Structural materials for nuclear reactors cooled with liquid Pb or Pb-Bi: behavior and damage of the metallic materials in contact with the liquid metal and under mechanical stress
Résumé
The durability of the structural materials at high temperature, under irradiation, under
mechanical stress and in the presence of the heat transfer fluid, is one of the
challenges in the development of 4th generation nuclear reactors, especially reactors
cooled by liquid lead (LFR) or for the development of Accelerator Driven Systems
which use liquid lead-bismuth eutectic (LBE). For these reactors, in addition of damage
due to irradiation, one of the major damages to structural materials could be due to
corrosion by liquid metal. Additionally, under stress (mechanical stress or stress due
to temperature fluctuations), structural materials can be susceptible to liquid metal
embrittlement (LME) or liquid metal accelerated damage (LMAD), i.e. partial or total
loss of ductility in the presence of liquid metal and therefore an earlier fracture. Thus,
although tough and ductile metallic alloys are selected, they may become brittle when
stressed in liquid metal exhibiting thereby the so-called LME. The objective of the
presentation is to summarize the knowledge acquired at UMET over more than 20
years on the mechanical behavior of coated or uncoated metallic alloys (martensitic
and austenitic) in the presence of liquid Pb or LBE and then to present the main
conclusions and the main issues for future researches. LME sensitivity of the materials
has been investigated by mechanical tests (monotonic tests (Small Punch Tests and
tensile tests) and Low Cycle Fatigue tests), in temperature, in air and in liquid LBE
or/and liquid lead and then the study of fracture surface, cracking and damage by SEM EDX-SEM-EBSD or ToF-SIMS analysis. In general, Body-Centered Cubic studied
materials are more sensitive to LME than Face-Centered Cubic (FCC) steels especially
at temperature lower than 450°C because of their low ductility while FCC alloys are
more susceptible at temperatures higher than 500°C. Note that, strain rate, roughness
of the surface, microstructure, temperature, oxygen content in the liquid metal appear
as parameters that influences LME sensitivity. Taking into account the mechanisms to
explain LME occurrence, challenges to avoid LME of the materials will be presented.