The reactor trip in emergency is done by simultaneous insertion of the control rods into the core by gravity following the drive mechanism de-energization, which is actuated by trip signals from the automatic control system. In the

case of failure to actuate the electromechanical protection system, the reactor shutdown is accomplished by the emergency boron injection system. Activation of the system is done by manually opening valves in the pipelines connecting the system to the reactor. Both shutdown systems ensures the reactor shutdown and its shutdown margin is sufficient enough to keep the cold clean reactor in a subcritical state.

2.3.2 Residual Heat Removal System

Normal decay heat removal when cooling down for maintenance and refueling, the steam generators with turbine bypass system are used and heat is rejected through the condensers. This can be achieved by natural circulation on the primary side but requires feed pumps and other equipments on the secondary system. If the secondary system is not available, active decay heat removal system with steam generators are used to remove decay heat and heat is rejected through the component cooling system.

Should there be no ac power available, decay heat is removed by natural convection system which only requires battery power to operate the initiation valves and passive decay heat removal system which is composed of heat pipe and heat exchangers. The heat is ultimately rejected to atmosphere by natural convection of air flow. Thus, there is theoretically infinite time of heat removal without operator intervention. One of the advantages of heat pipe passive decay heat removal system is that this system is continuously operating during normal plant operation to remove the heat loss from reactor vessel through the wet thermal insulation.