STA, TGA and EGA in Nuclear

Specific Heat and Transition Energetics

The capacity of a material to store energy is partly governed, by its specific heat (sensible heat). This is made up of lattice, electronic and defect components, depending on the material. This property is required for the design of any transient heat transfer process. It is also used to quantify surface OxidationOxidation can describe different processes in the context of thermal analysis.oxidation/reduction and the O/M ratio (defects) of fuels during processing. In some cases, the specific heat can be used as an indicator of the extent of damage in post-irradiation examination (PIE), e.g., stored energy. It is also required for calculating Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity from Thermal DiffusivityThermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.thermal diffusivity data.

Transition energetics (latent heat) are required to characterize solid-solid transitions, melting/solidification and Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition. Both the specific heat and transition energetics are most accurately and efficiently measured by differential scanning calorimetry (DSC).

The specific heat can also be measured using the laser flash technique, albeit with reduced accuracy and only with a reduced number of data points. (With DSC, the generation of a quasi-continuous set of temperature-dependent specific heat data is standard.) With the required expertise, DSCs can readily be adapted for hot work.

Mass Change and Evolved Gases

Temperature-dependent mass change coupled with evolved gas analysis provides valuable information to help quantify the O/M ratio, out-gassing during fuel processing, corrosion, reduction, volatile fission products/actinides during vitrification, impurities remaining from the separation process, etc. Thermogravimetric analyzers (TGA) or simultaneous TGA-DSC (STA) instruments, coupled to a quadrupole mass spectrometer (QMS) either directly or by a heated transfer line, or a TGA or STA, coupled to an FT-IR via a heated transfer line, are widely employed for these types of analysis. As with the other techniques previously discussed, these instruments can easily be modified for hot work.

Solidus and Liquidus Temperatures

Solidus and liquidus temperature as well as Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting temperature data are necessary to establish safe reactor operating conditions and to model accident scenarios such as coolant losses. These temperatures are greatly affected by impurities, radiation damage, O/M ratios, burnup and, of course, composition.

Surprisingly, solidus/liquidus temperatures are notoriously difficult to be accurately measured. DSC is the technique most often employed for these measurements, but care must be taken to avoid undercooling during solidification (especially critical for metal alloys). Sample time constants and temperature ramp rates must be carefully considered. Solidus/liquidus temperatures of most metal alloys can also be measured by the laser flash technique (via Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity/Thermal DiffusivityThermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.thermal diffusivity data), and dilatometry can be used for conductors and insulators alike. For materials that melt at ultra-high temperatures, thermal arrest is sometimes employed using optical pyrometers for temperature measurement. All things considered, DSC is the most versatile and accurate method.