SIRIUS: A Code on Fission Product Behavior under Severe Accident

The transport phases of the radioactive materials can be divided into gas and aerosol. The gas and aerosol phases of the radioactive materials move through the reactor coolant systems and containments as loaded on the carrier gas or liquid, such as steam or water. Most radioactive materials might escape in the form of aerosols from a nuclear power plant during a severe reactor accident, and it is very important to predict the behavior of these radioactive aerosols in the reactor cooling system and in the containment building under severe accident conditions. The suspended solid or liquid particles typically have a range of sizes of 0.01 to 20 μm. Aerosol concentrations in reactor accident analyses are typically less than 100 g/m³ and usually less than 1 g/m³. A SIRIUS (Simulation of Radioactive nuclides Interaction Under Severe accidents) code was developed to predict the behaviors of the radioactive materials in a reactor coolant system and in the containment under severe accident conditions. SIRIUS consists of an estimation of the initial inventories, species release from the core, aerosol generation, gas transport, and aerosol transport. The individual modules of the SIRIUS code, such as calculation modules for aerosol generation, sedimentation, impaction, thermophoresis, diffusion phoresis, and wall deposition, have been verified by simulating various virtual problems and an ABCOVE experiment. SIRIUS will be updated by adding some models related to the aerosol size tracking, aerosol elimination in the pool, iodine behaviors in the gas and pool, etc. In addition, the SIRIUS code may be used as a fission product module of the CINEMA code to analyze the severe accident phenomena in a nuclear power plant.