Full Core modeling techniques for research reactors with irregular geometries using Serpent and PARCS applied to the CROCUS reactor Daniel J. Siefman a,⇑ , Gaëtan Girardin a , Adolfo Rais a , Andreas Pautz a , Mathieu Hursin b a Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland b Paul Scherrer Institute (PSI), CH-5232 Villigen-PSI, Switzerland article info Article history: Received 20 January 2015 Received in revised form 30 April 2015 Accepted 4 May 2015 Available online xxxx Keywords: CROCUS reactor Serpent PARCS Research reactors Cross-section generation abstract This paper summarizes the results of modeling methodologies developed for the zero-power (100 W) teaching and research reactor CROCUS located in the Laboratory for Reactor Physics and Systems Behavior (LRS) at the Swiss Federal Institute of Technology in Lausanne (EPFL). The study gives evidence that the Monte Carlo code Serpent can be used effectively as a lattice physics tool for small reactors. CROCUS’ core has an irregular geometry with two fuel zones of different lattice pitches. This and the reactor’s small size necessitate the use of nonstandard cross-section homogenization techniques when modeling the full core with a 3D nodal diffusion code (e.g. PARCS). The primary goal of this work is the development of these techniques for steady-state neutronics and future transient neutronics analyses of not only CROCUS, but research reactors in general. In addition, the modeling methods can provide use- ful insight for analyzing small modular reactor concepts based on light water technology. Static compu- tational models of CROCUS with the codes Serpent and MCNP5 are presented and methodologies are analyzed for using Serpent and SerpentXS to prepare macroscopic homogenized group cross-sections for a pin-by-pin model of CROCUS with PARCS. The most accurate homogenization scheme lead to a dif- ference in terms of k eff of 385 pcm between the Serpent and PARCS model, while the MCNP5 and Serpent models differed in terms of k eff by 13 pcm (within the statistical error of each simulation). Comparisons of the axial power profiles between the Serpent model as a reference and a set of PARCS models using dif- ferent homogenization techniques showed a consistent root-mean-square deviation of 8%, indicating that the differences are not due to the homogenization technique but rather arise from the definition of the diffusion coefficients produced by Serpent. A comparison of the radial power profiles between the best PARCS model and full-core Serpent model showed largest relative differences in terms of power prediction at the core periphery, which is believed to be the product of the geometry simplifications made, the diffusion coefficients produced by Serpent, and the two-group energy structure used. The worth of a single control rod reproduced in PARCS showed a difference of 33 pcm from its 169 pcm worth simulated in Serpent. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The Laboratory for Reactor Physics and Systems Behavior (LRS) at the École Polytechnique Fédérale de Lausanne (EPFL) is conduct- ing a comprehensive three-dimensional dynamic analysis of its zero-power (100 W) teaching and research reactor CROCUS. The funding for the project is being provided by swiss nuclear and its ultimate goal is to update CROCUS’ safety report allowing for more flexible operation and expanded research applications. The current safety analysis report for CROCUS is based on diffusion theory and point kinetics for neutronics analyses and 1D thermal hydraulics for accident analyses (i.e. unprotected reactivity insertions). Recently, an effort has started within the LRS to increase the sophis- tication of CROCUS modeling by using more modern codes. The immediate goal of the project is the development of new and accu- rate models of CROCUS to perform static neutronics analyses and calculate the reactivity worth of the control rods. The long term objective is to perform dynamic, coupled neutronics-thermal hydraulics simulations of CROCUS. The new modeling methodology uses a deterministic approach for steady-state and transient neutronics analyses with the nodal simulator PARCS (Downer et al., 2006). For the work presented here, CROCUS was modeled with a pin-by-pin representation of http://dx.doi.org/10.1016/j.anucene.2015.05.004 0306-4549/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: daniel.siefman@epfl.ch (D.J. Siefman), gaetan.girardin@epfl.ch (G. Girardin), adolfo.rais@epfl.ch (A. Rais), andreas.pautz@epfl.ch (A. Pautz), mathieu.hursin@psi.ch (M. Hursin). Annals of Nuclear Energy xxx (2015) xxx–xxx Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene Please cite this article in press as: Siefman, D.J., et al. Full Core modeling techniques for research reactors with irregular geometries using Serpent and PARCS applied to the CROCUS reactor. Ann. Nucl. Energy (2015), http://dx.doi.org/10.1016/j.anucene.2015.05.004