In normal operation, a failure of the system or an operator’s error causes an abnormal transient state for which maximum linear power density is evaluated to assure that fuel rods do not exceed the allowable design limits (no melting of fuel, no occurrence of DNB, etc.). Two typical transients are evaluated here as an abnormal transient in operation.

(1) Abnormal withdrawal of control rods at power

An abnormal withdrawal of control rods by a malfunction of the control rod control system or an operator’s error causes an increase in power and a change in power distribution. However, the withdrawal of rod cluster control assemblies is stopped or the reactor is shutdown by the reactor protection systems, such as the high neutron flux, the over-temperature AT and over-power AT. Control rods could be withdrawn from an arbitrary position to the fully withdrawn.