IEC 60951-4:2009 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. The main technical changes with regard to the previous edition are as follows: - clarify the definitions; - up-date the references to new standards published since the first edition; - update the units of radiation. This publication is to be read in conjunction with IEC 60951-1:2009.