Reactor Primary System

    The MSBR primary system consists of the reactor, four primary heat exchangers that transfer heat from the fuel salt to the coolant salt, and four pumps that circulate the molten fluoride fuel-salt mixture. All of this equipment is contained within the reactor cell, as shown in Sect. 13. The fuel-salt drain tank and afterheat-removal equipment are considered to be a separate system and are described.
    The reactor primary system flowsheet is shown. About 94.8 X 106 lb/hr of fuel salt enters the bottom of the reactor at 1050°F. Fission energy within the graphite-moderated core raises the salt temperature to an average value of 1300°F at the reactor exit at the top. The salt then enters the bottom of the four fuel-salt circulation pumps. (For simplicity, only one of the four circuits is shown.) These centrifugal pumps force the salt through the tubes of the four shell-and-tube primary heat exchangers, where the fuel salt is cooled to about 1 050°F before returning to the bottom of the reactor.
    Each of the fuel-salt circulation pumps has a bypass in which about 10% of the total pump discharge flow is circulated. This loop contains a gas bubble injection section, where a sparging gas (principally helium) is introduced as small bubbles. The bubble generator is a venturi-like section in the pipe capable of generating bubble diameters in the range of 15 to 20 mils. The same bypass loop contains a gas separator, upstream of the bubble generator, which removes the inert gas and its burden of fission products with nearly 100% stripping efficiency. Downstream vanes kill the swirl imparted by the centrifugal gas separator. The removed fission products consist principally of xenon, krypton, tritium, and exceedingly small particles of noble metals. Based on 10% bypass flow, after a bubble is introduced it would make an average of ten passes through the reactor before being removed by the separator.
    The removed gases, along with a small amount of entrained salt, are taken to a small tank, where the off-gas is combined with that purged from the pump bowls and from the exit annulus at the top of the reactor. Since the off-gas leaving this tank is intensely radioactive, the line is cooled by a jacket in which there is a flow of 1 OS 0° F fuel salt taken from the reactor drain line just upstream of the freeze valve. This relatively small flow of fuel salt, which is subsequently returned to the pump bowl, also assures an open line between the drain valve and the reactor vessel.       Each fuel-salt pump bowl overflows about 150 gpm through the small tank, and this fluid flows with the off-gas to the drain tank. The overflow arrangement simplifies liquid level control and helps cool the drain tank head and walls. Salt-operated jet pumps at the bottom of the drain tank continuously return the molten salt to the circulation systems, as described in Sect. 2.4. The drain tank is provided with ample afterheat-removal capacity.
    The fuel-salt drain tank is connected to the bottom of the reactor vessel by a drain line having a freeze-plug type of "valve." At the discretion of the plant operator, the plug can be thawed in a few minutes to allow gravity drain of salt from the system into the drain tank. The freeze plug would also thaw in the event of a major loss of electric power or failure of the plug cooling system. The drain system is provided primarily in the event a leak develops in the fuel salt circulating loop and for safe storage of salt during maintenance operations. Although drainage is a positive reactor shutdown mechanism, it is not normally used as an emergency procedure since the reactor control and safety rods can quickly take the reactor subcritical while fuel-salt circulation is continued to remove fission product decay heat via the primary heat exchangers.
    A catch basin is provided at the bottom of the heated reactor cell in the unlikely event of a major spill of fuel salt from the system. The basin pitches toward a drain which would allow the salt to be collected in the fuel-salt drain tank.
    A fuel-salt storage tank is provided in addition to the drain tank in the event the latter requires maintenance. The heat-removal system for the storage tank has less stringent requirements and consists of simple U-tubes immersed in the salt. Water is boiled in the tubes and the steam condensed in a closed system by air-cooled coils located in the base of the natural-draft stack. A jet pump in this tank is used to return the fuel salt to the circulation system or to the drain tank.