To reduce the radiological consequences of postulated severe accidents, the design of future European nuclear reactors includes measures to avoid basemat penetration in case of a core meltdown. The considered retention schemes include a temporary retention of the debris in the reactor pit followed by the spreading of the accumulated molten corium with subsequent flooding and cooling.

To contribute to the verification of such concepts, numerical simulations of the spreading process were performed with the CORFLOW code. These are based on an extensive verification and validation effort, i.e., the code has also been applied successfully to several flow, heat transfer, and phase transition problems of water, glycerol, cerrotru- (low-melting Bi-Sn alloy), and thermite- and corium-melts.

Physical and numerical methods are described as well as code applications to analytical solutions, spreading experiments, and reactor corium-spreading processes.