In-situ calibration exercise for radiation thermometers used in fast laser-heating experiments


K. Boboridis1, L. Capriotti1,2 and R. Böhler1

1Joint Research Centre of the European Commission, Institute for Transuranium Elements, Karlsruhe, Germany
2Department of Nuclear Engineering, Politecnico di Milano, Milano, Italy

Keywords: radiation thermometry
property: melting point
material: niobium

At ITU a laser-driven pulse-heating technique is utilized for high-temperature investigations of nuclear fuel materials. Because of the high temperatures and high heating/cooling rates involved, often exceeding 3000 K and 10000 K/s, respectively, radiation thermometry is applied as the most suitable technique to record the surface temperature of the heated specimen. Traceability of the measured temperatures to the International Temperature Scale ITS-90 is ensured by periodic calibration using standard light sources. The validity of this calibration at higher temperatures and also under transient conditions similar to those existing during typical measurements is tested by frequently conducting in-situ checks using the radiance temperatures of selected metals at their melting points as reference values. This practice has long been suggested based on a large volume of experiments in which ribbon-shaped specimens were heated to their melting point in less than one second by the passage of a large electrical current-pulse through them [1]. It was found that irrespective of the initial surface conditions and of other operational parameters, notably the heating rate, the radiance temperature of the studied metals at melting was constant and highly reproducible. However, in laser heating experiments an additional detail may be of importance. Depending on the size and power density of the laser spot, the experiment duration, the specimen dimensions and its material properties, significant radial temperature gradients may develop across the area of interest on the disk-shaped specimen. The extent to which this can affect the temperature measurement, particularly in view of the radiation thermometer's size-of-source effect (SSE), is examined in the present work. Laser melting experiments were performed on specimens of Nb, Mo, Ta, and W. Specially prepared specimens were used to estimate the SSE. Combining the results of these measurements with information from numerical simulations for specific cases it is possible to evaluate the measurement accuracy of the employed radiation thermometers under various conditions.

References
  1. A. Cezairliyan, A.P. Miiller, F. Righini, A. Rosso, in Temperature: Its Measurement and Control in Science and Industry, J.F. Schooley, ed., (AIP, New York, 1982), Vol. 5, Part 2, pp. 377-381

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