Electrochemical methods have been developed in many countries as alternatives to traditional chemical ones for reducing the contents of salts in radioactive wastes and reducing the volume of the latter in reprocessing spent fuel [1][2][3][4][5]. Electrochemical methods have been developed in Russia to regulate the-valency states of U, Pu, and Np and in the extraction of noble metals from aqueous solutions, removing hazardous substances from radiochemical wastes, and so on. Here we report major results on electrolysis applied to the reductive re-extraction of plutonium at the stage of separation from uranium and stabilizing Pu(IV) at the affinity stage.We examined the electrochemical behavior of plutonium and neptunium ions in nitric acid. The reactions pu 4" + e --, PuJ';,',po~" + ,. ~ ~pO~ (2) are reversible at a platinum electrode, and the potentials of the Pu(IV)/Pu(III) and Np(VI)/Np(V) pairs in 1 M HNO 3 are 0.91 and 1,11 V respectively (here and subsequently, the potentials are quoted relative to a normal hydrogen electrode). The two reactions have first-order kinetics with controlled platinum-electrode potentials. In the presence of hydrazine, the product from (1) is Pu(III), but in the absence of hydrazine and with [HNO3] > 1 M, PU(III) is oxidized in the polarization to PU(IV) by the nitrous acid (oxide of nitrogen) formed by the electrochemical reduction of nitric acid. A difference from reactions (1) and (2) is that Pu(VI) and Np(V) are reduced in a more complicated fashion. In the presence of a stabilizer (sulfamic acid), a platinum electrode at 0.6 V reduces Pu(VI) to Pu(V) in solutions of low acidity (0.1 M) in the reaction PuO22+ + e --, PuO2 +. For [HNO3] > 1 M, there is further conversion of Pu(V) to Pu(IIl) by an autocatalytic mechanism that includes reactions at the electrode and in the bulk of the solution: 2PUO2 + + 4H + ~ PUO22+ + pu4+ + 2H20; Pu4+ + e._,. Pu3+; PuO2 + + Pu3+ + 4H + _., 2Pu4+ + 2H20; PuO2 + + pu4+ ~ PuO22+ + pu3+. The reduction of Np(V) in nitric acid is also autocatalytic, and the mechanism is the same as that found previously [6] for perchloric acid: NpO2 + + 4H + + e--,Np 4+ + 2H20;Np '*+ + e--,Np3+;Np 3+ + NpO2 + + 4H +-,.2Np 4+ + 2H20, wherethe first stage is the slowest one. However, a difference from perchloric acid, where the final product is Np(IlI), is that nitric acid leaves part of the neptunium in the quadrivalent state on account of the oxidation of NP(III) by nitrate ions.The oxidation of PU(III) to Pu(IV) and of Np(V) to Np(VI) by the reverse reactions in (I) and (2) occurs readily under potentiostatic conditions with a platinum electrode in the absence of hydrazine, but if hydrazine is present, this must first be destroyed. The oxidation of hydrazine N211 ~ -4e--N z + SH"at a platinum electrode in 1 M HNO 3 begins at 0.3 V (in an unstirred electrolyte) and involves a mechanism that includes the slow stage N2H5 + --2e ---N2H 2 + 3H +, which is followed by the fast reaction N2H 2 + 2e --, N 2 + 2H +.