Recent improvements in the coupled TRAC-PF1/NEM methodology

The latest released version of the coupled three-dimensional thermal-hydraulic/kinetics TRAC-PF1/NEM code has reached the level of direct industry application. To complete the development of TRAC-PF1/NEM in a production mode version, further improvements have been implemented and tested. To enhance the accuracy of neutronic nodal predictions, assembly discontinuity factors (ADFs) have been utilized in the coupled TRAC-PF1/NEM methodology. The ADFs are obtained by the CASMO-3 single-assembly spectrum/depletion calculations, and they are treated in the TRAC-PF1/NEM cross-section table generation procedure as additional homogenization parameters. Since the radial component of uncertainty (introduced by not using discontinuity factor) is dominant, the ADFs are introduced in the nodal expansion method (NEM) algorithm for each of the four radial surfaces of the node. Choosing axial node boundaries to coincide with material discontinuities allows unity discontinuity factors to be used at the two axial node surfaces. The programs developed, which extract the cross-section information from the CASMO-3 output files, have been extended to read ADF information from CASMO card image files. Subsequently, the ADFs are assembled into the NEM cross-section tables.