An integrated operator decision aid (IODA) system utilizing computers and cathode-ray tube (CRT) display devices has been developed by focusing attention on the importance of the operator’s role in a nuclear power plant. The developmental objective of this IODA is to improve safety, reliability, and the quality of nuclear power plant operation by helping operators handle abnormal plant conditions. Three support functions corresponding to the respective roles of operators are determined and developed: (a) a standby systems management function, (b) a disturbance analysis function for monitoring plant status and diagnosing disturbances, and (c) a posttrip operational guidance function for providing symptom-based operational guidance after a reactor scram. These functions have been integrated into an operator decision aid system for abnormal conditions in boiling water reactor (BWR) power plants. Information on plant diagnosis and guidance messages are provided through a three-level hierarchical structure of CRT display images. Results of functional verification tests of IODA using a BWR simulator show that the system effectively supports operators at all stages—from normal operation of the plant to safe plant shutdown after a reactor scram. The IODA can help operators quickly and easily understand a plant situation and take appropriate measures during abnormalities. It will also alleviate the burden on operators, thus reducing the potential for judgmental and operational errors in various plant operating modes.