Popular movements in the late 1950s pressed toward the Limited Test-Ban Treaty (LTBT), signed in 1963, which prohibited atmospheric testing. It was preceded by a voluntary test moratorium by the United States and the Soviet Union from 1958 through 1961. At the time, the development of nuclear weapons – and other things like a nuclear-powered airplane – was wild and woolly.
One of the points of competition was the size of explosion that a nuclear weapon could produce. This was a somewhat silly competition, because the amount of damage a bomb could wreak increases with the cube root of its energy. So ten 10-megaton (MT; that’s millions of tons of TNT equivalent) weapons would be much more damaging than one 100-MT weapon. But for some, size does matter.
The Soviet Union went for it. They built a 100-MT bomb, but they set it up so it would deliver only (only) 50 MT. They abandoned the voluntary test moratorium on August 30, 1961, and tested the bomb on October 30.
Alex Wellerstein has the whole story in the Bulletin of the Atomic Scientists. Wellerstein is a professor of the history of science at Stevens Institute of Technology and specializes in nuclear history. He’s also written a blog post with more information about Tsar Bomba. I recommend reading everything he writes.
In the article, he also tracks the American response, which was to think about building a still bigger bomb. The largest bomb ever built by the United States was 25 MT, but parts of the military wanted something bigger, as did some of the weapons designers. Somewhere around the mid-sixties, the United States lost interest. The article in the Bulletin contains a wealth of detail about both gigantic bombs.
We see the “size does matter” dynamic shaping up again in terms of numbers of nuclear weapons. China is building up its numbers and might reach United States and Russian numbers in a decade. To some, that means the United States must build more. And Russia has a long list of more or less plausible new weapons they say they are building.
There’s another dynamic that I’d like to footnote. During that 1958-1961 test moratorium, the United States found a problem in its deployed nuclear weapons: They might not be one-point safe; that is, a single-point detonation of the conventional explosives could set off a nuclear explosion. To check that out, hydrodynamic tests were necessary. Hydrodynamic tests involve exploding a partial weapons assembly to give a tiny nuclear yield.
Those tests were done at Los Alamos, on the Pajarito Plateau, underground. They provided practice to develop techniques that would be useful in conducting full-scale undergound nuclear tests after the LTBT came into force. A fuller story of these tests can be found here.
That site, not far from Bandelier National Monument, contains tens of kilograms of plutonium and enriched uranium. They would be difficult to recover, and when I was in charge of the site, we did some research that showed they weren’t moving through the dry tuff, even though some of the holes had accumulated water. We dried those holes out and emplaced covers that would prevent future accumulation. There’s a training facility there now, keeping people around so that nobody can sneak in and set up a drill rig.
When Khrushchev announced the end of the Soviet Union’s moratorium, he accused the United States of breaching the moratorium. Did he know about the hydrodynamic tests, or was that something he would have said anyway? The US now claims that Russia has been doing hydrodynamic tests with nuclear yields at the Novaya Zemlya test site. The Comprehensive Nuclear Test Ban Treaty, which Russia has ratified and the US has signed but not ratified (thank you, Republicans!) is interpreted by the United States to prohibit any nuclear yield at all from such tests and therefore claims that Russia is in violation of the CTBT.
The problem with that interpretation is that yields below about 1 kT (thousand tons of TNT equivalent) aren’t detectable off-site. The hydrodynamic tests of 1958-1961 couldn’t be detected with today’s improved diagnostics. So the US can keep accusing and Russia can keep denying until the cows come home.
We keep doing the same things over and over again.