Passive decay heat removal in advanced nuclear reactors

Passive safety systems are being considered in advanced reactor designs to provide inherent stability for the nuclear reactor and passive shutdown and to provide guaranteed removal of decay heat. The overall objective of this paper is to review the safety implications of this design trend and to provide a state-of-the-art commentary on related research. This paper compares different types of advanced decay heat removal systems and identifies typical key scenarios in which passive systems play an important preventative or mitigative role. The adequacy of the relevant experimental data-base, to aid the understanding of the essential physics and for code validation, is assessed. It is concluded that appropriate experimental programmes have been carried out by vendors for the purposes of design certification, but many of these data are not freely available. Further EU programmes which should add to the available data are now in progress. The current generation models/codes are also reviewed. The thermal-hydraulics codes for existing plant can be applied for predicting overall system behaviour in advanced plant, although these suffer from certain fundamental weaknesses e.g. in the modelling of thermal mixing. CFD codes are being increasingly used to overcome this shortcoming in modelling capability. These require further validation however, particularly for reactor safety applications where multi-phase flows are important.