Study on grain refinement for suppression of surface cracks in stainless steel used for nuclear reactor core (Thesis)

Hirota, Noriaki

JAEA-Review 2025-054, 132 Pages, 2026/03

JAEA-Review-2025-054.pdf:14.65MB

In recent years, extending the operational lifetimes of nuclear power plants and constructing new facilities have been conducted worldwide to achieve carbon neutrality. In Japan, following the Fukushima Daiichi Nuclear Power Station accident on March 11, 2011, only light water reactors (LWRs) that meet newly established safety standards have been permitted to restart, with most restarted plants being pressurized water reactors (PWRs), which differ from the boiling water reactors (BWRs) that were utilized at the Fukushima Daiichi Nuclear Power Station. Stress corrosion cracking (SCC) poses a serious safety concern in nuclear power plants. In particular, stainless steels such as SUS304 and SUS316 are susceptible to crack initiation due to oxide films and residual stresses. Countermeasures such as alloy composition modification and laser peening, which imparts compressive residual stresses, have been attempted; however, it remains uncertain whether compressive residual stresses can be retained for long periods under high-temperature conditions. Therefore, this study focuses on grain refinement processing as a novel approach for SCC mitigation and establishes a fabrication process for grain-refined stainless steels. The influence of grain refinement on SCC crack initiation was systematically analyzed under simulated reactor environments, and the mechanisms of SCC suppression were investigated. Furthermore, a surface-localized grain refinement technique was proposed and its effectiveness in suppressing SCC was evaluated. These results indicate that this technology can significantly contribute to SCC mitigation during long-term operation of nuclear power plants and is expected to play an important role in extending the service life of structural components such as shrouds.

Effect of grain refinement on cracks occurring in SUS304L stainless steel under nuclear reactor operating conditions

Hirota, Noriaki; Takeda, Ryoma; Ide, Hiroshi; Tsuchiya, Kunihiko; Kobayashi, Yoshinao*

Nuclear Materials and Energy (Internet), 45, p.102009_1 - 402009_10, 2025/12

https://doi.org/10.1016/j.nme.2025.102009

Using SUS304L stainless steel, which is employed in reactor structural components, the effects of grain refinement on stress corrosion cracking occurring under nuclear reactor operating conditions were investigated. As a result, after conducting slow strain rate testing (SSRT) in air and nuclear reactor operating environments, a comparison of the tensile properties of SUS304L with the same grain size revealed that elongation significantly decreased with increasing grain size under nuclear reactor operating conditions. In SSRT conducted in air, the -value obtained from the Hall-Petch relationship was lower than the conventional values. Observations showed the absence of cracks on SUS304L with 0.59 and 1.52 m grains; however, SUS304L with larger grains exhibited rougher fracture surfaces and side cracks. Thin oxide films were formed on SUS304L with 0.59 m and 1.52 m grains, while SUS304L with coarse grains of 28.4 m or larger enabled the formation of oxide films with over 2 m thickness. CrO films were formed on SUS304L with 0.59 m, 1.52 m, and 28.4 m, while CrO and Fe based oxides were formed on SUS304L with 39.5 m and 68.6 m. Crystal orientation analysis revealed linear surface layers without cracks in the -phase for SUS304L with 0.59 m and 1.52 m. In materials with Larger grain sizes, surface irregularities and cracks were observed in the -phase. In fine-grained SUS304L, lattice diffusion caused uniform O diffusion in the -phase, resulting in the formation of a thin CrO layer that suppressed cracks. In coarse-grained SUS304L, grain boundary diffusion caused Fe oxide formation at the grain boundaries, weakening them, and supersaturated O led to the formation of thick films comprising CrO and Fe-based oxides, resulting in peeling and cracking.

Evaluation of stability of precipitates under irradiation in 316FR steel used as fast reactor structural material

Toyota, Kodai; Onizawa, Takashi; Wakai, Eiichi*

Research & Development in Material Science (Internet), 21(5), p.2632 - 2637, 2025/06

Martensitic transformation-governed Luders deformation enables large ductility and late-stage strain hardening in ultrafine-grained austenitic stainless steel at low temperatures

Mao, W.*; Gao, S.*; Gong, W.; Kawasaki, Takuro; Ito, Tatsuya; Harjo, S.; Tsuji, Nobuhiro*

Acta Materialia, 278, p.120233_1 - 120233_13, 2024/10

https://doi.org/10.1016/j.actamat.2024.120233

Effects of grain size on ultrasonic attenuation in type 316L stainless steel

Wan, T.; Naoe, Takashi; Wakui, Takashi; Futakawa, Masatoshi; Obayashi, Hironari; Sasa, Toshinobu

Materials, 10(7), p.753_1 - 753_17, 2017/07

AA2017-0189.pdf:2.65MB

https://doi.org/10.3390/ma10070753

Effects of ultrastrong gravitational field on the crystalline state of a Bi-Sb alloy

Huang, X.*; Mashimo, Tsutomu; Ono, Masao; Tomita, Takeshi; Sawai, Tomotsugu; Osakabe, Toyotaka; Mori, Nobuo*

