Thermohydraulic behavior of the coolant in the initial phase of a loss-of-coolant accident

Thermohydraulic phenomena related to the issue of nuclear reactor safety have been focusing attention of researchers and engineers for many years. Nevertheless, physical phenomena occurring after a large-break or small-break loss-of-coolant accident (LOCA) are still poorly understood. In the first instants after the primary circuit pipe rupture there is a rapid depressurization of the system followed by explosive vapor generation in the superheated coolant and by the discharge of the two-phase mixture. The propagating depressurization wave imposes severe loads on the internal structures of the reactor. The paper summarizes the results of experimental investigations carried out at the National Research Institute for Machine Design during the past few years and concentrates on three important issues: critical two-phase flow; depressurization wave propagation and vapor generation in super heated coolant; interaction of the depressurization wave with the internal structures of the reactor