Fuel-salt drain system

    The MSBR drain system consists of the drain tank, the drain line and freeze valve, a pump and jet system to return salt to the circulation loop or to the fuel processing plant, the off-gas heat disposal system, an afterheat disposal system, and heater equipment which maintains the salt above its liquidus temperature. The drain system is housed in separate cells apart from the reactor cell.

    The drain tank serves several functions, the chief one being a safe storage volume for the fuel salt when it is drained from the circulation loop. A critical mass cannot exist in the tank because of insufficient neutron moderation, and the afterheat-removal system has assured reliability in that it is independent of the need for mechanical equipment, power supply, oi: initiating action by the operating personnel. Cell heaters assure that the tank and its contents remain above the salt liquids temperature of about 937°F.
    The drain tank serves as a 2-hr holdup volume for the highly radioactive fission product gases after they are separated from the circulating fuel salt in the processing system. Also, the drain tank acts as a sump for the overflow streams from the bowls of the salt-circulation pumps. The small stream of fuel salt which is sent to the fuel-processing cell for removal of fission products, protactinium, excess bred material, and impurities is taken from the drain tank and returned to it after treatment and adjustment of the uranium concentration. An additional use of the drain tank is that its storage volume, which is about 50% greater than the fuel-salt inventory, permits accommodation of some of the coolant salt in the unlikely event that a heat exchanger tube failure and pressure differential reversal permit coolant leakage into the primary system.
    The fuel-salt drain tank contains a liner to absorb gamma heat and to form an annular flow passage at the tank wall for about 600 gpm of overflow salt from the pump bowls. The salt stream passes along the bottom surface of the top head and down the sides to maintain metal temperatures within the design limits.
    A well in the bottom head of the drain tank contains five salt-actuated jet pumps. Four of the jets are provided with salt from the primary pump discharges to actuate the jets and return the overflow salt to the respective circulation systems. Siphon breaks prevent fuel salt from the pump bowl from draining back in the event a jet stops operating. The fifth jet pump is activated by about 100 gpm from a separate fuel-salt pump and is used to transfer salt to the fuel-processing cell or to fill the primary-salt circulation loop.
    Afterheat released in the drain tank is removed by a natural convection system employing an intermediate heat transport fluid. As shown in Fig. 2.3, 7 LiF-BeF2 coolant salt circulates through U-tubes immersed in the fuel salt to heat exchangers located at the base of a natural-draft stack. There are 40 separate and independent natural-convection circuits to afford a high degree of reliability. The heat exchangers transfer heat from tubes containing the transport salt to water-cooled plates which make no physical contact with the salt tubes. The steam generated in the plates is condensed in finned air-cooled coils in the natural-draft stack.