The reduction of Pu(IV) and Np(VI) with carbohydrazide (NH 2 NH) 2 CO in 1-6 M HNO 3 solutions was studied. The Pu(IV) reduction is described by a first-order rate equation with respect to Pu(IV). At [HNO 3 ] ≥ 3 M, the reaction becomes reversible. The rate constants of the forward and reverse reactions were determined, and their activation energies were estimated. Neptunium(VI) is reduced to Np(V) at a high rate, whereas the subsequent reduction of Np(V) to Np(IV) is considerably slower and is catalyzed by Fe and Tc ions. The possibility of using carbohydrazide for stabilizing desired combinations of Pu and Np valence states was examined.One of important lines of improving aqueous technology for reprocessing irradiated fuel, primarily highburn-up spent nuclear fuel, is search for new reagents, more convenient for performing redox operations involving Pu and Np ions. Organic compounds show promise for this purpose. Their use allows the amount of salt-containing radioactive wastes to be sharply reduced [1]. Carbohydrazide (NH 2 NH) 2 CO can be one of such reagents. Preliminary experiments showed that it rapidly reduces Pu(IV) and Np(VI) in nitric acid solutions at ambient temperature.In the literature, carbohydrazide was mainly examined as ligand for bi-and trivalent element ions [2][3][4][5]. Its reducing properties have been studied in considerably less detail. It is known that carbohydrazide reduces the Cu 2+ and Fe 3+ ions to Cu + and Fe 2+ , respectively. Both reactions occur at high rate and are accompanied by the evolution of gases: CO 2 and N 2 [6]. We found no data in the literature on interaction of actinide ions with carbohydrazide in nitric acid solutions. In this study we measured the rate of reduction of Pu(IV) and Np(VI) with carbohydrazide in HNO 3 solutions and examined the possibility of stabilizing Pu and Np in desirable valence state combinations using this reagent. Procedure.A temperature-controlled cell of a spectrophotometer was charged with the calculated amounts of stock solutions of HNO 3 and carbohydrazide and kept to attain the preset temperature. After that, a small aliquot of a Pu(IV) or Np(VI) solution was added, and the absorption spectra of the working solution were recorded at definite time intervals in the wavelength (λ) range 350-1100 nm. The concentration of the Pu and Np valence forms was calculated from the optical density (D) at λ = 475, 602, 980, and 710 nm, corresponding to the absorption maxima of the Pu 4+ , Pu 3+ , NpO 2 + , and Np 4+ ions, respectively.Chemicals and analysis. The Pu stock solution was prepared by dissolving PuO 2 in a boiling mixture of HNO 3 and HF. After the complete dissolution of PuO 2 , Al(NO 3 ) 3 was added to bind F -anions in a complex. The resulting solution was acidified with HNO 3 to a concentration of 7 M and purified by sorption on VP-1Ap anion exchanger. The desorbate was evaporated, and Pu was stabilized in the tetravalent state by adding H 2 O 2 . The solution thus prepared contained 440 g l -1 Pu and 6.6 M HNO 3 .

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