This part of IEC 60951 provides general guidance on the design principles and performance criteria for equipment for continuous in-line or on-line monitoring of radioactivity in process stream in nuclear power plants for accident and post-accident conditions. General requirements for technical characteristics, test procedures, radiation characteristics, electrical, mechanical, and environmental characteristics are given in IEC 60951-1. These requirements are applicable in this part unless otherwise stated. IEC 60951-4 is only applicable to continuous in-line or on-line measurement, i.e. monitors for which the detector measures radioactivity by being positioned in the process stream (i.e. immerged in) or adjacent to the process stream (i.e. viewing straight through a pipe or tank). It does not apply to monitors for which the detector measures a representative proportion of the stream at some remote location (sampling assembly), which are within the scope of IEC 60951-2. IEC 60951-4 is only applicable to monitors for accident and post-accident conditions. Process stream radiation monitoring equipment for normal and incident conditions are within the scope of IEC 60768.