The definition of safety as usually employed is the achievement of proper operating conditions, prevention of accidents or mitigation of accident consequences, resulting in protection of workers, the public and the environment from undue radiation hazards. (IAEA definition)
Nuclear security is the means and ways of preventing, detecting, and responding to theft, sabotage and unauthorized access to or illegal transfer of nuclear material and other radioactive substances, as well as their associated facilities. (IAEA definition)
The two notions are different but have in common some aspects and for some features a balanced and complementary approach between the two is necessary. For example, air plane crash in the design of an NPP used to consider the fall of the engine of a big aircraft and now since the 9/11, other additional measures are implemented in existing plants and new designs include the crash of a complete large aircraft on the containment. The measures taken for the 2 aspects are either technical or administrative ones. For security, the aspect of avoiding nuclear material theft or diversion is essential. For safety ensuring the main functions of the installation in all circumstances and maintaining the confinement of fission products is the main objective.