TOKYO — The United States’ top nuclear regulator said 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.

Her comment came as Japan finalizes a new energy policy that reverses a phase-out plan set by the previous government after the March 2011 Fukushima disaster.