Methodology for Calculating Minor Radioactive Releases From VVER 1000 Using TRACE Code

Abstract

After the Fukushima DAIICHI accident, new safety requirements were imposed in order to reduce risk of severe accident. One of the principles that have been adopted is the introduction of emergency action levels resulting from the expected consequences. They cover a wide range of component and system malfunctions resulting in emergency, incident, and/or accident conditions. To evaluate those emergency action levels, thermal hydraulic (TH) analyses simulating these malfunction/incident/accident conditions are required. This paper describes the simulation of a real operational incident scenario using a standard thermal hydraulic model of the power plant in the TRACE code that was originally intended for simulation of design basis accidents such as large break coolant accident or loss of flow accident. Special attention was given to the methodology, addressing a long duration of an incident with corrective actions of the operators, and to computational issues leading to model modifications caused by a long duration of the incident along with the necessary conservatisms in the estimated results of the simulated radioactivity release.