Analysis of LaSalle-2 neutron flux oscillation event with RETRAN code

A series of calculations were performed with a transient thermal-hydraulic calculation code RETRAN on the neutron flux oscillation event at LaSalle County Station Unit 2 on March 9, 1988, with the objectives to clarify the causes of the oscillation and effects of the power distribution on stability which were pointed out as a major difference between LaSalle-2 and typical BWRs in view of BWR stability. A method to determine instability threshold developed by the authors was used in the calculations. In order to clarify the causes of the oscillation, the event was simulated and a stability map under the conditions of the event was produced. By drawing the calculated locus of the event on the instability map, it was clarified that the neutron flux oscillation was caused by increase in core inlet subcooling during natural circulation condition due to recirculation pumps trip. In order to clarify effects of power distributions on the stability, sensitivity calculations were performed on the transient behaviors with the LaSalle-2 scenario changing the feedwater temperature transient. Also, the sensitivity calculations on the instability map assuming extremely stable power distributions were performed. Based on the calculated results it was concluded that the power distribution had large effects on the stability. It was pointed out that, in the extremely stable case, a neutron flux oscillation would not occur during a transient with the LaSalle-2 scenario and a margine in core inlet subcooling would be 10 K until oscillation would occur. (author)