UPDATE 1-Cardinal Health warns clients of isotope shortage

* Shortage this week was "most significant" yet - company

* Cardinal canceled all July 7-8 bulk orders of isotope

* Company cut customers' usual orders by half

* Shares down 0.34 percent (Adds background, details, stock move)

By Julie Steenhuysen

CHICAGO, July 9 (Reuters) - Cardinal Health Inc (CAH.N) warned customers it was "critically short" of a medical isotope used in scores of nuclear imaging tests due to the shutdown of a reactor in Canada that makes a third of the world's medical isotope supply.

In a July 7 letter to customers obtained by Reuters, the company's Nuclear Pharmacy Services unit in New York warned that on July 7-8 it would experience "the most significant shortage we have seen to date."

Nick Tucker, a supervisor in the Bronx, New York unit confirmed the shortage, which stems from the May 17 shutdown of a nuclear reactor in Canada that produces the large isotope supply.

Only five aging nuclear reactors produce molybdenum nuclear isotopes used in medical imaging tests.