The government eventually ended its program in 1968, after a long public debate, and Sweden joined a group of nations - including Switzerland, Ukraine and South Africa - that abandoned their nuclear weapons programs and showed the world that nuclear disarmament is possible.

The extension of Sweden's nuclear program was "uncomfortable" for politicians keen to reinforce the country's new anti-nuclear credentials. Until journalist Christer Larsson revealed the truth in 1985, forcing the nation to confront the secret of its nuclear past.

The veil of secrecy surrounding the program's history fueled speculation that Sweden still maintained a top-secret plan to develop its own nuclear weapons.

Decades later, Sweden is ending 200 years of neutrality by joining the NATO alliance, which has nuclear weapons, following Russia's invasion of Ukraine.

But why did the Swedes want to develop nuclear weapons? And why did they give up?

"Swedish line" to produce an atomic bomb
In Rusvik, a quiet suburb of Stockholm, there is a large school building that looks like a secret research institute. It does because it once was. The headquarters of the former Swedish National Defence Research Institute (FOA) is one of the few remaining physical vestiges of Sweden's nuclear weapons program.

Two weeks after reports and images of the devastated cities of Hiroshima and Nagasaki in 1945 reached Stockholm, the military commander of this nation, passionate about neutrality, asked the newly created FOA to prepare a secret report on the feasibility of Sweden building its own atomic bombs.

Sweden may have been a neutral country, but it was a nation whose leaders believed in armed neutrality. The price of that neutrality was a strong army, and its leaders understood that strategic atomic bombs might be needed in the future to preserve that neutrality. The country's long coastline and small population made it "easy prey" for an adversary like the USSR of the time.

The Nordic country had its own uranium deposits, albeit of low quality. It was a country with a solid infrastructure, thanks to its neutrality during the Second World War. The plan to develop an atomic bomb was not as far-fetched as it might seem today.

Three years after the atomic bombings of Hiroshima and Nagasaki in 1948, the FOA established "the Swedish line" for the production of a plutonium-based atomic bomb without the need for outside assistance. The plan was to produce plutonium by fusing Swedish uranium in Swedish nuclear reactors using heavy water.

Operating under secrecy, Swedish scientists were forced to start from scratch slowly and expensively, due to a lack of high-quality uranium supplies and a lack of information sharing with the United States. Out of necessity, the decision was also made to link the nuclear weapons program to the civilian program and disguise its true purpose.

"We had everything prepared", but...
"We had everything prepared to produce weapons-grade plutonium," says Thomas Jonter, author of The Key to Nuclear Restraint: The Swedish Plans to Acquire Nuclear Weapons Through the Cold War.

The plan called for two reactors. "One, Ågesta, a heavy water reactor south of Stockholm, and another, Marviken, built on the outskirts of the city of Norrköping, but which never went into production. The idea was to build 100 tactical weapons," explains Jonter.

"We knew exactly how it should be done. We had everything except the reprocessing plant and the weapons transportation system."

However, the slow pace of the weapons program eventually resulted in its collapse.

There was no public debate about the plans, as their existence was known only to a small circle of politicians, high-ranking military officers and scientists (and, presumably, Soviet spies).

The secret was revealed in 1954, when Swedish commander Nils Swedlund revealed the existence of the program and argued that these weapons were necessary to defeat the Soviet Union.

In April 1957, the US Central Intelligence Agency (CIA) estimated that Sweden had "a reactor program sufficiently developed to enable it to produce some nuclear weapons within the next five years", an assessment that accelerated the timetable to four years.

Sweden's prime minister at the time was Tage Erlander, who had a background in physics and regularly made a point of talking to world authorities in physics about atomic bombs, including Nobel laureate Niels Bohr.

The Dane made brilliant early contributions to nuclear physics and was smuggled out of occupied Denmark during the Second World War to join the Manhattan Project, which developed the first atomic bomb.

The more the prime minister spoke, the more he wavered in his support for the nuclear weapons program. Seeking consensus, he repeatedly postponed a final decision until the results of the arms control negotiations between the US and the Soviet Union were known.

His moral posturing - or shrewd political maneuvering, depending on who you believe - allowed critics of the nuclear weapons plan to organize. Many of them were women.

