The Reactor Protection System (RPS) is a system, computerized in later BWR models, that is designed to automatically, rapidly, and completely shut down and make safe the Nuclear Steam Supply System (NSSS – the reactor pressure vessel, pumps, and water/steam piping within the containment) if some event occurs that could result in the reactor entering an unsafe operating condition. In addition, the RPS can automatically spin up the Emergency Core Cooling System (ECCS) upon detection of several signals. It does not require human intervention to operate. However, the reactor operators can override parts of the RPS if necessary. If an operator recognizes a deteriorating condition, and knows an automatic safety system will activate, they are trained to pre-emptively activate the safety system.

If the reactor is at power or ascending to power (i.e. if the reactor is supercritical; the control rods are withdrawn to the point where the reactor generates more neutrons than it absorbs) there are safety-related contingencies that may arise that necessitate a rapid shutdown of the reactor, or, in Western nuclear parlance, a "SCRAM". The SCRAM is a manually triggered or automatically triggered rapid insertion of all control rods into the reactor, which will take the reactor to decay heat power levels within tens of seconds. Since ~ 0.6% of neutrons are emitted from fission products ("delayed" neutrons), which are born seconds/minutes after fission, all fission can not be terminated instantaneously, but the fuel soon returns to decay heat power levels. Manual SCRAMs may be initiated by the reactor operators; while automatic SCRAMs are initiated upon:

1. Turbine stop-valve or turbine control-valve closure.
   1. If turbine protection systems detect a significant anomaly, admission of steam is halted. Reactor rapid shutdown is in anticipation of a pressure transient that could increase reactivity.
   2. Generator load rejection will also cause closure of turbine valves and trip RPS.
2. Loss of offsite power (LOOP)
   1. During normal operation, the reactor protection system (RPS) is powered by offsite power
      1. Loss of offsite power would open all relays in the RPS causing all rapid shutdown signals to come in redundantly.
      2. would also cause MSIV to close since RPS is fail-safe; plant assumes a main steam break is coincident with loss of offsite power.
3. Neutron Monitor Trips – the purpose of these trips are to ensure an even increase in neutron and thermal power during startup.
   1. Source range monitor (SRM) / intermediate-range monitor (IRM) upscale:
      1. The SRM, used during instrument calibration, pre-critical, and early non-thermal criticality, and the IRM, used during ascension to power, middle/late non-thermal, and early/middle thermal stages, both have trips built in that prevent rapid decreases in reactor period when reactor is intensely reactive (e.g. when no voids exist, water is cold, and water is dense) without positive operator confirmation that such decreases in period are their intention. Prior to trips occurring, rod movement blocks will be activated to ensure operator vigilance if preset levels are marginally exceeded.
   2. Average power range monitor (APRM) upscale:
      1. Prevents reactor from exceeding pre-set neutron power level maxima during operation or relative maxima prior to positive operator confirmation of end of startup by transition of reactor state into "Run".
   3. Average power range monitor / coolant flow thermal trip:
      1. Prevents reactor from exceeding variable power levels without sufficient coolant flow for that level being present.
4. Low reactor water level indicative of:
   1. Loss of coolant contingency (LOCA)
   2. Loss of proper feedwater (LOFW)
   3. etc.
5. High drywell (primary containment) pressure
   1. Indicative of potential loss of coolant contingency
6. Main steam isolation valve closure (MSIV)
   1. Redundant backup for turbine trip
   2. Indicative of potential main steam line break
7. High RPV pressure:
   1. Indicative of MSIV closure.
   2. Decreases reactivity to compensate for boiling void collapse due to high pressure.
   3. Prevents pressure relief valves from opening.
   4. Serves as a backup for several other trips, like turbine trip.