[en] Severe accidents in nuclear power plants encompass a very wide range of interacting phenomena: • Thermal hydraulic behaviour in the vessel and primary circuit; • Degradation of the reactor core, including oxidation of fuel rod cladding, melt formation, relocation of material to the lower head, melt pool behaviour, lower head failure, ex- vessel corium recovery, molten core-concrete interactions; • Release of fission products from the fuel, structural material release, transport and deposition in the primary circuit, their behaviour in the containment (especially now with special emphasis on iodine and ruthenium), aerosol behaviour; Thermal behaviour, hydraulics in the containment, hydrogen molten fuel-coolant interactions, direct containment heating. This presentation summarises the main in-vessel phenomena involved.