(a) System codes have reached a highly developed modelling status and a large acceptance. They can reproduce accurately enough most existing safety related experiments, so far as the dominant physical mechanisms are known and understood;

(b) System thermal-hydraulic codes have been successfully applied for normal operation and accident conditions in the licensing of existing natural circulation BWRs. However, there is a consensus that current system thermal-hydraulic codes typically being used for safety analyses are not sufficiently validated to address all of the relevant conditions and phenomena of natural circulation based innovative designs (low pressure, low driving heads, increased effect of non-condensable gases, effect of buoyancy at low velocities, etc);

(c) Different codes are used for design and to study the different phenomena. The capabilities of the different codes to represent each case should be evaluated. More robustness is needed in the codes with regard to effectively addressing natural circulation phenomena;

(d) Consideration should be given to improvements in the code validation matrix with regard to natural circulation phenomena;

(e) Limitations exist whenever natural circulation phenomena are predominantly of a higher dimensional nature. 1D codes can be improved in this case by introducing special, component-related models that do not change the 1D structure (like reported for the core makeup tank modelling in APROS) or by introducing more-dimensional components (e. g. 2D downcomer). For development of more optimised designs, enhanced confidence in safety analyses, etc. There is a need for more accuracy of the codes and more detailed descriptions of new cooling concepts. This leads to a tendency to use, at least for local detailed analyses, 3D and time-dependent CFD codes. CFD codes are used even for designing experiments and their instrumentation arrangement. They can also be reliably used to study in a more qualitative manner the relevance of certain phenomena in flow and heat transfer problems, so far as the governing physics is included in the equations or models. Work is needed to improve CFD codes by appropriate modelling and related experiments, e. g. for the phenomena of non­condensable gases and instabilities as related to natural circulation;

(f) For more quantitative use in natural convection, especially with time dependent flow, improvements of the turbulence models are needed. Sometimes, LES is a necessary alternative. With two-phase flows, a quantitative use of CFD is currently limited to mainly homogeneous flows; for other flow regimes, the modelling of interfacial phenomena needs improvements. The turbulence modelling in two-phase flows can only be accepted as a first step. New correlations and models for two-phase flow should be developed, especially, in the turbulent flow regime. R&D is necessary to improve the models and to extend them for all flow regimes;

(g) Interfacial phenomena are still open to investigation. The interfacial heat transfer, interfacial mass transfer, and interfacial shear stress should be modelled and incorporated into the thermo-hydraulic codes;

(h) Theoretical and semi-theoretical studies could be considered international co-operative activities. This co-operation will provide better understanding of nuclear power technology and probably will reduce public reaction against nuclear technology.