A loss-of-coolant accident in a pressurized water, nuclear power plant is one which permits coolant to escape from the primary system. If such an accident were allowed to proceed uninhibited by corrective measures, the core may lose sufficient coolant such as to permit core heatup. In order to design a system to maintain the core cool, it is necessary to evaluate the coolant blowdown process which occurs after rupture and thereby establish the pressure-time and volume-time relationships of the primary coolant after rupture. The coolant blowdown process after rupture is complex because the two-phase expansion of water and steam obtains after saturation pressure is attained. The analysis of this process utilizes heat, mass and volume balances of the reactor coolant to establish the thermodynamic state of the reactor coolant at any time after rupture within conservative limits.