Study of complexing reactions causing the depletion of gadolinium ions from the moderator of CANDU‐PHW nuclear reactors

Abstract

Abstract The addition of gadolinium nitrate solution in the moderator of Hydro‐Québec's Gentilly 2 reactor led to an unexpected depletion of gadolinium (Gd) by precipitation. Analysis of moderator samples revealed that bicarbonate (HCO 3 − ) was the main anionic impurity. A study of the complexing reactions of Gd in concentrated solutions (0.0509M) of Gd(NO 3 ) 3 showed that in the absence of HCO 3 − and at pH < 6, the species in equilibrium are distributed as follows: predominant: Gd 3+ , NO 3 − and [GdNO 3 ] 2+ ; and traces: [GdOH] 2+ and [Gd(NO 3 ) 2 ] + . An increase in pH over 6.4 led to the formation of the solid phase: Gd 2 (OH) 5.1 (NO 3 ) 0.9 . In the presence of bicarbonate, the solid phase Gd 2 (OH) 5.0 (NO 3 ) 1.0 is visually detected at a pH as low as 5.8. In other respects, analysis of dilute solutions (0.00065M) of Gd(NO 3 ) 3 containing bicarbonate allows the solid phase precipitating between pH 5.5 and 6.6 to be ascribed to the formula Gd 2 (OH) 4 CO 3 ·3H 2 O. In the absence of HCO 3 − , no precipitation occurred up to pH 6.8. Finally, the loss of Gd from the moderator was established as being due to the formation of a hydroxide carbonate salt.