Answer
(What safety measures and systems exist to respond to unlikely events?)
Emergency plans and preparedness are essential tools for dealing with unlikely accident situations and for mitigating the accident consequences on the population and the environment. These plans are deduced from the accident scenarios studies which determine the releases. The possibilities of dispersion of the releases is envisaged by taking into account the weather conditions at the time or in the future, the population distribution and the evacuation routes if need be. The plans are regularly tested at plant level, or nationally or internationally.
Background
Nuclear safety includes in defence in depth the 5th level which studies the question of protecting the population and the workers, when all the barriers and measures failed to contain an (unlikely) accident. The previous question indicates the accident scenarios dealt with in design and in operation which could lead to losses of all barriers.
From these scenarios, one can deduct the most catastrophic ones and use them to determine what would be the consequences in terms of potential releases and effects on the health of the public and the environment. The plant workers are also part of the protection plan, except the control room operators in charge with their supervisors to contain and mitigate the accident (they are in the emergency control room well protected in the reactor area).
Emergency plans are prepared to prevent the exposure of the public or to decrease it as much as possible within prescribed limits. The preparation includes such measures as confinement in homes or buildings or evacuation of the dangerous zone determined by the nature and quantity of releases, the direction of wind/rain and the population repartition. Such emergency plans are tested at the installation site, internally to the site or with the local authorities or with the national ministries for emergencies so that people in the public and plant workers are well trained for implementing measures that most probably they will never have to be taken. Such exercises are performed between once every 3 months to once per year for the national one. It includes communication to the local population on the accident, on the measures to take and informs national government the media. International exercises are also carried out, their preparation and conduct being done by international organizations.
