Customary Internatinal Law is body of rules as a result of behaving to against the same problem and in the same way of states for long term. Terms of Nuclear plants; customary international rules areprevention of transboundary harm arising from hazardous activities, the principle of precaution and the cooperation rule. The prevention of transboundary harm arising from hazardous activities describes matter of a state not to be conduct and give not approve to conduct. The precaution, it means the precautionary principle stand for determining the risk and extent of the hazard and to be taken precautions that revealing its the quality and quantity in this way. Finally, cooperation rule is a rule that occur on many levels including national-international andvertical-horizontal. Thanks to this rule, cooperation into state and between the state and society comes true, more importantly, cooperation opportunity arises in the manner that increasing the availability to provide participation to decisions among states.
All of these customary international rules are to be applied on both configuration and operating phase of nuclear power plants. International agreements including these rules make the rules more effective. Because, no longer thestates which side of the agreements had been enter into an obligation in terms of positive law.
Key Words: Customary International Law, Nuclear Energy, International Law, Energy Law, Environmental Law.