In the past few decades, the contradiction between global climate change and the need to provide sufficient electricity for a growing global population has rekindled global interest in nuclear power. Gen-IV reactors, because of their safer, longer-lasting, proliferation-resistant, and economically viable characteristics, provide an excellent path for solving the contradiction and have attracted amounts of R&D investment from many countries.
For example, the Chinese Academy of Sciences has announced plans to invest $3 billion over the next two decades in the development of molten salt reactors of various designs. In Russia, a budget of $809 million has been allocated for the lead-cooled fast reactor and $550 million for the fuel cycle facilities.
Some main considerations in successfully developing and deploying Gen-IV reactor systems are the performance and reliability issues involving structural materials for both in-core and out-of-core applications. Because of the superior high-temperature strength, toughness, creep, and corrosion properties, nickel-based alloys have been proposed for various potential applications in Gen-IV nuclear reactor systems. For example, in molten salt reactor systems, Ni-based alloys are often utilized in reactor pressure vessels, heat exchangers, and other metallic parts in contact with molten fluoride salt.
However, in-service Ni-based alloys suffer higher fluences of neutron irradiation in the advanced nuclear reactors than in the current fission reactors. As a consequence, a high level of radiation-induced displacement damage in the form of vacancy and interstitial defects is generated and easily aggregates to form defect clusters (interstitial clusters and voids, among others) within the materials. In addition, a considerable amount of helium atoms, produced by a two-step nuclear reaction due to the large neutron absorption cross-section of Ni, are also introduced into the materials and trapped at radiation-induced vacancies to form He–vacancy clusters at elevated temperatures. Eventually, irradiation embrittlement, swelling, and phase instability have become the primary irradiation degradation pathways for Ni-based alloys. Given the extended design life of structural materials for the advanced nuclear reactors (60 years or more) relative to current nuclear reactors (~30 years), conventional Ni-based alloys rarely withstand the extreme irradiation environment.
It has long been known that grain boundaries and heterointerfaces can serve as sinks for radiation-induced defects and traps for helium atoms. Thus, designing a material from the perspective of grain boundaries and heterointerfaces has gradually become a consensus among the scientific community with which it is hoped to improve the radiation tolerance of materials. Graphene (Gr) is characterized by a high Young’s modulus (~1 TPa), high intrinsic strength (~130 GPa), and a low density. The two-dimensional nanomaterial exhibits potential as a reinforcing component to be incorporated into metal matrices. This disposition of the material can create a plentiful source of ultra-high strength metal–Gr interfaces and confer pure metals with novel functions. Excellent radiation tolerance may be one of the most outstanding features of metal–Gr nanocomposites. As a result, the solution of the above problems of material failure by Ni–Gr (NGNC) as a new radiation-tolerant material for the advanced nuclear reactors is highly possible.
At the beginning of the study, we investigated the self-healing mechanism of irradiation-induced defects in NGNC by atomistic simulations. We showed that NGIs can act as sinks to facilitate the recombination and annihilation of defects during collision cascades because energetic and kinetic driving forces promote the radiation-induced defects to bind to NGIs. However, another interesting phenomenon was also found by us: stacking fault tetrahedra are easily induced after relaxation when the size of the intrinsic defect of Gr of the composite exceeds a certain threshold. Moreover, several works have reported that high-density topological defects and structural disorder, originating from preparation and ion irradiation, may be introduced into the composite. Thus, it is doubtful whether the sink role of NGIs is still present when the Gr is not intact. And what’s worse is that neutron irradiation and elevated temperatures in the reactors can synergistically act on the composites, thereby causing a more complicated Gr surface morphology. Thus, some questions need to be answered. For example, can NGIs continue to exist given the above-mentioned conditions? How will the crystal texture of Gr and its metal matrices evolve?
Here we investigate the radiation tolerance of NGNC by using 300 keV He-ion irradiation with a fluence of 1 × 1017 ions/cm2 at 823 K. Pure Ni, which was used as reference material, was handled in the same condition. The analysis of Raman spectroscopy of NGNC suggests that the intrinsic disorder of Gr mainly stems from its nanocrystallization and that the disorder of Gr can be further aggravated by elevated temperature and irradiation. As for the results of GIXRD and TEM, the radiation damage of the Ni matrix in NGNC is not as significant as that of pure Ni. Less-disordered structures, such as lattice swelling and stacking faults, occur in NGNC. Coarse He bubbles are more difficult to form in NGNC than in pure Ni. The reason may be attributed to Gr’s own capability in maintaining two-dimensional structure and inhibiting the formation of large-size defects. Thus, the stability of NGIs is still able to be maintained by the disordered Gr, and the sink role of NGIs is still present.
With irradiation dose continuously increasing, high sp3 amorphous carbon may rapidly arise in the Gr. In consideration of the higher operating temperatures of Gen-IV nuclear reactors (i.e., above 873 K), an intrinsic self-repair mechanism keeping the Gr in a crystalline state may be activated. In addition, the crucial role of metal matrices for the healing of Gr defects has also been observed. Consequently, even if irradiation induces considerable damage, the Gr may partly recover its intrinsic structure; this speculation suggests that the sink role of NGIs may not completely disappear. All these results can provide a reference for the service life assessment of NGNC in advanced fission reactors.
These findings are described in the article entitled Radiation tolerance of nickel–graphene nanocomposite with disordered graphene, recently published in the Journal of Nuclear Materials. This work was conducted by Hai Huang, Xiaobin Tang, Feida Chen, Jian Liu, Xiangyu Sun, and Lulu Ji from the Nanjing University of Aeronautics and Astronautics.