Behavior of cobalt in iron- and steel-making processes

During the decommissioning phase of a nuclear reactor, most of the steel-reinforced concrete shielding around the pressure vessel is considered as low-level radioactive waste. It is very desirable to reduce the radioactivity of the low-level radioactive waste to below the clearance level. Normally, the radioactivity of steel after irradiation is found to be proportional to the cobalt content. In this study, for the production of low-activation steel, the mixing source and the partition behavior of cobalt in the iron- and steel-making processes are investigated. The results are summarized as follows: (1) the cobalt content in hot metal was found to be approximately 20 ppm, and the main source was serpentine. The ratio of the cobalt content to the iron content in the ore varied from 1.5 to 6.5 ppm, depending on the brand. In the charge materials for the BOF process and typical refractories, the cobalt content was negligible except for some ferroalloys. (2) By thermodynamic calculations, it can be deduced that the activity coefficient of CoO is very small in the slag.