It says the instant shutdown occured after a performance error was noted during a weekly test in the turbine system of the reactor's steam flow.
However the Finnish Radiation & Nuclear Safety Authority (STUK) says that plant employees performed the shutdown by hand.
The cause of the malfunction remains unclear.
Fortum says that the incident was not an accident and did not pose "any threat to people, the power plant or the environment" and that the second unit at Loviisa continues its normal operations.
Fortum spokeswoman Pauliina Vuosio says that similar 'instant shutdowns' occur from time to time because reactor security systems are so precise. The last such incident took place in March, she says.
STUK says the closure was carried out manually by staff members after they noticed an error during a performance test.