A scram or SCRAM is a quick shutdown of a nuclear reactor . A scram or SCRAM is an emergency shutdown of a nuclear reactor -- though the term has been extended to cover shutdowns of other complex operations , such as server farms and even large model railroads ( see Tech Model Railroad Club ) . In commercial reactor operations , this emergency shutdown is often referred to as a `` SCRAM '' at boiling water reactors ( BWR ) , and as a `` reactor trip '' at pressurized water reactors ( PWR ) .