In a related paper (L. Cheng et al., “TRACE/PARCS Analysis of Anticipated Transient Without Scram with Instability for a MELLLA+ BWR/5,” Nucl. Technol. Vol. 196), the results of TRACE/PARCS calculations for representative anticipated transient without scram (ATWS) events leading to core instability (ATWS-I) were presented. In that analysis, instability onset was observed in response to changing plant conditions of power, flow, and feedwater temperature. The baseline calculations were performed without using a PARCS feature to simulate noise in the reactor.

When a simulated reactor is unstable but is in a steady-state condition, an analytical tool may not show the onset of instability because there would not be a perturbation to excite oscillation. Such a condition of artificial stability could not persist in an actual reactor where subtle variation of local conditions (e.g., void fraction) would provide a constant source of perturbation, or “noise.” The regulatory purpose of the current work is to study the reliability of the TRACE/PARCS prediction of instability onset and oscillation growth during ATWS-I by providing a source of noise in the simulation. In addition, the results of this study support a generic methodology recommendation for any future studies.

PARCS has a feature that can simulate the reactivity effect of perturbations in the local void fraction. This feature, referred to as the white noise feature, is used to provide an artificial source of constant, local perturbation that would more closely mimic the actual reactor condition where local void fractions are constantly changing. Sensitivity of the onset timing and growth was studied by varying the magnitude, frequency, and contour of the perturbations applied by the white noise feature.

The study concludes that the onset timing and growth of both the initial corewide and subsequent bimodal oscillation stabilized at a certain combination of perturbation magnitude, frequency range, and frequency resolution. With the appropriate range of these parameters, the instability onset occurs ~20 s earlier, and peak oscillation amplitude is achieved ~15 s earlier when compared to the baseline calculations. Given the importance of oscillation onset and growth on potential fuel damage, this study recommends a specific methodology with respect to white noise to ensure reliable prediction with TRACE/PARCS for future studies.