Flashing and heat transfer during outsurge transients in a small pressurizer

The pressurizer behavior during the thermal-hydraulic transients of a pressurized water reactor (PWR) is a major issue of nuclear plant safety. During a loss of coolant accident of a sufficient extent, an outsurge from the pressurizer to the primary loop occurs and the system depressurizes notwithstanding the steam generated by the electrical heaters and by the flashing phenomena. Some complex thermal-hydraulic processes take place in the pressurizer such as the heat transfer between the wall and the fluid, the mass and energy transport through the liquid-vapor interface, the mixing of the subcooled and saturated water. The flashing and the wall heat transfer during an outsurge are important to evaluate the pressure transients. Here, experimental data and an analysis of outsurge transients in a small pressurizer at constant electrical power input are discussed. The test section is a vertical cylinder that is connected at the bottom to a cylindrical horizontal section where the steam is produced by electric heaters. The range of the pressure is from 3 to 6 MPa. The fluid and outer wall temperatures are measured at several elevations, and the flashing rate and the fluid/wall heat transfer are derived from the pressure and temperature time history data. The pressure transient is simulated by means of an extension of the Lu and Simpson prediction method