Numerical investigation of sodium melting in large storage tank

Sodium is used as a coolant in fast reactors. Before commissioning the reactor, the sodium is stored in large storage tank and allowed to freeze. During commissioning, it is melted and raised to higher temperature in order to fill the reactor systems. The methodology for heating the frozen sodium in large sodium storage tank (LSST) of capacity 240 tonnes, melt the sodium and raise the temperature to 150 °C by using surface electric heaters for filling to various sodium systems have been studied by using transient CFD analysis with melting simulation. Time required for various stages of heating process like melting of sodium raising the temperature to 150 °C have been estimated with all the heaters in healthy conditions as 176 hours and 195 hours respectively. The evolution of temperature with time is also verified with the similar sodium melting (37 tonnes) carried out at in-house experimental facility and found to be of identical pattern. Studies are also carried out with some of the heaters not working condition and additional time required for completion of heating operation have also been quantified. The results provide guidance for assessing the time required to switch on the subsequent heaters and melting of sodium in large storage tanks in Prototype Fast Breeder Reactor (PFBR). (author)