Acta Polytechnica CTU Proceedings doi:10.14311/APP.2018.14.0027 Acta Polytechnica CTU Proceedings 14:27–33, 2018 © Czech Technical University in Prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app NEUTRONIC ANALYSIS OF THE CANDIDATE MULTI-LAYER CLADDING MATERIALS WITH ENHANCED ACCIDENT TOLERANCE FOR WWER REACTORS Ondřej Nováka,∗, Martin Ševečeka,b a Department of Nuclear Reactors, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic, V Holešovičkách 2, Prague 8, Czech Republic b Department of Nuclear Science and Eng., Massachusetts Institute of Technology, Cambridge, USA ∗ corresponding author: ondrej.novak2@fjfi.cvut.cz Abstract. The paper summarizes preliminary results of neutronic analysis of candidate ATF cladding materials that are under development at the CTU in Prague. To evaluate basic neutronics-related characteristics the Serpent code was used which is a Monte-Carlo based simulation tool. A model of WWER fuel was developed and basic neutronic analysis performed. All coating materials entail certain reactivity penalty compared to reference uncoated cladding that was quantified. The coating of cladding affects also other neutron-physical parameters of cores that modify the performance of the WWER reactors that are discussed. Keywords: WWER, nuclear fuel, cladding, accident tolerant fuel, neutronics, serpent. 1. Introduction New types of nuclear fuels for Light Water Reactors with enhanced accident tolerance have been recently researched around the world. The concepts consid- ered as future Accident Tolerant Fuels (ATF) include a development of completely new fuel pellets (nitrides, silicides, FCM)[1], cladding materials (FeCrAl, SiC, Mo-alloys), non-fuel components or slight modifica- tions of current fuel system [2–6]. The modification of current fuel system is considered as a near-term concept which can be developed and adopted by the industry in less than 10 years [7]. Most of the ATF concepts entails a certain reactivity penalty that has to be precisely determined during the development due to economical and operational concerns. Many of the recent papers and reports focused on neutronic performance of ATF fuel concepts for tra- ditional PWR and BWR reactors [8–11]. However, the ATF fuels can and should be employed also in the WWER reactors that are in operation in the Czech Re- public, Bulgaria, Slovakia, Hungary, Finland, Ukraine, India, and Russia. The WWER reactors perform similarly to PWR reactors in the accidental and tran- sient conditions and therefore most of the material and thermal testing performed in PWR conditions is valid also for WWER reactors [12]. However, the WWER reactors have their unique specifics includ- ing hexagonal fuel assembly geometry, fuel assembly shroud, different coolant chemistry, different materials or harder neutron spectrum. These specific aspects affect in particular neutron-physical performance of the fuel. For that reason, models of neutronics performance of the reactor WWER-1000 were developed in the Serpent code, which is a well-known Monte-Carlo Figure 1. SEM micrograph of Cr cold-spray coated Zircaloy-4 cladding with nonuniform coating thickness. The top surface can be finished by polishing but the interface will remain nonhomogeneous due to high hardness of Cr particles. based code, developed in the VTT, Finland [13]. This model was used to analyze the performance of differ- ent multi-component claddings considered as potential candidates for the ATF development. Some of the concepts have been developed at the CTU [14] or MIT [15] and some are considered as alternative op- tions and will be investigated in future research. All studied concepts are based on the Zircaloy-4 alloy substrate (isotopic material composition is noted in Table 2). This alloy serves as a substrate on which different coating materials are applied. The coating materials include Chromium nitride, Chromium, Zir- conium silicate, FeCrAl or Molybdenum. An example of Zircaloy-4 cladding coated by pure chromium