Umbrella organisations representing coastal communities and councils with anti-nuclear policies have spoken out about forthcoming sea trials being held to establish whether to ship 26 tonnes of the so-called 'Dounreay exotics' - highly radioactive fuels - to the Cumbrian facility.
Other nuclear material currently goes by rail.
The Western Isles Council has already written to the Nuclear Decommissioning Authority, amid concerns a maritime option is being investigated when there is now only one emergency coastguard tug providing cover for the whole maritime area around Orkney, Shetland and the Inner and Outer Hebrides.
It is thought any ship to Sellafield would sail through the Pentland Firth, down through the Minch and past the Argyll islands.
Councillor Mark Hackett, chairman of the Nuclear Free Local Authorities, said: "Rail transports of such materials are bad enough, but at least there is the possibility of reasonably prompt emergency response with such an incident."
Norman McDonald, of Western Isles Council and who is president of Kimo International (The European Local Authorities Environmental Organisation) said the possibility of a fire, collision and subsequent radiation leak would have potentially devastating and harmful effects on one of the most sensitive parts of the north east Atlantic.
A spokeswoman for the Nuclear Decommissioning Authority said: "The date of the trial and details of the vessel can't be disclosed for security purposes. However, the transportation of nuclear materials is a practice that has been in place for a number of decades, without any major incidents."