Nuclear Engineering and Design 271 (2014) 348–351 Contents lists available at ScienceDirect Nuclear Engineering and Design jou rn al hom epage : www.elsevier.com/locate/nucengdes Stochastic arranging of CFPs in HTTR and criticality benchmark considering different modeling of CFPs Abbas Torabi Ardakani ∗ , M.B. Ghofrani, A.O. Ezzati Sharif University of Technology, Department of Energy Engineering, Sharif University of Technology, Azadi St., Tehran, Iran a b s t r a c t In this paper besides proposing a new method for stochastic arranging of coated fuel particles (CFPs) in graphite matrix of HTTR, the effect of homogeneous and heterogeneous (including stochastic and regular arrangement) modeling of CFPs in criticality calculations of this reactor is investigated using MCNPX. In regular arrangement, each CFP was modeled as a fixed sphere in the center of a cubic lattice; subsequently, depending on fuel compact’s packing factor, each fuel compact was filled with around 13,000 (approximate number of CFPs in fuel compact) of this cubic lattice. In stochastic arrangement, an algorithm was developed to distribute definite number of CFPs randomly in a cubic lattice; next each fuel compact was filled with the cubic lattice. Criticality calculations including effective multiplication factor and critical position of control rods in three core configurations (thin annular core, thick annular core and fully-loaded core) have been carried out. The effect of nitrogen impurity was further investigated in all calculations. Results were benchmarked with different simulation results presented in the IAEA-Technical Document No.1382. It was realized that heterogeneous modeling of fuel particles in both the regular and stochastic arrangements was more accurate and efficient approach than homogenous modeling .It was also observed that there is no significant difference between the results of regular and stochastic arrangements. In comparison with other simulations, presented in Tech-Doc1382, the simulated critical heights of control rods in the heterogeneous modeling were more consistent with the experimental data. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The High Temperature Reactors seem to be a promising tech- nology for the 4th generation of nuclear reactors. The High Temperature Engineering Test Reactor (HTTR) of the Japan Atomic Energy Agency (JAEA) is a 30 MWth, graphite-moderated, helium- cooled reactor that was constructed with the objectives to establish and upgrade the technological basis for advanced HTGRs as well as to conduct various irradiation tests for innovative high- temperature research (Saito et al., 1994). During the start-up core physics tests of the HTTR, various annular core configuration were formed to provide experimental data for verification of design codes for future HTGRs. In HTTR, there are 150 graphite blocks, each containing 31–33 fuel rods and each rod has 14 fuel compacts. The cylindrical fuel compacts contain large number of (around 13,000) coated microscopic fuel particles which are called TRISO. TRISO has a spherical uranium oxycarbide region surrounded by 4 layers of three isotropic materials (low-density pyrolytic carbon (PyC), high-density pyrolytic carbon, ceramic layer of silicon carbide and high-density pyrolytic carbon). The amount of fuel inventory depends on packing fraction (fraction of fuel compact volume filled with TRISO particles) of TRISO particles. ∗ Corresponding author. Tel.: +98 21 66166102. E-mail address: ab torabi@energy.sharif.ir (A. Torabi Ardakani). Moreover, there are 16 pairs of control rods in the HTTR which 9 pairs are in permanent reflector and 7 pairs in replaceable reflector which in criticality condition, three pairs are out of core and others are in the core with the same height from the active core bottom. CFPs’ heterogeneity and different modeling of them (homoge- nous and heterogeneous) in fuel compacts of HTTR is highly important issue for calculating parameters. Although this is important and interesting issue but few stud- ies have been addressed (Zakova and Talamo, 2008). It has some reasons such as codes limitations on stochastic geometry model- ing as well as large amount of data entries regarding HTTR’s unique feature of 12 different enrichments. In this work, the effect of homogeneous and heterogeneous (stochastic and regular arrangement) modeling of CFPs on the HTTR criticality calculations including effective multiplication factor and critical position of control rods were compared with one another. All calculations have been carried out by MCNPX version 2.6. Fig. 1 shows vertical cross section of HTTR and Table 1 shows main specifications of HTTR. 2. Methods of arranging cfps In this section, for the case of heterogeneous modeling, different arrangements of CFPs in graphite matrix of fuel compact will be described. It is obvious that in the case of homogenous modeling, CFPs and graphite were mixed to form a unit material. 0029-5493/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2013.11.060