The old control rods were replaced after similar cracks were found, according to the Oskarshamns-Tidningen newspaper.
Swedish radiation authorities are now uncertain if the reactor can be restarted before the problem is solved.
The control rods play an important in the plant’s safety, which is why reactor 3 was closed from October to December last year after the cracks were discovered.
The cause of the cracks is still unknown.
“It can be thermal fatigue, faulty material, or something else. Now it’s important that we find the definitive cause. Without that, we can’t eliminate the problem,” said Oskarshamns nuclear plant spokesperson Anders Österberg to the newspaper.