A NEUTRONIC FEASIBILITY STUDY OF THE AP1000 DESIGN LOADED WITH FULLY CERAMIC MICRO-ENCAPSULATED FUEL

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1 Engineering (M&C 2013), Sun Valley, Idaho, USA, May 5-9, 2013, on CD-ROM, American Nuclear Society, LaGrange Park, IL (2013) A NEUTRONIC FEASIBILITY STUDY OF THE AP1000 DESIGN LOADED WITH FULLY CERAMIC MICRO-ENCAPSULATED FUEL Chao Liang and Wei Ji Department of Mechanical, Aerospace, and Nuclear Engineering Rensselaer Polytechnic Institute th Street, Troy, NY ABSTRACT A neutronic feasibility study is performed to evaluate the utilization of fully ceramic microencapsulated (FCM) fuel in the AP1000 reactor design. The widely used Monte Carlo code MCNP is employed to perform the full core analysis at the beginning of cycle (BOC). Both the original AP1000 design and the modified design with the replacement of uranium dioxide fuel pellets with FCM fuel compacts are modeled and simulated for comparison. To retain the original excess reactivity, ranges of fuel particle packing fraction and fuel enrichment in the FCM fuel design are first determined. Within the determined ranges, the reactor control mechanism employed by the original design is directly used in the modified design and the utilization feasibility is evaluated. The worth of control of each type of fuel burnable absorber (discrete/integral fuel burnable absorbers and soluble boron in primary coolant) is calculated for each design and significant differences between the two designs are observed. Those differences are interpreted by the fundamental difference of the fuel form used in each design. Due to the usage of silicon carbide as the matrix material and the fuel particles fuel form in FCM fuel design, neutron slowing down capability is increased in the new design, leading to a much higher thermal spectrum than the original design. This results in different reactivity and fission power density distributions in each design. We conclude that a direct replacement of fuel pellets by the FCM fuel in the AP1000 cannot retain the original optimum reactor core performance. Necessary modifications of the core design should be done and the original control mechanism needs to be re-designed. Key Words: AP1000 reactor design, fully ceramic micro-encapsulated fuel, excess reactivity worth, reactivity control worth, power distribution 1. INTRODUCTION Seeking inherently safe light water reactor (LWR) designs by certain direct modifications of existing LWRs has been a pressing research area worldwide over the past 15 years [1-7]. The utilization of fully ceramic micro-encapsulated (FCM) fuel particles in current LWRs is one of many possible solutions and is currently being evaluated in the U.S. [5-7]. The FCM fuel has been shown to be significantly accident tolerant in the event of clad failure [10]. This excellent characteristic stems from two fuel design factors. The first is the usage of the TRISO fuel particles with multi-layer coatings as protection buffers where the fission products are virtually retained within the fuel kernel of each fuel particle [8-11]. The other factor is the usage of the ceramic silicon carbide (SiC) material as the matrix material, which shows excellent resistance to strong radiation, enhanced thermal conductivity and high melting point. In addition to enhancing Corresponding author, jiw2@rpi.edu Tel: +1 (518) ; Fax: +1 (518)

