Helium Induced Damage Mechanism and Performance Optimization of Nickel-based Alloy for Molten Salt Reactors

Hefei Huang*，Jie Gao，Guanhong Lei，Cheng Li，Jizhao Liu，Xingtai Zhou

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201800, China

ABSTRACT: Nickel-based Hastelloy N alloy is recognized as the promising candidate structure material for molten salt reactors (MSRs). However, the high temperature irradiation damage of Hastelloy N alloy will lead to its property degradation. In this work, the ion irradiation method was used to study this related mechanism. It is found that helium embrittlement is the main degradation mode of nickel-based alloy under high temperature irradiation. On one hand, the helium bubbles can act as obstacles for the free movement of dislocation lines, causing the hardening embrittlement of the alloy. On the other hand, the aggregation and growth of helium bubbles at the grain boundary will reduce the binding force of the grain boundary, resulting in the non-hardening embrittlement of the alloy. Considering that helium bubbles, especially large ones, are the main reason for the degradation of alloy property, the coarsening behavior and mechanism of helium bubbles in alloys were studied by in-situ TEM annealing. A new coarsening mechanism, deformation and coalescence, of helium bubble was found and proposed. This mechanism is an important supplement to the two known coarsening mechanisms of helium bubble. Considering that different types of irradiation damage defects exist simultaneously in the alloy under service, the coupling effect in the evolution process of different defects was studied by sequential ion irradiation method. The results showed that the presence of helium bubbles can enhance the damage caused by displacement atoms. In addition, the irradiation-corrosion synergistic damage mechanism of nickel-based alloy was also investigated by means of XAFS, EPMA and TEM. The results show that the growth and migration of helium bubbles will accelerate the dissolution of Cr element into molten salts, thus accelerating the molten salt corrosion of the alloy. This conclusion provides a basis for the service safety assessment of molten salt reactor core components under irradiation. On the basis of the above-mentioned research, a new type carbide nanoparticle dispersion strengthened (CDS) nickel-bases alloys has been successfully developed by powder metallurgy route, to improve the resistance to helium induced damage. This type of alloys provides a new idea for the development of high-temperature MSRs materials.

Keywords: Nickel-based alloy; Helium embrittlement; Irradiation-corrosion synergistic damage; Dispersed nanoparticles