Uncertainty and Sensitivity Analysis of the Heat Transfer Mechanisms in the Lower Head

This paper illustrates a comprehensive approach to investigate the heat transfer mechanisms between relocated core debris and the RPV wall, to quantify the modelling uncertainties and to identify the most sensitive parameters or models involved. The investigations are mainly oriented to the TMI-2 conditions. Moreover, a high Rayleigh number scenario has been investigated. Previous simplified simulations with the thermohydraulic code ATHLET confirmed that a comparatively small coolant flow passing through a gap between lower crust and the vessel wall can reproduce the thermal history of the TMI-2 vessel during the accident. A refined integral model for the interaction between debris and RPV wall has been developed which covers the major physical phenomena governing the long-term integrity of the RPV. An uncertainty and sensitivity analysis of this model has been performed which accounts for modelling and parameter uncertainties. Quantitative sensitivity statements were derived for the code results accounting for the combined influence of these uncertainties as well as for the possible impact of the sampling error. (author)