Thermal hydraulic investigations of an extended station blackout event in FBTR

Thermal hydraulic investigation into the capability of Fast Breeder Test Reactor (FBTR) in handling an extended station blackout event was carried out. The main concerns during this event are decay heat removal and prevention of sodium freezing in circuits. The means of removing decay heat in FBTR are (i) by natural convection of air in steam generator casing and (ii) heat loss through long sodium piping of the loop type reactor. The study is focussed towards the current operating power. The plant dynamics code DYNAM developed specific to this reactor system and validated against commissioning tests has been utilized for this purpose. It has been established that core cooling is not a concern even when the system cooling through steam generator casing is not activated. Piping heat loss is adequate to remove the decay heat. Cooling through SG casing is essential only for preventing heat up of cold leg sodium systems. Concern on sodium freezing arises when the decay power reduces to very low level below the natural heat loss through pipe lines. This situation arises only after a year for the current power level of the plant.