The multiphase coolant flow across the perforated immersion plate during a hypothetical core disruptive accident in a liquid-metal fast breeder reactor was simulated in a one-dimensional model. Extending from previous work with one-phase flow, water-air mixtures were used to test two-phase behavior. A large experimental matrix included systematic variation of the following parameters: geometry of the immersion plate (perforation ratio, number of the holes), height of the fluid head over the immersion plate, air volume fraction, size of the air bubbles, and acceleration of the fluid. The pressure drop across the immersion plate, the forces acting on the immersion plate and on the upper plate, acceleration and displacement of the piston, the air volume fraction, and the size of the air bubbles were measured in a wide range of Strouhal and acceleration numbers. The flow pattern downstream of the immersion plate was filmed with a high- speed camera. The following correlations were investigated:

  1. drag coefficients of the immersion plates as a function of the acceleration and Strouhal number
  2. time delay of the force on the upper plate as a function of the cavitation number
  3. forces and impulses acting on the upper plate compared with those acting on the immersion plate.
Finally, using high-speed film pictures, the formation of fluid jets downstream of the immersion plate was investigated. The following relations were obtained:
  1. displacement of the mixture surface and of the jets as a function of the perforation ratio and of the air volume fraction
  2. cavitation volume as a function of the cavitation number.