After the Fukushima nuclear accident in Japan, concerns have increased about radioactive releases from nuclear power plants (NPPs) into the environment. Analysis of annual radioactive effluent release reports (ARERRs) shows that from 2000 to 2020, abnormal releases of radioactive effluent occurred in 703 out of 1,323 Reactor·years in the United States, accounting for 53% of the total number of reactors in 63 PWRs. Furthermore, when examining incidents and malfunctions recorded in Korea’s Operational Performance Information System of Nuclear Power Plant (OPIS) during the same period, it can be estimated that abnormal releases occurred in 9 out of the 324 Reactor·years in PWRs and PHWRs. Meanwhile, database on radioactive releases from NPPs worldwide was collected, and events of abnormal/unplanned releases were investigated. Based on the data collected from 195 NPPs in 8 countries (South Korea, the United States, Japan, France, the United Kingdom, Germany, Spain, and Canada) over a period of 21 years, totaling 4,607 Reactor·years, a program called K-IRED (KHUIntegrated Radioactive Effluent Database) was developed using MS Access. Using K-IRED, three methodologies have been developed to predict abnormal events based on the annual radioactive releases for each NPPs and radionuclide (or radionuclide group). Three newly developed methodologies were applied to the 63 NPPs (1,323 Reactor·years) in the United States, categorized by radionuclides (or radionuclide groups). Assuming an increase in radioactive effluent due to abnormal events, the annual increase rate of radioactive effluent was calculated for each methodology and the results were analyzed. The optimal methodology among the three was derived, and the applicability of predicting abnormal events in other NPPs beforehand was examined. Therefore, by predicting abnormal or unplanned releases from NPPs to the environment in advance, it is possible to prevent accidents and reduce public concerns, as suggested by results of this study.