applied fracture mechanics

268Applied Fracture Mechanicsfor the identified spectrum of overcooling scenarios with the critical stress intensity factor ofthe RPV material obtained from the CVN test. In each one of the selected scenarios, theremust be enough margin to be sure that the vessel can withstand the selected loadsconditions.University of Pisa developed a methodology concerning the use of a chain of codes toquantify this margin by mean a deterministic approach to the PTS issue, see [17] [18].A preliminary analysis of the plant configuration and logics is required in order to identifythe spectrum of the scenarios that lead to an overcooling of the down-comer region.The methodology starts with the thermal hydraulic analysis of the Nuclear Power Plant(NPP) using a SYS-TH code such as RELAP5, CATHARE2, or equivalent, during a selectedtransient scenario. The goal of this step is to evaluate the plant response and to calculate thecooling load induced on the internal RPV wall surface by the Emergency Core Coolant(ECC) injection or by the cooling plug following a MSLB initiating event, to calculate theprimary circuit pressure and to provide boundary conditions for the next step.If the transient evolves in single phase, a more detailed analysis of the mixing phenomenaoccurring in the down-comer region can be performed by mean a CFD code. The result ofthis step needed for the PTS analysis is the temperature distribution inside the down-comer.Figure 2. UNIPI Methodology for PTS Analysis