using 27 http://dx.doi.org/10.14311/APP.2018.14.0027 http://ojs.cvut.cz/ojs/index.php/app Ondřej Novák, Martin Ševeček Acta Polytechnica CTU Proceedings Figure 2. SEM micrograph of E110 cladding coated using PVD with CrN after high temperature steam oxidation at 1200◦C for 9 minutes. Even after oxida- tion the coating is homogeneous and its thickness can be easily controlled. cold spray process is shown in Figure 1, other concept is a cladding coated by Physical Vapor Deposition (PVD) method with CrN. This coating is under test- ing at CTU in Prague and an example of CrN coated cladding exposed to 1200◦C steam fro 9 minutes is shown in Figure 2. As can be seen from both figure, different deposition technique produce coatings with different characteristics. Cold-spray coatings are more non-uniform and thicker. PVD coatings are generally thinner because the deposition is very slow. PVD can however produce very homogenous high-quality coatings, parameters of which can be easily controlled. Reactivity penalties and the effect of different coat- ings on cycle length has been studied and detailed studies are still on going. Changes of neutron spectra caused by deposited cladding coatings compared to reference uncoated case are also investigated as well as effects of coatings on change of reactivity coefficients. Different thicknesses of coating materials are studied and simple economic penalties for each material pre- sented. By changing the neutron spectrum in the core, the relative fission power distribution or Pu-239 and other actinides production might change for a partic- ular concept that has been also included in the study. All calculations and their results are directly com- pared to the uncoated reference Zircaloy-4 cladding alloy that has been by the industry for decades to clearly show potential benefits and negatives of the particular concept. 2. Model and methodology In order to obtain requested neutronics performance parameters, single 2D fuel pin model was developed using the Serpent code (version 2.1.26). WWER fuel pin model is based on real WWER-1000 fuel geome- try. Model of the fuel pin is shown in Figure 2. The boundary conditions were set to reflective. Fuel pin pa- rameters and materials used are noted in Table 1. The Parameter Value Outer cladding radius [mm] 4.572 Inner cladding radius [mm] 4.0005 Cladding thickness [mm] 0.5715 Cladding material Zircaloy-4 Fuel pellet outer radius [mm] 3.922 Pin pitch [mm] 12.75 UO2 density [g/cm3] 10.412 UO2 enrichment [wt.% ] 3.8 Zry-4 density [g/cm3] 8.00 Table 1. WWER-1000 fuel pin parameters used in the simulation. Isotope Weight fraction [-] 90Zr 0.49717 91Zr 0.10963 92Zr 0.16941 94Zr 0.17542 96Zr 0.028864 120Sn 0.015 56Fe 0.002 Table 2. Zircaloy-4 isotopic composition. fuel enrichment was optimized for better performance of the Monte Carlo calculation. Both transport and burnup calculations were used for studied materials. To obtain accurate results also for high burnup value, high number of burn steps1 and high number of neutron particles were simulated in many cycles2. Moreover, fuel pellet and cladding were divided into 5 radial regions that were burned separately to enhance depletion accuracy. B1 fundamental mode calculation was applied as well. The newest ENDF/B-VII.1 li- brary was used as a source of all nuclear data. Power density 0.03554 kW/g was used for all calculations. Main calculation parameters (temperature, boric acid concentration) simulated representative conditions of the WWER-1000 reactor core. These calculation pa- rameters are summarized in Table 1. Boric acid con- centration is based on the middle cycle value, temper- atures refer to average material temperature under nominal conditions (at the full reactor power). 