 19.580 5 C10 17.491 5 C10 19.580

6 D03 20.731 6 D03 19.3407 D06 20.566 7 D06 19.1478 D09 22.632 8 D09 21.1009 D12 20.653 9 D12 19.23710 D15 20.560 10 D15 19.150

Total burnup unit 98.79 Total burnup unit 194.814 Total burnup unit 196.764

3 1 C01 20.108 1 C01 19.527 1 C01 18.5282 C02 20.170 2 C02 19.893 2 C02 18.6803 C04 20.067 3 C04 19.538 3 C04 17.7384 C08 22.042 4 C08 19.720 4 C08 18.4815 C10 20.207 5 C10 19.799 5 C10 18.331

6 D02 20.912 6 D02 17.2157 D05 20.692 7 D05 18.0428 D08 21.449 8 D08 19.9039 D11 21.002 9 D11 18.24610 D14 20.440 10 D14 16.572

Total burnup unit 102.594 Total burnup unit 202.972 Total burnup unit 181.736

Z.I. Lyric et al. / Annals of Nuclear Energy 55 (2013) 225–229 229

Engineering Directorate at Japan Atomic Energy Agency (JAEA) forinstallation and demonstration of the computer codes in ReactorPhysics & Engineering Division of Institute of Nuclear Science &Technology (INST).

References

General Atomics. 1981. Safety Analysis Report for the 3000-KW Forced-Flow TRIGAMARK II Reactor, E-117-990.

Huda, M.Q. et al., 2008. Burnup analysis and in-core fuel management study of the3 MW TRIGA MARK II research reactor. Annals of Nuclear Energy 35, 141–147.

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Okumura, K. et al., 2000. Validation of a continuous-energy Monte Carlo burn-upcode MVP-BURN and its application to analysis of post irradiation experiment. J.Nucl. Sci. Technol. 37 (2), 128–138.

Rahman, M. et al. 2007. Evaluation of the MVP Model Developed for the TRIGAMark-II Research Reactor at AERE, Savar, through Criticality and ReactivityAnalysis. Internal Report, Institute of Nuclear, Science and Technology, INST-113/RPED-25.

Ravnik, M., Mele, I. 1988. Optimal utilization in TRIGA reactor with mixed core.International Reactor Physics Conference, Jackson Hole, Wyoming.

Ravnik, M. 1987. Burnup Calculations of TRIGA Reactor Core. IAEA InterregionalTraining Course on the Application of Small Computers to Research ReactorOperation Ljubljana.

Shibata, K. et al., 2002. Japanese evaluated nuclear data library, Version-3, Revision-3: JENDL-3.3. J. Nucl. Sci. Technol. 39, 1125–1136.

U.S. Nuclear Regulatory Commission. 1987. Safety evaluation report on high-uranium content, Low-enriched uranium–zirconium hydride fuels for TRIGAreactors, NEUREG-1282.