2 Chao Liang and Wei Ji the operational safety for LWRs that are operated for electricity generation, the FCM fuel has been demonstrated to be potentially utilized for transmuting transuranic (TRU) isotopes from LWR spent fuel by fabricating fuel kernels with thorium and plutonium fuels, which provides an inherently safe and advanced fuel cycle option for LWRs. To substantiate these aforementioned potential applications, a thorough assessment is needed to evaluate the reactor operation performance when FCM fuel is employed. The assessment should include a wide range of analyses, such as reactor core physics, thermal-hydraulics, fuel performance and other safety related characteristics. Some initiatives have begun and preliminary results have been obtained over the past few years. Recently, a group of researchers at the Idaho National Laboratory (INL) employed the lattice code DRAGON [12] to evaluate the neutronic performance for a LWR fuel pin loaded with FCM fuel [6-7]. Their results indicated an unacceptably short life cycle if standard uranium dioxide (UO 2 ) FCM particles are used and a modification of existing LWR design is needed. They also demonstrated the feasibility of burning transuranic FCM fuel from a reactivity viewpoint. Although neutronic analyses based on a fuel pin unit cell may provide evaluations with acceptable reliability for understanding the basic features of the FCM fuel in LWRs, a full core analysis is desirable, especially to the industry, and should ultimately be done to improve the understanding of the reactor performance from a full core perspective for the usage of FCM fuel in LWRs. Currently, such a full core neutronic analysis is lacking in the LWR research community. In addition, conventional full core analysis codes for LWRs lack the modeling capability to account for the realistic distribution of TRISO fuel particles. Therefore, the accurate prediction of the resonance integral in fuel kernels is limited. To address the present research gap, we perform a neutronic evaluation for a full core LWR using the high-fidelity Monte Carlo code MCNP [13]. Realistic configurations in LWRs with FCM fuel are accurately modeled, including the stochastic distribution of TRISO fuel particles in fuel pins. The analysis is for the initial fuel loading configuration of the existing commercial Westinghouse AP1000 LWR design. We aim to meet two objectives through the full core analysis work. The first objective is to provide a preliminary answer to the neutronic feasibility of the utilization of FCM fuel in LWRs without significant changes in the present core design, including the dimensions of fuel assemblies/pins and the configurations of fuel burnable absorbers (discrete/integral fuel burnable absorbers and soluble boron in primary coolant loop). The second objective is to provide a reference full core benchmark using high-fidelity Monte Carlo simulations to the developers of LWR analysis codes. Such data can be used to benchmark deterministic codes that are being developed for better analyzing LWRs with FCM fuel. To meet the above objectives, we first perform a full core analysis for the AP1000 design with nominal design parameters for the initial core fuel loading [14]. UO 2 fuel loadings with different enrichments in fuel assemblies at different locations are accounted for. Discrete and integral fuel burnable absorbers are included with exact design distribution patterns as documented in Ref. [15] in the full core geometry. Soluble boron in the primary coolant is also considered for excess reactivity evaluation calculations. Then, the original AP1000 full core analysis is used as a reference for the evaluation of the FCM fueled reactor core design by replacing the original fuel pellets with FCM fuel (i.e., a densely packed TRISO particle fuel in a SiC matrix background). The uranium oxycarbide (UCO) fuel is adopted in the TRISO fuel kernels. The excess reactivity worth and fission power density distributions in the modified design are evaluated at different 2/

3 An evaluation of full core AP1000 design loaded with FCM fuel FCM fuel fabrication parameters, including the fuel enrichment and TRISO fuel particle volume packing fraction. Meanwhile, the effects of fuel burnable absorbers on the full core performance are investigated. 2. CONFIGURATION OF FCM FUELED AP Configuration of AP1000 Reactor Core In the AP1000 design, fuel rods are fabricated from cylindrical tubes made of zirconium based alloy(s) containing UO 2 fuel pellets. Fuel assemblies are arranged in a pattern which approximates a right circular cylinder. Each fuel assembly contains a rod array composed nominally of 264 fuel rods, 24 rod cluster control thimbles, and an in-core instrumentation thimble, which are shown in Fig. 1(a). (a) Fuel assembly 17x17 array Figure 1. MCNP plot of AP1000 design. (b) Assembly loading pattern in full core For the initial core loading, the fuel rods within a given assembly have the same uranium enrichment. A total of 157 fuel assemblies of three different enrichments are used in the initial core loading to establish a favorable radial power distribution. Figure 1(b) is a plot of an MCNP full core model that shows the initial fuel loading pattern. Two regions consisting of the two lower enrichments, 2.35 w/o and 3.40 w/o, are interspersed to form a checkerboard pattern in the central portion of the core. The third region is arranged around the periphery of the core and contains the highest enrichment 4.45 w/o. To properly control the reactivity and power distribution throughout the reactor operating cycle, discrete and/or integral fuel burnable absorbers (FBAs) are used in the AP1000 design. The guide tubes in fuel assemblies are reserved for the discrete fuel burnable absorber (PYREX) rods that primarily consist of silica and boron oxide. Integral Fuel Burnable Absorber (IFBA) rods that consist of a thin boride coating layer are located in fuel assemblies with 5 different configurations [15]. In addition to PYREX and IFBA rods, the soluble neutron absorber is also utilized to compensate for reactivity changes due to fuel burn-up, fission product poisoning, burnable absorber depletion, and the cold-to-operating moderator temperature change. The concentration of soluble boron should vary as fuel depletes so this variation should be design dependent. It is another important reactivity control mechanism in addition to the other control strategies (discrete and/or integral burnable absorber, control rod) in the AP1000 design. 3/

