TRAC-BF1/NEM stability methodology for BWR core wide and regional stability analysis

摘要

A time--series analysis stability methodology is presented based on the TRAC-BFl/NEM coupled code. The methodology presented has a potential application for BWR core-wide and regional stability studies allowed by the 3D capabilities of the code. The stability analysis is performed at two different levels: using the TRAC--BFl point kinetics model and employing the three- dimensional neutronic transient capability of the NEM code. Point kinetics calculations show power fluctuations when white noise is applied to the inlet mass flow rate of each of the channel components. These fluctuations contain information about the system stability, and are subse- quently studied with time-series analysis methods. The analysis performed showed that the reactor core has a low-frequency resonance typical of BWRs. Analysis of preliminary three-dimensional calculations indicates that the power fluctuations do not contain the typical resonance at low frequency. This fact may be related to the limitation of the thermal-hydraulic (T H) feedback representation through the use of two-dimensiona1 tables for the cross-sections needed for 3D kinetics calculations. The results suggest that a more accurate table look-up should be used, which includes a three--dimensional representation of the feedback parameters (namely, average fuel temperature. average moderator temperature. and void fraction of the T H cell of interest). Further research is being conducted on improving the cross--section modeling methodology. used to feed the neutron kinetics code for both steady state and transient cases. Also a comprehensive analysis of the code transient solution is being conducted to investigate the nature of the weak dependence of the power response on T H variations during the performed 3 D stability transient calculations.