* Old reactor struggling along but unpredictable

* Ottawa to keep it alive until others come online

* Effort to design new reactor failed (Adds details and background)

By Randall Palmer

OTTAWA, June 10 (Reuters) - Canada will eventually stop making medical isotopes, Prime Minister Stephen Harper said on Wednesday amid a prolonged shutdown of an aging Canadian reactor that makes a third of the world's isotope supply.

The isotopes are valuable in medical imaging for diagnosing cancer, heart disease and other medical conditions, but Canada's nuclear reactor is 50 years old and efforts to engineer a replacement have failed.

"Eventually we anticipate Canada will be out of the business" of making isotopes," Harper told reporters.

The Chalk River reactor in eastern Ontario, run by government-owned Atomic Energy of Canada Ltd, was taken out of service in May because of a leak of a small amount of heavy water. AECL says it will be down for at least three months.

AECL and health sciences company MDS Inc (MDS.TO: Quote, Profile, Research) had poured large sums of money over 12 years into the Maple project to build replacement reactors but the government backed an AECL conclusion last year that the project should be terminated.