UPDATE 6-Russia, U.S. sign civilian nuclear pact

(Adds U.S. State Department spokesman paragraphs 12-13)

By Guy Faulconbridge

MOSCOW, May 6 (Reuters) - Russia and the United States signed a pact on Tuesday allowing the world's two biggest atomic powers to boost their nuclear trade and work on new ways to prevent the proliferation of nuclear weapons.

The civilian deal will open up the booming U.S. nuclear market and Russia's vast uranium fields to firms from both countries by removing Cold War restrictions that prevented bilateral trade potentially worth billions of dollars.

U.S. ambassador to Russia, William Burns, signed the deal with the head of Russia's state nuclear corporation, Sergei Kiriyenko, on the last full day of Vladimir Putin's presidency.

"The United States and Russia were once nuclear rivals -- we are today nuclear partners," said Burns.

At the 2006 Group of Eight summit in St Petersburg, President George W. Bush and Putin ordered ministers to reach a deal but it has faced opposition from some U.S. congressmen because of Russia's nuclear cooperation with Iran.

A 123 agreement, so-called because it falls under section 123 of the U.S. Atomic Energy Act, is required before countries can cooperate on nuclear materials.

It is critical to the Global Nuclear Energy Partnership, or GNEP, which the United States and Russia have discussed for more than a year as a way to expand peaceful nuclear energy development and mitigate proliferation risks.

"What this agreement allows us to do is to implement some very creative ideas that both Russia and the United States have put forward to deal with the growing challenge of proliferation of nuclear weapons," Burns said.

He said the deal would allow Washington and Moscow to move forward on proposals for international nuclear fuel centres, which would sell developing countries access to nuclear energy but remove the need for their own enrichment programmes.

NUCLEAR GIANTS

Russia and the United States control the largest arsenals of nuclear weapons in the world and both have ambitious plans to build hundreds of new reactors for power production.

Some U.S. politicians have said nuclear cooperation with Russia should be shunned because Moscow is helping Iran build an atomic power station, but the Bush administration is keen to have the pact approved this year.

State Department spokesman Sean McCormack said in Washington that now that the deal has been signed, it would be sent to Congress for lawmakers to review "in due course".