This paper describes a newly developed analytical method of evaluation of creep-fatigue strength of stainless weld metals. Based on the observation that creep-fatigue crack initiates adjacent to the interface of sigma-phase/delta-ferrite and matrix, a mechanistic model which allows the evaluation of micro stress/strain concentration adjacent to the interface was developed. Fatigue and creep damage were evaluated using the model which describes the microstructure after exposed to high temperatures for a long time. Thus it was made possible to predict analytically the long-term creep-fatigue life of stainless steel metals whose microstructure is degraded as a result of high temperature service.

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For long-term creep-fatigue of Type 304SS, intergranular failure is dominant when significant life reduction occurs. This phenomenon has its origin in grain boundary sliding as same as that in cavity type creep failure. Accordingly a simplified procedure to estimate intergranular damages caused by grain boundary sliding is presented in connection with the secondary creep.
In the conventional ductility exhaustion method, fracture ductility includes plastic strain, and damage estimation is based on primary creep recoverable during strain cycling. Therefore, the accumulated creep strain becomes a very large value, and is quite different from grain boundary sliding strain. As a new concept on ductility exhaustion, the products of secondary creep rate and time to rupture (ε₂t_R) is applied to fracture ductility, and grain boundary sliding is estimated using the accumulated secondary creep strain. It was shown that the time fraction rule and the conventional ductility exhaustion method can be derived analytically from the new concept.
Furthermore an advanced method on cyclic stress relaxation was examined. If cyclic plastic strain hardening is softened thermally during strain hold, softening of the primary and secondary creep can occur. And the unrecoverable accumulated primary creep strain causes hardening of the primary creep. The reduction of deformation resistance to the secondary creep accelerates the grain boundary sliding rate, and increases creep damage.
The new concept ductility exhaustion method based on the above consideration gave good life prediction for the intergranular failure mode.

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Creep-fatigue is a fatal failure mode of the high temperature structural materials of LMFBR. In this report, two important issues are discussed for the creep-fatigue evaluation of normalized and tempered Mod. 9Cr-1Mo steel which is a promising structural material for the steam generator of large scale LMFBR in Japan.
A new ductility exhaustion theory (NDET), a simplified conventional ductility exhaustion approach (DEA) and a time fraction approach (TFA) based on NDET are studied for the prediction of tension strain hold creep-fatigue damage of this material. Though Mod. 9Cr-1Mo (NT) steel has quite different basic material properties from those of austenitic stainless steel, NDET proposed for SUS304 is able to give a proper prediction for the creep-fatigue life of Mod. 9Cr-1Mo (NT) steel. The applicability of this evaluation method to Mod. 9Cr-1Mo (NT) steel is discussed based on micro-structural observations. It is considered that the creep-fatigue damage of this material under actual loading conditions is dominated by the creep-cavitation of grain boundaries in the same way as that of austenitic stainless steel. Finally, TFA based on NDET is proposed as the design creep-fatigue evaluation method of Mod. 9Cr-1Mo (NT) steel from the point of view of its appropriate conservatism for time-extrapolation and its simplicity.
The life reduction mechanism of low cycle fatigue of Mod. 9Cr-1Mo (NT) steel with strain hold at the compression side is discussed based on the data observed by a scanning type electron microscope. A new concept based on the location of oxidation on the test specimen surface can explain the reduction of low cycle fatigue life of Mod. 9Cr-1Mo (NT) steel.

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Strain controlled fatigue and creep-fatigue tests at 550°C were carried out on two kinds of SUS304 welded joints. The results obtained are summarized as follows;
(1) Fatigue strength reduction factor of welded joints, in the case without strain holding, shows a maximum value at the strain range of 0.3%, and decreases rapidly with a decrease in strain range.
(2) Creep-fatigue strength of welded joints, in the case of strain range of more than 0.5%, can be reasonably evaluated by strain concentration in a weld metal region, which is strongly affected by thermal softening and/or dynamic strain aging behaviors of weld metal and base metal.
(3) Creep-fatigue strength of welded joints in a low strain range region is considerably affected by creep damage, which increases due to elastic follow-up during strain holding period.

