Reactor Protection

Reactor Protection & Engineered Safety Feature Systems

Function

The Reactor Protection Systems are designed to shutdown the reactor and maintain it shutdown when needed. These systems are automatically actuated and may be manually actuated.

The Engineered Safety Feature Actuation Systems are designed to provide cooling for the reactor and to reduce the potential for offsite releases of radioactive materials. These systems are automatically actuated and may be manually actuated.

Terminology

Several terms that you may hear or see used are - Scram - Trip - Engineered Safety Feature Actuation - Containment Isolation - Anticipated Transient without Scram (ATWS) - Loss of Coolant Accident (LOCA). What does each of these terms mean?

Reactor Scram or Reactor Trip refers to the insertion of the control rods.
Turbine Trip refers to the shutting of the control valves that admit steam to the turbine.
Generator trip refers to the opening of the output breakers from the generator.
Engineered Safety Feature Actuation refers to the actions related to shutting down the reactor, mitigating the effects of a loss of reactor coolant accident, or minimizing offsite release.
Containment isolation refers to the automatic and built-in features that shut valves and shutdown ventilation systems to minimize offsite releases of radioactivity.
ATWS refers to the extremely low probability postulated event whereby it is assumed all rods do not automatically insert and cannot be manually tripped.
LOCA refers to the very low probability postulated event whereby it is assumed that a double ended guillotine break of the 3 feet (1 meter) pipe occurs and high pressure water discharges from both sections of the pipe. Small (1 to 2 inch) - intermediate and large break LOCAs are assumed and evaluated.

Common Design Features of Reactor Protection and Engineered Safety Feature Actuation Systems

These systems have the following features:

Multiple channels or trains of similar equipment - this is referred to as redundancy.
Sensors to detect an abnormal condition. Just as a person driving a car has indications of speed, oil pressure, and water temperature to warn of potential damage to the vehicle, the reactor operator has indications. Important ones are discussed below.
Multiple different techniques of sensing abnormal conditions - this is referred to as diversity
Devices with pre-set static or variable setpoints
Circuits that sense coincidence of several channels/trains reaching or exceeding the same setpoint. This practice is used to prevent false actuation.

In older plants, devices as relays and bistables are used. In newer plants, computerized equivalents may be used. The systems are designed to oerate if there is a loss of offsite power.

Parameters Sensed for Reactor Protection Systems (RPS)

Most plants use many of the diverse parameters listed below in their reactor protection systems. At specified setpoints, the reactor will shutdown by opening the circuit breakers that supply electrical power to control rods (or cause valves in the BWR hydraulic system to operate and accomplish the same purpose). The control rods will then insert. The circuitry is setup so that opening of any one circuit breaker will cause the rods to insert.

Reactor or Neutron power (can be viewed like speed in a car) sensors are monitored inside and outside the reactor. If a protection system uses external detectors, these are periodically recalibrated using detectors internal to the reactor. If power is excessive, fuel damage can result.
Reactor cooling system temperature (If temperature is excessive for a long time, fuel damage can result)
Pressurizer or reactor cooling system pressure (If pressure drops due to a leak or an open valve, voids may develop in the reactor, an undesireable condition).
Reactor cooling system flow (too low a flowrate through the reactor is undesireable)
Steam (secondary side) system pressure (If steam is drawn off too rapidly, steam pressure will drop; this can result in reactor cooling with the potential for damage to the reactor fuel, an undesireable condition)
Comparison of Feedwater (FF) and Steam (SF) flow rates ( if these are not matched for a long time, damage to the reactor fuel can result).

In the 1980's, plants in the United States were required to install diverse methods of shutting down the reactor to address the possibility of the anticipated transient without scram (ATWS) event. In the ATWS event, it is assumed the rods cannot insert. Other methods as chemical addition, steam generator heat removal, and pressure relief valve opening are provided.

Parameters Sensed for Engineered Safety Features Actuation Systems (ESFAS)

Functions provided by ESFAS include:

Emergency Core Cooling (ECCS) - start to provide cooling of the reactor core.
Emergency Feedwater - start to provide water to the reactor (if a BWR) or steam generatyor (if a PWR)
Containment Isolation - closure of valves in lines exiting the containment that do not serve an emergency safety function.
Containment Ventilation Isolation - closure of dampers in ventilation system exiting the containment to reduce potential for offsite releases
Emergency Ventilation System start and normal ventilation system shutdown.
Emergency Diesel Generator start to provide a backup power source.

Containment Spray start to reduce the containment pressure (only, if needed)

In some cases, depressurization of the reactor cooling system may be purposefully done:

BWR Safety-relief valves open to depressurize the reactor cooling system so that the high volume low pressure ECCS pumps can be used to cool the reactor core.
PWR Power operated relief valves may be opened to depressurize the reactor cooling system if a break in a steam generator tube occurs in order to reduce the leak between the reactor cooling and the secondary non-radioactive system.

Most plants use many of the diverse parameters listed below for ESF actuation:

Pressurizer or Reactor Cooling System pressure - start ECCS systems
Containment Pressure - start ECCS systems and containment at a pressure slightly above atmospheric.
Containment, Reactor Building, Auxiliary Building Ventilation Radiation Monitors

For more information on Reactor Protection and Engineered Safety Feature Actuation Systems, refer to one of the following:

Example of Palo Verde NGP PWR RPS and ESFAS setpoints - 1, 2, 3, 4
Example of Hatch NPP BWR RPS and ESFAS setpoints - 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20