Generation and study of Am(IV) by temperature-controlled electron pulse radiolysis.

First-of-a-kind temperature-controlled electron pulse radiolysis experiments facilitated the radiation-induced formation of Am(IV) in concentrated (6.0 M) HNO3, and enabled the derivation of Arrhenius and Eyring activation parameters for instigating the radical reaction between NO3˙ and Am(III).

Comparison of Americium(III) and Neodymium(III) Monothiophosphate Complexes.

Mixed-donor ligands, such as those containing a combination of O/N or O/S, have been studied extensively for the selective extraction of trivalent actinides, especially Am3+ and Cm3+, from lanthanides during the recycling of used nuclear fuel. Oxygen/sulfur donor ligand combinations also result from the hydrolytic and/or radiolytic degradation of dithiophosphates, such as the Cyanex class of extractants, which are initially converted to monothiophosphates. To understand potential differences between the binding of such degraded ligands to Nd3+ and Am3+, the monothiophosphate complexes [M(OPS(OEt)2)5(H2O)2]2- (M3+ = Nd3+, Am3+) were prepared and characterized by single-crystal X-ray diffraction and optical spectroscopy and studied as a function of pressure up to ca. 14 GPa using diamond-anvil techniques. Although Nd3+ and Am3+ have nearly identical eight-coordinated ionic radii, these structures reveal that while the M-O bond distances in these complexes are almost equal, the M-S distances are statistically different. Moreover, for [Nd(OPS(OEt)2)5(H2O)2]2-, the hypersensitive 4I9/2 → 4G5/2 transition shifts as a function of pressure by -11 cm-1/GPa. Whereas for [Am(OPS(OEt)2)5(H2O)2]2-, the 7F0 → 7F6 transition shows a slightly stronger pressure dependence with a shift of -13 cm-1/GPa and also exhibits broadening of the 5f → 5f transitions at high pressures. These data likely indicate an increased involvement of the 5f orbitals in bonding with Am3+ relative to that of Nd3+ in these complexes.

Multiscale Modeling of Plutonium Radiation Chemistry in Nitric Acid Solutions. 1. Cobalt-60 Gamma Irradiation of Pu(IV).

Careful manipulation of the plutonium oxidation states is essential in the study and utilization of its rich redox chemistry. To achieve this level of control, a comprehensive mechanistic understanding of radiation-induced plutonium redox chemistry is critical due to the unavoidable exposure of plutonium to ionizing radiation fields, both inherent and from in-process applications. To this end, we have developed an experimentally evaluated multiscale computer model for the prediction of gamma radiation-induced Pu(IV) redox chemistry in concentrated nitric acid solutions (1.0, 3.0, and 6.0 M). Under these acidic, aqueous solution conditions, cobalt-60 gamma irradiation afforded marginal net conversion of Pu(IV) to Pu(VI), the extent of which was dependent on the concentration of HNO3 and absorbed gamma dose. Multiscale calculations, which are in excellent agreement with experimental data, indicate that this observation is due to a combination of inherent plutonium disproportionation reactions and several radiation-induced processes, including redox cycling between Pu(IV) and Pu(III), as achieved by the reduction of Pu(IV) by nitrous acid and hydrogen peroxide, the oxidation of Pu(III) by nitrate and hydroxyl radicals, and the sequential oxidation of Pu(IV) to Pu(V) and Pu(VI) by the remaining available yield of nitrate radicals.

Effect of f-element complexation on the radiolysis of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]).

