The
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
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