Nodal kinetics model upgrade in the Penn State coupled TRAC/NEM codes

The Pennsylvania State University currently maintains and does development and verification work for its own versions of the coupled three-dimensional kinetics/thermal-hydraulics codes TRAC-PF1/NEM and TRAC-BF1/NEM. The subje ct of this paper is nodal model enhancements in the above mentioned codes. Because of the numerous validation studies that have been performed on almo st every aspect of these codes, this upgrade is done without a major code r ewrite. The upgrade consists of four steps. The first two steps are designe d to improve the accuracy of the kinetics model, based on the nodal expansi on method. The polynomial expansion solution of 1D transverse integrated di ffusion equation is replaced with a solution, which uses a semi-analytic ex pansion. Further the standard parabolic polynomial representation of the tr ansverse leakage in the above ID equations is replaced with an improved app roximation. The last two steps of the upgrade address the code efficiency b y improving the solution of the time-dependent NEM equations and implementi ng a multi-grid solver. These four improvements are implemented into the st andalone NEM kinetics code. Verification of this code was accomplished base d on the original verification studies. The results show that the new metho ds improve the accuracy and efficiency of the code. The verification of the upgraded NEM model in the TRAC-PF1/NEM and TRAC-BF1/NEM coupled codes is u nderway. (C) 1999 Elsevier Science Ltd. All rights reserved.