* Swiss-led plan sought post-Fukushima improvements
* Diplomats say amendment may have hampered safety reviews
* U.S. led resistance to changing treaty
* Diplomats adopt joint declaration, no amendment
By Shadia Nasralla
VIENNA, Feb 9 (Reuters) - The United States has derailed a proposal to toughen nuclear safety standards by amending a global atomic treaty, diplomats said, with opponents of the move arguing it would get mired in lengthy parliamentary ratification.
Months of wrangling about the future of the 77-nation Convention on Nuclear Safety (CNS) culminated at a Vienna meeting diplomats feared could expose divisions over safety standards four years after the Fukushima disaster in Japan.
Switzerland had put forward a proposal to amend the CNS, arguing stricter standards could help avoid a repeat of Fukushima, where an earthquake and tsunami sparked triple nuclear meltdowns, forced more than 160,000 people to flee nearby towns and contaminated water, food and air.
But Russia and the United States opposed an amendment of the CNS, diplomats told Reuters.
The gathering approved a Vienna Declaration on Nuclear Safety on Monday endorsing the main ideas of the proposed amendment, such as refitting old nuclear plants and minimising off-site contamination in case of accidents.
But it does not create a piece of international law.
Some opponents of amendments argued the lengthy process of pushing them through national parliaments would run counter to the aim of increasing safety standards quickly.
Changing the convention itself to be more punishing could have been off-putting for countries already reluctant to fully submit themselves to peer reviews, some diplomats said.
"The ratification process for an amendment would have distracted the focus of the contracting parties to pursue full participation," one senior U.S. official said.
Critics of an amendment say U.S. industry has already spent billions of dollars on improving nuclear safety since Fukushima.
"Opposition was mainly politically motivated as it would have been hardly possible for some countries to domestically ratify a changed convention," Hans Wanner, head of the Swiss nuclear watchdog, said in a statement posted on the internet.
"Many countries also fear massive cost increases by committing to refit older facilities," he said.
Rafael Grossi, Argentina's ambassador in Vienna and head of the conference, said that what might look like a weaker document to some is a practicable solution to others. "We tried to concentrate on what was achievable now," he told reporters. (Editing by Mark Heinrich)