France's nuclear plants are "highly vulnerable" to terror attacks, according to a new report by Greenpeace.

The spent-fuel pools of France's nuclear operator EDF, which contain one to three nuclear cores on average, have not been designed to withstand external threats, according to seven international experts who contributed to the report.

They noted that most of the nuclear plants were built before terrorists started posing a serious threat to European security.

"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," the report said.

An attack causing a loss of cooling water in the pools could spark a fire that spreads 250km, according to Yannick Rousselet, who leads Greenpeace's nuclear campaign.

"EDF must address this issue and reinforce its spent-fuel pools," he said.

Many of France's nuclear plants are built on its borders and could put citizens in Belgium, Germany, Switzerland and Luxembourg at risk, the campaign group warned.

Greenpeace estimates that it would cost EDF one billion euros per reactor to upgrade the pools.

EDF, which operates 58 nuclear plants in France, has denied that its spent-fuel pools are unsafe.

"Our nuclear fleet is safe and EDF, in close cooperation with the authorities, permanently evaluates the risk of terror attacks," an EDF spokeswoman said, as quoted by Reuters.

France's state secretary for the environment, Sébastien Lecornu, said the government would study the report and review recommendations.

"France has the most robust nuclear safety and security measures in the world," he told RTL radio.