The use of high power particle accelerators in various areas of applied nuclear science is presented with special emphasis on accelerator driven reactor systems (ADS) for transmutation of nuclear waste. National programmes for the development of spallation neutron sources are presented and the performance and reliability of existing or planned accelerators for use in ADS are discussed. Effects, such as thermal shocks and material resistance, on the reactor part of an ADS from loss of accelerator beam are discussed in greater detail.