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Radiochemical Processing Laboratory

Thermoanalytical Capabilities

Thermoanalysis can provide great insights into chemical reactions, chemical hazards analysis, and material processing by providing real-time information on reaction products, reaction enthalpies, mass changes, reaction kinetics, and heat transfer. This correlated information can be used to identify process operational parameters, assess chemical reactivity hazards, and provide fundamental chemical data that can be used for thermodynamic and kinetic calculations.

The Radiochemical Processing Laboratory (RPL) has, over the years, developed extensive thermoanalytical expertise and capabilities. Among the thermoanalytical instruments used in the RPL are simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA), evolved gas analysis (EGA), differential scanning calorimetry (DSC), accelerating rate calorimetry (ARC), Calvert microcalorimetry, near-adiabatic Reactive System Screening Tool, and thermal conductivity probes. In addition to the traditional analysis of non-radioactive systems, we also have systems that we have devoted to characterizing radioactive chemical systems.

DTA and DSC provide information about reaction heats or enthalpies. Using mg samples, DTA measures temperature differences between a sample and a reference as the temperature is increased at a known and constant rate 0.1 to 160°C/min) or held at isothermal conditions up to 1500°C. DSC measures reaction enthalpies as the sample is heated at a known and constant rate (0.1 to 320°C/min) or held at isothermal conditions up to 750°C.

The DTA and DSC are versatile methods providing important process information or information on the physical and chemical behavior using very small sample quantities. First and foremost, this method provides qualitative and quantitative information about whether a reaction requires heat to occur (endothermic) or produces heat (exothermic); you have to work a bit harder for quantitative results from the DTA. These methods provide insights about reaction onset temperatures and can be used to screen for reactive systems that may pose a potential risk when used in a process. DSC and DTA results can be used to obtain fundamental kinetic data either using non-isothermal methods or by studying reaction rates at different isothermal temperatures. In addition, DSC can be used to measure heat capacities.

The TGA complements the DTA and DSC. The TGA measures mass changes as the sample is heated at a known and constant rate (0.1 to 160°C/min) or at isothermal conditions. TGA results are used to determine reaction mass changes that can be used to support a postulated reaction mechanism particularly when coupled with EGA. When coupled with DTA and DSC, TGA results are used to identify DTA or DSC reaction temperature ranges to provide a more accurate measure of the reaction enthalpy. We have combined TGA/DTA for simultaneous measurement of heat and mass changes that we use for both radioactive and non-radioactive samples.

TG/DTA System
TG/DTA System

Examples of TGA/DTA use include determination of the effects of lanthanide- and transition metal-dopants on the reaction thermal sensitivity and kinetics of the air oxidation of uranium dioxide and the fluorination of uranium compounds to the volatile uranium hexafluoride.

Our capability to analyze the gases evolved during TGA/DTA analyses using mass spectrometry (MS) or infrared spectroscopy (IR) allows us to gain key insights into the reaction mechanism. We can learn much about complex reactions or a complex series of reactions. We have EGA for non-radioactive systems, but we have not yet developed this capability for radioactive samples.

The Reactive System Screening Tool, is a near-adiabatic calorimeter that was designed to aide in identifying potentially thermally reactive chemical systems using nominal 10 g samples. The RSST is similar to the DSC or DTA in operation. The sample is heated at nominally 1°C/min, and its temperature is monitored. For non-gas producing reactions, reaction enthalpies can be measured. Reaction kinetics can also be determined. We have used the RSST for non-radioactive samples and have done some development work to use the RSST to study radioactive samples.

Accelerating Rate Calorimeter
Accelerating Rate Calorimeter

Accelerating rate calorimetry measures temperature and pressure changes at adiabatic conditions using 1-10 g samples. The ARC was originally designed to assess the hazards of potentially reactive chemical processes. In its Heat-Wait-Search operational mode, the ARC is programmed to heat a sample to a target temperature and monitor heat changes at isothermal conditions and repeat the process until the instrument observes a self-heat rate greater than an operator-provided criteria. If the instrument observes a change in sample temperature, the instrument will maintain the calorimeter at the sample temperature up to 450°C and self-heat rates up to 15°C/min. This approach maintains adiabatic conditions and allows the sample to self-heat, which could lead to a thermal runaway reaction. In addition, to its obvious application for characterizing the thermal reactivity risk of process amounts of material, the ARC can be used to measure reaction enthalpies and determine Arrhenius kinetic parameters.

Because of the ARC’s versatility, we have used the ARC to investigate several different potentially reactive chemical systems reactions including, 1) potential organic Hanford tank waste constituents with nitrates and nitrates and nitrites that are also present in the wastes, and 2) simulated characteristic wastes arising from decontaminating plutonium glove boxes. The ARC has only been used for non-radioactive samples to date. We have several different ARC-type instruments with different designs.

Calvert Microcalorimeter
Calvert Microcalorimeter

The Calvert microcalorimeter provides a more sensitive method for measuring reaction enthalpies and reaction kinetics than the aforementioned methods. The Calvert calorimeter uses nominal 1-g samples and operates isothermally or in a dynamic temperature scanning mode (1°C/min). The Calvert is often used as the final screen for process operation if the ARC indicates reaction onset temperatures within 50°C of the temperature required for the process. Our Calvert microcalorimeter has only been used for non-radioactive samples to date.

Thermal conductivity is measured using a non-steady state probe system compliant with ASTM D 5334-92 and D 5930-97 standards. This heated wire technique is applicable to a wide range of materials including powders, particulate slurries, and weak solids. Studies can be conducted on radioactive and non-radioactive samples.

The RPL has a broad, versatile capability to determine the thermal behavior of a variety of chemical systems. The information gathered can be as fundamental as the reaction kinetics or the reaction pathway with identification of its products and stoichiometry or as application related as the reaction onset temperature or whether the reaction is endothermic or exothermic.

Point of Contact:

Randy Scheele, Separations and Radiochemistry Team
(509) 376-6911

RPL

Energy and Environment Directorate

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Solving Global Problems with Premier Staff and Facilities

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A National Asset with Multiple Missions
A National Asset with Multiple Missions

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