STOCKHOLM: Since the start of the nuclear era, highly radioactive waste has been crossing continents and oceans in search of a secure and final resting place.

Nearly all countries produce nuclear waste, some types of which can remain radioactive for thousands of years, but they cannot agree on the best way to store it.

At present highly radioactive waste is put into interim storage where it has to sit for 30-40 years for its radioactivity and heat production to decline. It is still hazardous and should be stored somewhere permanently.

In many countries it is unclear who will pay for the cost divided over hundreds, even hundreds of thousands of years. Utilities could end up with a bigger bill than expected.

Most high-level waste, the most dangerous kind, is spent fuel from the over 400 nuclear power reactors in more than 30 countries. The dismantling of nuclear weapons adds to the pile.

Even nuclear-free states produce waste from industry, hospitals providing radiation therapy, and research centres.

Experts say technology exists for secure underground deposits which could last millions of years. Most countries plan to seal the highly hazardous waste in containers and store it 500-1,000 metres underground.

Sceptics say it could be safe for decades or even centuries, but at some point it would be bound to leak or be attacked by terrorists.

“If there isn’t a responsible solution to deal with nuclear waste, it may be better to keep it above ground for a while longer when we are looking for technology that is safer,” said Martina Krueger, who works for the environmental organization Greenpeace in Sweden.

TO OPEN OR NOT?: Some politicians have demanded that the repositories are built so that future generations can open them and eliminate the waste with the help of new technology.