User:K4r45u

Scram 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. In commercial reactor operations, this emergency shutdown is often referred to as a "SCRAM" at boiling water reactors, and as a "reactor trip" at pressurized water reactors. In any reactor, a SCRAM is achieved by a large insertion of negative reactivity. 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. Any cutting of the electric current releases the rods. Another design uses electromagnets to hold the rods suspended, with any cut to electric current resulting in an immediate and automatic control rod insertion. A SCRAM rapidly releases the control rods from those motors and allows their weight and the spring to drive them into the reactor core, thus halting the nuclear reaction as rapidly as possible. In BWRs, the control rods are inserted up from underneath the reactor vessel. In this case a hydraulic control unit with a pressurized storage tank provides the force to rapidly insert the control rods upon any interruption of the electric current, again within four seconds. A typical large BWR will have 185 of these control rods. In both the PWR and the BWR there are secondary systems that will insert control rods in the event that primary rapid insertion does not promptly and fully actuate. The actual axe man at the first chain-reaction was Norman Hilberry. In a letter to Dr. Raymond Murray, Hilberry wrote: 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 ax and told that was it, "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. 'The term stands for Start Cutting the Rope Away, Man! This refers to the early safety mechanism of using an person equipped with an axe to cut the rope suspending the control rods over the reactor, at which point the rods would fall by gravity into the core, shutting the reactor down. This is according to my West High School General Chemistry teacher for my sophomore year, Mr. Steve Pike.'