Activity at the reactor in Petten was suspended following the discovery of air bubbles in the cooling system, its owners Nuclear Research and Consultancy Group (NRG) said in a statement on its website.
NRG produces technetium, a medical isotope, at the reactor in the north-west of the country. Medical isotopes are very small quantities of radioactive substance used for medical imaging and the treatment of diseases.
"The consequences of this unplanned stop will have an impact on the medical isotope market," the company said.
"NRG will not produce any medical isotopes in the coming month. We have informed our customers of the situation and are assisting in the search for alternative supply routes," the statement said.
Technetium is used in 80 percent of diagnoses that use radioactive technology.
Around seven million check-ups using technetium are made in Europe each year and eight million in the United States, NRG says.