A leaked draft of a report into safety measures at Europe's nuclear power plants has revealed the need for investment of €10-€25bn.
The draft report for the European Commission was ordered following a safety review designed to ensure a disaster such as Japan's Fukushima cannot happen.
It also reveals that some safety measures agreed 20 to 30 years ago still have not been implemented in some countries.
Fukushima showed that it was possible for two natural disasters to strike at the same time and knock out the electrical supply system of a plant completely, so it could not be cooled down.
The draft says nuclear plant safety regimes vary greatly and the amount that needs to be spent to improve them is estimated at €10-€25bn across all the reactors.
The commission is expected to finalise the stress test report tomorrow and it will be debated by EU ministers later this month.
France's nuclear watchdog has already said the country, which relies on nuclear power for about 75% of its electricity, needs to invest billions of euros.
The stress tests found that four reactors, in two different countries, had less than one hour available to restore safety functions if electrical power was lost.
By contrast, four countries operate additional safety systems fully independent from the normal safety measures and located in areas well-protected against external events.
A fifth country is considering that option.
The main finding, the draft says, is that there are "continuing differences" between member states' safety regimes.
It also says provisions to ensure the independence of national regulators are "minimal".
The stress tests are a voluntary exercise to establish whether nuclear plants can withstand natural disasters, aircraft crashes and management failures, as well as whether adequate systems are in place to deal with power disruptions.
All 14 member states that operate nuclear plants took part, as did Lithuania, which is decommissioning its nuclear units.
From outside the 27-member bloc, Switzerland and Ukraine joined in the exercise.
The tests were meant to have been completed around the middle of the year, but countries were given extra time to assess more reactors.
The draft report says the stress tests are not a one-off exercise and will be followed up.
Existing legislation also needs to be enforced, it said.
The deadline for passing the existing nuclear safety directive into national law was July 2011.
The commission started infringement proceedings against 12 member states that missed it.
To date, two have still not complied but the report did not specify which ones.