The behaviour of vapour channels traversing the upper part of a boiling, unconstricted debris bed is investigated. The presence of such discrete paths facilitates the vapour flow and therefore affects the coolability of the system. On the basis of an extensive experimental support the formation, population density, extent and stability of channels are discussed, leading to the development of a theoretical model for the dryout heat flux in a channelled bed. The latter can be categorized as shallow or deep according to the relative extent of the channelled and packed zones in it.