to the energy removal rate. Maximizing the area of energy exchange decreases the possibility of exceeding safe temperature limits. Problems that are encountered in space-related design are different in nature than those present in earth gravity. Some of the problems are the absence of gravity and the presence of vacuum outside the reactor vessel. The microgravity condition complicates the design since the traditional methods of fluid delivery and disposal are gravity assisted. Therefore the new method of coolant delivery to the vessel must be gravity independent. The second problem of vacuum presence outside the vessel can cause other serious problems. When a postulated rupture occurs, the coolant will be forced out of the vessel until the interior pressure of the vessel is at vacuum conditions. If vessel cooling is attempted using conventional methods, the liquid would flash as soon as it encountered a low pressure region. The vaporization process would not remove significant energy from the pins since the resulting two phase mixture would leak out of the core indiscriminately, with no assurance that all of the liquid was being converted to vapour by core decay heat. A new approach for energy rejection has been developed and is presented below. It attempts to maximize the energy rejection via vaporized fluid. Theory The principle behind which the SWEM operates is based on controlling the evaporation rate of lithium coolant at selected locations in the core. The controlled evaporation is based on the same principle under which the old kerosene lamps operate. Kerosene lamps have a porous cord, one side of which soaks in the kerosene reservoir and the other of which is located above the reservoir surface where it burns to generate light. The kerosene is transported by surface tension forces from the reservoir to the burning site. This example provides sufficient evidence of the capability of a wicked surface to transport fluid from one place to another. Therefore by using wicked surfaces inside the reactor vessel liquid can be supplied to the vessel from an outside reservoir. This method solves the problems associated with controlled fluid supply in the absence of gravity. The evaporation of the fluid is accomplished by exposing the surface of the wick to the radiant thermal energy. The energy is absorbed, used to change the phase of the liquid to vapour, and the vapour rejected from the vessel into space. The evaporated liquid is replaced by fresh liquid that is delivered by the wick to the evaporating surface. A current proposed method of implementing the SWEM heat rejection system for a LOCA is to replace fuel bundle walls with the wick structure. This wick structure serves a dual purpose. It provides structural support for the pins and acts as a melt down prevention device during the LOCA. A possible location of the wick is illustrated in Fig. 1. Figure 1 shows the location of the wick material throughout the core, where each hexagon represents a fuel bundle. This action was taken to maximize the surface area seen by the pins. The movement of the liquid in the wick is achieved by the difference in pressures that is generated by having different radii of curvature at selected locations. The smaller the pore size, the higher the pressure difference. Figure 2 shows a different view of the SWEM system with pore sizes, and flow directions. The small pore size is located in the core and a larger pore size in the reservoir. The vapour channel located at the center of the wick pressurizes the reservoir. This effect will force the liquid to flow from the reservoir to the evaporator.
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