*3.2. Specimen Testing Tube*

Figure 5 shows the testing tube. The testing tube is positioned downstream of the loop heater, as shown in Figure 4. The testing tube consists of an inlet orifice, a straight section, and an outlet orifice. The orifices are at 30◦ to the horizontal axis of the testing tube. The testing tube was machined from solution-annealed 316L SS plate as a tubing form of 2 mm thick, 40 cm long, and 13.8 mm outer diameter. The fabrication tolerance of the outer diameter and wall thickness is ±0.2 mm.

The chemical composition (wt %) of the type 316L austenitic SS used in this study is 16.84 Cr, 10.29 Ni, 0.73 Si, 1.02 Mn, 0.036 C, 0.030 P, and 0.005 S, balanced with Fe. The tube was solution heat-treated at 1080 ◦C for 1.5 h, and then cooled in water rapidly. The inner surface of the tube was polished to remove the rough layer after acid washing.

#### *3.3. Operation Conditions*

The temperature of the high- and low-temperature parts of the loop were 450 ◦C and 350 ◦C, respectively. The test duration was approximately 3000 h. The LBE flow rate was 5 L/min, corresponding to an average velocity of 0.7 m/s in the 9.8 mm diameter high-temperature testing tube. The flow rate was measured using an electromagnetic flowmeter (EMF). An oxygen sensor was placed in the loop, but did not work well, so the oxygen concentration in the LBE was neither measured nor controlled in this study. The literature [38] indicates that when the oxygen concentration is properly controlled to between 10−<sup>4</sup> wt % and 10−<sup>7</sup> wt %, an oxide film, with a thickness of several micrometers to tens of micrometers, will form on the surface of the stainless steel. Below that concentration range,

no oxide film is formed. We can thus deduce that the oxygen concentration in the present experiment is below 10−<sup>8</sup> wt %.
