UC Berkeley

The next generation of nuclear fission reactors is under development to lead to reactor designs that are more sustainable, more economical, safer, and more proliferation-resistant than current light water reactors (LWRs) (1). Each Gen-IV reactor design involves a unique heat transfer medium, which creates a host of novel nuclear materials challenges. One of these new reactor coolants is heavy liquid metal (HLM), i.e., pure Pb or Pb-Bi eutectic (LBE). HLM reactors are designed to operate in the fast neutron spectrum to reduce nuclear waste production compared to current LWRs. LBE specifically is an excellent HLM coolant candidate because of its low vapor pressure, low melting point (123.5 °C), and high boiling point (1670 °C), which allows for greater thermal efficiency in electricity production. However, steels in contact with LBE must withstand not only its corrosive nature but also the simultaneous exposure to radiation fields, high temperatures, and vibration.The interaction of two of these environmental extremes – LBE corrosion and radiation damage – is investigated in a newly developed simultaneous proton irradiation-corrosion setup. It consists of a corrosion chamber, in which sample disks (50 µm thick or less) can be exposed to corrosion and radiation simultaneously. A defocused proton beam is used to create point defects in the sample up to the metal-coolant interface. Pure Fe serves as a model system instead of more complex structural materials, such as steels, in order to get a better understanding of the interaction between radiation damage in the metal / the metal oxide and its influence on corrosion. The results show that the presence of the beam accelerates the degradation of the Fe-oxide layer formed on the sample, which allows LBE to penetrate sooner than in the absence of the beam. This penetration leads to the creation of pits underneath the oxide and a switch from the desirable oxide-forming regime to a dissolution-based corrosion mode, where the LBE dissolves the formerly metal-facing side of the oxide as well as the Fe sample itself. The resulting thinning of the samples was observed continuously during each irradiation-corrosion experiment with in-situ particle-induced x-ray emission spectroscopy (PIXE). PIXE also provides evidence for the accelerated corrosion in the beam spot being interrupted during extended periods of absence of the proton beam. This shows that corrosion is primarily accelerated in the simultaneous presence of radiation and corrosion. Ex situ microscopy analysis provides evidence that the thinning of the samples is strongest in the beam spot, but that surrounding areas are also affected. Thermocouple measurements during the experiments show that there is a temperature difference of 10-20 °C across the corrosion chamber when the beam is present. More work is needed to fully understand whether this temperature difference or the diffusion of radiation-induced defects are the primary cause of accelerated corrosion in and near the beam spot. To accelerate the study of structural materials even further and to increase the statistical significance of the results, a reduced-volume irradiation-corrosion setup was developed. In these experiments, several smaller irradiation-corrosion “chambers” with thin-film samples (1 µm or less) are mounted on a heater stage and exposed to a rastered proton beam to ensure identical irradiation-corrosion conditions. Preliminary results show that radiation accelerates the dissolution of Fe thin-films in LBE even in the absence of beam heating and at very low displacements per atom.