Thermal-hydraulic oscillations at LaSalle

This paper describes how, on March 9, 1988, LaSalle Unit 2 underwent a dual recirculation pump trip event. Trip of the recirculation pumps resulted in a flow decrease to natural circulation flow. As a consequence of the rapid power decrease which accompanied the flow decrease, the feedwater heater level control system began isolating extraction steam from the heaters. The cooler feedwater led to an increase in power and transition into a region of thermal-hydraulic instability. Approximately five minutes after the recirculation pump trip, large (25 percent peak-to-peak) neutron flux oscillations were observed. The oscillations increased in magnitude until a reactor trip occurred on high average power range neutron flux at 118 percent of full power. Although no fuel thermal or mechanical limits were exceeded during the event, the event caused concern relative to the adequacy of licensing analyses and potential violation of safety limits