Fifty-seven years ago on October 30, the most powerful nuclear weapon in the history of nukes was detonated by the Soviets. 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 together.
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 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 because of two reasons.
First: it was a three-stage nuke and weighed 27 tonnes and was only a little smaller than an American school bus. As a result, it couldn’t 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’ve meant Durnovtsev and his crew would’ve nearly certainly been killed.
To improve the odds of survival to 50%, engineers reduced the yield from 100 MT to 50 MT, and which they did by replacing a uranium-238 tamper around the bomb with a lead tamper. In a thermonuclear weapon – which the RDS-220 was – a nuclear fusion reaction is set off inside a container that is explosively compressed by a nuclear fission reaction going off on the outside.
However, the Soviets took it a step further with Tsar Bomba: 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 also included a uranium-238 tamper on the second and third stages, such that fast neutrons emitted by the fusion reaction would’ve 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’ve accounted for fully one quarter of all the radioactive fallout from all nuclear tests in history, gently raining down over Soviet Union territory. The modification resulting in 97% of the bomb’s yield being in the form of emissions from the fusion reactions 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 leverage the fusion of hydrogen atoms. In 1955, the Soviets, thanks to Sakharov’s work, won the race to detonate a hydrogen bomb that’d been dropped from an airplane, whereas 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. 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 undertake controlled nuclear fusion and so generate power. The ITER experiment uses this design.