3. Studied materials Several promising coating materials were tested in this neutron analysis study. The choice of coating materials is based on evaluation metrics for accident tolerant fuel concepts developed specifically for the WWER reactors [16]. Considered materials include namely: CrN, Cr2N, Cr, ZrSi2, SiC, Ti, FeCr10Al6, 1Following burnup steps were used: 0.0001, 0.09, 0.26, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 47, 50, 53, 56, 60, 63, 70 2400 000 neutrons per cycle, 400 active cycles and 50 inactive cycles 28 vol. 14/2018 Neutronic Analysis of the Candidate Multi-Layer Cladding Materials Figure 3. Fuel pin model from the Serpent code. The cladding and pellet are divided into several concen- tric rings to allow high precision of the simulation of isotopic changes. Mo. Other concepts that are under development elsewhere [17] are also considered, however results are not presented here: TiN, TiAlN, AlCrN, Ti2AlC, Ti–Si–C, Zr–Al–C. Most promising concepts (that are currently under development at CTU in Prague) were calculated with different coating thicknesses and with full burnup calculations. Calculation regime for each material is noted in Table 5. This selection re- sulted from very high computational requirements, especially the burnup calculation is extremely time consuming. All considered coating thicknesses used are noted in Table 4. The thicknesses of coatings considered are based on experience with high-temperature testing of re- spective concepts. The protective nature of coatings with thickness of 5 microns and lower is very limited [14]. Moreover, some deposition techniques, namely cold-spray process, produce non-uniform coatings and it is therefore necessary to consider higher thicknesses in order to ensure uniform layer of coating. An exam- ple of cold spray-coated sample is shown in Figure 1. The maximal thickness is based on potential coating stability and also economic consequences. It should be noted, that the optimal thickness of the coating is between 15-60 microns from the perspective of high- temperature testing. Because each coating entails certain reactivity penalty its thickness should be re- duced. The thickness of the final concept will be based on compromise between oxidation resistance and economical limitations. The coating layer generally negatively impacts mul- tiplication factor resulting in shortening of fuel cycle which has negative economic consequences on whole power plant. The decrease in effective power days is not easy to predict using only fuel pin calculation, due to the general influences of fuel handling and core de- sign, different fuel pin positions in fuel assemblies and reactor operation history. However, relative impact of Parameter Value Fuel temperature 1005 K Soluble boron concentration 525 ppm (3g/kg) Coolant density 0.7169 g/cm3 Number of radial pin regions 5 Coolant temperature 578 K Table 3. Stationary parameters of the Serpent calcu- lation chosen based on standard WWER-1000 opera- tional characteristics. coatings on fuel cycle length can be estimated. The methodology used is based on the multiplica- tion factor for final burnup value (70 MWd/kgU ) in case of fuel pin without coating. Then from the results for fuel pin with coating, burnup point with similar multiplication factor is estimated. Difference between estimated value and the final burnup value is calcu- lated and is used as factor describing the decrease in fuel cycle length. The breakpoint value depends mostly on fuel costs (especially additional enrichment costs). Final decision of economical acceptance should be based on deeper economical study and will be pre- sented in future. 