Radioactive waste is being flown to the US from a Scottish nuclear power plant.
The first in a series of flights which see about 700kgs of uranium from Dounreay flown across the Atlantic left John o' Groats Airport at the weekend.
In return, American uranium will be sent to the European atomic agency and converted into medical isotopes used in cancer treatment.
The exchange is part of the decades-long process of decommissioning the facility, which shut in the 1990s.
A spokesman for Dounreay Restoration Ltd, which is handling the decommissioning of the 65-year-old facility in Caithness, said: "All the nuclear material at the site is being removed. That process started in 2001 and is expected to take a number of years to complete.
"Our priority at all times is to comply with the regulations to ensure the safety and security of nuclear material."
Veronika Tudhope, assistant coordinator for the Scottish Campaign for Nuclear Disarmament, said moving the waste goes against the Scottish Government's own rules.
She said: "This is dangerous stuff and it shouldn't be flying - we've already got nuclear warheads on our roads and railways carrying waste around Scotland.
"The Scottish Government's policy is to leave nuclear waste where it is and to store it above ground so you can keep an eye on it. That's not happening in this case."
The vast majority of the nuclear waste left at Dounreay is being transported by road to places including Sellafield in Cumbria, where it is being stored or processed.
Earlier this year, John o' Groats Airport was expanded using £8m of funding from the Nuclear Decommissioning Authority to allow it to handle larger aircraft.
The airport, which is about 30 miles from Dounreay, was used in the 1990s to receive radioactive waste from abroad which was processed in Caithness.