Nuclear reactors that implement natural circulation passive safety systems may produce large temperature gradients in their working fluid as a result of local cooling caused by emergency core coolant (ECC) injection or local heating caused by steam condensation or heat exchanger heat transfer. Thermal stratification arises because the low flow condition typically encountered in a natural circulation system greatly reduces the amount of fluid mixing that can occur. Examples of thermal stratification during ECC injection include the formation of cold plumes in the downcomer, and liquid thermal stratification in the lower plenum, cold legs and loop seals.

ECC injection into horizontal piping partially filled with steam also results in liquid temperature stratification. The cooler liquid condenses the steam forming a saturated layer of liquid water on top of the sub-cooled liquid layer. This saturated layer is at a higher temperature than the sub-cooled layer, resulting in a stratified temperature condition. The formation of the saturated layer may mitigate occurrences of condensation-induced water hammer (CIWH) events.

Liquid temperature stratification can also arise in passive safety systems such as the natural circulation driven core make-up tanks (CMT) and the large liquid-filled tanks that serve as the heat sink for reactor core or containment passive cooling systems. Steam vented into the large safety tanks condenses in the cold liquid producing hot rising plumes that form thermal layers at the free surface of the tank. Thermal layers having different temperatures grow with time to create a large temperature gradient in the liquid.