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As a fundamental study of developing a long term creep-fatigue evaluation method of weldment of stainless steel that incorporates the degradation of weld metal, the authors investigated the mechanism of micro damage in the structure of 308 and 316 type weld metals that had been subjected to thermal and mechanical histories using transmission electron micrography. The results obtained are as follows. (1) M₂₃C₆ and Laves phase that precipitated in δ-ferrite phase were transformed into σ phase that is more stable under high temperatures after long time heating. (2) While δ-ferrite phase remained in the microstructure at 550°C even after supplying alloying elements to precipitates and changing to Fe-rich composition, at above 600°C it disappeared after transforming completely to σ phase or austenite phase. (3) The rate of dissolution of δ-ferrite phase under cyclic stresses for fatigue or creep-fatigue failure was higher than that under constant stress for creep failure, and was accelerated by 10 to 100 times compared with that under no load. (4) The dissolution of δ-ferrite phase led to initiation and propagation of cracks at the interfaces of σ phase and remaining δ-ferrite phase or austenite phase by making a difference of mechanical properties between the phases.

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The long term creep-fatigue life of type 304 stainless steel was evaluated by the creep-fatigue life prediction method based on a linear damage fraction rule. The displacement controlled creep-fatigue tests were carried out, and the time to failure of longer than 10000 hours was obtained. The creep damage of long term creep-fatigue was evaluated by taking into account the stress relaxation behavior with elastic follow-up during the hold period. The relationship between life reduction of creep-fatigue and fracture mode was provided by the creep cavity growth. The results of this study are summarized as follows;
(1) The long term creep-fatigue data can be reasonably evaluated by the present method. The predicted lives were within a factor of 3 of the observed ones.
(2) The present method provides the capability to predict the long term creep-fatigue life at lower temperatures as well as that at the creep dominant temperature.
(3) The value of creep damage for the long term creep-fatigue data increased by elastic follow-up. The creep-fatigue damage diagram intercepted between 0.3 and 1 can represent the observed creep-fatigue damages.
(4) The cavity growth depends on the hold time. The fracture of long term creep-fatigue is caused by the intergranular cavity growth. The intergranular fracture of creep-fatigue is initiated by the cavity growth and followed by the microcrack propagation along grain boundaries starting from creep cavities.

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  An application of a cathodic protection method with an impressed current system to control the corrosion of austenitic stainless steel in a boiling nitric acid solution was studied to improve corrosion resistance and to extend the operation life of components in a fuel reprocessing plant. Plate-type specimens made of ultralow carbon type 304 stainless steel (SUS304ULC) were immersed in 3 mol·dm⁻³ boiling nitric acid solutions including 10 and 1.7 g·dm⁻³ vanadium ions. Electrochemical potentiostatic tests and cathodic protection tests were performed using electrochemical test cells. The selected protective potential was below the transition potential between the passive and trans-passive states based on anodic polarization measurement. Corrosion rates in the solution with and without the protection were measured by potentiostatic tests. Additionally, the outer surface of the tube-type specimen of SUS304ULC was studied under the same condition. From the obtained results, corrosion rates of plate-type specimens with cathodic protection were observed to decrease by 1/40 and 1/10 those of the specimens without cathodic protection in the solutions including 10 and 1.7 g·dm⁻³ vanadium ions, respectively. In the case of tube-type specimens, outer surface thickness loss was decreased from 24 to 3 μm by the protection, and platinum was chosen as the anode because it showed no corrosion loss like gold and no cracking like zirconium. Authors concluded that the cathodic protection method can be expected as one of the methods of maintaining components in a fuel reprocessing plant.

