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3 Characterization Characterization requirements relate to all parts of the HLW man- agement process. Important parts of this process requiring characteriza- tion are the following: waste storage; retrieval; pretreatment; immobilization: tank closure; near-field monitoring; and state-of-the-equ i pment man itori ng. The successful performance of these HLW process activities depends to a great extent on characterization of the waste streams and of process equipment. Characterization requirements vary greatly depend- ing on the relevant operational area but generally include measure- ments of chemical, physical, and radiological properties. The main challenges in characterization consist of difficulties in retrieving and manipulating samples of radioactive material, with the related risk of radiation exposure to workers; high costs; long turnaround times; and poor data rel iabi I ity. Poor data reliability occurs when only a limited number of samples is available and these samples are not representative of the parameter measured. For example, samples from HLW tanks are known to be unrepresentative of the total contents because of the heterogeneity of waste distribution. Moreover, the complicated procedures necessary during retrieval, transport, and measurement of highly radioactive sam- ples significantly increase the chances of error. Research with the objective of improving characterization operations will contribute to increased safety, efficiency, and cost-effectiveness. H ~ G H - L E V E E W A S T E 28
Characterization issues The committee identified long-term basic research needs related to characterization activities throughout the HLW treatment process. The objective of the recommended long-term basic research for characteri- zation is to provide the scientific basis for developing innovative meth- ods acquiring real-time and, when practical, in situ characterization data for HEW and process streams that could be useful for all phases of the waste management program. Research needs for characteriza- tion are condensed into the two general ones listed at the end of this chapter. Analysis terms used in this report are defined in Sidebar 3.1. Table 3.1 provides examples of types of characterization activities sort- ed by process area. Characterization for Waste Storage High-level waste, whether stored in tanks as solids-liquid mixtures or in vaults as calcine, must be characterized to determine chemical (elemental and molecular), physical, and radiological properties to identify major safety concerns and to formulate subsequent treatment ·eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee SIDEBAR 3.1 DEFINITION OF ANALYTICAL TERMS . Sampling and analysis: Removal of a portion of the material to be characterized and determination of its properties, using various established methods. Given the state of current practices, sampling and analysis require a significant turnaround time (typically days to months). Turnaround time: Time between the beginning of a characterization process and the reporting of the data. In situ or on-line measurement: Measurement of a material's characteristics in place.The result can be displayed either at the site of the measurement or remotely. An example would be the determi- nation of a chemical composition by spectroscopic analysis. For instance, the source of electromag- netic radiation could be supplied by a fiber-optic cable inserted into the region of interest. Results are usually available in near real time. Real time: Instantaneous turnaround of result data. Near real time: Turnaround time of a few seconds to a few minutes. · Remote measurement: Determination of a material's characteristics from a distance without physi- cal contact with the material. An example would be the determination of chemical composition by analysis of gamma radiation emitted after interrogation using neutrons from a remote source. Aerial or satellite fly-by analysis is an extreme example of remote measurement. C h a r a c t e r i z a t i 0 n 29
TABLE 3.1 Desirable Types of Characterization anc Examples of Ana~ytes in Different Process Areas Process Area Type of Analysis Examples of Analytes Stored waste Chemical Physical Radiological Retrieval Physical Pretreatment Chemical Physical Radiological Immobilization Chemical Physical Radiological Tank Closure Chemical Radiological Elements and molecular species in condensed and gaseous phases of waste and chemical speciation Same properties as for the retrieval and pretreatment process area Foam or crust thickness in tanks Major radionuclides Density Rheology Percent solids Height of the liquids in concentrated salt solutions and sludges Major glass-forming components such as B. Na, Ca, Al, and Si associated with melter feeds; these analyses currently determine the glass product