BWR Thermal-Hydraulic Simulation project is concerned with advanced fluid mechanics and heat transfer analyses relevant to nuclear reactor safety applications. The project is supported by the major nuclear industry players in Nordic countries, to strengthen the modeling capability in the thermal hydraulics of nuclear power systems. The overall objective of the current work is to develop an advanced, CFD-based, computational tool with steady-state and transient capabilities to simulate nuclear fuel assemblies, with particular focus on prediction of safety margins and on supporting optimal assembly design. Another major safety issues of Swedish nuclear power plants is the ageing process of the equipment accelerated by an occurrence of thermal fatigue. This is particularly important in view of the planned upgrades of nuclear reactors to higher power levels. To address this issue, it is proposed to perform a development of numerical methods to predict the thermal mixing and fatigue phenomenon. The proposed project will help to develop CFD models to predict amplitudes and frequencies of temperature fluctuations in mixing turbulent flows. For this purpose an LES model, inclusive of conjugate heat transfer between solid walls and fluid, has already been implemented; however, longer computational times are required to better analyze the aforesaid temperature fluctuations. This model will be validated against experiments run at KTH.