Scram

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 modelrailroads (see Tech Model Railroad Club). In modern commercial reactor operations, however, it is often referred to as a reactor trip.[1]

In any reactor, a SCRAM is achieved by a large insertion ofnegative 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 ofreactor. 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 currentreleases 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 (lessthan 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 (byabsorbing 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 apressurized 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...