Scram

The core, which was built under the spectator seating at the University of Chicago's Stagg Field, had an actual control rod tied to a rope with a man with an axe standing next to it; cutting the rope would mean the rods would fall by gravity into the reactor core, shutting the reactor down.

And then the story.Leona Marshall Libby, who was present that day at the Chicago Pile, recalled[3] that the term was coined by Volney Wilson who led the team that designed the control rod circuitry: The safety rods were coated with cadmium foil, and this metal absorbed so many neutrons that the chain reaction was stopped.

The report includes a section written by Wilson's team shortly after the Chicago Pile achieved a self-sustaining chain reaction on December 2, 1942.

It includes a wiring schematic of the rod control circuitry with a clearly labeled "SCRAM" line (see image on the right and pages 37 and 48).

[5] The Russian name, AZ-5 (АЗ-5, in Cyrillic), is an abbreviation for аварийная защита 5-й категории (avariynaya zashhchita 5-y kategorii), which translates to "emergency protection of the 5th category" in English.

[6] In any reactor, a scram is achieved by inserting large amounts of negative reactivity mass into the midst of the fissile material, to immediately terminate the fission reaction.

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.

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, 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.

[8] Due to flaws in its original control rod design, scramming an RBMK reactor could raise reactivity to dangerous levels before lowering it.

For a reactor that has not had a constant power history, the exact percentage is determined by the concentrations and half-lives of the individual fission products in the core at the time of the scram.