0123 GMT December 18, 2018
France has the second-biggest fleet of nuclear plants in the world, after the United States, with 58 reactors providing 75 percent of the country's electricity. Belgium has two, AFP reported.
The seven experts from France, Germany, Britain and the US – specialists in nuclear safety, proliferation, economics and radiation – looked at various attack scenarios involving plants in both countries, some of which date back over three decades.
Noting the "very high level of threat to security in France and Europe" they said nuclear power plants were "without a doubt, a risk".
For security reasons, anti-nuclear group Greenpeace did not publish the full version of the report, which it said it would share with authorities in France, Belgium and neighboring countries.
In a public summary, the experts noted that most of France's reactors were built before the rise of modern-day threats from non-state terror groups such as Daesh or Al-Qaeda.
"For these historical reasons, reinforcement against heavy attacks on civil engineering works and protection systems for nuclear safety was not ?- or only marginally -? incorporated into the design of these facilities," they said.
The dangers were "even more pronounced in the case of spent fuel pools", which were not encased in confinement buildings like reactors, despite containing hundreds of tons of highly radioactive fuel.
'End the silence'
France has a total of 63 pools containing highly radioactive fuel rods that have been removed from reactors after their use.
The report said an attack on such a structure "could maximize the accident scenario in which fuel is uncovered, heats to the point of fusion and a significant fraction of its radioactivity is released," into the building and – given the building's lack of containment –into the wider environment.
The safety of France and Belgium's nuclear plants has been in the spotlight for years.
Belgian police investigating the November 2015 Paris terror attacks found 10 hours of video of the comings and goings of a senior Belgian nuclear official.
A year previously, the Doel 4 reactor, close to the Belgian port city of Antwerp, was shut down urgently after a leak in the turbine hall, caused by tampering.
In France, several mystery drone overflights were reported at various nuclear plants in 2014. No group ever claimed responsibility.
The head of Greenpeace France's anti-nuclear campaign, Yannick Rousselet, stressed the need to "end the silence on the risks that hang over nuclear plants.
"(France's state electricity supplier) EDF... cannot ignore the situation. It must take the security problem in hand by carrying out the work necessary to secure spent fuel pools," he said.
EDF, in a statement, assured its nuclear plants were "safe, properly monitored and very well protected" and that it was constantly evaluating their resistance to criminal acts or terrorism.