4 Chao Liang and Wei Ji The exact configuration of the AP1000 reactor design can be found in Ref. 14. The total active fuel height is cm. The core is surrounded by a cm thick core barrel with the inner radius of cm. Borated water flows between fuel assembly region and core barrel. Each fuel pin has a radius of 0.41 cm and the outer zirconium cladding has a radius of cm. The pitch of the fuel lattice is 0.61 cm. Fuel is assumed composed with g/cm³ mass density UO 2 at the initial core loading 2.2. Configuration of FCM Fueled AP1000 In the modified AP1000 design loaded with FCM fuel, we adopt TRISO fuel particles used by Very High Temperature Gas-cooled Reactors (VHTR) [16-17] for the FCM fuel. TRISO fuel particles are densely packed in the SiC matrix background to form a fuel compact. These fuel compacts replace the UO 2 fuel pellets in fuel pins. Zircaloy clad is still preserved in the fuel pin. Figure 2 shows a fuel assembly filled by lattice structure with TRISO fuel particles randomly distributed in the SiC matrix within each fuel pin. (a) 17x17 fuel assembly (b) Cross-sectional view of a fuel pin (c) A TRISO cell Figure 2. Fuel Pellet Filled with TRISO Fuel Particles. The specific geometric dimensions and material compositions/densities of the FCM fuel in each region are listed in Table I based on Ref. 15. Each TRISO fuel particle is composed of four coating layers as protection buffers. Table I. FCM fuel geometry and composition Regions Radius(cm) Isotope and atom density (atom/barn-cm) Fuel kernel UCO (UC.5 O 1.5 ) U: E-3; 238 U: E-2; C: E-2; O: E-2 Carbon buffer layer C: E-2 Inner pyrolytic carbon layer C: E-2 Silicon carbide layer C: E-2; Si: E-2 Outer pyrolytic carbon layer C: E-2 Silicon carbide matrix N/A C: E-2; Si: E-2 4/

