The mod. 9Cr -1 Mo steel is a candidate material for the main components of the fast reactor, and its design allowable stress is specified in Japan Society of Mechanical Engineers fast reactor standard. The reactor will be larger in size. For the design, welding is also required at a part to be a high-temperature and high-stress. Therefore, allowable stress for the weld joint is required. When the weld joint of the mod. 9Cr -1 Mo steel is used for a long time under high temperature, creep damage of the fine grain region in the heat affected zone remarkably progresses. The strength of the weld joint is lower than that of the base metal. Therefore, the allowable stress of the weld joint must be considered such a strength reduction. In this paper, by comparing the strength properties of mod. 9Cr-1Mo steel base metal and weld joint, allowable stress for weld joint by reduction of base metal strength in the restriction of primary stress of JSME fast reactor standard was examined.

The current buckling criteria given by the design standard of Class 1 vessels for fast reactors (FRs) power plant in Japan, “Design and Construction for Nuclear Power Plants, Division 2 Fast Reactors” by JSME, focuses primarily on plastic buckling of austenitic stainless steel vessel. For next-generation FRs, the higher-yield material such as modified 9Cr-1Mo steel, larger and thinner vessel due to capacity increase and seismic isolation system plan to be applied. In previous study, a series of buckling tests and analyses of modified 9Cr-1Mo steel vessels was conducted and the results showed that the previously proposed modified buckling strength equations are applicable to evaluate buckling strength under these conditions. In this study, the effects of initial imperfection and residual stress of large vessels manufactured by welding multiple plates on the buckling load were evaluated via a series of analyses. By these analyses, the global initial imperfection, from the superimposition of imperfection over the entire vessel due to accumulation of misalignment in buckling mode imperfection, had weak effect on the buckling strength. And the local initial imperfection and residual stress by welding also exerted weak effects on the buckling strength.

In current practices, stress analyses of Class 1 vessels with internal pressure of nuclear power plants have been carried out using the minimum thickness of the component. In this paper, influences of thickness on stresses have been investigated by comparing stress analyses for two kinds of 3D-FEA models of a typical nozzle of a pressure vessel with minimum thickness and nominal thickness. Since variations of stresses between minimum and nominal thickness models are small, nominal thickness can be employed in the stress analysis. In order to conduct more conservative evaluation, a correction method to multiply primary membrane stress by the thickness ratio and primary bending stress by the square of thickness ratio can also be used.

TBCs are frequently subjected to thermo-mechanical fatigue damages, because thermal stress is significant there. Here, understandings on not only the steady state thermo-mechanical fatigue (SS-TMF) failures but those on the non-stationary TMF (NS-TMF) failure have been essential. In the latter a special consideration should be taken to non-steady or transient thermal response of TBCs. In this work the failure behavior of NS-TMF were studied, in comparison with those of NS-TMF. The experimental works clearly demonstrated that the TBCs were subjected to additional damages under the NS-TMF. In order to insight this phenomenon, a simple mechanical model by which the transient thermal is passively taken into account has been proposed. The numerical calculation showed that the additional thermal stress is developed in the NS-TMF test, resulting in the additional damical and mechanical properties of TBS system and the Biot-number. Some suggestions were also discussed to avoid this type of new subject.