Optical system for real-time monitoring of nuclear fuel pellets at high temperature

Abstract

Understanding the behavior of nuclear materials regarding fission gas release in relation to the different thermal loads to which they can be subjected requires appropriated annealing tests in order to measure both the absolute level and the time dependence of the released fission products together with the corresponding fuel micro-structural changes during representative thermal transients. In this context, we describe in this paper the development and qualification of an experimental platform coupling heat treatment, gas release analysis and optical systems, for the study of fuel pellets at high temperature. This system, which is mainly devoted to power transient and LOCA (Loss Of Coolant Accident) type simulation, is based on an induction furnace to heat the pellet at high temperatures (up to 2000 °C with up to 50 °C/s temperature ramps) in controlled atmosphere. The device is coupled with dedicated analysis loops that are designed to identify and quantify on line the gaseous fission products released during the annealing test. The purpose of the optical system is the real time monitoring of the sample surface to provide additional information (for instance on the micro structure evolution and fuel fragmentation). It is coupled with non-contact temperature measurements by thermal radiation analysis to monitor and control the fuel temperature. Based on experiments conducted on non-irradiated Uranium dioxide samples, we demonstrate the performance of the system in the range 20–1800 °C with 10 μm resolution on a field of 1 cm2, with simultaneous temperature measurements obtained by multispectral pyrometry. We also discuss the limitations of the system as well as further developments for integration in a hot cell for application to irradiated fuels.