Sodium experiments in support of PFBR design safety

Full text: The pool type PFBR uses sodium as coolant. It has a roof slab of about 16 metres in diameter with large rotating plug, small rotating plug and control plug. The plugs rotate to orient. the fuel handling canal above the core and the operating and control instruments are supported in the control plug. The roof slab is maintained at 120 degC. The sodium temperature is 550 degC when the reactor is in operation. The axial temperature difference between the sodium and roof slab leads to intense convection of argon cover gas in the annular space of the rotating plugs causing large circumferential temperature difference and unwanted thermal stresses in the plugs and other components supported on roof slab. Experiments were carried out simulating reactor conditions in sodium facilities to control the circumferential temperature difference. The argon cover gas above sodium (550 degC) is at a temperature of about 350 degC. The sodium vapour generated, condenses as sodium mist in the cover gas. Due to convection currents the sodium mist is carried and deposited in cooler areas. The process creates problem for components like CSRDM which has long vertical annular gaps with a large axial temperature gradient. Experiments were carried out to reduce the convection currents inside the CSRDM annular space. A multiple sleeve was developed which reduces convection and hence the sodium deposition in the annular space