BANGKOK — China has built a land-based prototype nuclear reactor for a large surface warship, in the clearest sign yet Beijing is advancing toward producing its first nuclear-powered aircraft carrier, according to a new analysis of satellite imagery and Chinese government documents provided to The Associated Press.

China’s navy is already the world’s largest numerically and has been rapidly modernizing. Adding nuclear- powered carriers to its fleet would be a major step in realizing its ambitions for a true “blue-water” force capable of operating in seas far from China in a growing global challenge to the United States.

“Nuclear-powered carriers would place China in the exclusive ranks of first-class naval powers, a group currently limited to the United States and France,” said Tong Zhao, a senior fellow at the Carnegie Endowment for International Peace in Washington. “For China’s leadership, such a development would symbolize national prestige, fueling domestic nationalism and elevating the country’s global image as a leading power.”

Researchers at the Middlebury Institute of International Studies in California said they made the finding while investigating a mountain site outside the city of Leshan in the southwest Chinese province of Sichuan, where they suspected China was building a reactor to produce plutonium or tritium for weapons.

Instead, they concluded that China was building a prototype reactor for a large warship. The project at Leshan is dubbed the Longwei, or Dragon Might, Project and is referred to as the Nuclear Power Development Project in documents.

Neither China’s Defense Ministry nor Foreign Affairs Ministry responded to requests for comment.

There have long been rumors that China is planning to build a nuclear-powered aircraft carrier, but the research by the Middlebury team is the first to confirm that China is working on a nuclear-powered propulsion system for a carrier-sized surface warship.

“The reactor prototype at Leshan is the first solid evidence that China is, in fact, developing a nuclear-powered aircraft carrier,” said Jeffrey Lewis, a professor at Middlebury and one of the researchers on the project. “Operating a nuclear-powered aircraft carrier is an exclusive club, one that China looks set to join.”

Drawing on satellite images and public documents including project tenders, personnel files, environmental impact studies and even a citizen’s complaint about noisy construction and excessive dust, they concluded that a prototype reactor for naval propulsion was being built in the mountains of Mucheng township, about 70 miles southwest of Sichuan’s provincial capital, Chengdu.

The reactor, which procurement documents indicate will soon be operational, is housed in a new facility built at the site known as Base 909, which houses six other reactors that are operational, decommissioned or under construction, according to the analysis. The site is under the control of the Nuclear Power Institute of China, a subsidiary of the China National Nuclear Corp., tasked with reactor engineering research and testing.

Documents indicating that China’s 701 Institute, known as China Ship Research and Design Center, responsible for aircraft carrier development, procured reactor equipment “intended for installation on a large surface warship” under the Nuclear Power Development Project as well as the project’s “national defense designation” helped lead to the conclusion that the sizable reactor is a prototype for a next-generation aircraft carrier.

Satellite mages from 2020 to 2023 have shown the demolition of homes and the construction of water intake infrastructure connected to the reactor site. Contracts for steam generators and turbine pumps indicate that the project involves a pressurized water reactor with a secondary circuit — a profile consistent with naval propulsion reactors, the researchers say.

An environmental impact report calls the Longwei Project a “national defense-related construction project” that is classified “secret.”

“Unless China is developing nuclear-powered cruisers, which were pursued only by the United States and the Soviet Union during the Cold War, then the Nuclear Power Development Project most certainly refers to a nuclear-powered aircraft carrier development effort,” researchers wrote in a detailed 19-page report on their findings shared exclusively with AP.

Jamie Withorne, an analyst at the Oslo Nuclear Project who was not involved in the research and reviewed the findings, said Middlebury’s team made a “convincing argument.”

“From the identifying reports, co-location with other naval reactor facilities and correlating construction activity, I think it can be said that it is likely the Longwei Project is housed at Base 909, and it could potentially be located at the identified building,” she said.

The research does not, however, provide clues as to when a Chinese nuclear-powered carrier could be built and become operational, she said.

China’s first carrier, commissioned in 2012, was a repurposed Soviet ship, and its second was built in China but based on the Soviet design. Both ships — named the Liaoning and the Shandong — employ a so-called ski-jump type of launch method, with a ramp at the end of a short runway to help planes take off.

The Type 003 Fujian, launched in 2022, was the country’s third carrier and its first to be indigenously designed and built. It employs an electromagnetic-type launch system like those developed and used by the U.S. Navy. All three carriers are conventionally powered.

Sea trials hadn’t even started for the Fujian in March when Yuan Huazhi, political commissar for China’s People’s Liberation Army Navy, confirmed the construction of a fourth carrier. Asked if it would be nuclear-powered, he said at the time that would “soon be announced,” but so far it has not been.

There has been speculation that China may begin producing two new carriers at once — one Type 003 like the Fujian and one nuclear-powered Type 004 — something that it has not attempted before but that its shipyards have the capacity to do.

Matthew Funaiole, senior fellow at the Center for Strategic and International Studies’ China Power Project, said he doubts that China’s next carrier will be nuclear-powered. Instead, he said, he would expect the People’s Liberation Army Navy’s fourth carrier to focus on optimizing the design of the Fujian carrier with “incremental improvements.”