Pyroreflectometry as a technique for the accurate measurement of very high temperatures in molten materials

Experimental research into severe nuclear accidents often requires the accurate measurement of high temperatures of molten materials. Measurements of very high temperatures (1500-2500 °C) in liquid materials using standard pyrometry can entail uncertainties in the order of 5%-10%. Pyroreflectometry is a powerful technique with the potential to significantly reduce these uncertainties. A method is proposed to optimize pyroreflectometry temperature measurements in the 1500-2500 °C range and to allow more easily the detection of the solid-liquid phase transition. The originality of this research essentially relies on the use of pyroreflectometry based on two wavelengths (1.3 and 1.55 μm) and its application to liquid materials at high temperature, which implies to adapt technological elements and metrological procedures. The proposed procedure first requires temperature calibration, which is undertaken using three eutectic fixed-point cells, reducing temperature uncertainty. Second, precise settings are adopted to enable reflectivity measurements on specular surfaces, such as the surfaces of molten metals. Pyroreflectometry measurements on liquid surfaces have been validated on an iron sample. Subsequently, the application of pyroreflectometry at very high temperatures was validated on various materials: metal (iron and 18MND5 steel), oxide (alumina), and carbide (rhenium-carbon eutectic). For each of these samples, the uncertainties of temperature measurements in the 1500-2500 °C range were estimated in the range of 1%-2%, performing well against standard pyrometry measurements. The principal difficulties encountered during the pyroreflectometry characterization were the fine-tuning of parameters (optical head orientation and lens focusing) to enable measurements on highly specular surfaces and ensuring inert interactions between the samples and the crucible.