Du Guanghua, a researcher involved in the tantalum beam project at the academy’s Institute of Modern Physics in Lanzhou, confirmed that the project had military applications but declined to give details.

Du said the creation of a high-quality, high-output tantalum beam had long been a challenge because the metal’s unusually high melting point of nearly 3,000 degrees Celsius meant it was difficult to isolate the element and generate a particle stream big enough for experimental use.

Tantalum was also nearly as heavy as gold and highly advanced magnetic technology was needed to propel the ions to high speeds and control their movement, he said.

Two specialists not involved in the project said the project could be used to help China develop new weapons.

Han Dejun, a professor of nuclear science and technology at Beijing Normal University, said the experiment was very likely for the development of nuclear devices containing tantalum.

Such devices are called “salted bombs”, which take their name from a mediaeval practice of spreading salt on conquered lands as a curse.

On top of causing tremendous damage with a powerful blast, salted bombs use heavy metals such as tantalum to release a cloud of radioactive isotopes and spread them over a much larger area than a conventional nuclear device to render the space uninhabitable. Tantalum fallout can remain lethal for several months before decaying, while isotopes of other elements such as cobalt can be radioactive for decades.

“The most likely application that I can think of [for the tantalum acceleration experiment] is in nuclear research,” Han said.

“By generating a powerful beam of tantalum ions we can observe how the metal interacts with other elements and change form in high-speed collisions. It simulates what will happen in a real nuclear reaction.”

He said tantalum was used rarely in particle acceleration experiments because the heavy element was extremely difficult to handle.

China has stepped up the quantity and quality of its nuclear arsenals in recent years. A major mission of the nation’s nuclear weapon programme is to achieve high level of nuclear deterrence with a relatively small number of warheads, a strategy in which salted bombs could play a role.

Other countries have tested similar weapons. In 1957, Britain tested a 1 kilotonne cobalt bomb, another type of salted nuclear weapon, underground at Maralinga in Australia.

The idea of salted bombs was proposed by Hungarian-American physicist Leo Szilard early in the cold war. He warned that such weapons, if deployed, could wipe out humanity by producing enough fallout to radiate the entire surface of the planet.

So far no salted bombs have been tested in the atmosphere but the US Army did detonate a “dirty bomb” in an open field in 1953, according to a declassified US Defence Technical Information Centre document.

Dirty bombs involve conventional explosives – not nuclear devices – but are packed with radioactive material. The weapon tested by the US Army was loaded with more than 30kg of the same isotope used in Lanzhou, releasing a lethal dose of gamma rays over the target area, the document said.

Cai Minghui, an associate researcher at the National Space Science Centre in Beijing, agreed that the Lanzhou project could help China develop new weapons, possibly laserlike devices to do targeted damage. The heavy particles could not penetrate far into solid materials but they could do concentrated harm on the surface, he said.

“In theory, the particle beam of a heavy element such as tantalum can be used as a directed energy weapon,” said Cai, who was also not involved in the project.

Cai added that the technology could also be used to test China’s sensitive military equipment.

If the equipment could withstand the tantalum beam, it had a higher chance of enduring the harsh environment of electronic warfare, he said.

Retired Chinese major general Xu Guangyu, a senior researcher at the Beijing-based China Arms Control and Disarmament Association, said that as nuclear weapons continued to improve, China would make sure it did not fall behind in the technology to maintain strategic deterrence.

Xu said the experiments in Lanzhou could be for basic nuclear research, but the chance that China would actually build and stockpile salted nuclear warheads was “very low”.

“These are highly immoral weapons. In my opinion, they should never be developed or built,” Xu said.

“They can generate radioactive fallout that spreads around the globe. It would expose us to the risk of contamination, too.”