A reactor at Torness nuclear power station has been shut down due to a fault.
Operator EDF Energy said there was no health or environmental impact as a result of the automatic shutdown at the facility in East Lothian.
A spokeswoman said: "Reactor 2 was automatically shut down at around 1.15am due to a fault within the protection equipment on one of our reactor plant electrical systems.
"This equipment is designed to 'fail safe'; in other words a fault with protection will result in the reactor shutting down safely."
Reactor 1 at the plant was temporarily shut down in July due to an issue with the electrical system.
WWF Scotland director Lang Banks said: "With yet another unplanned shutdown this year at a Scottish facility, nuclear power has once again shown itself to be a totally unreliable source of energy. Thankfully Scotland has plenty of cleaner, safer energy alternatives it can call on to be able to deal with unplanned shutdowns at atomic facilities like this.
"Last month, wind turbines in Scotland alone generated enough electricity to supply three millions homes in the UK - equivalent to 126% of the electricity needs of every house north of the border."