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

A time-series analysis stability methodology is presented based on the TRAC-BF1/NEMcou- pled code. The methodology presented has a potential application for BWRcore-wide and regio- nal stability studies allowed by the 3D capabilities of the code. The stability analysis is performed at two di€erent levels: using the TRAC-BF1 point kinetics model and employing the three- dimensional neutronic transient capability of the NEM code. Point kinetics calculations show power ¯uctuations when white noise is applied to the inlet mass ¯ow rate of each of the channel components. These ¯uctuations contain information about the system stability, and are subse- quently studied with time-series analysismethods.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 ¯uctuations do not contain the typical resonance at low fre- quency. This fact may be related to the limitation of the thermal±hydraulic (T±H) feedback representation through the use of two-dimensional tables for the cross-sections needed for 3D kinetics calculations. The results suggest that amore 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). Fur- ther 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 3D stability transient calculations.