Novel Gadolinium Chemistry in Moderator Water

Abstract

During a recent outage at a Canadian CANDU plant, dissolved gadolinium levels in the moderator water began to decrease. Investigations were carried out to determine the cause for the loss of dissolved gadolinium. These showed that gadolinium had partially precipitated as oxalate, after the precipitate was characterized. The source of oxalate is believed to be a calandria tube leaking carbon dioxide into the moderator water. Various gadolinium oxalate removal options such as chemical cleaning through oxidation and/or complexation of the precipitate were investigated, involving a variety of organizations. A limited removal test run was conducted, and recommendations for a path forward were discussed.