A nuclear emergency starts when the plant monitors indicate that some operational systems do not operate properly and the situation cannot be controlled by the corresponding safety systems adequately. Upon failure detection, the plant operator evaluates the impact of the incident on the plant and identifies the affected systems and the availability of alternative systems to control the situation. Simultaneously, the plant operator inves­tigates whether the incident could lead to the escape of radioactive material within the plant or to the environment. Based on the results of its prelimi­nary evaluation, the operator initiates the mitigation actions, decides the level of the on-site emergency plan to be activated, and notifies the situation to the emergency coordinator who is responsible for off-site emergency plan activation.

Transition from normal operation to an emergency situation is a critical step that needs to be clearly established in emergency plans; for this reason, the operators are specifically trained in the use of procedures to identify abnormal situations within the plant, activate on-site emergency plans, and implement the emergency procedures to handle the situation. Similarly, activation of the off-site emergency plans requires specific training of the authorities in charge of response and intervention. The on-site/off-site inter­face also needs careful implementation to avoid any delay or disturbance in taking the necessary countermeasures.