60 years of 'Tsar Bomba', history's most powerful nuke

This post was originally published on October 31, 2018. I republished it once in 2020 after Rosatom, the Russian nuclear energy corporation, released 40 minutes of previously classified footage of RDS-220’s explosion on August 28, 2020 (embedded below). Watch this minute-long excerpt by Reuters of the explosion. I’m republishing it again, today, following the publication of a new report that examines the US’s reaction to the bomb.

Fifty-seven years ago, on October 30, 1961, the Soviets detonated the most powerful nuclear weapon in the history of nukes. The device was called the RDS-220 by the Soviet Union and nicknamed Tsar Bomba – ‘King of Bombs’ – by the US. It had a blast yield of 50 megatonnes (MT) of TNT, making it 1,500-times more powerful than the Hiroshima and Nagasaki bombs combined.

The detonation was conducted off the island of Novaya Zemlya, four km above ground. The Soviets had built the bomb to one-up the US and followed Nikita Khrushchev’s challenge on the floor of the UN General Assembly a year earlier, promising to teach the US a lesson. The B41 nuke used by the US in the early 1960s had a yield of half as much.

But despite its intimidating features and the political context, the RDS-220 yielded one of the cleanest nuclear explosions ever – and was never tested again. The Soviets had originally intended for the RDS-220 to have a yield equivalent to 100 MT of TNT, but decided against it for two reasons.

First: it was a three-stage nuke, weighed 27 tonnes and was only a little smaller than a school bus – too big to be delivered using an intercontinental ballistic missile. Maj. Andrei Durnovtsev, a decorated soldier in the Soviet Air Force, modified a Tu-95V bomber to carry the bomb and also flew it on the day of the test.

The bomb had been fit with a parachute (whose manufacture disrupted the domestic nylon hosiery industry) so that between releasing the bomb and its detonation, the Tu-95V would have enough time to fly 45 km away from the test site. But even then, the bomb’s 100 MT yield would have meant Durnovtsev and his crew would have nearly certainly been killed.

To improve their chances of survival to 50%, engineers reduced the yield from 100 MT to 50 MT – by replacing a uranium-238 tamper around the bomb with a lead tamper.

In a nuclear weapon, the material undergoing fission or fusion is typically surrounded by a layer called a tamper that serves two functions: keep the material from expanding due to the heat of fission/fusion so that it stays supercritical for longer, and to reflect neutrons emitted during fission/fusion back to increase the energy output.

But Tsar Bomba’s design was more complicated: the first stage nuclear fission reaction set off a second stage nuclear fusion reaction, which then set off a bigger fusion reaction in the third stage. The original design included a uranium-238 tamper on the second and third stages, such that fast neutrons emitted by the fusion reaction would have kicked off a series of fission reactions accompanying the two stages. Utter madness. The engineers switched the uranium-238 tamper and put in a lead-208 tamper. Lead-208 can’t be fissioned in a chain reaction and as such has a remarkably low efficiency as a nuclear fuel.

The second reason the RDS-220’s yield was reduced pre-test was because of the radioactive fallout. Nuclear fusion is much cleaner than nuclear fission as a process (although there are important caveats for fusion-based power generation). If the RDS-220 had gone ahead with the uranium-238 tamper on the second and third stages, then its total radioactive fallout would have accounted for fully one quarter of all the radioactive fallout from all nuclear tests in history, raining down over Soviet Union territory. The modification resulting in 97% of the bomb’s yield being in the form of emissions from fusion alone.

One of the more important people who worked on the bomb was Andrei Sakharov, a noted nuclear physicist and later dissident from the Soviet Union. Sakharov is given credit for developing a practicable design for the thermonuclear weapon – an explosive that could take advantage of the fusion of hydrogen atoms. In 1955, the Soviets, thanks to Sakharov’s work, won the race to detonate a hydrogen bomb that had been dropped from an airplane; until then the Americans had detonated hydrogen charges placed on the ground.

It was after the RDS-220 test in 1961 that Sakharov began speaking out against nuclear weapons and the nuclear arms race (one bomb didn’t change his mind, to be clear). He would go on to win the Nobel Peace Prize in 1975. One of his important contributions to the peaceful use of nuclear power was the tokamak, a reactor design he developed with Igor Tamm to conduct controlled nuclear fusion and generate electric power. The ITER experiment uses this design.

