Computational Fluid Dynamics

Computational dynamics (CFD) codes provide solutions to modelling more general thermal — hydraulics situations, which are not modelled adequately by the system codes. Examples of such codes are the CFX code (CFX 4.3, 1999) developed for general fluid flow applications; in particular, it can be applied for reactor safety analysis. Another example is the FEAT code including coupled thermal-hydraulic and structural modelling capabilities developed by British Energy.

CFD codes are used to model flows where 3D effects and/or turbulent mixing phenomena are important. They are also useful in modelling complex geometries with arbitrary boundary shapes and internal structures. They are used for detailed phenomenological modelling to gain understanding but also in supplying mixing models for benchmarking system codes. For LWR applications, they are used in transient analysis of boron dilution events, thermal mixing in overcooling transients, and cold water mixing in steam line breaks. They are also used for modelling pools in advanced reactor passive systems where thermal mixing processes are often important in modelling heat transfer mechanisms.

CFD codes are being validated for reactor safety applications in a number of different European research projects. The codes include CFX-5 and FLUENT for modelling flow mixing and flow distribution in the primary circuit (FLOMIX-R) (Weiss et al., to be published). CFX-5, CODE-SATURNE and TRIO-U (ECORA) (Scheuerer et al., to be published) are being validated for a range of applications including primary loop flow mixing, pressurised thermal shock (PTS) flow modelling and 3D containment analysis.

To date, CFD codes applications in reactor safety are largely concerned with single­phase applications. The ASTAR project (Paillere et al., to be published) has looked to extend the modelling limitations of the systems codes such as CATHARE, ATHLET, TRAC, RELAP5, etc. For example, a multi-dimensional model FLUBOX was coupled to ATHLET within this project. CFD codes are now being developed for multi-scale (termed CFMD (Yadigaroglu, to be published)) applications and these are being examined at the research level.

Fluid flow modelling in gas reactors where only single-phase flows are present, is a much more straightforward proposition than the modelling of two-phase flows in LWRs. CFD codes have been applied to gas reactor flow modelling in normal operation and accident conditions. They are particularly amenable for modelling such flows and they have also been coupled with neutronics codes to provide power variation feedbacks.

There have been substantial analytical methods developments for modelling sodium cooled LMFBRs (IAEA-TECDOC-1083, 1999). Codes have been developed with the support of extensive experimental facilities in Europe and the US. Codes have been produced for modelling decay heat removal under various accident conditions. The requirements have been to model forced and natural circulation in various components under steady-state and transient conditions. There has been particular attention paid to the development of multi-dimensional codes for modelling disturbed turbulent liquid metal flows. Much of this experience will be relevant to future liquid metal systems.

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