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 subject of this paper is nodal model enhance- ments in the above mentioned codes. Because of the numerous validation studies that have been performed on almost every aspect of these codes, this upgrade is done without a major code rewrite. The upgrade consists of four steps. The first two steps are designed to improve the accu- racy of the kinetics model, based on the nodal expansion method. The polynomial expansion solution of 1D transverse integrated di€usion equation is replaced with a solution, which uses a semi-analytic expansion. Further the standard parabolic polynomial representation of the trans- verse leakage in the above 1Dequations is replacedwith an improved approximation.The last two steps of the upgrade address the code eciency by improving the solution of the time-dependent NEMequations and implementing amulti-grid solver. These four improvements are implemented into the standalone NEMkinetics code. Verification of this code was accomplished based on the original verification studies. The results show that the new methods improve the accuracy and eciency of the code. The verification of the upgraded NEMmodel in the TRAC-PF1/NEMand TRAC-BF1/NEM coupled codes is underway