Journal of Applied Physics, 96(3), p.1336 - 1340, 2004/08

https://doi.org/10.1063/1.1763236

Mega-gravity field experiments were performed on the BiSb(atomic percent) alloy and pure Bi below their melting points, to investigate the change in crystalline state. For the alloy centrifuged at 191-205 C, no change in composition was observed, and the grain sizes of the crystals decreased from several mm to tens of m, while no distinct change in grain size was observed for the centrifuged pure Bi. The alloy centrifuged at 220-240 C consisted of two regions with different morphologies-fine-grained crystals with grain sizes around tens of m in the low gravity region, and large crystals with grain sizes several mm long and hundreds of m wide along the direction of gravity in the high gravity region, where sedimentation of atoms was confirmed. The large crystals with hexagonal structures were formed by preferential crystal growth roughly along the c axes, and a large strain that increased as the gravitational field increased existed inside these crystals. Formation of this anomalous crystal state might be correlated with the sedimentation of atoms.

Damage evaluation techniques for FBR and LWR structural materials based on magnetic and corrosion properties along grain boundaries

Hoshiya, Taiji*; Takaya, Shigeru*; Ueno, Fumiyoshi; Nemoto, Yoshiyuki; Nagae, Yuji*; Miwa, Yukio; Abe, Yasuhiro*; Omi, Masao; Tsukada, Takashi; Aoto, Kazumi*

Transactions of the Materials Research Society of Japan, 29(4), p.1687 - 1690, 2004/06

JAERI and JNC have begun the cooperative research of evaluation techniques of structural material degradation in FBR and LWR, which based on magnetic and corrosion properties along grain boundaries. Magnetic method has been proposed as the one of the non-destructive detection techniques on the early stage of creep-damage before crack initiation for aged structural materials of FBRs. The effects of applied stress on natural magnetization were investigated on paramagnetic stainless steels having creep-damages. On the other hand, corrosion properties and magneto-optical characteristics of ion-irradiated stainless steels in the vicinity of grain boundaries were estimated by AFM and Kerr effect microscope, respectively. These degradations were induced by changes in characteristics in the vicinity of grain boundaries. It is found that the initial level of progressing process of damage can detect changes in magnetic and corrosion properties along grain boundaries of aged and degraded nuclear plants structural materials.

Tensile and impact properties of F82H steel applied to HIP-bond fusion blanket structures

Furuya, Kazuyuki; Wakai, Eiichi; Ando, Masami; Sawai, Tomotsugu; Iwabuchi, Akira*; Nakamura, Kazuyuki; Takeuchi, Hiroshi

Fusion Engineering and Design, 69(1-4), p.385 - 389, 2003/09

https://doi.org/10.1016/S0920-3796(03)00079-6

In a fusion reactor, a blanket made of a low activation material like F82H is fabricated by solid Hot Isostatic pressing (HIP) joining method. In a previous study, blanket, grain coarsenings were found in a mock-up around HIP-joined region. To verify an effect of the coarsenings on a strength of the HIP-joined region, tensile test and hardness measurement were done. As the results, the tensile strength increased by about 50 MPa, and the elongation decreased by about 4 % in comparison with that of a standard alloy. Though the hardness was almost constant both in the coarsening and a non-coarsening regions, both of these hardness increased by about 5 %. Therefore, it could be judged that change of the tensile property is due to increase of the hardness. On the other hand, tensile and impact tests of a base metal without coarsening resulted in DBTT increase by about 40 K, although the tensile property was nearly equal to that of the joined-region with coarsening. It can be understood that this is the effect of the heat treatments in the fabrication process of the mock-up.

Evaluation of corrosion behavior of ion irradiated stainless steel using atomic force microscope

Nemoto, Yoshiyuki; Miwa, Yukio; Tsuji, Hirokazu; Tsukada, Takashi

Dai-12-Kai MAGDA Konfarensu (Oita) Koen Rombunshu, p.191 - 196, 2003/00

Development and research about analytical method for the study of corrosion behavior of austenitic stainless steel after irradiation was conducted from the point of view for basic study of IASCC (Irradiation Assisted Stress Corrosion Cracking). Ion irradiations were conducted with several irradiation conditions these were irradiation temperature, radiation damage, the contents of helium (He) implantation. AFM (Atomic Force Microscope) was used to evaluate surface condition of irradiated specimens after corrosion procedure. Corrosion condition was developed to obtain good surface condition of irradiated specimens to evaluate corrosion behavior by AFM. It was succeeded and corrosion behavior at inside of grains and grain boundaries of irradiated specimens was obtained. EBSP (Electron Backscatter Diffraction Pattern) was used to evaluate relation of corrosion behavior with grain boundary character. Moreover, relations of corrosion behavior with irradiation condition were discussed.

Understanding of mechanical properties of graphite on the basis of mesoscopic microstructure (Review)

Ishihara, Masahiro; Shibata, Taiju; Takahashi, Tsuneo*; Baba, Shinichi; Hoshiya, Taiji

Proceedings of OECD/NEA 2nd Information Exchange Meeting on Basic Studies in the Field of High-temperature Engineering, p.225 - 241, 2001/00

no abstracts in English