A Canadian nuclear reactor responsible for production of about a third of the world's medical isotopes has resumed operation after more than a year-long shutdown, officials said on Tuesday.

After low-power testing on the Chalk River reactor in Ontario proved successful, the 53-year-old facility returned to full power for the first time since a heavy water leak forced it offline in May 2009, Atomic Energy of Canada said.

The restart should provide some relief for Canadian isotope dealer MDS Nordion, which depends on Chalk River for much of its supplies. Shares of the company were up 19 Canadian cents at C$9.91 at midday on Tuesday on the Toronto Stock Exchange.

A medical isotope is a very small quantity of radioactive material used to perform imaging tests. Isotopes are mixed with solutions and injected into patients, where they give off energy that is read by a special camera.

Chalk River's shutdown caused headaches for doctors because isotopes have a short shelf life and cannot be inventoried. They are a critical tool in diagnosing and treating diseases and are used in some 50,000 procedures a day around the world.