A terror attack on a train carrying nuclear waste through Bangor could result in thousands of deaths, claim environment campaigners Greenpeace.
Their report also looked at potential accidents, warning of evacuations and that people as far away as Flint would have to find shelter.
But Direct Rail Service (DRS), which handles nuclear waste transportation, said "robust" measures were in place.
It defended its safety record and said security was the number one priority.
The Greenpeace report was compiled by nuclear expert John Large, who examined potential accidents and acts of terrorism which could severely damage a nuclear 'transportation flask', causing the release of radioactivity.
His review concluded that transporting the flasks containing spent nuclear fuel "provide no extraordinary safeguard against terrorist attack".
He said that the flasks would be at their weakest if caught in "the high and sustained temperatures involved in a tunnel fire".
As a result a list of six UK railway tunnels, including Bangor, were identified as potential problem areas.
"I was very wary of undertaking this review because of the sensitivity of the terrorist threat at this time," said Mr Large.
"However my reservations were quickly surpassed at an early stage of my research when it became obvious just how vulnerable these spent fuel flasks are," he added.
DRS, the company responsible for shipments of nuclear waste from Wylfa power station, said safety and security were a "number one priority".
"The safety record of moving used nuclear fuel by rail is exemplary - this material has been transported in this way since 1962, travelling over eight million miles without any incident involving the release of radioactive material," said a spokesman.
DRS worked within stringent safety rules, which were regulated independently added the spokesman.
The 'flasks' used to transport used nuclear fuel is constructed from forged steel, more than 30cm thick, with each flask weighing more than 50 tonnes.
"The flask-testing criteria - including those simulating a fully engulfing fire and a flask dropping from a bridge or similar structure - are designed to simulate very severe accident and incidents," said the DRS spokesman.
The regulator, the Office for Civil Nuclear Security (OCNS), said the nuclear generators and DRS made every effort to "randomise times and days of the week for movements of nuclear material by rail".
A spokesman said security was stringent and under constant review.
He said: "OCNS is satisfied with the thorough measures that have been taken to prevent the theft or sabotage of nuclear material in transit. It is not Government policy to disclose details of security measures or nuclear fuel movements, which may be of potential use to terrorists".