Lecture date: 13 April 2016
The analysis of nuclear reaction safety is of prime importance supporting the development of new infrastructure as well as the safe operation of existing reactors. These days, through the tremendous increase in computing power and the capability of modern numerical methods, it has become feasible to base the fluid-mechanical aspects of safety analysis on first principles. This requires one to solve the dynamics of the Navier-Stokes equations to fine spatial and temporal scales that characterise the fine details of the flow and corresponding heat transfer. While in some situations it is possible to approximate the full Navier-Stokes solution using (quasi-) Direct Numerical Simulation (DNS), the main emphasis is in the development of Large-Eddy Simulation (LES) in which a systematic coarsening of the governing equations is pursued that combines `sufficient’ spatial and temporal detail with a strong reduction of computing costs. This new approach to reactor safety analysis will be discussed and the use of DNS and LES for a number of examples will be critically reviewed. Attention will be given to basic configuration such as flow in a channel and flow over a backward facing step - this sets the expectation level for LES. A further example is drawn from turbulent flow in a mixing-tee geometry - this is a key junction of pipes transporting water at different temperatures. It will be shown that modern LES is capable of capturing fine details in a turbulent flow. The relevance of contributions due to small turbulent scales and contributions due to numerical discretisation errors are critically assessed.
NRG Nuclear Research & Consultancy Group, Arnhem