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# Cross Section Generation Guidelines for TRACE–PARCS (NUREG/CR-7164)

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## Publication Information

**Manuscript Completed:** November 2012**Date Published:** June 2013

**Prepared by:** D. Wang, B. J. Ade, and A. M. Ward

Oak Ridge National Laboratory P. O. Box 2008 Oak Ridge, TN 37831-6010

**Prepared for**: Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

C. G. Thurston, NRC Project Manager

NRC Job V6233

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## Abstract

This report documents a comprehensive comparison of cross sections calculated using different methodologies and codes, including CASMO, HELIOS, and TRITON XS. The conclusions from this study have resulted in this guidance document on how to choose cross section histories and branches for boiling water reactor (BWR) analysis, and the methodology to collapse the fine-energy and -space fluxes calculated by the detailed lattice calculation. The guidance herein is applicable to all BWR designs.

For BWR steady-state and transient analysis, the PARCS code uses two-energy-group cross sections for each computational node in the 3-dimensional grid. The PARCS cross sections are tabulated as a function of four instantaneous state variables: (1) control rod insertion, (2) fuel temperature, (3) coolant density, and (4) soluble poison concentration. The cross section values also depend on the isotopic mixture (i.e., concentration of 235U, 239Pu ...), which is characterized as a function of control rod and moderator density history variables.

In a typical calculation, the fuel temperature, moderator density, and soluble boron concentration are calculated by the TRACE code for a coupled TRACE/PARCS analysis. The instantaneous control rod insertion is provided by the user in the input deck. The historic control rod and moderator density values are provided by a steady-state core-follow simulator, which has followed the core operation since the initial loading up to the time of the transient to be calculated. All these parameters are taken into account to estimate the instantaneous cross section based on the tabulated values. This report documents the expected error in evaluating the instantaneous cross section as a function of the data table structure.

As a result of this study, guidelines for BWR cross section generation have been generated. The recommendations are the use of four instantaneous moderator density values at 0, 40, 70, and 90% void fraction at three different fuel temperatures of 500, 950, and 1500 K. For the history effect, three moderator density values at 0, 40, and 70% at a single 950 K fuel temperature provide sufficiently accurate results. For all cases, the user must generate these branches for controlled and uncontrolled bundles.

In addition, a coolant density branch with a moderator density of 1000 kg/m3 is needed to accurately model cold depressurized conditions. If boron injection is modeled in a BWR transient analysis, boron branches should be included.

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