Influence of axial coolant flow on fuel assembly damping for the response to horizontal seismic loads

The existence of a large damping increase under axial flow is clearly demonstrated by test results. Therefore this phenomenon concerns not only isolated tubes, but also rod bundles with multiple support such as fuel assemblies, and it may represent a large conservatism margin when not allowed for in their modelling. Test results also demonstrate that the mock-up behaviour is representative of that of a full-scale assembly, and that actual confinement conditions are not of much concern since their influence remains small. However, applying test results under axial flow to a core model can be envisaged only with caution, since no clear physical interpretation of this phenomenon has been found (at least for such large damping values). Further studies will comprise a complementary experimental program, in order to specify the physical nature and the application range of this effect, and to provide hints for a theoretical interpretation. (author)