NAFCON-SF: A sodium spray fire code for evaluating thermal consequences in SFR containment

Abstract

Analysis of postulated sodium fire events in the systems of Sodium cooled Fast Reactors (SFRs) is very important in the safety evaluation. Ejection of primary sodium into the Reactor Containment Building (RCB) and resultant sodium fire thermal consequences under very low probability energetic Core Disruptive Accident (CDA) forms the design basis of RCB, which is the final barrier for radioactivity reaching to the public. Among different possible modes of sodium fires generated by accidental leaks, the spray fire will cause severe thermal consequences due to divided and exposed burning of sodium in droplet form. This paper describes the sodium spray fire analysis code NAFCON-SF developed for the realistic evaluation of thermal consequences in enclosed air containment under postulated sodium leak scenarios. The code simulates two stages of sodium droplet combustion in the air using reaction rate based pre-ignition model and mass transfer based vapour phase combustion model to predict the ignition and combustion of individual sodium droplets. The code uses a distribution system to represent the possible sizes of sodium droplets in spray fire. The burning droplets are tracked from leak location to the floor of chamber by using the model for spherical droplet motion. The burning rate of sodium spray is evaluated as the sum of burning rates of all the droplets constituting the spray. The code employs a transient energy balance on chamber gas to evaluate its time varying temperature and pressure, by considering simultaneous heat addition from spray fire and heat loss to the chamber walls. The NAFCON-SF code has been validated for a wide range of influential parameters with different sodium spray fire test results available in the literature. The code predicted trends are in line with experimental results and the values of peak pressure rise and maximum gas temperature are in fairly good agreement with experimental results for short duration spray fire scenarios. The present code predictions are also compared with NACOM spray fire analysis code predictions and these results show the realistic evaluation of spray fire consequences by NAFCON-SF code compared to the experimental results. The validated code is a valuable tool for the safety analysis of postulated sodium spray fire scenarios in SFR. Further, the code will be validated with in-house large scale sodium spray fire experimental studies.