S. A. BRYAN, T. G. LEVITSKAIA, A. J. CASELLA, J. M. PETERSON, A. M. JOHNSEN, A. M. LINES, and E. M. THOMAS, Pacific Northwest National Laboratory, USA

Abstract: Separation processes for highly radioactive and chemically complex spent nuclear fuel require advanced safe technologies and process models based on large databases. Availability of advanced methodologies for on-line control and safeguarding of aqueous reprocessing flowsheets will accelerate implementation of the closed nuclear fuel cycle. This report reviews application of the absorption and vibrational spectroscopic techniques supplemented by physicochemical measurements for radiochemical process monitoring. In this context, our team experimentally assessed potential of Raman and spectrophotometric techniques for on-line real-time monitoring of the U(VI)/nitrate ion/nitric acid and Pu(IV)/Np(V)/Nd(III), respectively, in the solutions relevant to spent fuel reprocessing. Both techniques demonstrated robust performance in the repetitive batch measurements of each analyte in the wide concentration range using simulant and commercial dissolved spent fuel solutions. Static spectroscopic measurements served as training sets for the multivariate data analysis to obtain partial least squares predictive models, which were validated during on-line centrifugal contactor extraction tests. Achieved satisfactory prediction of the analytes concentrations in these preliminary experimentation warrants further development of the spectroscopy- based methods for radiochemical process control and safeguarding.

Key words: spectroscopic process monitoring, Raman, vis-NIR, uranium, plutonium, neptunium, nitric acid, nitrate, nuclear fuel reprocessing.

4.1 Introduction

There is a renewed interest worldwide to promote the use of nuclear power and close the nuclear fuel cycle under the Global Nuclear Energy Partnership (GNEP) Program, the Advanced Fuel Cycle Initiative (AFCI), and more recently under the Fuel Cycle Research and Development Program (FCRD). The long-term successful use of nuclear power is critically depen­dent upon adequate and safe disposal of the spent nuclear fuel, and imple­mentation of the closed nuclear fuel cycle recently regained attention in the

US community. Liquid-liquid solvent extraction is a separation technique commonly employed for the processing of the dissolved spent nuclear fuel. Availability of the closed nuclear fuel cycle requires precise solution control during processing. For example, extraction and recovery of minor actinides within an aqueous separation scheme necessitates precise control of aqueous redox conditions and acid concentrations at various places within the pro­cessing loop. Traditionally, the process performance of a given solvent extraction run is determined through sampling of the various process steams and subsequent laboratory analysis of those samples. Due to the highly radioactive nature of the process streams, this is a risky and time-consuming process. Remotely controlled on-line monitoring capabilities can help guide choice and adjustment of conditions to be used in processing of irradiated fuel and allow immediate feedback on changes made to the processing conditions. As a result, there is a renewed and urgent need for methods to provide on-line monitoring and control of the radiochemical processes cur­rently being developed and demonstrated. The instrumentation used to monitor these processes must be robust, require little or no maintenance, and be able to withstand harsh environments (e. g., high radiation fields and aggressive chemical matrices). The ability for continuous online monitoring allows the following benefits:

• accountability of the fissile materials

• control of the process flowsheet

• information on flow parameters, solution composition, and chemical speciation

• enhanced performance by eliminating the need for traditional analytical “grab samples”

• improvement of operational and criticality safety

• elimination of human error.

Sophisticated on-line monitor capabilities significantly enhance not only control over a process flowsheet but also accountancy of the inventory of a nuclear material. The increasing effectiveness of safeguards in spent fuel reprocessing plants is a great challenge to national and international communities. The International Atomic Energy Agency (IAEA) has estab­lished international safeguards standards for fissionable materials at repro­cessing plants to ensure that significant quantities of weapons-grade nuclear material are not diverted over a specified time frame. Because proliferant diversions are possible via deliberate modification of flowsheet chemistry, it is necessary to confirm proper operational performance to verify that facilities function under adequate safeguard-declared conditions. In any reprocessing facility, variability in process is expected under normal opera­tions, and currently, large-scale deviations can readily be detected and measured with certainty. Small-scale deviations in large facilities, however, are currently not well detected. Many of the methods useful for detecting deviations involve sending individual samples to a laboratory for process­ing, which can be time-consuming, thus delaying accurate appraisals of process conditions.

