For the first time, highly radioactive waste was shipped from Sellafield, UK to Japan.
It has been loaded onto a ship specifically designed to carry nuclear waste that will sail for Japan later. The waste is being transported inside a specially constructed container
The waste is a by-product of nuclear fuel spent by Japanese reactors that was sent to the UK for reprocessing during the 1980s and 1990s.
Some campaigners have criticized the shipments, saying they are dangerous.
"It is highly irresponsible for the industry to still be sending this kind of material across the world," said anti-nuclear campaigner Martin Forward.
But Sellafield spokesman Mike Johnson said the site was simply fulfilling its contractual obligations, and government policy, in transporting the waste safely.
Only two countries, the UK and France, have been engaged in the reprocessing of spent nuclear fuel from power stations.
The process involves extracting reusable uranium and plutonium from the fuel, leaving behind a liquid waste. Uranium and plutonium is extracted from the mix, and this can be reused to make new nuclear fuel, which is then returned to nuclear power plants.
The remainder is then waste in a liquid form.
Japan has storage facilities in place already, having received 12 shipments of high level nuclear waste from France in recent years.
Over the next decade, high-level waste will also be returned to European countries.
Returning waste to countries that have benefited from nuclear energy production is in line with both global agreements and UK government policy.
For Sellafield, it is also a source of revenue, as the company - which runs the site under a contract with the Nuclear Decommissioning Authority - is being paid by the government for meeting targets set as part of a complex contract that involves the decommissioning, reprocessing and waste management at the site.
"This is a performance-based milestone," Mike Johnson, executive director of Sellafield's waste and effluent disposition, told BBC News. "This is one of the ways we earn our fees."
Sellafield declined to reveal the payment details.
In about 10 years, high-level nuclear waste, equivalent to the waste created by the nuclear fuel received, will have been returned to all Sellafield's foreign customers.