Chapter 11
Advanced Testing Line for Actinide Separations Pollution Prevention in Industrial Processes Downloaded from pubs.acs.org by UNIV OF MASSACHUSETTS AMHERST on 09/25/18. For personal use only.
An Integrated Approach to Waste Minimization via On-Line-At-Line Process Analytical Chemistry L. R. Austin Los Alamos National Laboratory, Nuclear Materials Processing Technology, P.O. Box 1663, MS E-501, Los Alamos, NM 87545 A full scale Actinide Recovery Line has been designed and installed in the Plutonium Facility at Los Alamos National Laboratory. This system, called ATLAS (Advanced Testing Line for Actinide Separations) comprises operations ranging from dissolution, purification, and calcination, to waste stream polishing. The system is modular, such that new recovery technologies can be introduced into ATLAS and evaluated in an integrated manner with up-stream and down-stream recovery operations. A full spectrum of on-line and at-line analytical capabilities exist. The objective of this system is to demonstrate advanced technologies that can minimize or eliminate nuclear waste generation at the recovery site. The capabilities of the facility, as well as some of the development activities aimed at waste reduction and minimization, will be discussed. The principal Department of Energy (DOE) site for the production of nuclear weapons i s the Rocky Flats Plant, near Golden, Colorado. Residues from the production operations containing recoverable quantities of plutonium are processed as part of the Pocky Flats operations. Discharges from these secondary recovery operations comprise a significant part of the nuclear waste from Rocky Flats. Los Alamos National Laboratory, which operates a full-scale plutonium recovery f a c i l i t y , has been assigned an important technical support function to the recovery operations at Rocky Flats. The Los Alamos f a c i l i t y has been designated as the Lead Laboratory for developing and demonstrating improved processing technologies for the entire DOE complex. Therefore, aqueous recovery operations at Los Alamos have two objectives: (1) Improved recovery technologies are tested and demonstrated at f u l l scale before they are transferred to other DOE production
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f a c i l i t i e s , and (2) residues are processed to recover plutonium and to minimize the introduction of hazardous wastes to the environment. Previous Approach To Operations Mthough plutonium recovery operations have been i n use for over forty years, the specific chemical and technical details required to recover plutonium with high efficiency are not well known. Plutonium operations generally are far from optimal, and involve the addition of excessive chemical reagents that produce excessive liquid and solid waste volume. One of the major reasons for not operating processes in a manner that minimizes waste generation i s the complexity of the basic chemistry. A given impure feed material may contain a wide range of unknown metal œntaminants and anionic iitpurities that can interfere with the plutonium separation process. If the plutonium product i s not sufficiently pure, the entire batch must be recycled and reprocessed, with a corresponding increase i n processing time, personnel exposure, and waste generated. Thus, to assure a product of acceptable purity, we have focused on plutonium recovery at the expense of waste minimization. The lack of timely analytical data also contributes to process inefficiency. At present, almost a l l analytical samples must be removed from the process and sent to another building, several miles distant, for analyses. The normal delay before analytical results are available i s much too long to benefit our process operations. Moreover, the specific analysis needed for on-line process control often has either not been identified, or the analytical technique required for rapid assay does not exist. For example, the presence of excess fluoride i n the ion exchange feed solution directly affects process performance, yet, we are just beginning to appreciate the complex and competitive interactions of fluoride, aluminum, nitrate, and phosphate on the formation of the anionic plutonium nitrate complex. Until recently, the n i t r i c acid concentration of feed solutions was determined by a free-acid titration. This lengthy procedure yielded a value for hydrogen ion concentration, whereas what we need i s nitrate ion concentration. Likewise, because fluoride i s masked by elements such as silicon and aluminum, a total fluoride assay does not provide what we need, which i s the amount of fluoride that w i l l interfere with the formation of the sought plutonium nitrate complex. 1he concept of ATLAS provides a tool useful i n solving these complex problems. Process analytical instruments have been integrated into the unit operations required for purification. Full scale recovery can then be performed in an experimental mode to develop and optimize the processes. In addition, ATLAS can be used as the final full-scale testing and demonstrating of new recovery technologies as they are developed. ATLAS i s a modular system, and new unit operations can be inserted and evaluated in an integrated mode to optimize the entire flowsheet.
