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The direct and simultaneous spectrophotometric determination of Np(IV), Np(V) and Np(VI) at low concentrations (> 10" 7 M) in nitric acid (up to 3 M) and in the presence of uranyl nitrate (up to 1 M) without addition of a complexing agent is described. The splitting of the absorption peak of Np(V) at 981 nm due to the formation of a 1 : 1 binuclear complex with U(VI) and the overlap of this peak with the absorption peak of Np(IV) at 978 nm are taken into account. Limits for quantitative determination of Np(IV), Np(V) and Np(VI) are of the order of 10" 6 , 10" 7 and 10" 6 M, respectively.

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 (HCO3−) was the main anionic impurity. A study of the complexing reactions of Gd in concentrated solutions (0.0509M) of Gd(NO3)3 showed that in the absence of HCO3− and at pH < 6, the species in equilibrium are distributed as follows: predominant: Gd3+, NO3− and [GdNO3]2+; and traces: [GdOH]2+ and [Gd(NO3)2]+. An increase in pH over 6.4 led to the formation of the solid phase: Gd2(OH)5.1(NO3)0.9. In the presence of bicarbonate, the solid phase Gd2(OH)5.0(NO3)1.0 is visually detected at a pH as low as 5.8. In other respects, analysis of dilute solutions (0.00065M) of Gd(NO3)3 containing bicarbonate allows the solid phase precipitating between pH 5.5 and 6.6 to be ascribed to the formula Gd2(OH)4CO3·3H2O. In the absence of HCO3−, 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.

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