5 An evaluation of full core AP1000 design loaded with FCM fuel To model the stochastic distribution of TRISO fuel particles in fuel pins, we employ a Quasi- Dynamic Method (QDM) [18] that was developed for a general purpose of random sphere packing. The method first generates N spatial points that are uniformly distributed within a container for each sphere center. Overlaps may exist between spheres. Then, a normal contact force model is employed to eliminate sphere overlaps while constraining all the spheres within the container. The overlap elimination process is performed iteratively until no overlap present. The generated coordinates of the TRISO particles are stored in an external file. A MATLAB code is developed to read coordinates of particles and generate an MCNP input file for the modified AP1000 fuel core model. 3. NUMERICAL RESULTS In the neutronic evaluation of the modified AP1000 design loaded with FCM fuel, we primarily focus on two factors. The first one is the evaluation of excess reactivity that FCM fuel can provide without any control mechanism considered. To provide the same excess reactivity as the original AP1000 design for the initial core loading, the required minimum fuel enrichment and fuel particle packing fraction for the FCM fuel are determined by a series of static core simulations. A total of 24 full core simulations at the combination of different volume packing fractions from 30% to 60% and different UCO fuel enrichments from 10.36% to 20% are performed. The second factor is the evaluation of the control mechanism (fuel burnable absorbers) in the modified design. When FCM fuel is introduced, the boron concentration in burnable fuel absorbers and their distribution patterns that were carefully designed in the original AP1000 design may not be optimal for the new reactor design. The favorable fission power density distribution in the original design may be lost in the new design. As a preliminary study, we examine the effect of each type of fuel burnable absorber on the reactivity and power distribution change in the modified design. The following 4 cases are studied: (a) Fresh fuel loading without any fuel burnable absorbers; (b) Fresh fuel loading with PYREX rods in the reactor core; (c) Fresh fuel loading with IFBA rods in the reactor core; (d) Fresh fuel loading with soluble boron in the reactor core. The first factor evaluation focuses on Case (a) only. After a range of feasible enrichments and volume packing fractions are determined, the modified FCM fueled design with selected enrichment and fuel particle volume packing fraction is used for the second factor evaluation (Cases b-d) Excess Reactivity Evaluations without Burnable Absorbers (Case a) The multiplication factor for the original AP1000 design is first calculated in Case a. The calculated value is then used as a reference to assess the predicted k eff values obtained for the modified AP1000 design with FCM fuel under different fuel enrichments and particle fuel volume packing fractions. Values of k eff are summarized in Table II below. For simplicity, we denote fuel particle volume packing fraction as f and fuel enrichment as e in the remainder of the paper. Table II. Multiplication factor for fresh fuel loading w/o fuel burnable absorbers 5/

6 Chao Liang and Wei Ji Original AP1000 design Reference k eff = Modified AP1000 design with FCM fuel f e 10.36% 12% 14% 16% 18% 20% 30% % % % It is clear to see a range of design parameters (as highlighted in Table II) that meets neutronic feasibility of retaining at least the same excess reactivity as in the original design for the initial fuel loading. Next, we ll add each type of burnable absorber to the modified AP1000 design to assess their neutronic feasibility if the same amount and the same distribution pattern of these burnable absorbers are maintained as in the original AP1000 design Excess Reactivity/Fission Power Density Distribution Evaluations with Burnable Absorbers (Cases b-d) The effect of each type of burnable absorber on the reactivity and fission power density distribution in both the original and modified designs is calculated and compared. For the modified design, we select two different combinations of enrichment and packing fraction (f=40% + e=18% and f=60% + e=12%) as the FCM fuel options for the investigation. Evaluations on different combinations will be reported in a future publication. Table III shows the excess reactivity change due to each type of burnable absorbers. Table III. Excessive reactivity change due to each burnable absorber Original design Modified design Reference k * k * eff = eff = k * eff = at f=40%, e=18% at f=60%, e=12% (Case a) k eff Δρ=ρ-ρ * k eff Δρ=ρ-ρ * k eff Δρ=ρ-ρ * Case b (w/ PYREX rods) Case c (w/ IFBA rods) Case d (w/ soluble boron) Note ρ=(k eff -1)/k eff. Soluble boron and PYREX rods provide much larger worth of control than in the original design, while IFBA rods provide smaller worth of control. With all the other configurations kept the same in simulations for both designs, the control worth change of each burnable absorber is attributed to the difference of the fuel forms used in each design. In the original design, fissile materials are homogeneously distributed over the fuel pin region, while in the FCM fuel, fissile materials are clustered (contained within a TRISO fuel kernel) in the fuel pin region. Due to the clustering of fissile materials, even with a very high TRISO packing fraction (i.e. 60%), fuel kernels can only have 5.42% packing fraction. This opens many channels by which neutrons can 6/

