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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 (BWR), and as a "reactor ''trip''" at pressurized water reactors (PWR).〔(【引用サイトリンク】Reactor Protection & Engineered Safety Feature Systems )〕 In many cases, a SCRAM is part of the routine shutdown procedure as well. The term is usually cited as being 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, but NRC Historian Tom Wellock calls the axe-man story "a bunch of baloney." It could also stand for "Safety Control Rods Activator Mechanism" or "Safety Control Rods Actuator Mechanism", both of which are probably backronyms.〔(【引用サイトリンク】url=http://www.acronymfinder.com/SCRAM.html )〕〔(【引用サイトリンク】url=http://www.abbreviations.com/term/116485 )〕 'Scram' is also a verbal command used to tell someone or something to leave quickly and urgently, although its similarity to the technical term is most likely coincidental. == Mechanisms == In any reactor, a SCRAM is achieved by a large insertion of negative reactivity mass(es) 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. 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 mechanism is designed to release the control rods from those motors and allows their weight and the spring to drive them into the reactor core, in four seconds or less, thus rapidly halting the nuclear reaction by absorbing liberated neutrons. 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 (and often even tertiary systems) that will insert control rods in the event that primary rapid insertion does not promptly and fully actuate. Liquid neutron absorbers are also used in rapid shutdown systems for light water reactors. Following SCRAM, if the reactor (or section(s) thereof) are not below the shutdown margin (that is, they could return to a critical state due to insertion of positive reactivity from cooling, poison decay, or other uncontrolled conditions), the operators can inject solutions containing neutron poisons directly into the reactor coolant. Neutron poisons are water-based solutions that contain chemicals that absorb neutrons, such as common household borax, sodium polyborate, boric acid, or gadolinium nitrate, causing a decrease in neutron multiplication, and thus shutting down the reactor without use of the control rods. In the PWR, these neutron absorbing solutions are stored in pressurized tanks (called accumulators) that are attached to the primary coolant system via valves; a varying level of neutron absorbent is kept within the primary coolant at all times, and is increased using the accumulators in the event of a failure of all of the control rods to insert, which will promptly bring the reactor below the shutdown margin. In the BWR, soluble neutron absorbers are found within the Standby Liquid Control System (SLCS), which uses redundant battery-operated injection pumps, or, in the latest models, high pressure nitrogen gas to inject the neutron absorber solution into the reactor vessel against any pressure within. Because they may delay the restart of a reactor, these systems are only used to shut down the reactor if control rod insertion fails. This concern is especially significant in a BWR, where injection of liquid boron would cause precipitation of solid boron compounds on fuel cladding, which would prevent the reactor from restarting until the boron deposits were removed. In most reactor designs, the routine shutdown procedure also uses a SCRAM to insert the control rods, as it is the most reliable method of completely inserting the control rods, and prevents the possibility of accidentally withdrawing them during or after the shutdown. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Scram」の詳細全文を読む スポンサード リンク
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