A systematic study of the impact on the chemical reactivity of the oxidising n-dodecane radical cation (RH˙+) with f-element complexed 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) has been undertaken utilizing time-resolved electron pulse radiolysis/transient absorption spectroscopy and high-level quantum mechanical calculations. Lanthanide ion complexed species, [Ln((HEH[EHP])2)3], exhibited vastly increased reactivity (over 10× faster) in comparison to the non-complexed ligand in n-dodecane solvent, whose rate coefficient was k = (4.66 ± 0.22) × 109 M-1 s-1. Similar reactivity enhancement was also observed for the corresponding americium ion complex, k = (5.58 ± 0.30) × 1010 M-1 s-1. The vastly increased reactivity of these f-element complexes was not due to simple increased diffusion-control of these reactions; rather, enhanced hole transfer mechanisms for the complexes were calculated to become energetically more favourable. Interestingly, the observed reactivity trend with lanthanide ion size was not linear; instead, the rate coefficients showed an initial increase (Lu to Yb) followed by a decrease (Tm to Ho), followed by another increase (Dy to La). This behaviour was excellently predicted by the calculated reaction volumes of these complexes. Complementary cobalt-60 gamma irradiations for select lanthanide complexes demonstrated that the measured kinetic differences translated to increased ligand degradation at steady-state timescales, affording ∼38% increase in ligand loss of a 1 : 1 [La((HEH[EHP])2)3] : HEH[EHP] ratio system.

Radiolytic evaluation of a new technetium redox control reagent for advanced used nuclear fuel separations.

Technetium is a problematic radioisotope for used nuclear fuel (UNF) and subsequent waste management owing to its high environmental mobility and coextraction in reprocessing technologies as the pertechnetate anion (TcO4-). Consequently, several strategies are under development to control the transport of this radioisotope. A proposed approach is to use diaminoguanidine (DAG) for TcO4- and transuranic ion redox control. Although the initial DAG molecule is ultimately consumed in the redox process, its susceptibility to radiolysis is currently unknown under envisioned UNF reprocessing conditions, which is a critical knowledge gap for evaluating its overall suitability for this role. To this end, we report the impacts of steady-state gamma irradiation on the rate of DAG radiolysis in water, aqueous 2.0 M nitric acid (HNO3), and in a biphasic solvent system composed of aqueous 2.0 M HNO3 in contact with 1.5 M N,N-di-(2-ethylhexyl)isobutyramide (DEHiBA) dissolved in n-dodecane. Additionally, we report chemical kinetics for the reaction of DAG with key transients arising from electron pulse radiolysis, specifically the hydrated electron (eaq-), hydrogen atom (H˙), and hydroxyl (˙OH) and nitrate (NO3˙) radicals. The DAG molecule exhibited significant reactivity with the ˙OH and NO3˙ radicals, indicating that oxidation would be the predominant degradation pathway in radiation environments. This is consistent with its role as a reducing agent. Steady-state gamma irradiations demonstrated that DAG is readily degraded within a few hundred kilogray, the rate of which was found to increase upon going from water to HNO3 containing solutions and solvents systems. This was attributed to a thermal reaction between DAG and the predominant HNO3 radiolysis product, nitrous acid (HNO2), k(DAG + HNO2) = 5480 ± 85 M-1 s-1. Although no evidence was found for the radiolysis of DAG altering the radiation chemistry of the contacted DEHiBA/n-dodecane phase in the investigated biphasic system, the utility of DAG as a redox control reagent will likely be limited by significant competition with its degradation by HNO2.

Chemical Kinetics for the Oxidation of Californium(III) Ions with Select Radiation-Induced Inorganic Radicals (Cl2•- and SO4•-).

Despite the availability of transuranic elements increasing in recent years, our understanding of their most basic and inherent radiation chemistry is limited and yet essential for the accurate interpretation of their physical and chemical properties. Here, we explore the transient interactions between trivalent californium ions (Cf3+) and select inorganic radicals arising from the radiolytic decomposition of common anions and functional group constituents, specifically the dichlorine (Cl2•-) and sulfate (SO4•-) radical anions. Chemical kinetics, as measured using integrated electron pulse radiolysis and transient absorption spectroscopy techniques, are presented for the reactions of these two oxidizing radicals with Cf3+ ions. The derived and ionic strength-corrected second-order rate coefficients (k) for these radiation-induced processes are k(Cf3+ + Cl2•-) = (8.28 ± 0.61) × 105 M-1 s-1 and k(Cf3+ + SO4•-) = (9.50 ± 0.43) × 108 M-1 s-1 under ambient temperature conditions (22 ± 1 °C).