TOKYO -- The decision Jan. 16 to automatically extend a nuclear agreement with the U.S. came as a relief to a Japanese government worried about the prospect of renegotiating the basis for a cornerstone of its energy policy. But friction remains over a massive store of plutonium that highlights the problems with the nation's ambitious nuclear energy plans.

The nuclear fuel cycle pursued by Japan's government and power companies centers on recovering uranium and plutonium from spent fuel for reuse in reactors. This is made possible by the unique agreement with the U.S. that lets Japan make plutonium. The radioactive element can be used in nuclear weapons, so its production is generally tightly restricted.

"The agreement forms part of the foundation of Japan's nuclear power activities," said Hiroshige Seko, minister of economy, trade and industry, in comments to reporters Friday. "It's important from the standpoint of the Japan-U.S. relationship."

America began sharing its advanced atomic energy technology with other nations in the 1950s, aiming to promote its peaceful use. Washington remains hugely influential in setting ground rules for military applications of nuclear material, including with regard to reprocessing. Countries including South Korea have sought special arrangements like Japan's.

The lack of fuss over the renewal of the agreement, which had been due to expire this coming July, has masked concerns expressed behind the scenes. A Japanese official visiting Washington in December was asked by a U.S. nuclear policymaker about Japan's oversight of its plutonium stockpile.

Japan has amassed roughly 47 tons of plutonium stored inside and outside the country -- enough for some 6,000 nuclear warheads. With the nation's nuclear power plants gradually taken offline after the March 2011 Fukushima Daiichi disaster, and progress on restarting them sluggish, Japan has been left with no real way to whittle down a pile drawing international scrutiny.

Washington ultimately did not ask to change the nuclear agreement, which after the expiration date can be terminated by either side with six months' notice. Given the tense regional security situation, including North Korea's missile advances, "Japan and the U.S. apparently didn't want the world to see friction between them over nuclear power," said a Japanese government insider in contact with Washington.

Tokyo's relief at the lack of American demands is dampened by the awareness that the deal could be scrapped at any time. "It's more unstable than before," an industry ministry official acknowledged.

The best-case scenario for Japan would have been securing an agreement that set a new expiration date. But any such change would have had to go through the U.S. Congress, where lawmakers supporting nuclear nonproliferation might not have welcomed giving Japan -- which already has no prospect of using up its existing supply -- carte blanche to keep reprocessing. This risk is likely why Washington opted for automatic extension of the existing agreement.

The precursor to the current deal, signed in 1955, let Japan use American technology to kick-start its own atomic energy industry. A new agreement in 1968 permitted reprocessing of spent fuel with U.S. consent. A 1988 revision gave blanket permission for reprocessing for peaceful applications.

But the nuclear fuel cycle policy this enabled has stalled amid chronic problems at key facilities. The Japanese government decided in 2016 to scrap the Monju plutonium-fueled experimental fast breeder reactor. And a reprocessing facility in northern Japan that would be critical to producing plutonium fuel usable by conventional reactors has faced repeated delays that have pushed back the completion date from 1997 to 2021.

Reducing Japan's plutonium stockpile will be vital to assuaging international concerns. Seko asserted that plutonium consumption will pick up again as the Nuclear Regulation Authority clears more reactors to restart.

But this may not work as well as Tokyo hopes. Just five reactors have met the stricter safety standards imposed in the wake of the Fukushima Daiichi meltdowns, and not all of these use plutonium.

The nuclear watchdog said Jan. 16 that it will devise new guidelines to better adhere to the government's principle of not possessing plutonium without a specific purpose. Critics of Japan's plutonium production will likely not be satisfied without a convincing, reality-based plan to deal with the issue.