A Canadian nuclear reactor that produced about a third of the world's supply of medical isotopes has stopped operation for three months because of a heavy water leak, authorities said.

The reactor produced isotopes, which are radioactive substances used to help doctors diagnose and treat diseases by entering directly into affected cells in the body, rather than by external radiation.

"Based on our preliminary findings to date, we now believe that the NRU (National Research Universal) reactor will be out of service for at least three months," said senior vice president Bill Pilkington of Atomic Energy of Canada Ltd (AECL).

"Sophisticated diagnostic procedures are required to determine the exact nature and extent of the repairs before returning the NRU reactor safely to service," he said.

The Chalk River reactor, located about 200 kilometers west of Ottawa, produces between 30 and 40 percent of the world's supply of medical isotopes used mainly in the diagnosis and treatment of cancer, heart disease and bone ailments.

As concerns mounted over an isotope shortage while the Canadian reactor undergoes repairs, Canada appealed for other isotope producers to step in to help.

"I continue to lead productive discussions with my counterparts in other isotope-producing countries that have some ability to assist with this supply issue," said Minister of Natural Resources Lisa Raitt.

"These countries all have different constraints and capacity that we are working through, but we will continue using all resources available to us to manage this shortfall."