The Petten reactor in northern Holland, which produces a third of the world's medical isotopes, was shut down for safety reasons at the end of August.
Last month its owner, the Nuclear Research and Consultancy Group (NRG), said it hoped the reactor would be back in operation by late November.
But on Tuesday NRG spokesman Leontien Zuurbier said the repair work was "more complex" than originally thought and gave a new re-opening date of February 16.
Activity at the reactor in Petten was suspended following the discovery of air bubbles in the cooling system in August.
The closure has led to a shortage of technetium, an isotope made of small quantities of radioactive substance that is used for medical imaging and the treatment of diseases.
Technetium is used in 80 percent of diagnoses using radioactive technology.
Around seven million check-ups using technetium are made in Europe each year and eight million in the United States, according to NRG.
The four other nuclear reactors that produce isotopes for the pharmaceutical industry, in Canada, France, Belgium and South Africa, have filled the gap left by the suspension of Petten reactor, Zuurbier said.