Thermal convection loop

Tests in thermal convection loops with a maximum temperature of 800 C showed that nickel and nickel-rich alloys with chromium and other additions were relatively very susceptible to the mass-transfer type of attack by lead. On the other hand nickel-rich alloys with molybdenum were among the better of the alloys tested (Table 7.35). 

Table 7.36 Results in thermal convection loop tests of material in contact with molten lead...

Loop Tests Loop test installations vary widely in size and complexity, but they may be divided into two major categories (c) thermal-convection loops and (b) forced-convection loops. In both types, the liquid medium flows through a continuous loop or harp mounted vertically, one leg being heated whilst the other is cooled to maintain a constant temperature across the system. In the former type, flow is induced by thermal convection, and the flow rate is dependent on the relative heights of the heated and cooled sections, on the temperature gradient and on the physical properties of the liquid. The principle of the thermal convective loop is illustrated in Fig. 19.26. This method was used by De Van and Sessions to study mass transfer of niobium-based alloys in flowing lithium, and by De Van and Jansen to determine the transport rates of nitrogen and carbon between vanadium alloys and stainless steels in liquid sodium. 

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. 

Figure 3. Hot leg and cold leg sections from Inconel-sodium thermal convection loop...

The simplest nonisothermal flowing system where processes associated with dissolution and deposition occur is one in which flow is induced by thermal convection. This is accomplished by heating one leg of a closed loop and cooling another leg. The flow rate is dependent on the height of the heated and cooled sections, on the temperature gradient, and on the physical properties of the liquid. Both single-phase (all-liquid) and two-phase (liquid-vapor) loops have been tested. In some cases, thermal convection loops are destructively examined after operation [55,67]. In others, specimens are removed and replaced numerous times for cumulative periods of 10 000 h or more [22,54,68],... 

The thermal convection loop is also useful for studying dissimilar-metal mass transfer. The bimetallic loop design shown in Fig. 13 was used by DeVan and Jansen [86] to determine the transport rates of nitrogen and carbon between vanadium alloys and stainless steels in a sodium circuit. Mass transfer rates and carbon and nitrogen effects on mechanical properties were monitored by means of insert specimens in the hot and cold legs. The effects of surface area ratios of the two materials were determined by adding or subtracting insert specimens. 

Still another use [87] of thermal convection loops has been the study of corrosion effects in two-phase (vapor and liquid) potassium systems. In this case the lower half of the loop is filled with liquid and the upper half with vapor. The data derived from such a system are basically similar to... 

FIG. 13—Vanadium-stainless steel-sodium thermal convection loop (1.0 In. = 25.4 mm) [84]. (Reprinted with permission from the Oak Ridge National Laboratory, which is managed by Martin Marietta Energy Systema, Inc. for the U.S. Department of Energy.)... 

Romano, A. J., Klamut, C. J., and Guiinsky, D. H., The Investigation of Container Materials for Bi and Pb Alloys, Part I. Thermal Convection Loops, Brookhaven National Laboratory Report, BNL-811, 1963. 

Pawel, S. J., DiStefano, J. R., and Manneschmidt, E. T., "Corrosion of Type 316L Stainless Steel in a Mercury Thermal Convection Loop, Oak Ridge National Laboratory report, ORNL/TM-13754, April 1999. 

FIG. XXX-7 Weight change versus time of Hastelloy N specimens exposed to fuel salt in thermal-convection loop NLC-19A (ORNL, USA) [XXX-31]. 

In the thermal convection loop operated with molten Li,Be,Th,U/F salt system the HN80MTY alloy specimens have shown the maximum corrosion rate at 6 pm/year (see Table 5.5), as for the HN80MT alloy it was two times lower [16,58]. The corrosion was accompanied by selective leaching of chromium into the molten salt, which is evidenced by the 10-fold increase in its concentration with reference to the initial one for 500 h of exposure. Similar oxidizing conditions characterized by the same content of Fe and Ni impurities in the salt took place in testing a standard Hastelloy N alloy on... 

Table 5.6 Mechanical characteristics of alloy HN80MTY, MONICR, and HN80M-VI before and after corrosion tests in thermal convection loop [59]...

