It erupted in an electrical transformer, triggering the reactor protection system and "the third reactor was turned off," Ilona Zayets, a spokeswoman for the state nuclear agency Energoatom, told AFP on Tuesday.
"Radiation levels have not risen," she said, adding that no one was hurt as a result of the incident.
She said the causes are being investigated.
The level of seriousness of the incident on the international scale for emergencies at nuclear stations is zero, the nuclear agency said in a statement.
The third reactor remained switched off on Tuesday because the transformer needs to be replaced.
Ukraine was the scene of the world's worst nuclear disaster when one of the reactors at Chernobyl power station, which is about 100 kilometres (60 miles) north of Kiev, exploded in 1986 during testing.
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