A senior adviser to the Biden administration and other experts say that deepening cooperation between China and Russia could overturn decades of stable international nuclear arms control.

Pranay Vaddi, the U.S. National Security Council’s senior director for arms control, said that China and Russia have been deepening cooperation on key technologies for nuclear weapons while strengthening ties with Iran, according to Bloomberg.

“We’re entering a different period,” Vaddi said. He told Bloomberg that during the Cold War, the U.S. and the Soviet Union agreed to maintain a nuclear balance and limit certain types of weapons. The Cold War lasted from 1947 through 1991, and since then more countries have been developing the technologies and materials needed for weapons of mass destruction.

According to a report earlier this month in the South China Morning Post, citing the Chinese site of Russian state media, Sputnik News, a subsidiary of Russia’s state atomic energy corporation, Rosatom, will be allowed to export highly enriched uranium to a power plant in southeast China over the next three years. The CFR-600 power plant in Xiapu, Fujian province has two fast neutron reactors, each capable of producing 600 megawatts of power.

The uranium-235 that Russia will supply to China for the project is mainly used to fuel nuclear reactors. However, it can also be used for manufacturing nuclear weapons.

Tian Li, vice president of the nuclear power branch of the China Electric Power Promotion Council, was quoted by the South China Morning Post as saying the Fujian plant would not be used for military purposes.

Military experts believe that Russia’s aid may help China expand its nuclear arsenal faster.

Anthony Cordesman, emeritus chair in Strategy at the Center for Strategic and International Studies, told VOA Mandarin in an interview, “We now live in a nuclear world once again. And it is not something that seems likely to go away over the next 10 to 20 years.”

Most people alive today “didn’t grow up at a time when basically the United States and Russia seemed to be on the edge of an actual possible nuclear conflict. The last real test of whether nuclear weapons were likely to be used was the Cuban Missile Crisis,” he said, referring to the 1962 face-off over Moscow’s placement of nuclear missile sites in Cuba.

When Chinese President Xi Jinping visited Russia in March, President Vladimir Putin announced that Rosatom and the China Atomic Energy Agency had signed a long-term cooperation project contract to develop fast neutron reactors and closed nuclear fuel cycle systems.

Cordesman said while the agreement does not necessarily mean the two countries will cooperate to develop nuclear weapons or delivery systems, “it certainly does empower China” at a time when it is estimated to be wanting to expand its nuclear weapons arsenal from 250 to more than 1,200. He said China also has three new missile sites under construction and is producing nuclear submarines that can launch nuclear missiles.

Alex Wellerstein, an historian of science and nuclear technology and professor at the Stevens Institute of Technology, told VOA Mandarin in an email, “The International Panel on Fissile Materials estimates that China already has some 14 metric tons of HEU (highly enriched uranium) and around three metric tons of separated plutonium (see here for a full report from a few years ago). It is enough to build as many warheads as they could ever desire, even if those estimates were significantly off.”

Patty-Jane Geller, a former senior policy analyst for Nuclear Deterrence and Missile Defense at the Heritage Foundation, published an analysis in March saying nuclear cooperation between China and Russia goes back to the 1950s when the Soviet Union, consisting of Russia and 14 surrounding countries, provided material and technical assistance to China’s nuclear program. Rising tensions between the two countries during the Cold War led to a halt of nuclear aid, but their cooperation resumed in the 21st century, years after the Soviet Union fell in 1991.

“This development means that the more fuel Russia provides, the more plutonium China can produce. And the more plutonium China can produce, the more nuclear weapons it can build,” she said.

Three senior GOP lawmakers said in a March letter to U.S. national security adviser Jake Sullivan that cooperation between China and Russia is a “direct threat to U.S. security” and called on the Biden administration to “use all tools at its disposal” to stop the “dangerous” cooperation.

A Pentagon report released in 2020 predicted that China would “at least double” the size of its nuclear warhead stockpile over the next decade. But by the end of 2022, it had already done so. Geller predicted that with help from Russia, China might be able to accelerate this buildup even further.

Some military experts worry that Xi might use China’s nuclear weapons in a conflict related to Taiwan, a self-governing island that China considers its own territory. The U.S. is expected to announce $500 million in direct military assistance for Taiwan, money that would come from tapping into a congressional authorization in the 2023 budget allocating $1 billion for Taiwan, according to The Hill.

James Stavridis, a retired U.S. Navy admiral and former commander with NATO Supreme Allied Commander Europe, told The Washington Post that if the U.S. and China enter a conventional war, there’s a high risk of escalation to a nuclear war.

“Two great powers who face each other in combat are unlikely to avoid using tactical nuclear weapons, at least at sea,” Stavridis said. “Once that threshold is crossed, it is but a short step to a much broader nuclear conflict.”