Theoretical and Experimental Analyses of the Impact of High-Temperature Surroundings on the Temperature Estimated by an Optical Pyrometry Technique

The light-water one-rod equipment for loss-of-coolant accident (LOCA) experimental investigation (LORELEI) device, one of the experiments of the future experimental nuclear reactor Jules Horowitz of Commissariat à l’Energie Atomique et aux Energies Alternatives, will be dedicated to the study of the LOCA. For a better understanding of the temperature conditions during a LOCA, the surface temperature of the fuel cladding has to be monitored. For this purpose, we are designing a noncontact temperature measurement device based on multispectral pyrometry, adapted to the extreme conditions of LORELEI. In this paper, we present a radiative transfer model, taking into account the direct flux from the measured surface and contributions from the surroundings, to simulate the impact of high-temperature surroundings on the temperature estimated using the multispectral radiation thermometry (MRT) method. The hypothesis of a constant emissivity profile in the spectral ranges of calculation is used to estimate the surface temperature. Experimental pyrometry measurements are performed on Zircaloy-4 and Inconel surfaces in high-temperature surroundings to validate the model. As in LORELEI, an optical fiber is used to carry the infrared thermal flux from the cladding to the spectrometer. Thermocouples are used to give a reference temperature and to provide a complete temperature monitoring of the device. A temperature difference is observed between the “true” cladding temperature and the MRT temperature estimation, when the temperature of the surroundings is between 50 °C and 150 °C below the cladding temperature, and this difference increases with the cladding temperature. This provides a correction value to apply on the temperature estimation to approach the real cladding temperature.