4. Results As was previously mentioned, several coating materials were simulated (CrN, Cr, ZrSi2, SiC, Ti,. . . ) Most of these materials underwent precise burnup calculation. Comparison of all calculated cases for zero burnup is noted in the Table 6. The column Difference shows difference in reactivity in pcm between the reference case without coating material and coated concept. As it is shown in the table, decrease in multiplication factor was observed for all materials and the absolute value of the decrease is closely linked to the higher absorption cross section of coating materials. Decrease in multiplication factor results in shorter fuel cycle which has direct impact on power plant profit. The 1% decrease in fuel cycle length can be considered as acceptable when consequently increas- ing safety of the power plant. Moreover, 1% decrease in fuel cycle length offers sufficient thickness of coat- ing material so that coating material can provide requested safety enhancement. Some materials re- sulted in only small decrease in keff , which means that impact on fuel cycle length is not crucial. How- ever, Molybdenum coating has significant impact on fuel cycle length. All materials, except Mo, has de- crease in fuel cycle length around 1% for 20 µm thick coating. For 50 µm thick coating is the decrease above 2%. Molybdenum reaches 4% for 50 µm. The best material with the lowest fuel cycle length influence can be considered ZrSi2. This coating material even increased multiplication factor for high burnup values when compared to reference case (see the Figure 5). The data summarized in Table 6 are shown also in Figure 3. The decrease in reactivity corresponds 29 Ondřej Novák, Martin Ševeček Acta Polytechnica CTU Proceedings Outer Cladding Radius [mm] Coating Thickness [µm] 4.572 0 4.578 6 4.581 9 4.587 15 4.592 20 4.602 30 4.622 50 4.672 100 Table 4. Coating thicknesses used in calculations and corresponding outer cladding radius. The inner cladding radius is constant. Figure 4. Relative reactivity penalty for considered coated claddings with various thicknesses Material Burnup Coating thicknesses CrN yes 6, 9, 15, 20, 30, 50, 70,100 Cr yes 15 ,30, 50, 70, 100 SiC yes 15, 20, 100 Ti no 9, 15, 20, 30, 50, 70, 100 ZrSi2 yes 15, 20, 30, 50, 70, 100 Cr2N no 20, 30, 50, 70,100 Mo no 20, 30, 50, 70,100 Table 5. Coating material and tested parameters. clearly to absorption neutron cross section. The penalty of coatings also increases with thickness of coatings as expected. The changes in neutron flux for considered materi- als are shown in Figure 6. Decrease in the thermal part of neutron spectrum was observed. This change may lead to changes of reactivity coefficients and ef- fective cross sections of materials in the core. This decrease suggest the decrease in reactivity of whole system described above. On the other hand, an in- crease in neutron flux in epithermal and fast neutron spectrum affect isotopic composition of materials due to different effective absorption. This results need however further confirmation and additional full core calculations. The change of reactivity coefficients sug- gest that core behavior in reactivity-related events (RIA, MSLB) will differ from the reference case. Ad- Figure 5. Multiplication factor dependency on bur- nup value for 70µm coating ditionally, different neutron spectrum in the core will lead to different material composition including minor actinides that are crucial from the point of reactor operation. Isotopic changes of coating materials were mostly dependent on neutron capture probabilities of involved isotopes in coating. Isotopic composition of pellets differs due to changes in neutron spectrum described above. As an example the production of Pu-239 dur- 30 vol. 14/2018 Neutronic Analysis of the Candidate Multi-Layer Cladding Materials Coating material Coating Thickness [µm] Outer Cladding Radius [mm] Keff [-] Keff uncertainty Difference [pcm] Reference case 0 4.572 1.28949 3.60E-05 0 Cr 15 4.587 1.28424 3.90E-05 -409 Cr 30 4.602 1.27897 3.50E-05 -823 Cr 50 4.622 1.27209 3.70E-05 -1368 Cr 70 4.642 1.26513 4.00E-05 -1925 Cr 100 4.672 1.25482 3.80E-05 -2763 Cr2N 20 4.592 1.28163 3.50E-05 -613 Cr2N 30 4.602 1.27777 3.70E-05 -917 Cr2N 50 4.622 1.27005 3.60E-05 -1531 Cr2N 70 4.642 1.26224 4.00E-05 -2159 Cr2N 100 4.672 1.25073 4.10E-05 -3099 CrN 6 4.578 1.28357 3.60E-05 -461 CrN 9 4.581 1.28592 3.70E-05 -278 CrN 15 4.587 1.28707 3.70E-05 -188 CrN 20 4.592 1.28179 3.80E-05 -601 CrN 30 4.602 1.27783 3.60E-05 -912 CrN 50 4.622 1.27005 3.80E-05 -1531 CrN 70 4.642 1.26233 3.80E-05 -2152 