acceptability Metals associated with melter life and product quality, such as noble metals, Mn, Fe, Ni, and Cr in melter feeds Anions associated with glass product quality and melter corrosion, such as sulfates, phosphates, fluorides, and chlorides RCRA materials in the secondary waste streams Organics that may affect the redox potential of the glass melt Liquid levels in concentrated salt solutions and sludges Particle size distribution Density Radionuclides associated with high-activity fields and HLW such as Cs-137, Sr-90, U-235,Tc-99, Np-237, and other transu ranic elements Redox potential of the melter feed Organic materials that affect melter operation such as tri-N-butyl phosphate, multi-ring aromatics Volatile species present in off-gas stream such as RCRA materials, NOx,and SOx; toxic gases;and Hg and other semivolatile metals such as B,Cs, and Na Viscosity Density Foam or crust thickness in the melter Radionuclides in the off-gas stream, such as Tc, Cs, and I Metals associated with environmental concerns such as Pb, Hg, Cr. Np, and Pu Residual radionuclides remaining in tanks and process facilities, such as Cs, Np,Tc, and Pu NOTE: RCRA = Resource Conservation and Recovery Act H I G H - L E V E L VV A s T E ~ A ~ 30
steps. Determination of the presence of foam or crust layers in tanks is also important for safe storage, because such layers may trap hazardous gases, which are then released in "burps" causing potential explosion hazards. Data have been acquired for HEW stored in tanks at Hanford and SRS, but very few are available for INEEL calcine. Core, liquid "grab" (using the "bottle-on-a-string" method), and vapor-phase sam- pling are the methods currently used to characterize waste in the tanks. Sampling from tanks is very costly and has a lengthy turnaround time for analyses. The cost varies from thousands of dollars for limited analy- sis of liquid or gaseous samples to hundreds of thousands of dollars for core samples. Turnaround times vary from a few days for vapor-phase samples to typically 180 days for core and liquid grab samples, depending on the extent of characterization required and the difficulty of acquiring the sample. These long turnaround times are due in part to administrative procedures. In practice, the operator must develop sam- pling plans, obtain radiation safety approval, and set up the equipment to retrieve the sample and send itto the laboratory. However, most of the delays still relate to technological limitations in manipulating highly radioactive samples. Characterization for Retrieval Retrieval from tanks requires physical data on the properties of solid materials transport and chemical data on dissolution and compatibility of different waste streams. Pipeline transfers require data on physical properties, such as density, particle size, viscosity, and fraction of solids, primarily to facilitate pumping and to avoid plugging the pipelines (Table 3.1 ). Characterization for Pretreatment As HEW from multiple tanks is blended together for subsequent treatment, its composition changes, thus requiring new chemical, physi- cal, and radiological characterization (Table 3.1~. There is also a need to characterize feeds and process streams to avoid unwanted reactions and secondary products when blending. A formal review process takes place at Hanford for every waste transfer resulting in blending composi- tions to ensure that certain waste compatibility criteria are met. Characterization is an obvious key problem with regard to the quality of this review process. These analyses usually involve determination of the major glass-forming components such as boron, sodium, aluminum, and silicon. These compositions must be known to percentage levels. Minor components that are detrimental for the immobilization process (see Chapter 5), such as sulfate (SO42-), phosphate (PO43-), fluoride (F-), C h a r a c t e r i z a t i 0 n
and chromium (Cr3+), must be measured to the tenths of a percent level. These analyses ideally are obtained from a well-blended batch of feed material of uniform and consistently known composition, over some sign if icant i nterval of ti me. Radiological characterization of cesium, strontium, and other transuranic elements during the pretreatment steps is necessary to verify that the requirements of radionuclide separations processes have been met. Residual levels of radioactive materials in LLW streams are in the order of microcurie-to-millicurie per gram (3.7 x 104 becquerel per gram and 3.7 x 107 becquerel per gram, respectively). Secondary sys- , ... . . . . . . . . . . . . . . tem effluents from pretreatment must also be characterized for environ- mental purposes. The release of hazardous chemicals into the environ- ment is controlled by the EPA. Some elements that must be quantified to comply with EPA's regulations are lead, mercury, and