Abstract
The prevention of excessive deformation by thermal ratcheting is important in the design of high-temperature components of fast breeder reactors (FBR). This includes evaluation methods for a new type of thermal ratcheting caused by an axial traveling of temperature distribution, which corresponds to moving-up of liquid sodium surface in startup phase. Long range traveling of the axial temperature distribution brings flat plastic deformation in wide range. Therefore, at the center of this range, residual stress that brings shakedown behavior does not accumulate. As a result, repeating of this temperature traveling brings continuous accumulation of the plastic strain, even if there is no primary stress. In contrast, in the case with short range traveling, residual stress is caused by constraint against elastic part, and finally it results in shakedown. Because of this mechanism, limit for the shakedown behavior depends on distance from the elastic part (i.e. half length of region with plastic deformation). Igari et al. proposed a mechanism-based evaluation method that focuses the traveling range of the temperature distribution. In this method, temperature difference was assumed to constant in the traveling phase, in other words, the temperature distribution moves subsequently to temperature rise. According to this assumption, the traveling range is equal to the plastic deformation range. However, in the actual design of the fast reactor vessel nearby liquid sodium surface, the temperature distribution moves up synchronizing with the temperature rise, without any intentional control. Because the moving up of the liquid sodium surface results from the heat expansion of the liquid sodium the assumption to isolate the temperature increase rise and traveling may be too conservative. In the actual design, the plastic deformation range becomes smaller than the traveling range of the coolant level. In this paper, we examined characteristics of the accumulation of the plastic strain caused by realistic heat transients, namely, traveling of temperature distribution synchronizing with temperature rise. This examination was based on finite element analyses using elastic-perfectly plastic material. As a result, we confirmed that the shakedown limit depends on not the traveling range of the temperature distribution but the plastic deformation range, which was predicted by the elastic analysis. We can control the plastic deformation range by changing rate of the moving-up of liquid sodium surface.