CrN 100 4.672 1.25083 3.60E-05 -3091 Mo 20 4.592 1.27511 3.80E-05 -1128 Mo 30 4.602 1.26880 3.60E-05 -1631 Mo 50 4.622 1.25727 3.90E-05 -2563 Mo 70 4.642 1.24646 4.00E-05 -3452 Mo 100 4.672 1.23096 4.10E-05 -4755 SiC 15 4.587 1.28850 3.70E-05 -77 SiC 20 4.592 1.28817 3.80E-05 -102 SiC 100 4.672 1.28237 3.60E-05 -555 Ti 9 4.581 1.28814 3.70E-05 -105 Ti 15 4.587 1.28725 3.70E-05 -174 Ti 20 4.592 1.28641 3.80E-05 -239 Ti 30 4.602 1.28481 3.90E-05 -364 Ti 50 4.622 1.28170 3.70E-05 -608 Ti 70 4.642 1.27846 3.70E-05 -863 Ti 100 4.672 1.27391 3.60E-05 -1223 ZrSi2 15 4.587 1.28832 3.80E-05 -91 ZrSi2 20 4.592 1.28793 3.70E-05 -121 ZrSi2 30 4.602 1.28711 3.60E-05 -185 ZrSi2 50 4.622 1.28542 3.90E-05 -317 ZrSi2 70 4.642 1.28384 4.00E-05 -440 ZrSi2 100 4.672 1.28123 3.90E-05 -645 Table 6. Multiplication factors for different coating materials and thicknesses. 31 Ondřej Novák, Martin Ševeček Acta Polytechnica CTU Proceedings Figure 6. Flux difference between the reference case (cladding without any coating) and claddings with fully dense 15 microns thick coatings. Figure 7. Change in Plutonium-239 concentration for different coating materials ing burnup is shown in Figure 7 for different cladding materials. The effect increases with increasing bur- nup as expected. Different breeding ratio and change in reactivity coefficients affect also other reactor per- formance parameters as axial offset, radial peaking, material irradiation damage or performance during accidents e.g. RIA[10]. For that reason, full core calculations need to be performed before concluding the evaluation of different ATF concepts including the precise definition of effective cross sections. 5. Conclusion Vast selection of coating materials considered for ATF cladding was studied in area of neutronic performance of a WWER reactor. Impact of coating layer to multi- plication factor and reactivity during whole fuel cycle was studied. In addition, changes of material itself caused by neutron interaction were calculated and dis- cussed. Moreover, neutron flux changes were studied as well. Multiplication factor in all cases decreased due to higher neutron absorption of coating materials. This causes reducing of cycle length having economic consequences for utilities. For that reason, the thick- ness of the coatings need to be optimized not only from the perspective of safety enhancement but also from economical perspectives. By using coated claddings, the neutron spectrum in a reactor will change. This brings new questions related to operation of a reactor and its safety. Detailed full-core calculations need to be performed in order to precisely quantify the effects of particular coatings. However, this study re- veals only neutronics-related performance of the ATF claddings. To conclude and select the most appropri- ate coating material enhancing the accident tolerance of WWER reactors, other studies from different field must be included. Acknowledgements This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS16/252/OHK4/3T/14 and IAEA CRP 21065 “De- velopment and Testing of Coated Fuel Cladding for VVER Reactors with Enhanced Accident Tolerance”. References [1] N. R. Brown, M. Todosow, A. Cuadra. Screening of advanced cladding materials and UN–U3Si5 fuel. Journal of Nuclear Materials 462:26–42, 2015. doi:10.1016/j.jnucmat.2015.03.016. [2] R. B. Rebak, K. A. Terrani, R. M. Fawcett. FeCrAl Alloys for Accident Tolerant Fuel Cladding in Light Water Reactors p. V06BT06A009, 2016. doi:10.1115/PVP2016-63162. 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Corrosion Reviews 35(3):141–165, 2017. doi:10.1515/corrrev-2017-0010. 33 http://dx.doi.org/10.1016/j.anucene.2016.09.033 http://dx.doi.org/10.13182/NT14-22 http://dx.doi.org/10.1016/j.net.2017.12.011 http://dx.doi.org/10.14311/AP.2016.4.0089 http://dx.doi.org/10.1515/corrrev-2017-0010 Acta Polytechnica CTU Proceedings 14:21–27, 2018 1 Introduction 2 Model and methodology 3 Studied materials 4 Results 5 Conclusion Acknowledgements References