Criticism has changed opinions
The Social Democratic Women's Federation (Sveriges Socialdemokratiska Kvinnoförbund, SSKF), led by Inga Thorsson "has become the strongest voice against the nuclear project," says Jonter.

"Social Democratic women argued very early on that Sweden should not develop nuclear weapons for various reasons," says Emma Rosenberg, a doctoral researcher in International Relations at Stockholm University.

"Instead of offering protection, these weapons could actually make Sweden a target. It would therefore reduce security rather than increase it."

"They also argued that it would be completely immoral, given the humanitarian consequences of using nuclear weapons. A peaceful country like Sweden could never contribute to the kind of suffering caused by nuclear weapons."

The contribution of women like Thorsson to the debate was generally unwelcome. "They were looked down upon as basically emotional women who shouldn't talk about things they didn't understand," says Rosenberg: "And defense policy was considered something that at that time only men were capable of addressing."

When other groups joined the SSKF women, such as the Swedish Nuclear Weapons Action Group (AMSA), public opinion began to change. It was a shift helped by the collapse of military support for the weapons. The Swedish Army, Air Force and Navy realized how expensive they were and cuts would be needed in all three branches to pay for them.

The US's negative attitude towards Sweden's nuclear plans was also important, given the growing defense cooperation between the two countries in other areas, including the use of Swedish runways to receive US bombers.

The Swedish military and civilian nuclear energy program came to depend on American technology for things like missile systems, the design of new light water for civilian nuclear reactors and even nuclear fuel - which actually hampered Sweden's attempt to develop its nuclear weapons.

At one point, Sweden even explored acquiring American nuclear weapons.

There was also a growing opinion among the Swedish elite that the country didn't need to develop its own nuclear weapons because it was protected under the US nuclear umbrella, even though it wasn't a member of NATO.

"It's important to point out that there was no formal agreement," says Jonter. "I read the prime minister's diary and he doesn't mention it anywhere, because it would have been very difficult for either party to sign such an agreement."

What the author did find were American political documents stating that Washington would be "prepared to provide assistance to Sweden as part of a NATO or UN response" against Soviet aggression.

"But for that kind of agreement to really mean anything, it has to be formalized," says Rosenberg. Jonter's investigation found no evidence of an agreement.

During the 1960s, under the leadership of politician and diplomat Alva Myrdal, Sweden became closely linked to international efforts to prevent the spread of nuclear weapons, with a redoubled campaign against Swedish weapons themselves. Even supporters of the original plan now only wanted research to continue, but not production.

This change was reflected in public opinion. In 1957, 40% of the public supported the acquisition of nuclear weapons, with 36% against and 24% undecided. Eight years later, only 17% agreed, with 69% against and 14% undecided.

So it was no surprise when the Swedes abandoned their nuclear weapons project in 1966, nor when they signed the Non-Proliferation Treaty in 1968 and parliament voted to end the program completely, although limited research was carried out in the 1970s.

Jonter points out that Sweden's experience can serve as a lesson in today's world. "One lesson is that producing nuclear weapons is not that easy," says Jonter, "even if the country has an internal nuclear infrastructure. It's very complicated."

This means that a country that wants to produce nuclear weapons needs to cooperate with other, more technologically advanced nations - which can create a relationship of dependency.

In addition, there is the importance of setting aside enough time for public debate, so that citizens can truly understand what it means for their country to acquire nuclear weapons. "I think this is a very important lesson," says Rosenberg.

Of course, just because there are lessons doesn't mean that political leaders will change their behavior.

"Unfortunately," Jonter wrote in Physics Today in 2019, "the decision to withdraw from a nuclear deal with Iran suggests that [then US President Donald] Trump and his advisers did not learn this... [first] lesson."

In 2012, Sweden transferred what was left of the plutonium it had produced for its nuclear weapons program to the US.

"There was a kind of discussion in the 1960s about a reserved option, but as far as we know, the program was terminated," says Jonter. "Of course it's secret, but politically it would be impossible for a party to defend the production of nuclear weapons."

Rosenberg is more direct. "There is no material or political will left. We don't produce nuclear weapons."