Editing Scram (section)

==Mechanisms==
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In any reactor, a scram is achieved by inserting large amounts of negative [[reactivity (nuclear)|reactivity]] mass into the midst of the fissile material, to immediately terminate the fission reaction. In [[light-water reactor]]s, 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 [[pressurized water reactor]]s 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.

In [[boiling water reactor]]s, 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. 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.

[[File:NS Savannah SCRAM MD1.jpg|thumb|right|SCRAM button in the control room of the [[NS Savannah|NS ''Savannah'']] ]]
Liquid neutron absorbers ([[Neutron poison|neutron poisons]]) are also used in rapid shutdown systems for heavy and light water reactors. Following a 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 poison solutions are [[aqueous solution|water-based solutions]] that contain chemicals that absorb neutrons, such as [[borax|common household borax]], [[boric acid|sodium polyborate]], [[boric acid]], or [[gadolinium nitrate]], causing a decrease in [[effective neutron multiplication factor|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 [[boiling water reactor safety systems#Standby liquid control system (SLCS)|standby liquid control system]], 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 (chemistry)|precipitation]] of solid boron compounds on fuel cladding,<ref>{{cite book |last= Shultis |first= J. Kenneth |author2=Richard E. Faw |title= Fundamentals of Nuclear Science and Engineering |publisher= Marcel Dekker |year= 2002 |isbn= 0-8247-0834-2}}</ref> 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.