This International Standard defines the requirements for instrumentation needed to detect leakage from reactor coolant systems of light water nuclear reactors. Methods of leak detection are described, and characteristics of different methods of detection and of differentiating between allowable and abnormal leakages are given. The sensitivity of systems required to ensure early detection of developing leaks and to give warning of possible leak-before-break situations is considered, and recommendations are given to reduce spurious alarms. Recommendations are given for data display which will allow presentation in a clear and concise manner to enable operators to take appropriate actions to minimize the risk of hazard to plant equipment and to personnel. The requirements are given for recording and logging, as well as for summation of individual signals to detect trends. The object of this standard is to define the requirements for leakage detection data, for arranging it in a logical manner, and for displaying information to enable the operator to take appropriate action. A secondary objective of this standard is to standardize criteria, methods and procedures for assuring the design and operational adequacy of the reactor coolant and associated systems (RCAS) leakage detection systems used in light water cooled nuclear reactors.