Equilibrium evaporation behavior of polonium and its homologue tellurium in liquid lead-bismuth eutectic

Ohno, Shuji  ; Kurata, Yuji; Miyahara, Shinya  ; Katsura, Ryoei*; Yoshida, Shigeru*

Experimental study using the transpiration method investigated equilibrium evaporation behavior of radionuclide polonium (Po) generated and accumulated in liquid lead-bismuth eutectic (LBE) cooled nuclear systems. The experiment consisted of two series of tests: preliminary evaporation tests for homologue element tellurium (Te) in LBE, and evaporation tests for Po-accumulated LBE in which test specimens were prepared by neutron irradiation. The evaporation tests of Te in LBE provided the suggestion that Te would exist in a chemical form of PbTe as well as the information for confirming the validity of technique and conditions of Po test. From the evaporation tests of Po in LBE, we obtained fundamental data and empirical equations such as Po vapor concentration in the gas phase, Po partial vapor pressure, thermodynamic activity coefficients, and gas-liquid equilibrium partition coefficients of Po in LBE in the temperature range from 450C to 750C. Additionally, radioactivity concentration of Po and Bi vapor in a cover gas region of a typical LBE-cooled nuclear system was estimated based on the obtained experimental results, and the importance of Po evaporation behavior was demonstrated.