The flow and heat transfer phenomena taking place in reactor core and containment problems or during postulated transients in Light Water Reactors have long been studied using system codes. Today, various factors plead for the development of more specialized, precise CMFD tools. ASCOMP is a partner in large?scale developments in this direction within the European research community (NURESIM project).
TransAToffers the opportunity to simulate large?, medium?and small?scale, 2D and 3D, thermal?hydraulic two?phase flows featuring phase change and conjugate heat transfer using interface tracking methods (VOF and Level Set).
TransAT(Transport phenomena Analysis Tool) can simulate a wide range of single and multi?fluid/component flows including conjugate heat transfer. TransATis particularly designed to handle fluid?fluid and fluid?solid multiphase flows (CMFD), featuring large density and viscosity ratios. TransAT has unique capabilities in treating wetting (e.g., of superheated surfaces by droplets), film flows, etc.
TransATprovides medium?scale two?phase thermal?flow behaviour in specific components of a system (examples shown below), or detailed phenomena, e.g., flow around grid spacers in a fuel bundle.
Pressurized Thermal Shock (PTS) during an assumed LOCA: cold water is injected into the cold leg, thermal shocking of the wall under pressure is a concern.
The injected cold water mixes with the hot fluid in the cold leg and flows towards the downcomer, poor mixing may lead to extreme thermal gradients in the structures. The figures show the water free surface deformations (obtained with Level Sets) and diffusion of the heatsubsequent to coolant injection. Use was made of the IST/BMR technique (left), where the
Fuel bundle analysis
Simulation of Sadatomi’s experiment (IJMF, 2004) of two?phase flow distribution in a 9?rod fuel assembly, without phase change. Here, too, the simulations used the IST/BMR technique (left). Conjugate heat transfer between the rods and the fluid is simple to tackle, as the fully?coupled heat transfer equations (solid?gas?liquid) are solved simultaneously. The phase distribution coloured for interface temperature is shown on the right; heat conduction in the tube is not shown.
Steam injection in a BWR containment pool
A typical example in NPP technology is the venting of vapour and gas mixtures into pressure suppression pools to control containment pressure. TransATusing VOF is a excellent tool for the predicting the behaviour of the large bubbles and their breaup, their rate of condensation in the pool, the currents induced in the surrounding liquid and the consequent mixing. The LES?VOF model was used to capture topology deformations and large?scale turbulence.