* Companies have already set aside provisions
* Fund would ringfence money for decommissioning
* Germany to exit nuclear power by 2022 (Adds economy ministry comment)
BERLIN, Dec 17 (Reuters) - Germany plans to set up a fund to ringfence 17 billion ($21.2 billion) euros nuclear power firms have set aside to cover the long-term costs of decommissioning plants, Sueddeutsche Zeitung newspaper reported on Wednesday.
There has been concern in Germany that taxpayers could foot part of the bill to shut the plants because the money put aside by the firms to cover decommissioning costs could be lost in the event of a bankruptcy, or used for something else.
Citing an internal document from the economy and environment ministries, the German newspaper said the utility companies should pay 17 billion euros into a fund in instalments to meet the longer-term costs, while about 19 billion euros of their provisions would pay for dismantling and waste storage.
"We can't be sure that all nuclear power generators will survive the changes on the energy market and meet their long-term obligations in decommissioning," the ministry paper said.
The document said the 36 billion euros already set aside by Germany's four nuclear operators E.ON, RWE, EnBW and Sweden's Vattenfall was neither "bankruptcy proof" nor "committed to a particular cause".
Germany decided to close all its nuclear plants by 2022 after the Fukushima nuclear disaster in Japan three years ago.
A spokeswoman for the economy ministry said the paper was an internal document from October and one of several proposals. She said the government had looked at various options after media reports earlier this year said the utility companies were considering restructuring their provisions.
"One of the possible options was the creation of a fund, but this idea has not been pursued up to now in the economy ministry," she said.
The newspaper suggested that in case of insolvency, the costs for nuclear decommissioning should be met first. It said the sole objective of the fund would be to ensure all costs are met and would not absolve utilities of their responsibilities.
$1 = 0.8010 euros Reporting by Alexandra Hudson; editing by David Clarke