Calibration and optimization of pressurized thermal shock for benchmarking direct numerical simulation

Pressurized Thermal Shock (PTS) is considered as an important issue that challenges the integrity of the Reactor Pressure Vessel (RPV). PTS denotes the occurrence of a rapid cooling of the internal RPV surface under pressurized conditions. This overcooling may induce the criticality of an existing or postulated defect inside the vessel wall. The most severe PTS scenario concerns the Emergency Core Cooling (ECC) injection during a LOCA (Loss Of Coolant Accident). The injected cold ECC water mixes with the hot primary water present in the cold leg and the mixture flows towards the downcomer, where further mixing takes place, leading to large temperature gradients at the vessel surface. PTS is identified as one of the safety issues where CFD can bring real benefits complementary to the system codes' analyses. However, to gain the confidence in CFD codes' results, a comprehensive validation programme is necessary to demonstrate the capability of such codes to reliably predict PTS-related phenomena. In absence of detailed experimental data for the RPV cooling for CFD codes validation, DNS databases constitute a valid alternative for experiments. The aim of the work is to assess the capability of the spectral element code NEK5000 to perform a high quality DNS of a PTS scenario. For this purpose, a single phase PTS scenario is considered, which is based on the ROCOM facility. The main idea is to design a numerical experiment, in the form of a DNS, which takes into account the jet impingement on the RPV wall, the resulting turbulent mixing in the downcomer and the evolution of the temperature distribution for both structures and fluid in an ECC scenario. However, such a DNS analysis represents a demanding application, including the simulation challenges of T-junction thermal mixing and conjugate heat transfer. The present work consists of two parts. In the first part, a calibration study has been performed in order to obtain a feasible and well representing PTS computational domain. This calibration analysis has been performed using RANS simulations and the computational domain has been optimized in terms of boundary conditions, properties and dimensions. In the second part, the DNS capabilities of NEK5000 have been tested for a well-known channel flow configuration. Different numerical parameters have been tested and their influence has been studied to obtain high quality turbulence statistics.