A computational model, based on a rigorous application of the first law of thermodynamics together with heat and mass transfer laws, makes no a priori assumptions concerning the thermodynamic path of the process. This method has been reduced to practice in a digital computer program called TOPS. Transient primary pressures for three load-drop transients at Shippingport agree well with calculations using the TOPS model. The model is particularly useful in predicting the effects of spray systems and heater operation on plant pressure transients.