Trace code validation for natural circulation during Small Break LOCA in EPR-type reactor

Abstract

The PWR PACTEL test facility was built in Lappeenranta (Finland) to gain experience in thermal-hydraulics behavior of vertical steam generators used by EPR (European Pressurized Water Reactor) during SBLOCA (Small Break Loss of Coolant Accident) transient, which involves natural circulation phenomenon. The benchmark, which consisted of blind and open part, offered a unique opportunity for code users to improve and test their knowledge and skills in developing the input deck models and performing calculations. For a purpose of this investigation, Royal Institute of Technology (KTH) has developed two TRACE code models. The main point of this thesis is to study TRACE code performance during SBLOCA transient and sensitivity of the developed TRACE models for the time and space convergence, which is very important for transients involving natural circulation phenomenon. Four different nodalizations coarse, inter-medium, fine and fine-sliced (space convergence), are designed for both designed models, which are calculated with different maximum time steps (time convergence). The results assessment was made by comparisons of the main parameters e.g.: Pressure of upper plenum, Inlet/outlet temperature of Core/SGs, Collapsed water level in the core, among others. In addition, discussion about vertical SGs performance during natural circulation phenomenon and conclusions for both, code users and developers, are provided.