Reactor Protective System

A Reactor Protective System (RPS) is a set of nuclear safety components in a nuclear power plant designed to safely shutdown the reactor and prevent the release of radioactive materials. The System can "trip" automatically (initiating a Scram), or it can be tripped by the operators. Trips occurs when the parameters meet or exceed the limit setpoint. A trip of the RPS results in full insertion (by gravity in pressurized water reactors or high-speed injection in boiling water reactors) of all control rods and shutdown of the reactor.

Pressurized water reactors

Some of the measured parameters for US Pressurized Water plants would include:

  • "High Power", auctioneered between high nuclear power and high differental temperature (delta T) between the inlet and outlet of the reactor vessel (a measure of the thermal power for a given RCS flowrate).
  • "High Startup Rate" (active below 10-4 percent power) at low power levels.
  • "High Pressurizer Pressure"
  • "Low Reactor Coolant Flow"
  • "Thermal Margin/Low Pressure" (reactor power versus RCS pressure)
  • "High Containment Pressure"
  • "Low Steam Generator Level"
  • "Low Steam Generator Pressure"
  • "Loss of Load" (Main Turbine Trip)

Each parameter is measured by four separate independent channels such that actuation of any two channels would result in an automatic SCRAM or reactor shutdown. Failure of any one channel would not cause or prevent an automatic shutdown. The system also allows manual actuation by the operator.

Boiling water reactors

Boiling water reactors have two channels to their RPS. Both channels must be grounded to automatically activate a scram, and each criteria for shutdown is measured twice for each channel - each criteria need fail only once per channel to ground the channel. The manual scram buttons are also part of the RPS. (The reactor can also be shut down by other means than the RPS.)

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