Large thermal-hydraulic system codes have been developed for the analysis of various fault conditions and initiating events. Examples of such codes include TRAC (Guffee et al.), RELAP5 (RELAP5/MOD3 Code Manual, 1995), CATHARE, ATHLET and RETRAN together with other industry system codes. Recently the TRAC-M or TRACE code is being developed which constitutes an amalgamation of the TRAC and RELAP5 codes (Spore et al., 2001). For the PWR, these codes calculate the flow, temperature and pressure in the primary circuit and secondary side. They include modelling of the reactor vessel, hot and cold legs, pressuriser and steam generators and safety systems using fundamental components of pipes, vessels, valves, etc. Most of the system codes can be adapted to other water reactor systems, e. g. BWR, VVER and RBMK.

In addition to thermal-hydraulics models, these codes typically contain point kinetics models to model the reactor power, and also 1D (radial) fuel rod models. Many have now been coupled to 3D neutronics codes of the type described above. In the UK, for example RELAP5 has been coupled with the PANTHER code, e. g. using the TALINK code (Page et al., 1998). RELAP5 has also been coupled with other neutronics codes. Generally, a few individual fuel rod models are coupled to a single thermal-hydraulic channel, e. g. an average rod and a hot peak rated leading rod. The fuel rod/coolant heat transfer exchange includes cladding to coolant heat transfer correlations, a gap conductance model between the fuel and clad, and thermophysical properties for the fuel. Ballooning, oxidation and rupture models are also required for the clad for LOCA analysis.