Reactor one at Torness power station, near Dunbar, East Lothian, was taken "off-line" just before 2pm yesterday.
Operator EDF Energy said there were "no safety, health or environmental impacts" as a result of the development.
It is understood station bosses switched the reactor off after an alarm went off during testing.
In a statement posted on the energy company's website, station director Paul Winkle said: "During testing of the reactor protection system, reactor one automatically shut down.
"We test this system, which is very sensitive, on a regular basis to prove the reactor will always shut down when required.
"The reactor will be returned to power as soon as testing is satisfactorily completed."
Torness provides enough electricity to power more than a million homes across Scotland, according to the company.
Reacting to the shutdown, Lang Banks, the director of environmental group WWF Scotland, said: "Thankfully, Scotland has plenty of cleaner, safer energy alternatives it can call on to deal with unplanned shutdowns at atomic facilities like this."