An emergency shutdown in which all control rods are driven into the core at once, by gravity in most pressurized water reactors and by hydraulic pressure from below in boiling water reactors. It is triggered automatically by the protection system on signals such as high flux, low coolant level or turbine trip, or manually by the operator, and takes roughly two to four seconds. Fission stops within moments, but decay heat continues and must be removed.
Reactor scram at 1043 on low level, all rods in, entering the scram procedure.
Every unplanned scram is counted and reported, so plants work hard to remove single-point vulnerabilities that can trip the reactor.
Also heard as
- reactor trip
- trip
- SCRAM
Do not mix it up with
- Shutdown margin: A scram is the act of inserting the rods; shutdown margin is how far subcritical the reactor ends up afterwards.
Related terms
Control rod
A movable rod or blade of neutron-absorbing material used to control reactor power and shut the reactor down.
Decay heat
Heat produced by radioactive decay of fission products after a reactor shuts down, which must be removed.
Shutdown margin
How far subcritical a reactor would be with all rods inserted except the single most valuable one stuck out.
Diversity
Using different designs, technologies or physical principles for the same safety function to guard against common-cause failure.