* Tests cover natural disasters, aircraft crashes, human error
* All 14 member states operating plants participating (Adds detail throughout)
By Barbara Lewis
BRUSSELS, March 6 European Union safety tests on nuclear plants should be completed by around the middle of the year as time is needed to ensure they are thorough enough, EU Energy Commissioner Guenther Oettinger said.
In comments ahead of the anniversary of the Fukushima nuclear disaster on March 11, Oettinger said stress tests would be completed "not later than summer".
"Thoroughness is more important than delivering quick results. Our multinational teams still need some time to finish the tests. Not later than summer we will publish the results," he said in a statement.
A June council meeting of energy ministers is expected to receive a report on the stress tests, as has long been planned. The full process might not be completed by then.
Oettinger said the tests were strict and objective and sought to establish whether nuclear plants could withstand natural disasters, aircraft crashes, management failures and what systems were in place to deal with power disruptions.
The EU embarked on the tests among the 14 member states which operate nuclear plants as a first stage of its response to the Fukushima tragedy.
Green groups and politicians have questioned whether the EU is doing enough and Germany last year said it would phase out all its atomic plants by 2022, while Italy voted to ban nuclear power for decades.
Oettinger has said the stress tests offer the potential to reassure on nuclear safety well into the future and that cooperation across the bloc on a sensitive issue was significant progress.
The tests are expected to be followed up later this year by a Commission proposal, which could seek to reform EU nuclear safety legislation, and officials have been debating questions, such as whether regulatory authorities are sufficiently independent.
Under the Lisbon Treaty, which defines the power of the EU governing bodies, nuclear power comes under the remit of member states.
As a result, the stress tests are voluntary, but all 14 member states, which together operate the bloc's 143 nuclear plants, as well as Lithuania, which is decommissioning its nuclear units, are taking part.
Outside the 27-member bloc, Switzerland and Ukraine have also joined in.
"In the past, it would have been unthinkable that multinational expert teams would be granted access to nuclear power plants," Oettinger said in Tuesday's comments.
"Nuclear safety was and is primarily a national competence. The fact that multinational teams conduct the tests increases objectivity and adds a true European dimension to the tests." (Reporting by Barbara Lewis; editing by Rex Merrifield and Keiron Henderson)