To support the emergency operation of the nuclear power plants (NPP), Korea Atomic Energy Research Institute (KAERI) has been developing an operation supporting system called KOSSN (KAERI Operation Supporting System for NPP). KOSSN provides the operator with very detailed action items which are derived from the Emergency Operating Procedures (EOPs). The artificial intelligence (AI) and the probabilistic safety assessment (PSA) techniques are used in developintg KOSSN. It consists of three parts: The operator action items for the emergency operation of NPP are provided in the first part. The second part of KOSSN generates the success paths of the safety systems which can be used for the operator to restore the challenged safety functions and rank them according to the respective operability. In the last part, according to the present plant status, the appropriate event tree is suggested to the operator in order to show the possible future courses of accident progression. For the detailed man-machine interface, a color graphic display is utilized. KOSSN is built on a workstation using the IF-Prolog language and the X11 graphic tool. (Author).