Reduction of Pu(IV) and Np(VI) with carbohydrazide in nitric acid solution

Abstract

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 .

INTERACTION OF Pu(VI) AND Np(VI) WITH CARBOHYDRAZIDE IN NITRIC ACID SOLUTIONS

“…As it was found, an earlier [2] Np(IV) formation from the Np(V) was observed at [HNO3] ≥3 mol/l and relatively high concentration of CO(N2H3)2 (more than 0.2 mol/l).…”

Insights into the Reduction Mechanisms of Np(VI) to Np(V) by Carbohydrazide

“…17,19,21−23 Volk et al reported that carbohydrazide can quickly reduce Np(VI) to Np(V), while it further reduces Np(V) to Np(IV) very slowly. 17 23 These works suggest that the slow reduction of Np(V) to Np(IV) by carbohydrazide is probably the higher energy barrier, which is kinetically hindered. Although the reaction of Np(VI) to Np(V) by carbohydrazide was investigated experimentally, their mechanism is still unclear, so it is essential to explore the reaction mechanism of Np(VI) to Np(V) by CO(N 2 H 3 ) 2 from the perspective of theory.…”

“…It has been suggested to remove Np from the SNF reprocessing process using advanced PUREX processes such as the Partitioning Conundrum Key (PARC) process and SUPERPUREX process . Some free-salt reductants such as hydroxyurea, , hydrazine, − and their derivatives have been experimentally applied to reduce Np­(VI).…”

Reduction of Np(vi) with hydrazinopropionitrileviawater-mediated proton transfer

“…Therefore, the more efficient separation of Np from U and Pu in the PUREX process is the reduction of Np(VI) to Np(V). Experimental results have been reported for several highly efficient and environment-friendly Np(VI) reductants, including oximes, [4][5][6] aldehydes, [7][8][9] hydroxylamine, [10][11][12][13][14] urea, 15,16 hydroxamic acid, [17][18][19][20] hydrazine, [21][22][23][24][25][26][27][28] and their derivatives. Acetaldoxime reduces Np(VI) to Np(V) only at less than 2 M nitric acid concentration and low temperature (308 K).…”