The application of multiple online monitoring capabilities provides a unique ability to rapidly identify unwanted/suspect deviations from normal operating conditions. The feasibility of this kind of on-line control of nuclear fuel reprocessing streams via analytical techniques was investigated as early as the 1970s (Parus 1977), and researchers have examined both the direct measurement of actinides, via spectrophotometry, and the use of physico­chemical measurements, such as temperature, density, and dielectric proper­ties, to indirectly measure actinide concentrations. These analytical methods can also be used to measure other solution components, such as NO3- or organic solvents that control actinide behavior, providing another method for actinide quantification.

Raman spectroscopy (Madic 1983,1984; Guillaume 1982) and ultraviolet — visible (UV-vis) spectroscopy (Schmieder 1972; Ertel 1976, 1985; Baumgartner 1980; Yamamoto 1988; Burck 1991; Colston 2001) are analyti­cal techniques that have been used extensively to measure concentrations of various organic and inorganic compounds, including actinides (Bryan 2007). Additionally, measurement of dielectric properties has also been proposed for on-line monitoring of fuel reprocessing systems (Yamamoto 1988).

The spectroscopic signatures of U, Pu, and Np analytes in different oxida­tion states have been extensively measured in solution and widely known (Madic 1983; Guillaume 1982; Maya 1981; Nguyentrung 1992). Even though Pu(VI) (Madic 1983) and Np(VI) (Guillaume 1982) species are Raman active, they are expected to be present in the dissolved fuel at low enough concentrations to prohibit their quantification by the Raman method. In our work, we employ Raman spectroscopy for the determination of U(VI), HNO3, and NO3- in various aqueous and organic streams. Plutonium species can be measured by visible absorption spectroscopy using multiple wave­lengths for its quantification (Ryan 1960; Cleveland 1979) and neptunium species can be monitored by vis-NIR spectroscopy (Burney 1974; and Stout 1993).

Armenta et al. reviewed the most recent literature (2000-2006) concern­ing the general (non-actinide applications) combination of flow-injection techniques and vibrational spectroscopy analysis and noted that both tech­niques have significant advantages. Flow-injection analysis is the technique in which a sample aliquot is injected into a reagent stream, where it dis­perses and the mixture flows past a detector. Flow-injection analysis offers “automated sample processing, high repeatability, adaptability to micro­miniaturization, containment of chemicals, [and] waste reduction,” while vibrational spectroscopy allows for “(i) fast monitoring of the whole spec­trum; (ii) high resolution and wide wavenumber range; (iii) many bands that can be employed for determination of each single compound; [and] (iv) simultaneous control of several compounds in the same sample.” These characteristics have allowed several commercial instruments to be applied successfully to quality and process control in the dairy, wine, gasoline, diesel, and lubricating oils industries. The authors also note that new flow-injection techniques in development have the potential to lower sample sizes and increase sensitivity (Armenta 2007).

Multiple laboratories have successfully constructed systems for remote in-line photometric measurements of actinides using spectrometers capable of scanning the entire visible and NIR range quickly enough for process measurements (Burck 1991). On a larger scale, Lascola et al. (2002) success­fully constructed a system for the on-line measurement of uranium from process tanks. Tank samples were routed through an initial vial to ensure proper mixing and were then sent through a flow-through optical cell con­nected via fiber optics to a diode-array spectrophotometer measuring in the 350-600 nm range. Partial least squares (PLS) models allowed for concen­tration measurements up to 11 g/L uranium, as well as uncertainties (2 a; concentration dependent) no greater than 0.30 g/L.

Several authors have conducted density studies of typical actinide repro­cessing solutions for use in criticality and process monitoring. Sakurai and Tachimori (1996) analyzed published density data for solutions containing plutonium(IV), uranium(VI), and nitric acid. Using regression analysis, they correlated solution density with analyte concentration that expanded the range of actinide concentrations to Pu < 173 g/L and U < 380 g/L and pro­vided standard error (0.00294 g/cm3) that improved upon the error associ­ated with previously established relationships. Kumar and Koganti (1998) extended this work to include mixed organic solutions, reporting density for UO2(NO3)2 and HNO3 in TBP/n-dodecane solutions ranging from 0 to 100% tri-butyl phosphate (TBP).