7 An evaluation of full core AP1000 design loaded with FCM fuel directly pass through a fuel pin without interacting with the fissile material. For resonance energy neutrons, the channeling effect would increase the resonance escape probability and more neutrons are thermalized in the water. For thermal energy neutrons, the channeling effect would increase the probability for the neutrons to return to the water without interacting with the uranium fuel. If soluble boron is present in the water, the returned thermal neutrons can be absorbed and the reactivity is decreased. This is why we see a large increase in the worth of control of soluble boron in the water. This increase is even strengthened by the use of moderation material carbon as the TRISO particle coating and matrix materials. Carbon increases the neutron slowing power in the modified design, in addition to the moderation by water. It predicts that a much higher thermal neutron spectrum would occur than in the original design. Therefore the worth of control from soluble boron becomes much higher in the modified design. The high increase of the control worth for PYREX rods can also be attributed to the increased thermal neutron spectrum. However, IFBA rods present a different change. Their worth of control is decreased in the modified design. This implies that other factors may have a dominant effect on the control worth of IFBA rods. The possible factors can be related to the spatial distribution of IFBA rods, as well as the heterogeneous fuel enrichment distribution. To clearly understand those factors, further investigations will be performed. In addition to the control worth, radial and axial fission power density distributions in both original and modified designs in Cases a-d are also investigated. Figures 3-5 show the radial distributions. For the original design, fission power density is calculated in each fuel pin (Fig. 3). For the modified design, fission power density is calculated in each fuel assembly (Figs. 4-5). From Fig. 3, both PYREX and IFBA rods flatten the fission power density distribution over the core so the core temperature distribution can be more uniform in the radial direction. The soluble boron in water is a strong absorber that can significantly depress the total power. Case (a) Case (b) Case (c) Case (d) 7/

8 Chao Liang and Wei Ji Figure 3. Radial Fission Power Density Distribution in Original AP1000 Design. Case (a) Case (b) Case (c) Case (d) Figure 4. Radial Fission Power Density Distribution in Modified Design f=40% & e=18%. Case (a) Case (b) Case (c) Case (d) Figure 5. Radial Fission Power Density Distribution in Modified Design f=60% & e=12%. 8/

9 An evaluation of full core AP1000 design loaded with FCM fuel From Figs. 4 and 5, the fission power density distributions in the modified design with the two FCM fuel options show similar features at each studied case. This indicates that the effect of burnable absorbers on the power distribution is not sensitive to the fuel particle packing fraction and the fuel enrichment. We also calculate the radial power peaking factors and they are shown in Table IV. The power peaking factor for FCM design is larger than the original design for all the four cases. This is primarily due to the constant fuel enrichment setting in the modified design. While in the original design, fuel assemblies with three different fuel enrichments are designed. In both designs, the utilization of PYREX rods can decrease the power peaking factor, while the IFBA rods and the soluble boron in water increase the peaking factors. Table IV. Radial power peaking factor (assembly level) Case a b c d Original AP FCM fueled f=40%, e=18% FCM fueled f=60%, e=12% Axial fission power density distributions are plotted in Fig. 6, where reference denotes the original AP1000 design. In Fig. 6, fission energy deposition is normalized to one source particle so the distribution curves can reflect the relative power level in each design. We can see that there is a decrease of overall power in the modified FCM fuel design compared with the original design, indicating a higher neutron flux is needed in the modified design if the same total power level is retained as the original AP1000 design. Therefore, a shorter fuel depletion cycle is expected. For all the four cases, the two FCM designs are very close with each other. This implies that the overall core performance may not be sensitive to the fuel enrichment and TRISO packing fraction. Case (a) Case (b) 9/

