M&C 2017 - International Conference on Mathematics & Computational Methods Applied to Nuclear Science & Engineering, Jeju, Korea, April 16-20, 2017, on USB (2017) This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for the United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). Development of the CASL-VERA V4.2m5 MPACT 51-group Libraries with ENDF/B-VII.0 and VII.1 Kang Seog Kim 1 , Mark L. Williams 1 , Dorothea Wiarda 1 , Kevin T. Clarno 1 , and Yuxuan Liu 2 1 Oak Ridge National Laboratory, 1 Bethel Valley Rd., Oak Ridge, TN 37831, USA kimk1@ornl.gov; williamsml@ornl.gov; wiardada@ornl.gov; clarnokt@ornl.gov 2 Department of Nuclear Science and Engineering, University of Michigan: 2355 Bonisteel Blvd. Ann Arbor, MI, 48105 yuxuanl@umich.edu Abstract - The MPACT neutronics module of the Consortium for Advanced Simulation of Light Water Reactors (CASL) core simulator is a 3-D whole core transport code being developed for the CASL toolset, Virtual Environment for Reactor Analysis (VERA). The v4.1m3 MPACT 47-group library with ENDF/B- VII.0, which was developed previously by using Oak Ridge National Laboratory AMPX/SCALE code packages, includes deficiencies, especially for burnt fuel cases. New v4.2m5 MPACT 51-group libraries with ENDF/B-VII.0 and VII.1 have been developed to have better accuracy by improving the library generation methodology. This study discusses a detailed procedure to generate the MPACT 51-group libraries and results for various benchmark problems. I. INTRODUCTION The MPACT [1] neutronics module of the Consortium for Advanced Simulation of Light Water Reactors (CASL) [2] core simulator is a 3-D whole core transport code being developed for the CASL toolset, Virtual Environment for Reactor Analysis (VERA). MPACT is under development for neutronics simulation coupled with the CTF code for thermal- hydraulics simulation for pressurized light water reactors. Key characteristics of the MPACT code include (1) a subgroup method for resonance self-shielding and (2) a whole-core transport solver with a 2-D/1-D synthesis method. Thus the MPACT code requires a cross-section library to support all the MPACT core simulation capabilities. The MPACT cross-section library has been generated based on the AMPX/SCALE code packages [3] developed at Oak Ridge National Laboratory with additional programs and procedures that are being developed to generate subgroup data, transient data, epithermal upscattering resonance data, and the final MPACT format library, which is not supported by the AMPX/SCALE code packages. New AMPX and MPACT 51-group libraries with the ENDF/B-VII.0 and VII.1 nuclear data have been generated for the CASL MPACT neutronics module. The new 51- group structure was developed to be a subset of the SCALE- 6.2 [3] 252-group structure and to be applicable for both pressurized water reactor (PWR) and boiling water reactor (BWR) applications. Currently, the primary scattering calculation option of the VERA-CS MPACT module is a transport-corrected P0. Therefore, the 51-group structure has been developed for group widths to be evenly distributed to minimize convergence issues in whole-core transport calculations. Several deficiencies have been identified in the previous MPACT 47-group library [4]. Deficiencies include a very large reactivity bias for high-void BWR fuels, relatively large group-wise reaction rate errors, large reactivity bias at moderate and high burnup points, sensitivity to the number of radial rings, cold-to-hot reactivity swings and no available ENDF/B-VII.1 library. The new MPACT 51-group libraries have been developed to resolve these issues. This paper describes the detailed procedure used to generate the new v4.2m2 MPACT 51-group libraries with ENDF/B-VII.0 and VII.1 and provides various benchmark results that demonstrate improvements in the new libraries compared with the v4.1.m3 MPACT 47-group library with ENDF/B-VII.0. II. LIBRARY GENERATION 1. MPACT Library Generation The MPACT multigroup (MG) library generation procedure includes eight steps. (a) The AMPX MG library is generated by using temperature-dependent pointwise (PW) weighting functions, which are obtained from the CENTRM [3] PW slowing-down transport calculations. Resonance data are generated by narrow resonance (NR) approximation. (b) Intermediate resonance (IR) parameters are generated, and resonance data are updated with new data, which are obtained by the CENTRM homogeneous slowing-down calculations using LAMBDA and IRFFACTOR-hom [3], as shown in Figure 1. (c) Resonance data are updated with new data calculated by the CENTRM heterogeneous slowing-down calculations for important resonance nuclides by using IRFFACTOR-het [3]. (d) Subgroup data are generated for the selected nuclides by using SUBGR [3]. (e) Epithermal upscattering resonance and subgroup data are generated for 238 U by using the continuous energy SCALE Monte Carlo code CE-KENO [3], the embedded self-shielding method [5] and SUBGR.