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S p e c i a l Constraints
Nuclear materials processing i s subject to a unique set of constraints that set i t apart from other manufacturing and chemical processing operations. Among these constraints are: (1) nuclear materials control and accountability, (2) radioactive contamination control, and (3) exposure of personnel to radiation. Technologies such as continuous processing, automatic process control, robotics, and other forms of automation are well developed and routinely integrated into the design of most modern processing f a c i l i t i e s . Although a few good examples of automatic process control have existed for many years (PUREX at Hanford and Savannah River) i n general, advanced control and automation technologies have not found wide spread use within the DOE complex because of these cited constraints. Although these technologies could contribute significantly to nuclear materials process efficiency and waste minimization, the ability to integrate them into facilities within the complex today represents a very d i f f i c u l t and challenging technical goal. For example, reduced exposure for operators at the cost of increased exposure for maintenance personnel i s not necessarily a good trade-off. Nuclear materials accountability poses a particularly demanding set of problems and constraints. Almost by definition, all process operations must be batch, or at best, semi-continuous/batch in order to keep an accurate account of the nuclear material inventory. Quite often, accountability requirements, process efficiency considerations, and safety regulations conflict with each other. Good nuclear materials accountability practice requires that two people oversee each nuclear material transfer. Such required duplication of effort prevents efficient use of personnel. Likewise, this i s in direct conflict with our policy of keeping radiation exposure of personnel to levels as low as reasonably achievable (ALARA). A large fraction of our total effort i s devoted to accountability activities. The general accountability approach i s simple and straight forward. Each nuclear material item i s to be measured at every step i n the process, while maintaining accurate and up-to-date accountability records. Although nuclear materials accountability i s not a direct waste minimization issue, obtaining accurate accountability measurements on a wide variety of waste presents a formidable technical challenge. Sound technical solutions to these d i f f i c u l t accountability problems could produce significant gains i n worker productivity and a simultaneous reduction i n radiation exposure. Improved accountability measurement techniques therefore could contribute significantly to process efficiency. At the Los Alamos Plutonium Facility we have just begun an integrated program to apply our best technical effort toward these accountability problems. Reliable on-line and at-line process analytical chemistry i s expected to play an important role i n the solution to this problem.
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Our challenge i s to address the important technical needs, while simultaneously meeting the cited constraints, i n an integrated development program. The goal i s to provide the DOE complex with a combination of technologies that will significantly improve recovery efficiencies, reduce waste, and comply with a l l of the accountability, safety, and security regulations. The Approach The approach used at the Plutonium Facility at Los Alamos National Laboratory i s to develop processes that produce the least amount of waste when evaluated on a total integrated facility. Our comprehensive waste minimization program has three elements: 1. Optimize existing processes to minimize the waste generation, including recycling when possible. 2. Develop additional treatment or polishing operations that w i l l convert a significant fraction of the total waste volume to effluents that can meet discharge limits for the plutonium f a c i l i t y , or can be discharged directly to the environment without harm. 3. Develop revolutionary new technologies that could result in significantly lower total waste generation. The ATLAS concept embodies a l l three aspects of waste minimization described above. The goal i s to provide process optimization and waste minimization for the f a c i l i t y as a whole. Often what may appear best for one operation may have a net over-all negative effect on the entire f a c i l i t y . Process modeling i s an important tool that can provide this over-all assessment. Wiat i s ATLAS? Approximately four years ago, we recognized a need within the DOE Complex for a f a c i l i t y capable of reprocessing many of the unusual test items that were collecting within nuclear materials storage vaults. These items continue to be stored because of the uncertainty of recovery success, as insufficient processing knowledge exists. Also, insufficient material exists to warrant development of a recovery flow sheet, and then scale up for the recovery campaign. In addition, we recognized the need to evaluate new full-scale recovery flow sheets i n a fully integrated manner. Oambining these two needs the idea of an Advanced Testing Line for Actinide Separation (ATLAS) was born. ATLAS i s an aqueous recovery f a c i l i t y that includes a l l operations ranging from dissolution to waste stream polishing. Two of the glove boxes are Kynar lined so both nitrate and chloride based flow sheets can be evaluated. The f a c i l i t y i s f u l l scale, that i s kilogram quantities of plutonium can be
