Analysis of a Transient Over-Power in KALIMER-600

KALIMER-600 is designed to satisfy the safety principle of defense-in-depth and also the safety design objectives which have been established to implement the safety principle in the design. The most important two primary safety design objectives are the accident resistance and the accident mitigation. The main purpose of these objectives is to detect any failure threatening the safety functions and to control the accidents within the design basis by maintaining the required fundamental safety functions. Highly reliable diversified shutdown mechanisms are equipped for the reactivity control function during an accident or abnormal transients in KALIMER-600. The reactivity is also controlled by the inherent reactivity feedback mechanisms incorporated in the design. In addition, a uniquely designed passive decay heat removal circuit provides the heat removal function. Due to these passive and inherent safety characteristics, the safety of KALIMER-600 is much improved than the existing PWR designs. Therefore, the events whose frequencies are higher than 10-7 per reactor-year are categorized as design basis events (DBEs). A transient over-power (TOP) is one of the most important DBEs in the design of a liquid metal-cooled reactor. A TOP event is initiated by an inadvertent withdrawal of one or more control rods from the initial inserted position due to a malfunction of the drive motor systems. The rod control system of KALIMER-600 allows only one rod to move at a time, thus, a failure in shim motor control results in a maximum reactivity insertion at the maximum shim motor speed. The TOP postulates that a malfunction in the reactivity controller causes the shim motor to continue to withdraw the control rods to the top of active core. Single rod withdrawal is the most likely rod withdrawal accident, but it is assumed that the limiting postulated DBE TOP is the withdrawal of all primary control rods at the maximum shim motor speed. This event bounds other reactivity insertion events considered as potential DBEs. When the reactor power reaches the high neutron flux trip setpoint, the reactor trip occurs. The high core outlet temperature trip is the other competing reactor trip mechanism. The acceptance criteria for the safety analysis are determined to guarantee the public safety by assuring the integrity of fuel rod and primary structures. The integrity of the vessel structures and boundaries is assured by limiting their average core exit temperatures. The clad temperature is limited to exclude the stress rupture and the eutectic formation between the metallic fuel and the fuel. The criteria for the fuel and the coolant are the fuel solidus temperature and the sodium boiling temperature