Dual Atmosphere Corrosion of 15-15Ti Cladding Material by Molten Pb and JOG Phases

Licentiate thesis, 2024

High temperature corrosion of cladding materials is still an issue hindering the deployment of Pb-cooled Generation-IV reactors. The combined effects of Heavy Liquid Metal coolant (molten Pb or Pb-Bi eutectic) and interaction with simulated fission products (JOG phases) have not been investigated thoroughly. In this work, the focus is on the corrosion of 15-15Ti cladding tube. The system was composed of molten Pb on the outer side of the cladding tube; while on the inner side, the selected JOG phase was Cs2MoO4. A reaction capsule was modified for experimental studies. The design minimized the excess oxygen ingress and made sure that JOG phases and molten Pb were not in initial direct contact. The flow of the coolant and irradiation was disregarded at this stage, while the chemistry of the cladding corrosion was the focus. A notch geometry was introduced to thin the cladding in certain locations, mimicking a pre-damage scenario and simulating a cladding breach accident scenario; creating a controlled interaction of the HLM coolant and the JOG phases. The capsule was tested between temperatures of 600 °C to 1000 °C. The corrosion process was investigated as: the corrosion on hosting tube, cladding material (inner and outer side), and the pre-thinned surfaces of the cladding material. A kinetic model was drawn up to define the thickness losses and predict the limits of the capsule. Regarding the molten Pb side, a severe delamination effect was identified as one of the main reasons for thickness losses, which eventually led to cladding breach around thinned areas at ≥900 °C. Beneath the delaminated layer, intergranular attack was observed mainly as Fe-Cr spinel scales. Even though the characteristics of the spinel changed with the temperature, intergranular oxidation was identified as the eventual starting point for the cladding breaches. Around the breached locations, the interaction of molten Pb and Cs2MoO4 was seen but characterization of this interaction requires further study, and a more specific analysis campaign. On the Cs2MoO4 side, similar spinel scales were observed, while their intensity was much lower, and delamination was seen as a local event.