chromium. Characterization for immobilization Vitrification in borosilicate glass is the current baseline method cho- sen by DOE for immobilizing HEW. Assurance that the waste glass is of sufficient quality to meet the waste acceptance criteria (see Sidebar 5.3 in Chapter 5) rests entirely on control of the vitrification feed composi- tion and process conditions (for instance, feed flow rates and melter temperature). That is, instead of sampling the final glass product, the amounts of HEW and glass-forming materials (vitrification feed) are maintained within a predetermined composition envelope before they are melted to ensure that the product is in compliance with the desired end waste form. Once the glass log is made, there is no provision for analytical verification of its acceptability or for recovery of the final glass waste form if its properties are unacceptable. Therefore, chemical and physical characterization of the vitrification feed is an extremely important parameter in immobilization operations (for additional details, see Chapter 5~. An important factor in determining the rate of wasteglass production at the SRS and WVDP is the time required to analyze the melter feed and to verify that the correct amount of glass-forming material has been mixed in the waste stream. The current in-cell analytical methods involve mixing samples of the waste sludge and frit, melting, redissolv- ing the resulting glass in hydrofluoric acid, and analyzing the sample by inductively coupled plasma-emission spectrometry. The entire process is very time consuming (typically 36 hours per sample) and, hence, very expensive. The cost of analyzing a glass sample at the SRS is approximately $2,000 Canteen, Personal Communication, 2001 ). H ~ G H - L E V E E W A S T E
Characterization of the glass stream as it is poured from the melter would provide significantly more information and shorter turnaround times. Glass stream characterization presents a challenging task, because the measurements would have to be performed in the presence of high background radiation fields. However, characterization of the glass stream would confirm that the glass falls within the envelope of acceptable compositions and would determine whether species insolu- ble in the glass, such as crystalline spinels and noble metals, are trapped in the melter or are leaving with the glass product. Therefore, on-line characterization of the glass stream is a highly desirable process control option. One method to achieve this goal might be to develop or adapt a remote analysis method based on the absorption or emission of electromagnetic radiation by the glass stream. Since the glass stream poured from the melter to the canister is hot and luminous, the absorp- tion or emission of electromagnetic radiation could be analyzed and calibrated against different glass composition and/or glass properties. Characterization for Tank Closure Characterization for tank closure is a topic more fully discussed in Chapter 6, in a section with the same title; therefore, it is briefly sum- marized here. As the HEW is retrieved from the tanks, a fraction of radioactive solids may remain in the tanks as "crust" or dense "hard- heel" solids. Characterization of tanks, crusts or heels is necessary to determine their chemical and radionuclide (actinide content, alpha, gamma, and beta emitters) composition. Residual HLW left in the tanks must also be characterized to demonstrate that the waste has been ade- quately removed and that the residues can be considered "incidental waste" (see Sidebar 6.1 ) and be left in the tanks. Characterization research needs for tank closure are very similar to characterization needs for deactivation and decommissioning activities. These are addressed by a different NRC committee (NRC, 2000c, 2001 b). Characterization for Monitoring the Near Field The area immediately around and below the tanks (the "near field") must be characterized to monitor potential waste leaks from the tanks. This issue is addressed in Chapter 6 as well. The quantity of waste that has leaked into the environment (i.e., soils) and the tank residuals are used as the collective source term in risk analyses for the site. The leaked wastes may contain hazardous chemicals such as lead, mercury, and chromium, as well as radionuclides. Knowledge of the chemical speciation of these hazardous materials in the tank, in the soil near the tanks, or in the LLW fraction left on site is crucial to track and control C h a r a c t e r i z a t i 0 n 33
