Nuclear reactors for power production require essentially two control regimes-one for start-up and the other about design point power. The start-up regime requires controls and instrumentation for neutron flux. The power regime is one in which temperatures must be controlled. The Bell-Strauss simulator has been used successfully for development of neutron flux control equipment. Nuclear power plant simulation requires a more elaborate analog computer such as the present ORNL nuclear plant simulator. This computer has been used successfully for designing servo mechanisms for rod control, temperature regulation and heat dump control for power reactor designs. Some of these applications are described.