UPDATE 2-Cardinal Health warns clients of isotope shortage

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

* Cardinal says sees no financial impact from shortage

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

* Company cut customers' usual orders by half

* Shares down 0.44 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."

Cardinal Health spokesman Troy Kirkpatrick told Reuters the company has experienced sporadic shortages stemming from the May 17 shutdown of a nuclear reactor in Canada, but he said the company sees no financial impact from the shortage.

"Obviously, with the Canadian reactor down, it does impact the supply. But because of the arrangements we've made with our suppliers, we're looking at near-normal levels," he said.

Only five aging nuclear reactors produce molybdenum-99, the most commonly used medical isotope. Molybdenum-99 has a shelf life of just 67 hours, making it impossible to stockpile.

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

Atomic Energy of Canada Ltd said on Wednesday it now expects its Chalk River nuclear reactor to be off line until late 2009, knocking out a key supplier of medical isotopes to North America. The United States has no domestic supply.

The shutdown has forced hospitals in the United States and Canada to ration supplies and delay medical tests.

Kirkpatrick said that aside from the shortages this week, the company expects supply problems on July 27, August 24 and 25 due to planned maintenance at two reactors in Europe.

Cardinal gets isotopes from two companies: privately held Lantheus Medical Imaging of North Billerica, Massachusetts, and Dublin-based Covidien Plc (COV.N), another major U.S. supplier.