their subsequent migration and assess the threat posed. For instance, it is important to know the oxidation state of chromium, because its hexa- valent state is more hazardous to the environment than its trivalent state. These species have to be measured at part-per-million (ppm) levels relative to the soils and groundwater. Activity levels for radio- nuclides in the environment are in the order of picocuries per gram (3.7 x 1 o-2 becquerels per gram) to microcuries per gram (3.7 x 104 becquerels per gram). Characterization research needs for the EMSP in near-field monitoring are also addressed in a previous NRC report (NRC, 2000a). Characterization of State of the Equipment Characterization also includes determination of the state-of-the- process vessels and equipment to ascertain their continued viability in performing their operations. Examples of characterization of equipment properties that influence the HLW cleanup process are the following: tank integrity; stress corrosion cracking and pitting of melter electrodes, valve boxes, and pipelines; and the level of obstruction and location of plugs in transfer lines. Currently, evaluation of the process vessels is carried out by manual inspection techniques, if at all. These techniques are time consuming and may increase the radiation exposure of operat- ing personnel. Therefore, knowledge of the operating state of equipment reduces the potential for personnel exposure to HLW and the planning required for maintenance intrusions in high-radiation areas. However, the equipment must be characterized frequently to ensure good operat- ing conditions, to provide early warning of malfunctions, and (possibly) to increase public confidence in DOE's ability to operate the sites with- out generating additional environmental hazards. Finally, if process conditions could be adjusted in real time in response to actual equip- ment status, the likelihood and consequences of serious equipment or process fai I u res cou Id be reduced. General Long-Term Basic Research Needs for Characterization Long-term basic research is needed to provide the underlying princi- oles for the characterization of chemical and ohvsical Properties of HLW, and to monitor the condition of the equipment. A long-term basic research program should contribute to development of the following: H ~ G H - L E V E E W A S T E
Remote-sensing instruments. These instruments will reliably pro- vide data on bulk properties or condition of the equipment, in a reasonable time frame (minutes to hours), at distances of tens of meters, in the order of picocuries per gram (3.7 x 1 o-2 bec- querels per gram) for radionuclides and ppm levels for chemical compositions. Examples of current technologies that perform measurements at a distance in reasonable time frames are prompt gamma neutron activation analysis that measures ele- mental compositions; radiation monitors; infrared temperature indicators; and sonar measurements for wall thickness. 2. On-line or in situ instruments. These instruments will reliably provide data on bulk properties or condition of the equipment in real time or almost real time without retrieving samples, at ppm to percent levels for chemical compositions and in the order of picocuries per gram (3.7 x 1 o-2 becquerels per gram) for radionuclides. Examples of current technologies that perform measurements in real time are pH probes, radiation monitors, thermocouples and pressure gauges, electrolytic corrosion moni- tors, and fiber-optic spectrometers. Of these techniques, remote monitoring is technically the more diffi- cult. Because of the distance requirement, it will likely necessitate mea- su remeet of d ifferent properties th an wi I I on- I i ne, contact measu re- ments (see Sidebar 3.1~. However, remote monitoring would allow more analyses on waste prior to retrieval, providing real statistics on waste composition, reduced radiation exposure to workers, and quicker turnaround times. Desirable types of characterization and examples of analyses in dif- ferent process areas are listed in Table 3.1. Examples of possible fields in which research is needed to develop improved methods for perform- ing physical, chemical, and radiological characterization are the follow- lng: . communication systems and signal processing; · acoustic, microwave, and electromagnetic sciences for monitor- i ng pu rposes; analytical chemistry using electromagnetic radiation for remote analysis; · material sciences to identify radiation-resistant materials used for characterization purposes (probes, sensors, monitors, robots); and . · quantum physics and optical materials for optical communica- tion and optical signal processing. C h a r a c t e r i z a t i 0 n
It is important to emphasize the need for developing long-term basic research for characterization purposes across the spectrum of the waste management process activities. Different DOE's focus areas involved in cleanup activities such as subsurface contamination and deactivation and decommissioning have similar characterization needs. Therefore, it is necessary to coordinate the characterization efforts among different programs and across the DOE complex. H ~ G H - L E V E E W A S T E 36
