Thermal-convection loops, liquid metals

The thermal-convection loops are limited to flow velocities up to about 6 cm s . Where higher velocities are required, the liquid must be pumped, either mechanically or electromagnetically the latter is usually preferred as it avoids the problem of leakage at the pump seal. Basically, these forced-convection systemsconsist of (c) a hot leg, where the liquid metal is... 

Thermal-convection loops. A sealed loop filled with a liquid metal is heated in certain sections and cooled in others to cause circulation due to changes in density of the liquid metal... 

The mechanism of temperature-gradient mass transfer is illustrated in Figure 1. This type of corrosion may be studied in a thermal-convection loop test (Figure 2). Because the solubility of most container materials in a particular liquid metal is temperature-dependent, solution in the hot section and subsequent deposition in a cooler section may occur. The results of this type of corrosion may be seen in Figures 3 and 4. 

It should be noted that, since the leading concept of the fusion power-plant system with V-alloys uses liquid Li as coolant and tritium-breeder, the majority of the corrosion and compatibility studies have been oriented to those with liquid Li. Although V is susceptible to oxidation at high temperature, oxidation is not an issue in liquid Li because of high O affinity of Li. A corrosion test using a mono-metallic thermal convection Li loop made of V-4Cr-4Ti pipes, conducted at 700°C for 2355 h, showed the corrosion loss rate corresponded to only <1 pm/year [26]. 

Thermal convection loop tests at BNL. A typical thermal con- ection loop that lias been used at BXL is shown in Fig. 21-3. The loop provided with a double-valve air lock at the top of the vertical section which permits taking liquid metal. samples while the loop is running without contaminating the protective atmosphere. The hot leg is insulated and h(-at is supplied to that section of the loop while the cold leg is exposed and two small blowers are utilized to extract heat. The hottest point in the loop is at the "tee at the upper end of the in.sulated section, and the coldest in tlie bottom of the expo.scd section. The total height of the loop proper is... 

The time-dependence of void formation in Inconel, as observed both in thermal-convection and forced-circulation systems, indicates that the attack is initially quite rapid but that, it then decreases until a straight-line relationship exists between depth of void formation and time. This effect can 1)0 explained in terms of the corrosion reactions discussed above. The initial rapid attack found for both types of loops stems from the reaction of cliromium with impurities in the molt [reactions (13-1) and (13-2)] and with the FF4 constituent of the salt [reaction (13-3)] to establish a quasi-etiuilibrium amount of CrF2 in the salt. At this point attack proceeds linearly with time and occurs by a mass-transfer mechanism which, although it arises from a different cause, is similar to the phenomenon of temperature-gradient mass transfer observed in liquid metal corrosion. 

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