UPDATE 1-Canada still inspecting shuttered isotope reactor

* Supplies a third of medical isotopes

* Repair needs still undetermined

* Extended shutdown not ruled out

* Supply shortages feared (Updated throughout)

TORONTO, May 21 (Reuters) - Canadian authorities said on Thursday it is too early to predict how long an aging nuclear reactor that produces much of the world's medical isotope supply will be idled, but said they could not rule out an extended shutdown.

The 50-year-old Chalk River nuclear reactor in eastern Ontario, which supplies a third of the world's medical isotopes, was taken offline last week after Atomic Energy of Canada Ltd discovered a small leak of heavy water, used in the nuclear reaction process.

Officials downplayed a media report on Thursday that the facility was likely to be shut for at least a year, but later said that could not be ruled out until they determine what kind of repairs are required.

The only official estimate is that the plant will down for a month or more.

"As to what it might be beyond that, that really depends on what we find in our inspections," Bill Pilkington, AECL's vice-president in charge of the Chalk River plant, told reporters.