General Discussion

Nuclear fission takes place in the realm of nanoseconds. Even though the process, thermal fission, is self-regulating, automatic protection functions are still required. The purpose of the Reactor Protection System is to receive inputs from plant instrumentation, determine if an emergent condition exists and automatically take action to protect the health and safety of the public.
Perhaps the most important trip to detect is the over power, or high flux trip. The importance of this trip lies not in the fact that the over power condition would damage the reactor, but with the fact that the higher the power, the more decay heat is produced. As previously discussed, a shutdown reactor will produce the equivalent of 17 percent reactor power immediately after a trip.
The block diagram on the right outlines the components that are utilized for detecting and correcting a high flux trip condition. Each component receives and transmits information to protect the core. After reading the functions of each of the components below, return to the diagram and click on each component for more in depth and interesting information.

Power Range Nuclear Instrument Detectors

The Power Range Nuclear Instrument Detectors sense thermal neutrons coming from the core. The neutrons interact with material in the detector and create electron pulses which are transmitted to the equipment in the Instrument Rack Room. Four channels of detectors are used to provide core averaging when measuring Reactor Power.

Nuclear Instrumentation Cabinets

The Nuclear Instrumentation Cabinets convert the pulses received from the detectors and converts these pulses to usable voltage levels for metering and protective functions. When a High Flux Condition is detected, the bistables located within the instrumentation cabinets trip and sends that trip condition to the Solid State Protection System.

Solid State Protection System

The Solid State Protection System receives its inputs, not only from the Nuclear Instrumentation Cabinets, but from the Westinghouse 7300 cabinets as well. The 7300 cabinets monitor various temperature, pressure and flow parameters throughout the primary plant. If the Nuclear Instrumentation Cabinets are they eyes of the Reactor Protection System, (They see what is happening) the Solid State Protection System is the brain. The SSPS monitors the conditions of the various bistables within the plant and acts accordingly as these bistables trip. In the case of a High Flux Trip, in a non-test condition, if two out of the four bistables are tripped, a reactor trip occurs.

Reactor Trip Switchgear

The Control Element Drive Mechanisms (CEDMs) are powered by the Rod Drive Motor Generator Sets via the Reactor Trip Switchgear. The Reactor Trip Switchgear consists of four Westinghouse DS-416 Metal Clad Circuit Breakers. These circuit breakers are arranged such that there are two sets of two breakers in series. The A Train Reactor Trip Breaker is in series with the B Train Reactor Trip Breaker. The A Train Reactor Trip Bypass Breaker is in Series with the B Train Reactor Trip Bypass Breaker. Each Trip Breaker is in parallel with its Bypass Breaker. These breakers are kept closed by a 24VDC signal, which is supplied by the Solid State Protection System. During a Reactor Trip, the Solid State Protection System removes the 24VDC signal causing the breakers to open, removing power from the CEDMs.

CEDMs and Control Rods

The CEDMs grip and move the Control Rods. During normal power operations, the Control Rods are fully withdrawn from the core. Core reactivity is controlled by the Boron concentration within the Reactor Coolant System. During a Reactor Trip, the Reactor Trip Breakers remove power from the CEDMs allowing the Control Rods to fully insert into the core. This event adds enough negative reactivity to immediately shutdown the reactor.