A parametric study of variables affecting BWR instability

Calculations were performed to investigate the effect of ranging several key variables on the power response of a BWR core subjected to periodic flow oscillations, such as those that might be exhibited during an instability event. It was observed that flow oscillation amplitude, its frequency, and core inlet flow subcooling have the greatest impact on the magnitude of resultant power oscillations. In particular, when flow is oscillated at a frequency of about 0.235 Hz for a typical 8 X 8 BWR rod bundle, it may produce power peaks more than 100 times the initial power. Further, based on some experimental data and supporting calculations, it is shown that increasing feedwater subcooling leads to both increased flow amplitude and decreasing frequency in a boiling natural circulation loop. It was concluded that the degree of subcooling of feedwater is the single most important parameter affecting the magnitude of reactor power oscillations. In summary: A single channel TRAC -BF1 model of a BWR core to study the effect of individual parameter variations, on reactor peak and average power during flow oscillations. These included flow amplitude, flow oscillation frequency, inlet subcooling, gap conductance, number of parallel channels and several others. In addition, the effects of temporal and spatial discretization of the problem were assessed. Among these, the inlet flow conditions have the strongest effect on the magnitude of power oscillations. In particular, the power peaks are very sensitive to the frequency of the flow oscillation and there is a critical frequency of the flow oscillation that produces very high peaks in the power. In the second part of this study it was shown that reduction of the feedwater temperature in a natural circulation boiling system reduces the flow oscillation frequency and increases its amplitude. It is concluded that in a BWR, during natural circulation, it is critical to maintain feedwater temperature control to prevent high subcooling of the core synergism of three separate effects, namely: higher moderation, increased amplitude and decreased frequency of the flow oscillations. (authors)