A Step Towards Rules to Identify Bounding Conditions in Terms of Fatigue Damage for NPP Operational Transients

Abstract

Mechanical components of nuclear power plant are subject to variations in temperature and pressure, which cause stress and ultimately induce fatigue damage. Usually, these variations, called operational transients, are well identified through the use of sensors and can be easily classified into design transients and associated to different reactor states and functional situations. However, a number of operational transients cannot be covered by the corresponding design transients due to temperature or pressure parameters exceeding the defined surveillance thresholds. In that case, an alternative is to find a more severe design transient to cover the operational transient, and then justify the existing design fatigue analysis remain valid. This process is today not existent in design standards and partially mentioned in standards such as RSE-M, yet it is critical for the safe operation of plants especially in the domain of Long Term Operation (LTO). In this paper, a set of mechanical criteria is developed to compare the severity between transients, which include four aspects: maximum stress state, minimum stress state, alternating stress intensity and fatigue damage. A three-dimensional rain flow method is used to identify extreme stress state and to detect multiaxial stress peak and valley. As an example, a typical nozzle is assumed to endure a non-classified operational transient. The envelope analysis for this transient classification is demonstrated and a design transient is selected to cover the operational transient. The numerical simulation results show that if a design transient and an operational transient satisfy the mechanical criteria strictly, then the operational transient can be bounded by the design transient. The proposed mechanical criteria are consistent with the requirements of the RCCM and RSE-M codes in term of preventing fatigue damage.