The Ontario nuclear reactor that supplies 30 per cent of the world's medical isotopes will be shut down for at least three months to repair, the facility's operator said Wednesday.

The 52-year-old National Research Universal reactor at Chalk River, Ont., was shut down May 15 after inspectors detected a heavy-water leak inside the facility.

The shutdown left doctors and medical researchers scrambling for a scarce supply from the world's four other isotope-producing reactors.

Atomic Energy of Canada Ltd., which runs the reactor, said in a release that an internal inspection has identified the source of the leak at the base of the reactor vessel where there is corrosion on the outside wall.

The statement said the inspection also showed additional corrosion points on the outer wall, but AECL insisted there was no risk to workers, the public, the environment or nuclear safety as a result of the incident.

"Based on our preliminary findings to date, we now believe that the NRU reactor will be out of service for at least three months," said Bill Pilkington, AECL senior vice-president and chief nuclear officer.

Used in treatment, imaging

Medical isotopes — tiny radioactive particles that can be injected into the body — have become the standard treatment for some cancers and have also brought medical imaging to new levels.

The federal government has faced repeated questions in Parliament over its handling of the crisis, as well as a previous shutdown in 2007.

Natural Resources Minister Lisa Raitt has said Canada is taking action by working with international partners in Europe and Australia to stem the isotope shortage.

The two-month closure in 2007 followed the Canadian Nuclear Safety Commission's discovery that the reactor had been operating for 17 months without two cooling pumps hooked up to an emergency backup power system capable of withstanding a severe earthquake.

Facing an isotope shortage, Parliament voted to overrule the commission's safety objections and the reactor was restarted in December 2007 with only one pump connected to the emergency power supply.

In January 2008, Gary Lunn, natural resources minister at the time, fired the head of the Canadian Nuclear Safety Commission, Linda Keen, for what Lunn said was the government's loss of confidence in her leadership over the way she handled the shutdown.

Prime Minister Stephen Harper shifted Lunn out of the natural resources portfolio after the fall federal election.