What is a Reactor Trip and How Does it Protect the Plant?

nppmapThe Salem nuclear power plant’s Unit 1 “tripped” on Dec. 21st. Brown’s Ferry Unit 2 tripped the following day. In both cases, something happened that caused the reactor to automatically shut down to ensure safety. In other words, a trip means a plant is doing what it’s supposed to do. Let’s look at the term a bit more closely.

Key operating parameters of a nuclear power plant, such as coolant temperature, reactor power level, and pressure are continuously monitored, to detect conditions that could lead to exceeding the plant’s known safe operating limits, and possibly, to damaging the reactor core and releasing radiation to the environment.

If any of these limits is exceeded, then the reactor is automatically shut down, in order to prevent core damage. In nuclear engineering terms, the automatic shutdown of a nuclear reactor is called a reactor trip or scram . A reactor trip causes all the control rods to insert into the reactor core, and shut down the plant in a very short time (about three seconds).

How do control rods do their job?

The control rods are composed of chemical elements that absorb neutrons created by the fission process inside the reactor. They are placed methodically throughout the nuclear reactor as a means of control. For example, as the control rods are moved into the reactor, neutrons are absorbed by the control rods and the reactor power is decreased. Inserting them all at the same time shuts down the reactor. Control rods can also be inserted manually, if necessary.

The plant operator then determines the reason for the trip, remedies it and, when it’s determined to be safe, restarts the reactor. So, while not common, a reactor trip is an important way to protect the components in a nuclear power plant from failing or becoming damaged.

Samuel Miranda
Senior Reactor Systems Engineer