The potential of using optical spectroscopic techniques, such as Raman and visible/near infrared (Vis/NIR), for on-line process control and special nuclear materials accountability applications at a spent nuclear fuel reprocessing facility was evaluated. The availability of on-line, real-time techniques that directly measure process concentrations of nuclear materials will enhance the performance and proliferation resistance of the solvent extraction processes. Further, on-line monitoring of radiochemical streams will also improve reprocessing plant operation and safety. This paper reviews the current state of development of the spectroscopic on-line monitoring techniques for such solutions. To further examine the applicability of optical spectroscopy for this application, segments of a spent nuclear fuel, with approximate burn-up values of 70 MW d/kg M, were dissolved in concentrated nitric acid and adjusted to varying final concentrations of HNO 3 . The resulting spent fuel solutions were batch-contacted with tributyl phosphate/n-dodecane organic solvent. The feed and equilibrium aqueous and loaded organic solutions were subjected to optical measurements. The obtained spectra showed the presence of quantifiable Raman bands due to NO 3 − and UO 2 2+ and Vis/NIR bands due to multiple species of Pu(IV), Pu(VI), Np(V), the Np(V)-U(VI) cation-cation complex, and Nd(III) in fuel solutions. This result justifies spectroscopic techniques as a promising methodology for monitoring spent fuel processing solutions in real-time. The fuel solution was quantitatively evaluated based on spectroscopic measurements and was compared to inductively coupled plasma-mass spectroscopy analysis and Oak Ridge Isotope Generator (ORIGEN)-based estimates.

A portable spectroelectrochemical sensor has been designed, evaluated, and demonstrated on a complex sample of radioactive waste. The sensor consisted of a black delrin sample compartment with a total internal sample volume of 800 microL, attached to an indium tin oxide coated glass multiple internal reflection optical element. Detection was by total internal reflection of light from a blue light emitting diode source. After a 10 min uptake for each standard, the sensor showed a linear response in absorbance change for 5 x 10(-5) to 5 x 10(-3) M ferrocyanide with electrochemical modulation by scanning at 20 mV/s from -0.30 V to +0.55 V vs a Ag/AgCl reference electrode. Due to the complex nature of Hanford radioactive tank waste samples containing ferrocyanide, a standard addition method was developed for analysis. The spectroelectrochemical sensor determined a concentration of 9.2 mM ferrocyanide for U-Plant-2 simulant solution containing 9.38 mM ferrocyanide that was prepared according to Hanford process flowsheets. A radioactive tank waste sample from Hanford Tank 241-C-112 was determined to be 1.0 mM in ferrocyanide using the spectroelectrochemical sensor. A value for the ferrocyanide concentration in the sample of 0.61 mM was determined by FTIR spectroscopy.

Like the Re analogue, the ligand-to-metal charge transfer (LMCT) excited-state of [Tc(dmpe)3]2+ (dmpe is bis-1,2-(dimethylphosphino)ethane) is luminescent in solution at room temperature. Surprisingly, both [M(dmpe)3]2+* species have extremely large excited-state potentials (ESPs) as oxidants-the highest for any simple coordination complex of a transition metal. Furthermore, this potential is available using a photon of visible light (calculated for M = Re(Tc); E1/2* = +2.61(2.52) V versus SCE; lambdamax = 526(585) nm). Using a Rehm-Weller analysis with a series of aromatic hydrocarbons as electron-transfer quenchers, E1/2(Re2+*/Re+) has been determined to be 2.58 V, in good agreement with the calculated value. Both [M(dmpe)3]2+* species are quenched by chloride ion and both can function as excited-state oxidants in water solution.

A distinct need exists for real time information on an acid concentration of industrial aqueous streams. Acid strength affects efficiency and selectivity of many separation processes, including nuclear fuel reprocessing. Despite the seeming simplicity of the problem, no practical solution has been offered yet, particularly for the large-scale schemes involving toxic streams such as highly radioactive nuclear wastes. The classic potentiometric technique is not amiable for online measurements due to the requirements of frequent calibration/maintenance and poor long-term stability in aggressive chemical and radiation environments. Therefore, an alternative analytical method is needed. In this work, the potential of using Raman spectroscopic measurements for online monitoring of strong acid concentration in solutions relevant to dissolved used nuclear fuel was investigated. The Raman water signature was monitored for solution systems containing nitric and hydrochloric acids and their sodium salts of systematically varied composition, ionic strength, and temperature. The trivalent neodymium ion simulated the presence of multivalent f metals. The gaussian deconvolution analysis was used to interpret observed effects of the solution nature on the Raman water O-H stretching spectrum. The generated Raman spectroscopic database was used to develop predictive multivariate regression models for the quantification of the acid and other solution components, as well as selected physicochemical properties. This method was validated using independent experiments conducted in a flow solvent extraction system.

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The electrochemical and spectroelectrochemical behavior of europium(III) chloride in a molten salt eutectic, 3LiCl–2KCl, over a temperature range of 643–1123 K using differential pulse voltammetry, cyclic voltammetry, potential step chronoabsorptometry, and thin-layer spectroelectrochemistry is reported. The electrochemical reaction was determined to be the one-electron reduction of Eu3+ to Eu2+ at all temperatures. The redox potential of Eu3+/2+ shifts to more positive potentials, and the diffusion coefficient for Eu3+ increases as temperature increases. The results for the number of electrons transferred, redox potential, and diffusion coefficient are in good agreement between the electrochemical and spectroelectrochemical techniques. This research extends our ability to develop a spectroelectrochemical sensor for lanthanides and actinides into molten salt media.

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A spectroelectrochemical sensor consisting of an indium tin oxide (ITO) optically transparent electrode (OTE) coated with a thin film of partially sulfonated polystyrene-blockpoly(ethylene-ran-butylene)-block-polystyrene (SSEBS) was developed for [Tc(dmpe)(3)](+) (dmpe = 1,2-bis(dimethylphosphino)ethane). [Tc(dmpe)(3)](+) was preconcentrated by ion-exchange into the SSEBS film after a 20 min exposure to aqueous [Tc(dmpe)(3)](+) solution, resulting in a 14-fold increase in cathodic peak current compared to a bare OTE. Colorless [Tc(dmpe)(3)](+) was reversibly oxidized to colored [Tc(dmpe)(3)](2+) by cyclic voltammetry. Detection of [Tc(dmpe)(3)](2+) was accomplished through emission spectroscopy by electrochemically oxidizing the complex from nonemissive [Tc(dmpe)(3)](+) to emissive [Tc(dmpe)(3)](2+). The working principle of the sensor consisted of electrochemically cycling between nonemissive [Tc(dmpe)(3)](+) and emissive [Tc(dmpe)(3)](2+) and monitoring the modulated emission (λ(exc) = 532 nm; λ(em) = 660 nm). The sensor gave a linear response over the concentration range of 0.16-340.0 μM of [Tc(dmpe)(3)](2+/+) in aqueous phase with a detection limit of 24 nM.