After the Fukushima accident, it is necessary to develop some technique that can monitor the progression of severe accidents in nuclear power plants (NPPs). It is therefore very important for an operator to monitor safety related parameters for the diagnosis of severe accidents and to manage it properly. So to monitor and to check the availability of plant instrumentation during severe accidents, this paper presents quantitative and qualitative analyses of safety parameters by using online risk monitor system (ORMS). An ORMS considers the increasing potential for failure for a working component due to aging, which appears in the form of component's performance degradation. ORMS therefore requires a continuous feedback regarding performance and failure probabilities of components, which directly or indirectly contributes to the failure of a system. ORMS has been designed to automatically update the online risk models and reliability parameters of equipment. A case study of emergency diesel generator (EDG) of Daya Bay NPP has been performed, and operational failure rate and demand failure probability of EDG have been calculated with the help of ORMS. The results of ORMS are well matched with data obtained from Daya Bay NPP. ORMS can support in decision-making process for operators and managers at NPPs. Copyright (c) 2013 John Wiley & Sons, Ltd.