While Sakurai and Kumar have reported the most recent empirical density relations that are considered to be the most accurate empirical equations thus far, several authors have pursued theoretical calculations of actinide/nitrate solution densities in order to clarify the density functions at higher concentrations where the empirical equations begin to deviate from experimental data. Thermodynamic modeling for binary (or isopies — tic) solutions have been applied to make small but important corrections for solutions with high concentrations of the following components: UO2(NO3)2, U(NO3)4, Pu(NO3)4, Pu(NO3)3, Th(NO3)4, Am(NO3)3, and

HNO3 (Charrin 2000a, 2000b; LeClaire 2003).

Enokida and Suzuki (1992a, 1992b) used a set of computer models to theoretically test the feasibility of using temperature profiles to determine the uranium(VI) concentrations during solvent extraction processes from nitric acid into 30% TBP/n-dodecane organic phase. Using literature values for the heat of the TBP complexation, along with mass and heat balance equations, the authors created a model that calculated both steady — and transient-state uranium(VI) concentrations when given a temperature profile and a set of flow rates and feed concentrations for a typical set of counter-current mixer-settler extractors. Their model compared favorably with other models that calculated uranium(VI) concentrations using dis­tribution coefficients, which showed that using temperature profiles at various process stages is a feasible means of uranium(VI) concentration measurement.

Yamamoto (1988) used a flow-through cell system to measure the dielec­tric properties of the 30% TBP/n-dodecane-HNO3-H2O and 30% TBP/n — dodecane-HNO3-H2O-UO2(NO3)2 systems commonly used in spent nuclear fuel reprocessing. The estimated dielectric constants were found to vary much more significantly with HNO3 than with equivalent molar additions of H2O or UO2(NO3)2. The measurements were found to be sufficient to accurately measure the HNO3 concentration in the (30% TBP/n-dodecane)- HNO3-H2O system and could be used to measure the HNO3 concentration in the (30% TBP, n-dodecane)-HNO3-H2O-UO2(NO3)2 system if the UO2(NO3)2 concentration is known.

On-line monitoring of nuclear waste streams was successfully demon­strated by combining spectroscopic measurements with physicochemical measurements (conductivity, density, and temperature) in real-time quanti­tative determination of chemical components in the waste (Bryan et al. 2005; Bryan 2008). This new on-line monitoring system, which features Raman spectroscopy combined with a Coriolis meter and a conductivity probe, was developed by our research team to provide immediate chemical data and flow parameters of high-level radioactive waste streams. This process moni­toring system was used to measure the concentration of components of high brine/high alkalinity waste solutions, such as nitrite, chromate, aluminate, phosphate, sulfate, carbonate, and hydroxide, during retrieval from Hanford waste storage tanks. The Raman bands of interest for these species are well resolved and have been easily incorporated into a chemometric model for quantitative analysis of the solution components.

By inclusion of visible/near-infrared (vis-NIR) spectrocopy, this system was modified to monitor spent fuel reprocessing streams. A fiber-optic Raman probe allows monitoring of various species encountered in both aqueous and organic phases. Raman active species include: 1) metal oxide ions, such as uranyl, neptunyl, and pertechnetate ions, 2) organics, 3) inor­ganic oxo-anions, and 4) water. The trivalent and tetravalent actinides and lanthanides in both the aqueous and organic phases are monitored by vis-NIR spectroscopy, as are pentavalent or hexavalent Np and Pu complexes that are expected to be at concentrations too low to be deter­mined by Raman spectroscopy. Process monitoring and control is feasible at various points within the fuel reprocessing streams. Notably, process monitoring/control is not specific to any single flowsheet because alternate flowsheets also contain Raman and/or UV-vis-NIR active species that can be measured spectroscopically. In addition, Coriolis (for density and flow) and conductivity instruments can be used on most process streams.

The Raman and vis-NIR spectrometers used under laboratory conditions are easily convertible to process-friendly configurations to allow remote measurements under flow conditions using different sampling capabilities, such as fiber-optic probes, dip probes, and flow-through cell geometries as have been demonstrated by our research team in the centrifugal contactor flow tests. Spectroscopic data collected during the flow test served for the validation of the chemometric predictive models developed under static batch conditions as described in Sections 4.2 and 4.3.