Several conclusions can be drawn from the survey of experimental data and the identification

of needs; they are listed below:

a) The development and validation of CFD codes increased the requirements on the quality and details of experimental data. This has, e. g. led to a new instrumentation like tomography of two-phase flows to get knowledge about the spatial distribution of different fluids or phase. In addition, local turbulences on a micro scale should be modelled. New challenges result for the quality of experimental results from the required knowledge of detailed interfacial phenomena. The required knowledge as listed above should be available for steady state phenomena and transient flows.

b) For sequences with phase changes, especially for the condensation process, the influence of non-condensable gases on interfacial interactions, heat transfer and friction losses has to be investigated in more detail as compared with existing knowledge and with ongoing research.

c) For reactors using the natural circulation mode for core cooling, the neutronic-thermal hydraulic interactions are of vital importance because these might lead to core and flow instabilities. Instabilities with large amplitudes may influence stable operation.

d) A validation of models in computer codes should be based on different experiments in different facilities — if possible. Nevertheless the necessity to have well-based scaling laws to extrapolate from smaller scales to real plant sizes is of high importance. In addition, data from plant transients performed intentionally or unintentionally should be used more often for code validation.

e) There is a large amount of data from Separate-Effect tests and Integral tests selected systematically by CSNI for the purpose of code validation; see [Validation Matrices] for
the requirements on the selected data and the matrices. Nevertheless, some special tests focusing on NC-relevant phenomena and system behavior should be incorporated into these matrices. In addition, it should be evaluated whether further tests are needed to increase the confidence in the experimental database.