After the 2011 accident at the Fukushima Daiichi Nuclear Plant, Japan stopped all nuclear power production. Today, the country is restarting reactors, rewriting its energy law, and recasting nuclear power as a pillar of national and regional security rather than a liability to be managed. South Korea, its neighbor and rival in reactor exports, is moving along a parallel track, despite a change of government that briefly appeared to threaten the opposite. 

Together, the two countries are reshaping the nuclear politics of Northeast Asia and doing so increasingly in concert with Washington, as energy policy folds into the broader architecture of trilateral security cooperation.

The unresolved question behind both countries’ restarts is what to do with the spent fuel piling up at reactor sites. I explored one effort to solve that issue earlier this month in a visit to Japan’s Horonobe Underground Research Laboratory. There, hundreds of meters below the surface, engineers are testing whether the geological disposal methods every nuclear country is counting on actually hold up over time. Horonobe is a research site, however, not a repository, and Japan still has no designated location for permanent waste disposal. That gap has not slowed the political momentum above ground.

Before 2011, nuclear power supplied roughly 30 percent of Japan’s electricity. After the meltdown at Fukushima Daiichi, every commercial reactor in the country was taken offline, and public opinion turned sharply against nuclear power. What followed was over a decade of dependence on imported liquefied natural gas and coal, at a cost Japan, which imports about 90 percent of its primary energy, could not sustain indefinitely, especially as global gas markets tightened after Russia’s invasion of Ukraine in 2022.

The return to nuclear power began in earnest with Prime Minister Kishida Fumio’s Green Transformation (GX) policy, launched in 2022. The GX Basic Policy, approved by the Cabinet in February 2023, explicitly committed the government to restarting reactors already cleared by regulators and to building next-generation designs. 

The Seventh Strategic Energy Plan, approved by the Cabinet in February 2025 along with the accompanying GX2040 Vision, cemented nuclear power’s return as government policy rather than a temporary emergency measure. In early 2026, that policy produced its most tangible result yet: the restart of Unit 6 at Tokyo Electric Power Company’s Kashiwazaki-Kariwa plant, the world’s largest nuclear facility by capacity. Perhaps more pointedly, it’s run by the operator directly responsible for Fukushima Daiichi. Such a restart would have been politically unthinkable a decade ago.

The motivation is obvious. New public funding for data center and semiconductor facilities, worth over a trillion yen, requires that the electricity used come entirely from renewable or nuclear sources. That sends an unambiguous signal that Tokyo now considers nuclear energy to be indispensable to industrial policy, not just decarbonization.

South Korea is moving along a similar trajectory, even if it once looked to be headed in a different direction. President Lee Jae-myung took office in June 2025 after having campaigned on skepticism toward nuclear power. Lee argued that the technology was inherently dangerous and that the country’s reactor-heavy generation mix needed re-examining in favor of renewables. Seven months later, confronted with polling showing strong public support for nuclear energy and with surging electricity demand from an AI-driven data center boom, his government reversed course.

Seoul confirmed it would proceed with two new large reactors under the 11th Basic Plan for Electricity Supply and Demand, which targets a nuclear share of nearly 32 percent by 2030 and over 35 percent by 2038. In August 2026, Lee’s government went further, naming small modular reactors and nuclear fusion among seven technologies it wants to establish as future growth engines for the South Korean economy.

Korean reactor technology is already competitive with Western vendors. Around the same time Lee took office, Korea Hydro & Nuclear Power signed a contract to build two APR-1000 reactors in Dukovany, Czechia, a deal worth roughly $18.6 billion. The company has since signed cooperation agreements covering nuclear development in the Philippines, small-reactors studies with Singapore, and full fuel-cycle cooperation with France’s Orano. South Korea, which imports roughly 90 percent of its own energy needs and around 30 percent of its low-enriched uranium from Russia, is treating nuclear exports and domestic energy security as two sides of the same policy.

In July 2026, nuclear energy became an area of interest for trilateral cooperation between the United States and its two East Asian Allies. Japan, South Korea, and the United States agreed to jointly develop and export small modular reactors across the Indo-Pacific, explicitly extending their trilateral security partnership into commercial nuclear cooperation. 

With Russia and China as the only realistic nuclear vendors for many developing countries, Washington has a clear interest in ensuring that Japan and South Korea enter the race. It would benefit Washington if its two Northeast Asian allies, rather than Beijing or Moscow, become the default suppliers of reactor technology to the rest of the region. 

For Tokyo and Seoul, folding nuclear commerce into the trilateral security relationship also offers insurance against the awkward fact that both countries’ export ambitions still run through Washington’s non-proliferation rules and, in South Korea’s case, the 123 Agreement governing nuclear cooperation with the United States.

Japan’s own export potential remains constrained precisely because its domestic restart is still incomplete. Barely a handful of reactors are back online out of the roughly three dozen that existed before 2011. That leaves South Korea, for now, as the more credible reactor exporter of the two. 

What the restart narrative tends to leave out is that neither country has solved the problem I found researchers still puzzling over at Horonobe: Japan has no operating deep geological repository and no site formally designated to host one. That means the used fuel accumulating at reactor sites, including at the restarted Kashiwazaki-Kariwa plant, has nowhere permanent to go. South Korea faces the same problem, with domestic opposition to any proposed storage site consistently more intense than opposition to reactors themselves.

That gap matters for the trilateral SMR push, too. Exporting reactor technology to Southeast Asia or the Pacific Islands without a credible regional answer on waste management invites the same criticism that has dogged nuclear power at home in both countries: that the industry is better at building political consensus for new construction than for dealing with what construction leaves behind. 

For now, Northeast Asia’s nuclear revival is a story about energy security, industrial competitiveness, and alignment with Washington against Chinese and Russian reactor diplomacy. But underneath all these developments lurks the unanswered question of what to do with the increasing amounts of nuclear waste. That may end up mattering as much to the region’s energy future as any restart or export deal announced so far.