Boiling Water Reactor Stability Analysis by TRACE/PARCS: Modeling Effects and Case Study of Time Versus Frequency Domain Approach

The TRACE/PARCS code was applied in this work to examine the validity of the coupled three-dimensional thermal-hydraulics and neutronics system analysis codes for boiling water reactor stability analysis. The evaluation was performed against the Ringhals-1 stability tests and compared with the frequency domain analysis using the code STAB. A comprehensive assessment of modeling choices for the TRACE stability analysis has been made, including effects of time-space discretization and numerical schemes, thermal-hydraulics channel grouping, neutronics modeling, and control system modeling. It was found that with careful control of numerical diffusion, the predictions from TRACE agree reasonably well with the Ringhals-1 test results and the predictions from STAB. The benchmark results of both codes against the Ringhals stability test are found to be at the same level of accuracy. The biases for the predicted global decay ratio are [approximately]0.07 in TRACE results and -0.04 in STAB results. However, the standard deviations of their decay ratios are both large, [approximately]0.1, indicating large uncertainties in both analyses. The uncertainties in both modeling approaches are identified. Although the TRACE code uses more sophisticated neutronics and thermal-hydraulics models, the modeling uncertainty is not less than that of the STAB code.