This paper presents technical aspects of large-scale direct numerical simulation (DNS) using high performance computing (HPC) cluster. It is divided in two parts. The first part contains a detailed description of HPC infrastructure used for the task, located in ?wierk Computing Centre (CI?), Poland. The description includes hardware configuration, software used for on-demand deployment of dedicated subclusters, and finally queuing and storage systems. The second part presents the large-scale simulations performed using this HPC cluster. In this regard, a highly scalable CFD code NEK5000 has been used to perform the DNS of two important thermal-hydraulic problems within the nuclear industry, i.e. pressurized thermal shock and inter-channel mixing in a bare rod bundle configuration.