Corrosion Studies of 15-15Ti Cladding Tubes in High Burn-up Conditions of Lead-cooled Fast Reactors

Doktorsavhandling, 2026

Cladding corrosion remains a major challenge to the deployment of Lead-cooled Fast Reactors (LFRs). The behavior of 15-15Ti cladding steel under dual environment exposures to molten Pb and fission products, particularly at temperatures exceeding operational limits, remains insufficiently investigated. This thesis examined the corrosion of 15-15Ti under conditions relevant to high burn-up in LFRs, with molten Pb at the outer cladding surface and selected Joint Oxyde-Gaine (JOG) compounds at the inner surface. Cs₂MoO₄, Cs₂TeO₄, CsI and a mixture of these compounds were selected as representative JOG phases. A reaction capsule was designed for simultaneous exposure of the outer and inner 15-15Ti cladding surfaces to molten Pb and either Cs₂MoO₄ or Cs₂TeO₄ at 600 – 1000 °C for 52 – 168 h. Wall thickness loss increased with temperature and exhibited Arrhenius-type behavior. Similar trends in the Pb/Cs₂MoO₄ and Pb/Cs₂TeO₄ systems indicated that molten Pb was the main contributor to thickness loss, primarily through oxide scale delamination. Internal oxidation on the Pb-side produced Fe‑Cr spinel grains adjacent to Ni‑enriched metallic grains. Single environment exposures complemented the capsule exposures by enabling phase identification regarding JOG-induced corrosion. Cs₂MoO₄ resulted in Fe-oxides and Fe-Cr spinel scales, while Mo reduction was observed at ≥900 °C. Cs₂TeO₄ exhibited stronger temperature-dependent changes, including Te ingress and telluride formation at 700 °C, combined Cs and Te ingress at 800 °C, and complete consumption of the cladding tube at 900 °C capsule exposure. CsI exposures showed no measurable thickness loss but substantial evaporation at ≥700 °C. Thermal analysis of the JOG Mixture indicates a eutectic melting point at ~577 °C, coinciding with a considerable increase in thickness loss between 500 and 600 °C. Lastly, cladding breaches during dual environment exposures were investigated, which were initiated by pre-drilled notches on the cladding tubes. Pb-Mo-O compositions were interpreted as Pb₂MoO₅ at 900 °C and PbMoO₄ at 1000 °C. Cs-Pb-O compounds were also detected but their phases could not be resolved, while no distinct Pb-Te compound was detected. In addition, Cs₂O, MoO₃ and metallic Te were identified as potential species to be released upon cladding breach. Overall, Cs₂TeO₄ was deemed to be the most aggressive among individual JOG compounds under these conditions; while JOG Mixture results highlighted the importance of multicomponent interaction in JOG-focused exposures.