Fretting of fuel cladding materials for Pb cooled fast reactors—Approach to long term prediction using fretting maps

• Favourable load–amplitude combinations can minimize the fretting severity.
• Suitable material improvement and corrosion counter-measures improve the wear resistance and mitigate the corrosion enhancement of fretting.
• Surface treatment and Al-alloying (e.g. GESA-T91) improves remarkably the wear and corrosion resistance.
• Possible design solutions are the installation of spacer grids and supports in the non-active area.

Numerous fretting tests at severe conditions (accelerated tests) were performed using a new designed facility (FRETHME) on candidate steels such as the f/m T91 steel, the austenitic 15-15Ti steel and Al surface alloyed T91 (GESA-T91). The fretting damage increases with the increasing number of cycles/time and temperature. Fretting interacts with the corrosion mechanisms occurring in liquid Pb alloys (fretting corrosion) and destabilizes the corrosion barriers, favouring e.g. dissolution attacks. Due to the favourable wear and corrosion resistance properties of the surface alloyed layer, GESA-T91 steel showed the best fretting corrosion behaviour up to 550 °C. On the contrary, due to the high Ni content, the 15-15Ti steel is affected by dissolution enhanced fretting; while oxidation enhanced fretting characterizes T91 steel at temperatures higher than 500 °C. In most of the tests under accelerating conditions, 10% of fuel clad thickness was penetrated after quite short times already. To extrapolate the obtained results to conditions (load and amplitude) that allow long term use of the respective component in Pb cooled reactors, the concept of fretting maps was applied. Fretting maps were constructed using the obtained experimental data especially the fretting wear coefficient, which is a characteristic of a specific fretting regime. The obtained fretting maps were used to determine the tolerable amplitude and load up to which the tested materials can be used in a Pb cooled nuclear reactor.