Causes of instability at LaSalle and consequences from postulated scram failure

The March 9, 1988 instability event at the LaSalle County-2 BWR power plant was simulated on the BNL Engineering Plant Analyzer (EPA) in order to assess its ability to simulate oscillatory transients, to identify the causes of the instability, to determine the maximum power amplitude of limit-cycle oscillations after postulated scram failure and to rank leading modeling parameters which affect instability. The EPA was found capable of simulating the LaSalle instability, which was found to have been caused by the combination of low core flow, moderately high power, low feedwater temperature and strong bottom power peaking. After postulated scram failure, the power was found to peak at 1300% of normal full power. Void reactivity, power peaking and core exit flow impedance were found to be the leading phenomena affecting thermohydraulic instability. 3 refs. 17 figs., 3 tabs