Combined Raman Imaging and ¹⁸O Tracer Analysis for the Study of Zircaloy-4 High-Temperature Oxidation in Spent Fuel Pool Accident

Oxidation at high temperature (HT) of cladding materials is expected to be the primary cause of the fuel assemblies' degradation in spent fuel storage pool loss of cooling accidents. Unlike a loss-of-coolant accident (LOCA) in a reactor vessel, the presence of air in the atmosphere is expected in a spent fuel pool accident and is known to be an aggravating factor because of the “catalytic” role of nitrogen on the oxidation. In steam LOCA conditions, a low-temperature (LT) oxidation scale simulating the corrosion oxide formed during in-service use of the assemblies has been observed to have a protective effect regarding subsequent HT oxidation. In the presence of nitrogen, HT oxidation kinetic experiments show that the protective effect of an LT corrosion scale also exists but is lost much earlier than in pure steam. Oxygen transport through LT oxide layers has been studied using the ¹⁸O tracer technique. We performed experiments in ¹⁸O₂ alone as well as in mixed ¹⁸O₂ + N₂ atmospheres, at 850°C. Micro-Raman imaging, both at the specimen surfaces and on cross sections, gave clear evidence for different characteristic distributions of ¹⁸O in the scales. Some of these distributions have been correlated with the presence of cracks and porosity in the LT oxide, which allows oxygen to locally penetrate in the scales. In cases in which no radial crack is present, apparent oxygen diffusion coefficients in the oxide were derived from fitting ¹⁸O diffusion profiles. Nitrogen appears to have no or only a limited influence on the oxygen diffusivity, but it is observed to reach the metal-oxide interface faster than oxygen, at which point it reacts with the metal and the sub-stoichiometric oxide to form zirconium nitride. The later conversion of the ZrN into ZrO₂ might lead to the destabilization of the LT scale and to the loss of its protective effect.