The United States’ top nuclear regulator said on Friday that atomic energy users, including Japan, must figure out how to ultimately store radioactive waste.

Allison Macfarlane, head of the Nuclear Regulatory Commission, said in Tokyo that finding an underground repository remains a challenge despite a global consensus on the need for such a facility to deal with waste coming out of nuclear power plants.

Japan has no final waste repository, not even a potential site. The U.S. government’s plan for building a repository at Yucca Mountain in Nevada has been halted by strong local opposition due to safety concerns.

“In the nuclear community, we of course have to face the reality of the end product spent fuel,” Ms Macfarlane told reporters.

She urged countries that are contemplating or embarking on a nuclear power programme to formulate back-end plans at an early stage.

The new policy under Prime Minister Shinzo Abe’s pro-nuclear government is pushing to restart as many reactors as possible if deemed safe under the new, stricter safety standards that took effect this past summer. The new policy, whose draft was discussed on Friday by a government panel, is also expected to stick to Japan’s shaky fuel cycle programme despite international concerns about the country’s massive plutonium stockpile.

Japan is stuck with 44 tons of plutonium at home and overseas after unsuccessfully pushing to establish a fuel cycle, with its fast breeder reactor and a reprocessing plant never fully operated. Experts say Japan’s plutonium stockpile poses a nuclear security threat and raises questions over whether Japan plans to develop a nuclear weapon, which Tokyo denies.

Japan also has more than 14,000 tons of spent fuel in cooling pools at its 50 reactors, all of which are offline. Some pools are expected to be full in several years, and are expected to be moved to a dry cask facility just completed in northern Japan.

Ms Macfarlane said moving them to dry cask storage is a safer option but still only a temporary measure.

More In: Comment | Opinion