This publication is a companion to the IAEA Safety Report, Accident Analysis for Nuclear Power Plants. It is specifically devoted to the analysis of severe accidents. Although the publication does not explicitly differentiate between various reactor types, it has been written on the basis of the available knowledge and databases developed for light water reactors (LWRs). Its application is therefore oriented mainly towards pressurized water reactors (PWRs) and boiling water reactors (BWRs) and, to a more limited extent, water cooled water moderated power reactors (WWERs) and pressurized heavy water reactors (PHWRs).