10 Chao Liang and Wei Ji Case (c) Case (d) Figure 6. Comparison of Axial Power Distribution. Table V. Axial power peaking factor (assembly level) Case a b c d Original AP FCM fueled f=40%, e=18% FCM fueled f=60%, e=12% The axial power peaking factors in both designs are summarized in Table V. We can see a very close power peaking factor (around 1.5) between the two FCM options and the original design in all the four cases, indicating a similar power distribution shape. Figure 7. Neutron Spectra in the Original and FCM AP1000 Designs. As a further investigation, we calculate the neutron spectra over the core for both designs under the condition that fuel burnable absorbers are all present (PYREX, IFBA and soluble boron) with f=40%, e=18%. The spectra are shown in Fig. 7. A much larger thermal spectrum peak can be seen in the modified FCM fuel design compared to the original design. This is consistent with the analysis of the excessive reactivity changes from the original design to the modified design. Compared with the original design, the neutron spectrum in the modified design does not show a strong resonance absorption induced depressions in the energy region of 10-6 ~ 10-2 MeV, indicating a higher escape probability in FCM fuel design for neutrons in the range of heavy 10/

11 An evaluation of full core AP1000 design loaded with FCM fuel metal resonance energy. Also, carbon in FCM fuel design provides additional moderation in addition to the water. Therefore, more thermal neutrons and less fast neutrons are present in the core. Considering a higher thermal spectrum peak and the k eff value calculated as at the normal operation (much smaller than 1.0 predicted for the original AP1000), this indicates a high absorption rate of the neutrons in boron absorbers if they are present. 4. CONCLUSIONS This paper presents a static neutronic feasibility study of loading fully ceramic microencapsulated (FCM) fuel in the AP1000 reactor design at BOC. A reactivity worth from each boron absorber (including discrete/integral fuel burnable absorber and soluble boron in water) is studied in the FCM fueled design with full core MCNP modeling. Our investigation shows the change of fuel design from homogeneous distribution to random packed TRISO particles can have significant impacts on the overall performance of the reactor. The first impact is due to the clustering of fissile material in FCM fuel compacts. Thermal and resonance neutrons are more likely to pass through the compact without interacting with fissile/fissionable materials and enter the water region. These neutrons are more likely to be absorbed in PYREX rods and soluble water, thus increase the control worth. The second impact is due to the existence of carbon in silicon carbon matrix and TRISO coatings. The carbon provides additional moderations in addition to the water and more thermal neutrons to be present in the reactor. With the significant changes of each control mechanism s role in the FCM fueled AP1000 design, the original control mechanism needs to be re-designed. Axial/Radial power density distributions are also calculated. With one enrichment region modeled in the FCM fuel design, the favorable radial power distribution of the AP1000 cannot be retained with a much larger peaking factor, but a good standing of axial power distribution is shown in the modified design. With all these calculations, we conclude that a direct replacement of fuel pellets by the FCM fuel in the AP1000 cannot retain the original optimum reactor core performance. Necessary modifications of the core design should be done. These may include keeping the fuel loading strategy of assembling different enrichments in the radial directions, and/or reducing the boron concentration in water and burnable absorbers. Moreover, the current work is only limited to a steady-state simulation at BOC. A burn-up calculation to study the performance of the reactor over time can provide more information to design a reactor. All these work will be investigated in the future. REFERENCES 1. M. H. Kim, K. M. Bae, Y. J. Kim, Use of carbon-coated particle fuels in PWR assemblies, Transactions of the American Nuclear Society, 77, pp (1997). 2. J. Porta, P. Lo Pinto, M. Bonnet, K. Kugeler, Z. Alkan, R. Heuss, W. von Lensa, Coated particle fuel to improve safety, design, economics in water-cooled and gas-cooled reactors, Progress in Nuclear Energy, 38, pp (2001). 3. Y. Shimazu, H. Tochihara, Y. Akiyama, K. Itoh, PWRS using HTGR fuel concept with cladding for ultimate safety, Advanced Reactors with Innovative Fuels Workshop Proceedings, Chester, United Kingdom, Oct , pp (2002). 11/

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