A Parametric Study of Fuel Lattice Design for HTR-10 . Meng-Jen Wang Institute of Nuclear Engineeting and Science, National Tsing Hua University, Hsinchu, Taiwan 30013, R.O.C, ' Jinn-Jer Peir' Nuclear Science and Technology Development Center, National Tsing Hua University, - • • ,,- Hsinchu, Taiwan 30013, R,O,C. .K. Chen-Wei Chi Jenq-Horng Liang Institute of Nuclear Engineering and Science, National Tsing Hua University, ' ' Hsinchu, Taiwan 30013, R,O.C, In this study, the multiplication factor and neutron spectrum be- haviors were investigated against the moderator-to-fuel ratio, the fuel loading height, and the detector location in high-temperature gas-cooled reactor (HTR)-IO. The MCNP5 computer code (version 1.51) was employed to perform all the simulation computations. The results revealed that the multiplication factor varies signifi- cantly depending on the moderator-to-fuel ratio and the fuel load- ing height due to the competition among the neutron moderation and absorption abilities of the moderator as well as the netitron production ability of the fuel. Due to its inherent stability, HTR-10 is deliberately designed such that the multiplication factor de- creases and the neutron spectrum softens as the moderator-to-fuel ratio increases. The average neutron energy level in the HTR-10 fuel balls is approximately 240 keV and ranges from smallest to largest at the middle, bottom, and top of the reactor core, respectively [DOI: 10.1115/1.4002879] Keywords: fuel lattice, HTR-10, MCNP5, moderator-to-fuel ratio, multiplication factor, neutron spectrum 1 Introduction ' ' Recently, high-temperature gas-cooled reactors (abbreviated as HTGRs or HTRs) have attracted increasing interest and are con- sidered as the most promising candidate for generation IV reactors due to their superior advantages including inherent safety, great modularity, low cost, easy financing, high efficiency in electricity generation, and short construction time [1,2], particularly HTR- 10, a well-known experimental 10 MW pebble-bed type HTR that was built in China and achieved its first criticality on Dec. 1, 2000 [2]. The key reactor and fuel lattice design parameters are listed in Tables 1 and 2, respectively. A detailed description of HTR-10 can be found in Refs. [1,3], among others. Basically, the original Corresponding author. Contributed by the Nuclear Division of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINE.S AND POWER. Manuscript received July 2. 2010; final manuscript received July 7, 2010; published online April II, 2011. Editor: Dilip R. Ballal. weight of heavy metal per fuel ball is 5 g, while the volumetric filling fraction of fuel balls in the reactor core is 0.61. In addition, the amount of fuel and graphite balls in the reactor core is the same and is approximately 13,500. Each fuel ball contains about 8335 tiny triple-coated isotropic (TRISO) particles comprising uranium dioxide kernels coated with three layers of pyrolitic car- bon (PyC) and one layer of silicon carbide. Notably, the PyC layers are of various densities in order to accommodate fission products while the SiC layer is designed to serve as a cladding in order to avoid the release of fission products. As can be clearly seen, the huge amount and complicated design of fuel balls makes analysis of the nuclear analysis more complex. Furthermore, in order to enhance availability in reactor operations, the fuel balls roll and recycle continuously by means of being discharged from the bottom and refreshed at the top of the reactor core according to their bumup. Moreover, the on-line refueling design prevents neutron monitors from being installed in the reactor core. There- fore, a thorough theoretical understanding of the neutronic char- acteristics in regard to the variety of fuel lattice designs is of prime importance in order to enhance the smooth operation of HTR-10 and promote further improvements in it. However, to the best of our knowledge, no research has been conducted involving a parametric study of fuel lattice design for HTR-10. Therefore, this constitutes the main objective of this study. This parametric study of HTR-10 was conducted by investigat- ing the dependence of the multiplication factor on the concentra- tion ratio of graphite to fuel (i.e., abbreviated as Nc/N^j but re- ferred to as the moderator-to-fuel ratio herein) and the fuel loading height. Variations in the neutron spectrum in different locations in the reactor core as well as the moderator-to-fuel ratios were also studied in depth. 2 Simulation Models In essence, the Monte Carlo N-Particle (MCNP) is a general- purpose, continuous-energy, generalized-geometry, time- dependent, coupled neutron-photon-electron, Monte Carlo trans- port computer code [5] and is widely used in nuclear reactor analysis. The neutron energy levels employed in the code nor- mally range from 10~" MeV to 20 MeV. In this study, the MCNP.5 computer code (version 1.51 ) [4,5] was adopted [6] to perform all the simulation computations. Notice that the MCNP5 computer code (version 1.20) was also utilized in order to make compari- sons. The MCNP5 computer code consists of three parts: cell, sur- face, and data cards. The data library employed was ENDF/B-6, while the arrangement of reactor core and TRISO particles was based on the "lattice" and "universe" functions given in MCNP5. Following the simulation model proposed by Lebenhaft [1], the fuel and TRISO particles' lattice designs employed in this study were body-centered-cubic (bcc) and simple cubic (sc) structures, respectively. A schematic representation of the fuel and TRISO particles' lattice designs is depicted in Fig. 1. As can be seen, the ball located at the center of the bcc structure is the fuel ball, while the balls located at the comers of the bcc structure are graphite balls. Also notice that the temperature utilized in this study for the MCNP5 cross section data library was 293 K for simplicity. In order to validate the MCNP5 model of HTR-10, this study conducted the computations for the multiplication factor as a function of moderator-to-fuel ratio for various fuel loading heights. The fuel loading height with a multiplication factor of unity is defined as the critical fuel loading height. In this study, the various moderator-to-fuel ratios were obtained by varying the sizes of the TRISO particles, fuel region, fuel balls, and graphite balls as well as the number of TRISO particles. These results are tabulated in Table 3. The original fuel lattice design (i.e., the so- called design point herein) suggested by Lebenhaft [1] is shown in case 4. Notably, cladding thickness (i.e., the thickness between the fuel ball radius and the fuel region radius) is normally 0.5 cm for the fuel lattice design shown in cases 1-6. However, the cladding thickness for the fuel lattice design shown in cases 7-13 is re- Journal of Engineering for Gas Turbines and Power Copyright © 2011 by ASME AUGUST 2011, Vol. 133 / 084503-1