The incident occurred at 11.30 am on Monday at the Paluel nuclear installation in northern France for reasons not immediately known, the operator of the plant Electricité de France (EDF) said in a statement.
The defect in the emergency circuit automatically shut down reactor number four, the power company said.
It marked at least the second time in less than two months that the number four reactor was shut down, following a previous outage reported on October 9.
Technicians at the plant were looking at the possible causes of the problem during the middle of the day before restarting the reactor at a time that was not disclosed.
Later in the day staff detected a defect in a pump for a secondary cooling system in a non-nuclear part of the plant.
This led to a slowdown in the production of electricity from reactor number two.
The Paluel nuclear power plant, which entered into service between 1984 and 1986 on the Normany coast, near Dieppe, has a total of four reactors with a 1,300-megawatt production capacity.