Experimental Evaluation of Release Rates of Cobalt Ions from Cobalt Oxides at 285°C

Abstract

A major radiation source in boiling water reactors (BWR) for personnel exposure during shutdown maintenance has been identified to be activated corrosion products, mainly ⁶⁰Co, deposited on the primary system piping wall. Consequentry, information on Co transport mechanisms is vital to reduce this buildup in the primary system. It is generally considered that Co ions, which are corrosion products, are adsorbed on oxides deposited on the fuel rod surface, activated by neutrons, and then released as radioactive ions. As cobalt ferrite (CoFe₂O₄) is a thermodynamically probable compound under BWR conditions⁽¹⁾, ⁶⁰Co is considered to be released from it. Cobalt oxide (CoO) is also formed on the fuel rod surface when the concentration of Co ions in the coolant exceeds its solubility limit. Water pH might play an important role in reactions of adsorption and release because protons involve in these processes. However, effects of water pH on adsorption and release at high temperature have not been experimentally clarified yet.
In this study, release rates of Co ions from CoFe₂O₄ and CoO particles were measured at 285°C and various pH values. Then 9 mechanism for their release was oronosed.