Nuclear reactors work on a principle that from fission reaction large amount of heat is produced and it heats up the coolant. According to European Nuclear Society, there are 438 operable nuclear power plants around the

There is two categories for fuel.
Fissile: These elements is fissionable
- U235 : It is the only natural fissile fuel can be found in enviroment
- U233: It comes from Th232
- Pu239: It comes from U238

The fundamental objective of nuclear safety is to protect people and the environment from harmful effects of ionizing radiations. This objective applies to all circumstances that give rise to radiation risks. It implies the following:
- Control of radiation exposure of people and control of releases to the environment
- Prevention of events that could lead to a loss of control over nuclear reactor core or any other source of radiations;

A radionuclide is an isotope of one element wich turns into another one during time with emissions of radiations:charges particles (α, β, e) and/or neutral particles (γ, n). Activity is the unit of this disintegration per time. Unit is Becquerel (1 Bq = 1 dis..s-1) or Curie (1 Ci = 3,7 1010 Bq, activity of 1 g of Ra, used till ≅ 1990). The interactions of particles with matter lead to ionizations, excitations, and thermic transfers. Absorbed dose is energy depositon to unit mass.

ACCIDENTS
- Kyshtym Accident
- Windscale Fire
- SL-1 Accident
- Three-Mile Island Accident (TMI)
- Browns-Ferry Accident
- Chernobly Accident
- Davis-Besse
- Fukushima

IAEA Functional Requirements for Response
Establishing Emergency Management and Operations
- On-site / Off-site
- At the Local / National / International levels
- Coordination between all responding organizations is necessary Identifiying,Notifiying and Activating
- Operator shall promptly determine the emergency class and initiate appropriate actions
- The notified off-site body shall promptly initiate the appropriate preplanned and coordinated response