It has been shown before that a number of plant situations can only be simulated with computer codes.

The computer code development started together with the development of nuclear reactor systems. With increasing computer capabilities, a sound experimental basis and the set-up of validation matrices the computer code development for system codes has now practically reached its final stage. As examples, short descriptions for ATHLET, RELAP 5 and CATHARE are given below.

There is growing interest in CFD computer codes that are capable of modelling 2-D and 3-D fields, and which have the potential to simulate complex geometries. However, more developments and validation is necessary. The FLUENT code will be described below as an example of a CFD code.