Background
The molten salt reactor (MSR) is a type of nuclear reactor in which the primary coolant is a molten salt mixture. These types of reactors operate at near-atmospheric pressure while at much higher temperatures (up to 700-750°C) than light water reactors. Identifying corrosion resistant materials that can perform acceptably under these conditions is challenging. For example, although austenitic stainless steel possesses excellent high temperature strength and lower cost, it exhibits poor corrosion resistance in molten fluoride salts. Corrosion of metallic alloys by molten salts is one of the technical issues and R&D priorities that need to be addressed to assess the viability of molten salt reactors. The "Technical Gap Assessment for Materials and Component Integrity Issues for Molten Salt Reactors" by Oak Ridge National Laboratory indicated that for use in structures inside a reactor vessel, Ni-based alloys and alloys with dense Ni coatings were effectively inert to corrosion in fluorides. Furthermore, in the absence of comprehensive knowledge of the effects of neutron irradiation on these new corrosion resistant alloys, a database must be developed of the thermal, physical, and mechanical properties of these materials under intense neutron irradiation.
Approach
The research comprised three tasks: (i) applying surface engineering methods on an Alloy 617 substrate to develop bi-layer and tri-layer structures that combine high corrosion resistance in fluoride molten salt and high strength at high temperatures; (ii) investigating the corrosion resistance of Ni, Mo, W, and Ta coatings when exposed to fluoride molten salt at high temperatures and assessing the effectiveness of a diffusion barrier in mitigating inter-diffusion between the surface layer and substrate; and (iii) performing calculations to determine the radiation resistance of the materials, then using MCNP to evaluate neutron damage reaction rates inside the corrosion resistant material.
Accomplishments
High-temperature molten salt exposures over a temperature range of 610°C to 850°C showed that the bi-layer coatings provided some corrosion resistance, with the Ni-Alloy 617 structure showing the best corrosion resistance. The W-Alloy 617 and Mo-Alloy 617 coatings had good resistance at temperatures below 750°C. The Ta-Alloy 617 samples, despite having a predominately intact Ta coating, showed intergranular attack at all temperatures tested.
The tri-layer materials all had some amount of delamination of the outer coating. In the case of the Ni tri-layer system, the Ni outer layer provided protection against intergranular corrosion below 680°C. The Mo and W tri-layer samples, though delaminated, also provided some protection against intergranular corrosion but at lower temperatures. MCNP calculations showed that neutron effects such as neutron embrittlement and helium embrittlement of the tested materials, might limit their useful life.