Passive Initiators (PI) are small heat exchangers outside the Reactor Pressure Vessel (RPV); one line of this heat exchanger is connected to the RPV well above the normal water level and the other line well below the water level. Both lines are always open to the RPV. The secondary side of the Passive Initiators is filled with water. In case of a substantial water level decrease in the RPV steam will condense inside the PI and heat up the secondary side resulting in a pressure build-up. This can be used for control purposes. In Fig. 13a the four designs tested are schematically shown. In Fig. 13b the pressure build-up on the secondary side and the outflow is given for water and ethylacetate (ESEE), which has a lower boiling point than water.

a)

FIG: 13. Designs (a) and experimental results (b) of the different passive initiators.