UPDATE 1-Hospitals brace for shortage of medical isotopes

* Shuttered reactor in Canada a big U.S. supplier

* Companies scrambling to make up for shortfall

* Many medical diagnostics tests "imperiled" (Adds comments from reactor official, paragraphs 17-18)

By Julie Steenhuysen

CHICAGO, May 21 (Reuters) - Makers of medical isotopes used in scores of diagnostic imaging tests are scrambling to find new suppliers after Canadian health officials shut down a nuclear reactor last week that produces a third of the world's supply.

Atomic Energy of Canada Ltd closed down its 50-year-old reactor at Chalk River, Ontario, after a small leak of heavy water, used as part of the nuclear reaction.

It expects the reactor to remain out of operation for more than a month, but some analysts think it could be many months.

Only five nuclear reactors in the world produce molybdenum-99, or Mo-99, which is used in diagnostic tests for cancer, heart disease and a host of other ills.

"It's going to cause a shortage and it's going to cause a price rise. Those are unavoidable negative consequences," Stephen Brozak, president of WBB Securities in New Jersey, said in a telephone interview.