The reason Tsar Bomba or any weapon like it, with a yield in excess of 50 MT of TNT, was never tested again was the Limited Test Ban Treaty, signed two years after the Soviets tested the weapon. In a new study of historical records, nuclear historian Alex Wellerstein has reported that while then US President John F. Kennedy responded outwardly by minimising the bomb’s importance, closed-door discussions among government officials, including Kennedy himself, suggest that the bomb had much more of an impact on American thinking than they cared to admit.

Wellerstein’s report is long and full of new details about many aspects of Tsar Bomba. (Its lead image offers a view of the test explosion from Maj. Durnovtsev’s Tu-95V.) Look out for Arzamas-16 (the ‘closed city’ that was the Soviet Union’s first nuclear research and production centre), Project 49, the engineers Yuri Trutnev and Yuri Babaev, Edward Teller’s plans and the weapon-concept known as RIPPLE. It’s also interesting how the physics of the bomb itself began to have a say what the US government could, and couldn’t, do next.

For example, the American military establishment wanted to respond to the Tsar Bomba test with a bigger detonation of their own (of course). Defence secretary Robert McNamara told Atomic Energy Commission chairman Glenn Seaborg, and Seaborg subsequently relayed to the upper echelons, that this could be in the form of a scaled up Mark 41 with a possible maximum yield of 65 MT. But at 5.8 feet wide, 25.4 feet long and weighing 15.8 tonnes, it would just about fit in the bomb bay of the B-52, a.k.a. the “Stratofortress”, and would require five years to make. That was too far away in the future.

For another, and much earlier in the narrative, a Soviet weapons scientist named Leonid Feoktistov, belonging to Chelyabinsk-70 – a rival establishment of Arzamas-16 – expresses disappointment that the Tsar Bomba represented not a new frontier of weapons so much as ‘just’ a bigger version of weapons that already existed. Trutnev and others disagreed with this assessment, saying that scaling up a multi-stage nuclear reaction wouldn’t guarantee success and that, in their words, “many things could have happened, including a failure to achieve the desired explosive yield” of 100 MT.

The contention is fair, but to my mind Feoktistov’s argument also seems directly connected to what Robert McNamara would later tell US Congress: that there wasn’t much to be gained in terms of the science itself by detonating weapons of even higher yield.

Indeed, Wellerstein delves into the immediate political, diplomatic and military response to the Tsar Bomba test to conclude, with good reason, that the US was left with two mutually exclusive choices: start a programme to build bigger, badder nukes or double down on and ratify the Limited Test Ban Treaty (‘limited’ because it wouldn’t prevent underground testing, which was strictly for low-yield weapons because of the need to fully contain the fireball).

Secretary of Defense McNamara would be called before Congress to defend the military implications of the treaty before they ratified it. He was emphatically in favor of it—the only area where the United States was not ahead of the Soviets in testing was “very high-yield” weapons, but he now argued that the United States had “no great interest” in those. It was a return to the public rhetoric that had proliferated after the first announcement of a 100-megaton test by the Soviets: Such weapons were wasteful and ridiculous. Lower-yield weapons, which were still quite powerful (a megaton or two is nothing to sniff at!), could be even more destructive if deployed in quantity. The security gained from a treaty that would not only reduce global fallout but would also guarantee a trend toward lower yields, would be worth anything that could be gained from multi-megaton tests.

But neither the US nor the Soviet Union, but especially the US, entered the treaty blind. The US inserted a “readiness” clause, which meant that it would stay ready to resume above-ground tests if the USSR violated the treaty. This was deemed necessary because the two power blocs had agreed to a ‘Test Ban Moratorium’ in 1959 that Nikita Khrushchev violated in 1961 with Tsar Bomba – to intimidate then president-elect Kennedy and to compensate for the lack of strength implied by the USSR not having any missile launch points close to the American mainland, a lacuna that also led to the Cuban Missile Crisis.

Source for many details (+ being an interesting firsthand account you should read anyway): here. Featured image: The RDS-220 weapon seconds after detonation, as its fireball develops overhead unto to its maximum width of 8 km and height of 10 km. Source: Reuters/YouTube.