Experimental Studies on Pb-Bi Flow Technology and Steel Compatibility With Pb-Bi

For the development of the Pb-Bi cooled FBR and the ADS with Pb-Bi target, the compatibility of steels for core, structural and window materials with high temperature Pb-Bi is one of the critical issues. The effect of corrosion on the mechanical strength of steels should be also concerned. An oxide layer which is formed and self-healed on the steel surface in Pb-Bi is expected to improve the compatibility if oxide potential in Pb-Bi is controlled and monitored adequately to form stable oxide layer. Therefore, monitoring technology of oxygen concentration in Pb-Bi is required. In the present study, a performance test of oxygen sensor, a steel corrosion test and a steel mechanical strength test, or a pipe rupture test, were performed as follows: (1) Test of oxygen sensor: For the monitor of the oxygen potential in Pb-Bi, a thermal stress proof type oxygen sensor made of electrolyte conductor (MgO-ZrO₂ and Y₂O₃-ZrO₂) with the reference fluid of oxygen saturated bismuth was developed, and the performance test was conducted using the corrosion test loop. The performance was stable and reliable in the 1000-hour operation. The electromotive forces (EMF) of the sensor cells of MgO-ZrO₂ and Y₂O₃-ZrO₂ were nearly the same as each other, and they were not destructed during the 1000-operation. (2) Steel corrosion test: High Cr steels including heat resisting steels were exposed to a liquid Pb-Bi flow at the temperature of 550°C, the velocity of 1m/s, the oxygen concentration of 1.7×10⁻⁸wt% and the temperature difference of 150°C for 1000 and 500 hours. It was found that weight losses were lower in general in the steels with higher Cr content. The steels with high Cr, Si and Al formed thin oxide layers and exhibited better compatibility with Pb-Bi. (3) Steel mechanical strength test (pipe rupture test): Metallurgical analysis for ruptured pipe made of SS-316 was performed. The pipe had experienced the exposure to Pb-Bi at 400°C for 3440 hours, at 350°C for 4 hours, at 300°C for 50 hours, and at 250°C for 622 hours. Pipe rupture occurred possibly due to thermal expansion of Pb-Bi at heat-up processes. The results of the analysis indicated that Pb-Bi penetration to steel matrix occurred more seriously near the ruptured part than the other part of the pipe. The analytical result suggested that a brittle fracture might occur in the inner part of the ruptured pipe wall by liquid metal embrittlement because of Pb-Bi penetration, whereas dimples observed suggested that ductile fracture might occur in the outer part of the ruptured pipe wall.