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processed. If additional capacity i s required, the line would be duplicated rather than scaled up. ATLAS i s designed in a modular way such that new technological developments can be easily incorporated. A l l unit operations are integrated so that the effects of a single unit on other up and down stream operations can be measured. Each unit i s linked together with an advanced on line data gathering and control computer. ATLAS has a f u l l spectrum of on-line/at-line process analytical instrumentation. Instruments include a Quantum 1200 UV-VTS spectrophotometer, gamma monitor, alpha monitor, ion chrcmatograph, X-ray fluorescence unit, and acid sensor. Analytical capabilities included plutonium valence, actinide concentration, trace metal and anion concentrations, and acidity determination. A chemical sensor program augments the process analytical program. After the fundamental technical understanding of the process i s obtained, the next step i s being able to obtain real-time data from the appropriate control point. Often one finds that at certain c r i t i c a l control points, a sensor i s not available to extract the required information. Thus, to augment the process analytical effort, a program on sensor development exists at the Laboratory. We have recently demonstrated an on-line acid sensor that has the capability to measure acidity up to 11 M n i t r i c acid concentrations. ATLAS provides one of the keys to linking most of our aqueous development programs. Aqueous development activities include electrochemical dissolution, improved ion exchange, solvent extraction, and precipitation. In addition, we have an extensive program in the area of organic ligand development. This program i s directed at understanding the mechanism of selective metal ion capture with organic ligands, and custom building ligands for specialized selectivity. There i s a strong t i e between the ligand extraction and the sensor programs. If a ligand can be synthesized that i s very selective for a given species, then a theoretical basis exist for sensing that species. Technical Developments The concept of ATLAS embodies several new technical developments. These cover such activities as dissolution, precipitation, purification using advanced separation technologies to include membrane development and synthesis of extraction ligands for heavy metals separation. The two development activities that w i l l be discussed here are focused on process analytical chemistry. The Cn-line Acid Sensor. One recent development involves a new sensor to measure acidity below a pH of 1, a crucial measurement for many high acid chemical processing operations. We have developed an optical sensor that can make high acidity measurements on-line automatically. The sensing material utilizes a polymer (polybenzimidazole) and an indicator (chromazurol-S). The polymer i s chemically bound to the surface of a fused s i l i c a lens. The indicator i s
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physically entrapped within the polymer. The absorbance band of chromazurol-S at 554 nm increases i n intensity with increasing acidity starting at 4M hydrogen ion concentration. The peak sensitivity of this material i s between 4 and 12M hydrogen ion concentration. The sensor i s very selective for hydrogen ions because i t uses an acid/base indicator. Furthermore, the polymer's small pore size prevents most other cations from reaching the indicator. This sensor has proven to be quite stable i n high acid solutions. We have observed a signal degradation of less than 10% over a two month period. Acid concentrations obtained with this sensor were within 3% of values obtained by titration. The sensor uses fiber optic spectrophotometry. Light i s focused through a lens, from a fiber optic cable. The light passes through the acidic solution, and i s refocused with a second lens into another fiber optic cable. One or both lens are coated with the sensing material. The attenuated light proceeds to the spectrometer's fiber optic detector. Acidic solutions are pumped perpendicular to the light path. The acidic solution i s in direct contact with the sensing material. The sensor absorbance equilibrates rapidly, usually within one minute. Alternatively, a probe configuration can be used where the sensing material i s directly deposited onto an optical fiber. Phenol red i s an indicator that responds well between .1 and 4M hydrogen ion concentration. I n i t i a l indications show that by properly combining the phenol red and chromazurol-S indicators into the same coating, the range of sensing can be extended from .1 to 12 M hydrogen ion concentration. Advanced Neural Net Pattern Recognition. Another development involves the use of "intelligent" process control using neural networks. A neural net pattern recognition system i s being used to obtain process control parameters from abstract knowledge and complex spectral data. Spectral data from concentrated n i t r i c acid solutions containing a wide array of anions and cations can readily be obtained. The highly varying concentrations of these other anions can complex dissolved species and produce dramatic shifts i n the spectral peaks, thus making the interpretation of the data extremely d i f f i c u l t . The thrust of this work i s to use the entire spectral range rather than focus of specific peaks. This presents a complex data set, which w i l l be feed into the pattern recognition algorithm. Once the system i s trained using a known data set with predictable recovery results, parameters w i l l be set that can be used to extract information from spectra obtained on each new item to be purified. The data extracted w i l l then provide key parameters that w i l l be used to optimize feed treatment and ion exchange operations to optimize plutonium recovery and minimize waste generation. Sunmary
The DOE faces an enormous task of rebuilding the weapons complex with cost effective and efficient operations. Some of the base
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technologies required to accomplish this task also apply to the environmental remediation of the DOE site, which i s even a larger and more costly project. The technologies being developed here are focused on aqueous recovery of nuclear materials for the DOE Complex, however, the broader application to other industrial problems i s also readily apparent. At Los Alamos National Laboratory a systematic approach has been developed to integrating unit operations and providing total process optimization for waste minimization. On-line and at-line process analytical chemistry i s one of the major keys to realizing this goal. Many other developments under investigation at Los Alamos, such as membrane and organic ligand synthesis, sensor development, process control, modeling, and a r t i f i c i a l intelligence/neural net data analyses and process control are part of the total integrated approach. RECEIVED December 9,1991
