Scram

A scram or SCRAM, also known as AZ-5 (Russian: АЗ-5), is an emergency shutdown of a nuclear reactor. It is a type of kill switch. In commercial reactor operations, this type of shutdown is often referred to as a 'SCRAM' at boiling water reactors (BWR), a 'reactor trip' at pressurized water reactors (PWR) and EPIS at a CANDU reactor. In many cases, a SCRAM is part of the routine shutdown procedure as well.When I showed up on the balcony on that December 2, 1942 afternoon, I was ushered to the balcony rail, handed a well sharpened fireman's axe and told, 'if the safety rods fail to operate, cut that manila rope.' The safety rods, needless to say, worked, the rope was not cut... I don't believe I have ever felt quite as foolish as I did then. ...I did not get the SCRAM story until many years after the fact. Then one day one of my fellows who had been on Zinn's construction crew called me Mr. Scram. I asked him, 'How come?' And then the story.The safety rods were coated with cadmium foil, and this metal absorbed so many neutrons that the chain reaction was stopped. Volney Wilson called these 'scram' rods. He said that the pile had 'scrammed,' the rods had 'scrammed' into the pile. A scram or SCRAM, also known as AZ-5 (Russian: АЗ-5), is an emergency shutdown of a nuclear reactor. It is a type of kill switch. In commercial reactor operations, this type of shutdown is often referred to as a 'SCRAM' at boiling water reactors (BWR), a 'reactor trip' at pressurized water reactors (PWR) and EPIS at a CANDU reactor. In many cases, a SCRAM is part of the routine shutdown procedure as well. The etymology of the term is a matter of debate. United States Nuclear Regulatory Commission historian Tom Wellock notes that scram is English language slang for leaving quickly and urgently, and cites this as the original and mostly likely accurate basis for the use of scram in the technical context. A persistent alternative explanation posits that scram is an acronym for 'safety control rod axe man', which was supposedly coined by Enrico Fermi when the world's first nuclear reactor was built under the spectator seating at the University of Chicago's Stagg Field. It could also stand for 'Safety Control Rods Activation Mechanism' or 'Safety Control Rod Actuator Mechanism'. Both of these are probably backronyms from the original, non-technical usage. The Russian name, AZ-5, derives from Аварийной Защиты 5-й категории (АЗ-5) (Avariynoy Zashchity 5-y kategorii (AZ-5)), 'Emergency Protection of the 5th Category'. In any reactor, a SCRAM is achieved by inserting large amounts of negative reactivity mass into the midst of the fissile material. In light-water reactors, this is achieved by inserting neutron-absorbing control rods into the core, although the mechanism by which rods are inserted depends on the type of reactor. In PWRs, the control rods are held above a reactor's core by electric motors against both their own weight and a powerful spring. A SCRAM is designed to release the control rods from those motors and allows their weight and the spring to drive them into the reactor core, rapidly halting the nuclear reaction by absorbing liberated neutrons. Another design uses electromagnets to hold the rods suspended, with any cut to the electric current resulting in an immediate and automatic control rod insertion.