In this paper, we describe the state of computational fluid dynamics simulations (CFD) of deflagration and detonation processes in hydrogen-air mixtures, using vector- and parallel computing systems, which have been provided in the Institute for Safety Research and Reactor Technology (ISR) at the Forschungszentrum Julich (FZJ). The R&D work is performed within the scope of an EC project on hydrogen safety that is addressed to the verification of models and criteria for the prediction of Deflagration-to-Detonation Transition (DDT) in hydrogen-air-steam systems under severe accident conditions. Particularly, we report on the present state and recent progress made in the establishment of the GRAY hardware cluster (T90, T3E, J90) with vector and parallel processing capabilities, as well as on the current achievement of the CFD software cluster (CFX, ERCO, DET, IFSAS), including test cases for verification and validation, with some illustrating examples. Emphasis is put on the multi-dimensional simulation of fast turbulent hydrogen flames, for instance, using the general purpose field code CFX from AEA. The numerical results are compared with experimental results, which have been obtained for various conditions in the Russian large scale RUT test facility. Specifically, we outline deflagration-detonation processes concerning the numerical resolution of reacting flows in complex geometries, applying mesh refinement or massively parallel processing. First test cases indicate that our modern field code cluster (MFCC) with high-performance supercomputer networking (HPCN) will be a suitable constellation to resolve DDT processes in safety enclosures of innovative nuclear reactor containments or other industrial plants, e.g. solar hydrogen demonstration facilities. (C) 1998 Elsevier Science Inc. All rights reserved.