Fio. 13-1. Diagram of a standard thermal-convection loop, showing locations at which metallographic sections are taken after operation. 

Fig. 13-3. Hot-leg section from an Inconel thermal-convection loop which circulated the fuel mi.xture NaF-ZrF4-UF4 (50-46-4 mole %) for 1000 hr at 1500°F. (250 X)...

Fig. 13-23. CCN graphite (A) before and (B) after exposure for 1000 hr to NaF-ZrF4-UF4 (50-46-4 mole %) at 1300°F as an insert in the hot leg of a thermal-convection loop. Nominal bulk density of graphite specimen 1.9 g/cm .

Surface reactions. Experimental evidence has shown that the corrosion resistance of steels in Bi is in part due to the formation of insoluble films on the steel surfaces. The effect of these films on the corrosion behavior of different steels is not readily determined by thermal convection loop experiments because of the relatively low temperatures (400 to 550°C) and long times associated with such tests. The comparative behavior of different... 

Solution rale tests. The solution rates of Fc into Bi, and Bi- - Zr and Mg, were measured in crucibles of a carbon steel, a 2 % Cr-1% Mo, a 5% Cr-1 Mo, and an AISI typc-410 steel. The crucible, Bi, and additives were equilibrated at 400 to 425°C, the temperature rapidly rahsed to 600°C, and the coiiceiitration of Fo in solution measured as a function of time. Results are shown in Fig. 21-2. In the presence of Zr-fMg, the 5% Cr-1 2 c I Io and the AISI type-410 steels dissolved at approximately th( same rate, while the 2 % Cr-1% Mo steel dissolved more slowly. No detectable dissolution of Fe from the carbon steel was measured in 44 hr at blO°C. These results are parallel to the thermal convection loop results, and consi,stent with the film-formation studies in that the measured solution rates are inversely proportional to the ability and rate at w hich the steels form ZrX films. At present no data are available on rates of solution for ZrC-forming steels. 

The beha vior of steels in U-Bi is studied in three types of tests. Thermal convection loops are used to test materials under dynamic conditions. In these, the fuel solution is continuously ( irculated through a temperature differential in a clo.scd loop of pipe. Variables such as material composition, maximum temperature, temperature differential, and additive concentrations are studied in this test. More than si.xty. such loops have now been run at BXL. The principal limitation in these tests is that the velocities obtained by thermal pumping are extremely low when compared with the lAIFR design conditions. 

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... 

Fig. 21-3, Thermal convection loop. A. Air lock. B. Hot leg. C. Cold leg. D. Fans. E. "Tee connection. F. Melt tank with AISI t5rpe I0 steel filter...

Summary of Thermal Convection Loop Data All loops were fabricated from 1/2 IPS Sch 40 pipe of the steel indicated... 

In Table 21-4, data from 29 thermal convection loop experiments at BNL are summarized. The first three loops were fabricated from 2 % Cr-1% Mo steel pipe and the U-Bi solution was not inhibited. Loops No. 4 to 17 inclusive were made with 2 % Cr-1% Mo steel and inhibited with Mg and Zr. Loops No. 18 to 25 inclusive were fabricated from various types of steels, ranging from Bessemer to 18% Cr-8% Ni austenitic steels. Loops No. 26 to 29 were made from 2 % Cr-1% Mo steel pipe. The purpose of these tests was to study the effectiveness of Ca, Th, and Ti as inhibitors. 

Dynamic tests of the reaction between Bi and steel in the presence of a radiation field must be completed before a final selection can be made of materials for the LMFR. The effect of velocity on corrosion is not certain from the out-of-pile studies, so that no exact analogy can be made between out-of-pile forced circulation loops and in-pile capsules. There has been limited work done at Harwell [6] with thermal convection loops in and out of a radiation field. These loops had no U but did contain Ca and Zr inhibitors. The data suggest that pile radiation may have induced some acceleration of mass transfer. 

R. Cygan, Lead-Bismuth Eutectic Thermal Convection Loop. USAEC Repi it N.. V-SR-1060, North American Aviation, Inc., Oct. 15, 1954. 

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