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It was evaluated creep damage by Electromagnetic Acoustic Resonance (EMAR), Noise Energy Method and Supersonic Spectroscopy (First Moment) Method on SUS 304 fabricated by Hot Isostatic Press (HIF) at 1050°C. Attenuation coefficient of electromagnetically excited acoustic wave in HIP has been investigated with frequency between 1.1 MHz and 5.4 MHz. Attenuation coefficient of the shear wave of as low as 2.5×10^-4/μs at 1.1 MHz has been detected by reducing diffraction loss and the loss due to transmission of the ultrasonic wave to the electromagnetic acoustic transducer through the mechanical contact. The attenuation coefficient increases in proportion with frequency up to 5.4 MHz on the samples (specimen) fabricated with the pressure of 170 MPa. On the other hand, on the samples (specimen) fabricated with pressure lower than 80 MPa, the attenuation coefficient increases rapidly above 4 MHz. The void area fraction has been found to increase as the pressure decreases, which may be responsible for the rapid increment of the ultrasonic attenuation of the samples (specimen) fabricated with pressure lower than 80 MPa. As a result, it was clear that EMAR is more sensitive one to detect creep voids than the other methods. So We hope it is possible to use the actual facilitles in future.

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Modified 9Cr-1Mo steel is the principal candidate material of a steam generator in a fast breeder reactor because of its superior high-temperature property. In this study, the influence of long-term strain hold on the failure life of Mod. 9Cr-1Mo steel was investigated and the applicability of existing creep-fatigue life evaluation methods was disccussed based on the experimental results. Creep-fatigue tests with hold time up to 10 hours per cycle at either tension or compression maximum strain were conducted under strain control of 0.5% and 1.0% at 550°C. Although failure life reduction occurred by introducing short hold period in the compression hold tests, the degree of reduction became smaller with increasing hold period. On the other hand, the failure life reduction became larger with increasing hold period in the tension hold tests. It was found that the failure life reduction in the compression hold tests was mainly due to the accumulation of tensile strain at the center of a specimen and that in the tension hold tests it was caused by intergranular damage in terms of creep cavity nucleation and growth from observation of the failure specimens. The time fraction rule adopted in a design code gave longer life compared to the experimental life, especially at low strain level. The failure life was well predicted by the ductility exhaustion method and the nonlinear damage accumulation model.

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東日本大震災の時点で福島第一原子力発電所1～4号機の使用済燃料プール（SFP）に保管されていた燃料集合体は海水注入や建屋の損傷により生じたガレキの混入によって通常とは異なる環境履歴を経ている．そのため，SFPから取り出された使用済燃料を使用済燃料共用プールで長期保管するためには，それらの環境履歴が燃料集合体の健全性に及ぼす影響を評価することが不可欠である．燃料集合体について，構造健全性及び被覆管密封性の観点から健全性が維持されると評価された．

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低線量率での塩化物水溶液中での炭素鋼の腐食挙動を，500 Gy/hのガンマ線照射下で塩化物イオン濃度の異なる塩化物水溶液を用いた腐食試験により調査した．照射により腐食速度は増大し，腐食速度が極大となる塩化物イオン濃度が存在した．腐食速度増大には塩化物水溶液の放射線分解で生成する酸化性化学種が関与していた．主な酸化性化学種は酸素と過酸化水素であり，放射線下での炭素鋼の腐食は酸素と過酸化水素の拡散過程に支配されていた．腐食速度の塩化物イオン濃度依存性と酸化性化学種濃度の塩化物イオン濃度依存性には良い対応関係が認められた．

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　Distributed database system named "Data-Free-Way" for advanced nuclear materials has been developed by National Research Institute for Metals (NRIM), Japan Atomic Energy Research Institute (JAERI) and Power Reactor and Nuclear Fuel Development Corporation (PNC) under cooperation agreement between these three organizations. In this paper, feature and functions of the system including input data are described together with method to share database among the three organizations as well as example of the easy accessible search of material properties. Results of analysis of tensile properties data on type 316 stainless steel collected by the different organizations and stored in the present system are also introduced as an example of attractive utilization of the system.

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