Spatio-temporal evolution of sodium aerosol and temperature distribution during sodium pool fire in a confined environment

In case of a Core Disruptive Accident in sodium-cooled fast reactor, sodium will be ejected from the reactor vessel into the Reactor Containment Building (RCB) and results in sodium fire. Liquid sodium combustion generates aerosol, which gets dispersed and remains suspended in RCB based on geometry of the building and degree of turbulence. The Environment Source Term (EST) primarily depends on the dynamics of sodium aerosols inside the containment. Hence, it is essential to carry out a safety analysis of SFR using computational tools and models shall be validated with experimental data, particularly aerosol dynamics inside the containment along with thermal hydraulics. Besides, all codes assume uniform aerosol concentration in large containment buildings and the evolution of suspended concentration was predicted. However, in actual case a non-uniform concentration profile exists, which leads to stratification of aerosol concentration inside the containment. Towards assessment of aerosol dynamics in realistic scenario, spatial-temporal characteristics of sodium aerosol and temperature distribution were studied by conducting sodium pool fire experiments in a large containment. The sodium fire experiment was conducted by using similar to 2.0 kg of sodium in a pool area of similar to 0.25 m(2). The measured maximum pool and gas temperatures are 685 degrees C and 126 degrees C respectively, and the temperature gradient at the wall boundary layer was 7.5 degrees C/cm. The estimated average sodium burning rate and aerosol release rate are 17.3 kg/m(2)hr and 0.24 g/s, respectively. The spatial variation of aerosol characteristics was measured for both horizontal and vertical planes. The measured maximum aerosol concentration in the horizontal plane ranges from 2.76 to 5.32, 2.16-3.30 and 2.09-4.21 g/m(3) for the bottom, middle and top elevations respectively. For the vertical plane, aerosol concentration varied from 2.16 to 3.65 g/m(3) for central positions and 2.53-5.32 g/m(3) for off-center positions. The trend of average suspended aerosol concentration at each elevation showed that the concentration was higher at the top elevation than at the middle and bottom for up to 15 min i.e., during aerosol generation period and after that concentration was found to be higher at the bottom elevation for the next 25 min. Based on the measurements, it is observed that the aerosol distribution is not well mixed within the experimental period. A significant difference in aerosol mass concentration was observed between each elevation, between estimation and observations based on a zero-dimensional model. The evolutions of aerosol median size by theoretical simulation capture the overall trend reasonably.

Automated summary

In the event of a Core Disruptive Accident in a sodium-cooled fast reactor, sodium will be released and result in a sodium fire in the Reactor Containment Building (RCB). The dispersion and suspension of sodium aerosols inside the containment are crucial factors in determining the Environment Source Term (EST). Therefore, it is important to conduct safety analysis of the reactor using computational tools and validate the models with experimental data on aerosol dynamics and thermal hydraulics within the containment.