The shutdown of the country's nuclear plants was "in line with their design" when confronted with an unexpected power outage, the Spanish Nuclear Safety Council (CSN) said in a statement.

That happened when the country suffered a national blackout at 1030 GMT, halting trains, fouling traffic and delaying flights.

Four reactors at three nuclear power plants were in operation when the outage struck and immediately went offline.

Reactors at two other nuclear plants were already stopped before the blackout and generators were also maintaining basic functions safely, the CSN said.