Experimental results on hydrogen embrittlement by the present author were re-analyzed systematically and were discussed with the prevailing theories, reminding the fracture mode. The changes in the states of hydrogen during embrittlement were also considered.
In metal lattice, hydrogen is trapped by various kinds of defects, namely trap sites. The dislocation is the representing one. It traps hydrogen in competition with carbon and becomes mobilized. This helps to make the dislocation interact with each other so as to produce the vacancy. This vacancy can successively trap hydrogen and be stabilized by forming the vacancy-hydrogen compound (VHC). By slow deformation, this VHC is transported by dislocation motion to form vacancy cluster inside the grain. When the VHC is transported toward to grain boundary and reacts with it, more hydrogen becomes trapped there.
The, vacancy cluster is shown to be an essential factor for quasi-cleavage (QC) fracture that is a kind of ductile fracture. The oxide inclusion promotes the formation of vacancy cluster and enhances QC fracture more with hydrogen absorption. These facts support that the HESIV (Hydrogen Enhanced Strain Induced Vacancy) theory is applicable to QC fracture.
In tempered martensitic steels, hydrogen that is trapped by grain boundary causes the inter-granular (IG) fracture, which is co-existing with the QC fracture. When the prior grain boundary bears more phosphorus segregation and/or film like cementite precipitation, the grain boundary facet become prevailing in fracture surface. Along the grain boundary region where cementite is lacking, there appears “tear ridge” in between intergranular (IG) facet. In the extreme case with scares globular grain boundary cementite, the fracture surface is covered with fine wrinkling (tear striation), which is finely arranged multiple tear ridge. This is sometimes referred as IG-like or IQC (or intergranular QC) fracture.

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Recently, Carbon Fiber Reinforced Plastic (CFRP) composite material has been applied to large-scale structures such as large scale wind turbine blade and large pressure vessel not only aircraft structure. In addition, when considering the application of the CFRP composites also to large-scale structures in sea water it is needed to investigate the influence of sea water absorption characteristics of the CFRP composites on the strength properties.
In this study, we conducted tensile strength tests and fatigue strength tests in conditions of air atmosphere and sea water atmosphere by using specimens with a circular hole made of CFRP laminate composite plate and we have investigated the following strength properties of CFRP laminate composite, that is, (1) the influence of artificial sea water absorption on the tensile strength, (2) the fatigue strength in air and in artificial sea water at room temperature.
From these test results, we have obtained the basic data of CFRP laminate composite to be utilized to various large-scale structures.

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To improve the reliability against stress corrosion cracking (SCC) of stainless steel welds by using surface crack harmless technology via peening, an analytical study was conducted assuming residual stress, material properties (K_sc(l)）and four crack aspect ratios (A_s). The results of the numerical analysis were as follows.
(1) The maximum crack depth that can be rendered harmless (a_hlm) increases with increasing compressive residual stress and the K_sc(l). However, the effect of As was very small.
(2) The calculated values of a_hlm are 0.5-1.5 mm. A crack at 80% of the a_hlm is defined as a crack that should be detected accurately by nondestructive inspection (NDI).
(3) If large and deep residual stresses are introduced by peening after weld repair, not only a_hlm can be increased and necessary cracks are accurately detected, but also the number of detected cracks can be reduced, leading to a reduction of the number of weld repair areas. This will ensure the improvement of reliability and rationality against SCC.
(4) If a crack detected during NDI is not subjected to peening after weld repair, it is necessary to detect even shallow cracks by the NDI method with extremely high crack detection probability. Therefore, the NDI time becomes longer, and the number of weld repair areas increases. However, it is difficult to improve the reliability and rationalize the maintenance.

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Several types of Small Modular Reactors are under development. They have advantages in short construction period with less investment risk. Usually small nuclear reactor is not cost effective. The problem can be solved by factory production and simplification of the design utilizing passive safety features. The demand of market will give a lot of influence on the possibility of practical application.

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