Yasuhide Asada

Simplified analysis and design for elevated temperature components of Monju

Abstract The Japanese Prototype Fast Breeder Reactor Monju is now at a stage of submitting applications to the government for getting Construction Permission. Detailed structural design of components is now in progress. As well understood, a number of components of Monju have to be subjected to elevated temperature design. For this need, Power Reactor and Nuclear Fuel Development Corp. has developed “Elevated Temperature Structural Design Guide for Class 1 Components of Prototype Fast Breeder Reactor” (ETSDG). It is now incorporated into “Construction Code for Prototype Breeder Reactor”. The basic concept of ETSDG is originated from ASME Boiler and Pressure Vessel Code Section III, Case Interpretation N-47, ETSDG prepares some extended simplified inelastic analysis methods in addition to the basic concept of N-47. This extension is necessary to make easy the complicated inelastic analyses. The simplified methods have been developed through many case studies with detailed inelastic analyses, elastic analyses and experimental verifications.

Construction codes developed for prototype FBR Monju

Abstract The paper describes the regulation system for LWRs in Japan and discusses some typical points which are attained in the newly developed CCPBR (Construction Code for Prototype Breeder Reactor). Revised welding standards are briefly introduced for the Monju construction.

Exploratory research on creep and fatigue properties of 9Cr-steels for the steam generator of an FBR

Abstract Research on the applicability of 9Cr-steels to the steam generator of the demonstration fast breeder reactor was performed by the Subcommittee of the Japan Welding Engineering Society as a four-year program from 1985. In this program, exploratory tests, which included tensile, creep rupture and low-cycle fatigue tests, were conducted on three kinds of 9Cr-steels (Mod.9Cr-1Mo, 9Cr-1Mo-V-Nb, and 9Cr-2Mo) and their weldments. This paper describes the summary of results obtained in this program. Among the tested 9Cr-steels, Mod.9Cr-1Mo steel shows the best creep rupture strength and its weldment indicates almost the same level of creep rupture strength and the base metal at 500 and 550°C. The low-cycle fatigue properties of Mod.9Cr-1Mo steel is also discussed from its relation to the tensile properties.

Effects of strain rate change on fatigue life of carbon steel in high-temperature water

In high temperature waters that contain dissolved oxygen (DO) to certain content, the fatigue life of carbon steel is strongly affected by strain rate. A formula has been advanced to quantify this effect when the strain rate is held constant. However, the strain rate changes continuously in most of transients of actual plant operation. There is no way currently to assess the effects of strain rate when the strain rate is varied as in the actual plant transients. To find a solution to this problem, a series of strain controlled fatigue tests have been conducted with the strain rate changed stepwise or continuously. It is shown that a method, in which the product of the environmental effect and the strain increment within a unit time interval in a transient period is integrated from the minimum strain to the maximum, evaluates the environmental effect with satisfactorily high accuracy. This method is called the modified rate approach method. It is shown also that the procedure of taking the strain rate as averaged over the minimum to peak of the strain change as giving rise to more conservative evaluations than the ones the modified rate approach method produces.

Creep-fatigue damage assessment by subsequent fatigue straining

Abstract A series of creep-fatigue tests has been conducted with modified 9Cr-1Mo steel at 873 K in a high vacuum environment of 0.1 mPa. In order to investigate the accumulation of creep-fatigue damage, the creep-fatigue test programme includes changes in strain waveform during the test: from creep-fatigue type to fatigue type and from fatigue type to creep-fatigue type. The conventional linear cumulative damage rule for fatigue and/or creep-fatigue damage fails in evaluating the creep-fatigue life under the present complicated strain wave history. The linear summation of the life fraction is smaller than unity when the prior loading is creep-fatigue type and larger than unity when the prior loading is fatigue type. Scanning electron microscope (SEM) observation of the fracture surface was also conducted. In the case where the strain waveform changes from prior creep-fatigue type to subsequent fatigue type, the crack mode changes from transgranular to intergranular with an increase in the prior creep-fatigue loading history. In the case where the strain waveform changes from prior fatigue type to subsequent creep-fatigue type, the primary crack mode is generally intergranular regardless of the prior fatigue loading history.

Formulation of air environmental effect on creep—fatigue interaction

Abstract A series of creep-fatigue experiments has been conducted with 304 stainless steel at 650 °C, 2 1 4 Cr-1Mo steel at 550°C and modified 9Cr-1Mo steel at 600°C in air and in a very high vacuum environment of 0.1 μPa. A damage model based on the overstress concept was employed to evaluate the “pure” creep-fatigue life observed in this high vacuum environment which is completely free from the environmental effect of air. This damage model for the vacuum data was also applied to data obtained in air in order to evaluate the environmental effect of air on the creep-fatigue interaction. It was found that the fatigue damage is highly affected by the air environment, resulting in a time-rate-dependent life reduction. This life reduction is mainly controlled by the strain rate or the time duration of the compression loading stage. The environmental effect of air on the creep damage is complicated. The analyses based on this damage model suggest that the air environment accelerates the creep damage in 304 stainless steel, gives no effect in modified 9Cr-1Mo steel and reduces the creep damage in 2 1 4 Cr-1Mo steel.

Creep-fatigue interaction: data base analysis and applications

Abstract Creep-fatigue tests were conducted with a Type 304 stainless steel at 650°C using a wide variety of strain wave forms. Wave shape and hold-time effects were of special interest. Two distinct approaches were developed for analyzing the above test results in 10−9 mbar vacuum. Both approaches are based on a concept of “pure” creep-fatigue interaction. Data obtained in various environments such as air, 10−6 mbar vacuum and sodium are also listed. Creep-fatigue behavior in different environments are compared and the role of environmental effect is qualitatively discussed.

Creep-fatigue crack propagation tests and the development of an analytical evaluation method for surface cracked pipe

Abstract This paper shows test results and 3D/FEM estimations of the surface crack growth in a straight pipe and elbow under creep-fatigue conditions. Simplified estimation schemes such as CEGB/R6, CEA and GE/EPRI were also applied to straight pipe tests. The electrical potential method was successfully applied to measure the surface crack geometry; so crack propagation rates both for surface and thickness direction were measured. Predicted growth rates by 3D inelastic FEM analyses were compared with test data and the coincidence between test results and predictions was confirmed. Crack growth rates evaluated by the simplified method were also compared with test results and FEM results. The applicability of the simplified estimation scheme is discussed.

Ratcheting deformation of advanced 316 steel under creep-plasticity condition

A series of mechanical ratcheting tests under tension-torsion biaxial conditions has been conducted with an advanced 316 stainless steel at 923 K. Accumulation of torsional ratcheting strain was measured with a cyclic axial strain ranges of 0.005-0.02, cyclic axial strain rate of 10 -5 to 10 -3 S -1 and steady torsional stresses of 24.5/√3 to 73.5/√3 MPa. The accumulation of ratcheting shear strain is mainly affected by the cyclic axial strain range and the steady shear stress, and increases with an increase of both these parameters. A simple evaluation of the accumulation of ratchet strain is proposed. Although this procedure is based on the separation of creep and plasticity and uses experimental data to skip the plasticity analysis, the obtained results show good agreement with the experimental results.

System Based Code: Basic Structure

This paper introduces the basic theoretical structure of the “System Based Code” which has initially been proposed by the authors intending to give nuclear industry a leap of progress in the system reliability, performance improvement and cost reduction. The System Based Code intends to give a theoretical procedure to optimize the reliability of the system by administrating every related engineering requirement throughout the life of the system: from design to decommissioning.Copyright