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 (see Tech Model Railroad Club). In modern commercial reactor operations, however, it is often referred to as a reactor trip. 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 pressurized water reactors (PWR), 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 (less than four seconds, by test on many reactors) 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 (by absorbing neutrons) as rapidly as possible. In boiling water reactors (BWR) 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. Liquid neutron absorbers are also used in emergency shutdown systems.

During SCRAM the operators can inject solutions containing neutron poisons directly into the reactor coolant. Various solutions, including sodium polyborate and gadolinium nitrate, are used. Normally, these poisons are stored in pressurized tanks that are kept isolated from the reactor coolant system.

When needed, valves or pumps are actuated to rapidly introduce the boron from these tanks into the primary loop. 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 on fuel cladding, which would prevent the reactor from restarting until the boron deposits were removed.

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