The priority for every nuclear power plant is the safe, reliable operation of the plant. The industry’s commitment to this principle is absolute – a cornerstone of the nuclear safety culture is “conservative decision making.” What this mean is if there is ever any threat to the safe, reliable operation of a plant, no matter how remote or unlikely, the decision is always to take the most cautious course of action, which may be to immediately shut down the plant.
The automatic or manual shutdown of the plant is referred to as a “trip” or “scram.” The shutdown occurs when the reactor is shutdown by rapidly inserting control rods into the fuel core to instantaneously stop the fission chain reaction. Trips can occur either automatically or manually when certain predetermined parameters indicate the plant is not operating as expected. The parameter values, often referred to as trip points, are selected with enough margin to ensure safety and protect plant equipment. The concept is much like a circuit breaker in your home. The circuit breaker will trip when a predetermined level is reached so the design limits of things like wiring, receptacles and other devices are not exceeded.
Control rods are designed to insert into the core to stop the fission process in less than two seconds. The time required to return the power plant to service is determined by the time required to assess and correct the cause of the trip limit being exceeded. A normal time to recover from a minor problem would be roughly 48 – 96 hours.
Plant trips and the startups are practiced regularly in every nuclear plant’s training organization. Exact replicas of plant control rooms are used to simulate different scenarios that could lead to a trip. It’s an approach and environment that’s very similar to an airline flight simulator. In the event of a plant trip, any improvement opportunities and best practices are captured during post-trip reviews. These reviews of operator and equipment performance are evaluated for inclusion in future simulator training and also shared with the nuclear industry.Nuclear plants are operated with multiple redundant safety barriers in place to protect public safety. Operators work to avoid trips by constantly monitoring, assessing and correcting any negative trends in plant performance. Trips are an excellent safeguard that ensure the continued safe operation of nuclear plants.