Protection of Drain Pumps Against Transient Cavitation

It has been known that drain pumps in nuclear power plants may suffer cavitation under transient turbine load reductions [1]. Although increasing the size and height of the drain tank can prevent it, such provisions are often not practical, and in some instances even not possible. Some components, such as moisture separators, normally drain through drain receivers into the surface heater or drain tank. Since the drains are at the saturation condition, the drainage is generally accomplished by gravity. This necessitates locating the moisture separators considerably above the drain tank. With a limited physical elevation of the moisture separators relative to the main turbine in connection with a low profile of nuclear power plants, it is impractical to raise the heater-drain tank system somewhat similar to the deaerator in fossil power plants. This paper explores some conceptual protective methods, and briefly discusses their pros and cons as applied to both drain pumping forward and backward systems. The method of quantitative determination of design parameters required for each protective method is either referred to or derived. Based on simplicity, economy, and reliability, this paper concludes that the drain tank pressure decay control system appears to be the most promising protective method for the drain pumping forward system, whereas either the continuous feedwater injection system or the continuous drain subcooling system is the optimum method for the drain pumping backward system.