A Devon dockyard said it has changed its procedures after it emerged a flask carrying radioactive fuel was not transported correctly.
Spent fuel rods from nuclear subs are removed at the dockyard
A valve plate on the flask was fixed the wrong way round as it was moved from HMS Talent, a nuclear submarine at Devonport Dockyard, in April 2005.
The fault was reported to the Nuclear Installation Inspectorate.
It concluded that no radioactivity had been released and that the safety risk was "minor".
But DML, which manages the dockyard, has been criticised for the mistake.
Spent fuel rods from the reactors which power the Royal Navy's nuclear submarine fleet are removed at Devonport Dockyard.
They are then transported by rail the 400 miles to the Sellafield reprocessing plant in Cumbria.
Independent nuclear expert John Large said the April incident was "a very serious potential event".
"Since these flasks travel through cities, high density and high populations you could affect quite a considerable number of members of the public," he said.
Nathan Argent from Greenpeace called for an end to nuclear waste being transported in this way.
"The fact the public are being put at risk in this manner is totally outrageous," he said.
"Greenpeace believes this is an unnecessary, irresponsible and unacceptable practice and should be stopped in its tracks today."
In a statement on Friday, DML said a range of measures were immediately put in place as a result of a comprehensive investigation into the incident.
It said: "DML regards safety in all aspects of its operations as being of paramount importance. "This is reflected in the highly detailed reports and remedial actions that resulted from these incidents."