Neptunium has been previously shown to present challenges within a used nuclear fuel reprocessing scheme due to its tendency to exist in the (IV), (V), and (VI) oxidation states simultaneously. In order to control this neptunium speciation, and informed by relevant work in the literature, we are currently engaged in a study of nitric/nitrous acid redox chemistry with Np(V) and Np(VI). To minimize radiological exposure risks, we are also exploring the validity of using vanadium as an analogue for the study of the kinetics of the Np(VI)/Np(V) reduction by nitrous acid. The kinetics of the reduction of vanadium(V) by nitrous acid in solutions of nitric acid was investigated spectrophotometrically by the method of initial rates. Orders of reaction with respect to V(V), and HNO2 were previously found to be 0.90, and 1.25 respectively, in reasonable agreement with the analogous reaction orders for the reduction of Np(VI) by nitrous acid previously reported by Precek and Paulenova - suggesting that, for this particular reduction, V(V) can serve as a good kinetic analogue for Np(VI). Within this study orders of reaction with respect to [H +], [NO3 -], [SO4 2-], and [ClO4 -] have also been found to be 0.1, -0.2, 0.1, and 0 respectively. Preliminary experiments have also been conducted on the reduction of V(V) by the known reducing agent NO which has hitherto not been considered in the reduction of Np(VI) to Np(V).

Neptunium has been previously shown to present challenges within a used nuclear fuel reprocessing scheme due to its tendency to exist in the (IV), (V), and (VI) oxidation states simultaneously.In order to control this neptunium speciation, and informed by relevant work in the literature, we are currently engaged in a study of nitric/nitrous acid redox chemistry with Np(V) and Np(VI).To minimize radiological exposure risks, we are also exploring the validity of using vanadium as an analogue for the study of the kinetics of the Np(VI)/Np(V) reduction by nitrous acid.The kinetics of the reduction of vanadium(V) by nitrous acid in solutions of nitric acid was investigated spectrophotometrically by the method of initial rates.Orders of reaction with respect to V(V), and HNO2 were found to be 0.90, and 1.24 respectively, in reasonable agreement with the analogous reaction orders for the reduction of Np(VI) by nitrous acid previously reported by Precek and Paulenovasuggesting that, for this particular reduction, V(V) can serve as a good kinetic analogue for Np(VI).The value of the rate constant k for the rate law -d[V(V)]/dt = k[V(V)][HNO2] 1.24 /[H + ] was found to be 7.5 × 10 -4 M -0.24 s -1 at 20 °C, two orders of magnitude smaller than that for the reduction of Np(VI) by HNO2, a difference that is attributable to the loss of one of the oxygens during the VO2 + to VO 2+ reduction reaction.

Routing neptunium to a single product in spent nuclear fuel reprocessing is a significant challenge. In this work, we have further improved the simulation of neptunium extraction in an advanced PUREX flowsheet by applying a revised model of the Np(V)-Np(VI) redox reaction kinetics, a new nitric acid radiolysis model, and by evaluating various models for the nitrous acid distribution coefficient. The Np disproportionation reaction is shown to have a negligible effect. The models are validated against published cold test' experimental results; the hot test' simulation suggests that high neptunium radiolysis could help to achieve high recoveries using this flowsheet.

To support the development of an Advanced PUREX process that fully recovers neptunium with uranium and plutonium for recycle, an enhanced understanding of neptunium behaviour in nitric acid and tributyl phosphate phases is essential. The most important reaction affecting neptunium extraction in the PUREX process is the oxidation of Np(V) by nitric acid. This paper reports new data on Np(V) oxidation in nitric acid under conditions relevant to the development of a flowsheet for the primary extraction/scrub contactor in an Advanced PUREX process.

Hydroxamic acids (XHAs) are organic compounds with affinities for cations such as Fe3+, Np4+ and Pu4+ and have been identified as useful reagents in nuclear fuel reprocessing. Acid catalyzed hydrolysis of free XHAs is well known and may impact negatively on reprocessing applications. The hydrolysis of metal-bound XHAs within metal ion-XHA complexes is less understood. With the aid of speciation diagrams, we have modelled UV-visible spectrophotometric kinetic studies of the acid-catalyzed hydrolysis of acetohydroxamic acid (AHA) bound to the model ion Fe(III). These studies have yielded the following information for the hydrolysis of AHA in the Fe(AHA)2+ complex at 293 K: (i) the order with respect to [H+] during the rate determining step, m=0.97, is the same as for the free ligand, indicating a similarity of mechanisms; and (ii) the kinetic rate parameter, k 1=1.02×10−4 dm3⋅mol−1⋅s−1, is greater than that for the free ligand, k 0=1.84×10−5 dm3⋅mol−1⋅s−1 for pH>−0.5, a result that is consistent with a Hammett analysis of the system.