Before revision: upright=1.2|Core of CROCUS, a small nuclear reactor used for research at the EPFL in Switzerland A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reaction .
After revision: upright=1.2|Core of CROCUS, a small nuclear reactor used for research at the EPFL in Switzerland A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction .