Several advanced reactor designs implement core make-up tanks (CMTs) to provide natural circulation cooling to the core. CMTs are elevated tanks connected to the reactor vessel and primary loop at the top and bottom of the tank. Special lines connect the bottom of the tank with the vessel, and are termed direct vessel injection (DVI). In connection to this, an important interaction occurs between the CMT, the accumulator and the IRWST also considering the actuation signal for automatic depressurization. The tanks are filled with cold borated water and can provide coolant injection at system pressure. The tanks are normally isolated from the reactor vessel by an isolation valve located at the bottom of the vessel. The fluid is always sensing full system pressure through the top connection line. In the event of an emergency, the bottom isolation valve is opened to complete the natural circulation loop and permitting cold borated water to flow to the core. The relative elevation between the core and the CMT and the density difference between the hot primary system water and the cold CMT water creates a buoyancy-driven natural circulation flow that eliminates the need for a pump. Decay heat is removed from the core by convective heat transfer from the fuel to the single-phase liquid in the reactor vessel. CMT behaviour includes natural circulation, liquid thermal stratification in the tank, and liquid flashing during plant depressurization.