The evaporation rate is calculated by evaluating the temperature difference between the wick and the fuel. The temperature of the liquid metal in the wick is calculated by determining the absolute pressure that the wick pores exert on the fluid. This pressure is determined by assuming that the space between the pins and the wick is a vacuum, and that there is a pressure difference across the vacuum-liquid interface caused by surface tension effects. Since the pressure rises from a vacuum to a level determined by capillary forces, the absolute pressure of liquid will be the pressure that is maintained by the capillary forces at the meniscus. Since the liquid saturation temperature depends on the absolute pressure of the liquid, maximum liquid temperatures can be controlled for the wick. Thus, maximum temperatures can be determined for various pore sizes, since they determine the capillary pressure. This maximum temperature setting can be used for controlling the energy transfer rate, i.e. evaporation. One of the SWEM design criteria is to take into account possible entrainment of the fluid from the wick during the initial blowdown transient in a manner similar to the concerns addressed Ref. [1], Entrainment of the liquid from the wick walls can be caused by high velocity fluid that flows next to the wick during the initial transient at the time the rupture is initiated. The wick pore must be sufficiently small so that the surface tension force can prevent the liquid from being striped by high liquid velocities. Entrainment [2] can generally be said to occur when the ratio of inertia forces to the surface tension exceeds one. When the inertia forces approach surface tension forces there is an increased probability of liquid becoming entrained. The ratio of the two forces is given by the Weber number. For the purpose of determining the Weber number, the maximum velocity has to be known in the core. The rate at which the coolant is lost from the break will be governed by the sonic limit of the fluid. This sonic limit will dictate the velocity of the liquid that will flow next to the wick in the reactor core. Implementing this theoretical development for a typical space reactor design, certain conclusions were reached. The results of evaluating the maximum liquid supply rate and the entrainment limit for the wick were in agreement with what the saturation
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