"Repair work will last in any case until the end of November, which means that the November irradiation cycle is cancelled," the Nuclear Research and Consultancy Group (NRG) said in a statement on Wednesday.
"The consequences will be felt on the medical isotopes market," said NRG, which produces a third of the world's medical isotopes at the reactor in Petten in the country's northwest.
Medical isotopes are used for medical imaging and the treatment of diseases. They have a short life span and cannot be stored for long.
European hospitals have been forced to reduce the number of examinations being carried out because of a shortage of one widely-used isotope produced at Pettin, technetium.
Officials said they had been unable to run the reactor in August as planned because of gas bubbles found in a cooling system, which have been blamed on corrosion.
NRG said it is "in touch with clients on progress (at the reactor) almost daily and is helping them in their search for possible alternative providers."
It has also been in regular contact with other reactors about adapting their production to the market's needs, NRG said.
Some seven million examinations are carried out each year in Europe with technetium, while eight million are performed annually in the United States, according to NRG.
Only four other reactors worldwide -- in Canada, France, Belgium and South Africa -- produce isotopes for the pharmaceutical industry.