Comparison of lifetime assessments between ASME and RCC-MRx in liquid metal nuclear research reactors surfing from thermal striping

Abstract

The thermal striping phenomenon is a random temperature fluctuation that may cause thermal fatigue damage to components until the significant reduction of their lifetime or their fast break. It leads to creating some hazardous situations, which can cause the failure of mechanical systems. In the nuclear industry, components with a level of safety should be designed to prevent fatigue damage. In case of the occurrence of fatigue crack initiation on the component during service, it is required to analyse the causes and the consequences, most of the time, leading in the replacement of the component and modifying the causes (bad surface finish, residual stresses, etc.) to avoid future appearance damage. Indeed, in the case of liquid metal-cooled nuclear research reactors, the reactor core’ component integrity can be threatened and that should push engineers to be more meticulous in their job.Engineering industry standards and computation software provide engineers with efficient rules to study this phenomenon and find suitable solutions. Among them, RCC and ASME, which are widely used in the world. Globally, codes are similar but some differences exist in their approaches that lead to bringing additional technical difficulties for engineers.In this paper, the methodologies of surface stress calculation in pipes are compared between RCC-MRx and ASME III subsection NB. The goal is to estimate the gap and identify the main causes of differences in the risk assessment of thermal striping in research reactors cooled by liquid metal.Based on the lifetime computations from two code procedures, comparison between the main parameters is done. Results show an acceptable similarity between linear temperatures and tangible differences for second temperature and stresses. These results prove the existence of a gap between codes. It reveals that the risk assessment of thermal striping is approached more conservatively by the RCC-MRx than by ASME, in this studied case.