Did scientists have a rough idea of how much energy would be produced when the theory was developed?
Sure. They knew the masses of the raw materials and the products, and they knew E = mc^2.
As @Chronos says, they certainly knew the order of magnitude.
But even in 1954 the Castle Bravo thermonuclear test yield was famously 3 times what they expected, because the theory had missed a reaction pathway.
The story that the Manhattan project theorists thought there was a non-zero probability that a fission bomb would ignite the atmosphere and destroy the Earth appears to be urban legend.
This is a bit of a hijack, but is there any possibility that a fusion reaction could be ignited without a fission starter?
Maybe a really energetic chemical explosive where all the hydrogen atoms have been replaced by deuterium, or even tritium? Mix in some lithium and… ??
Yeah, I know, I should be able to calculate this myself by finding the energy spectrum of hydrogen atoms in a chemical explosion and comparing that with the threshold required for fusion reactions. Maybe I’ll crank up Wolfram Alpha tomorrow and have a closer look. Too lazy tonight… 
My guess is probably not…?
It’s been done before. Just start with a big pile of molecular hydrogen gas, and watch it light up! No need for human intervention, even.
But bear in mind that this doesn’t work if you do it at night.
Ha Ha. I don’t think I can amass that much hydrogen in my lab, it isn’t big enough.
But seriously: on a human scale?
As I said, I doubt if it’s possible. But damn, I’m going to have to start doing some real scientific calculations tomorrow! Oh well, keeps the brain working…
How much of the nuclear payload is exposed to the initial detonation?
Stick it in a tokamak? People use deuterium-tritium plasma for this, though. Now you are looking at Lawson’s figure of merit in terms of the product of density, temperature, and confinement time.
It’s actually really, really easy to get some fusion. You can make a device at home (if you’re handy with electronics and such) that’ll do it on a tabletop. What’s tough is to get a reaction that’s even briefly self-sustaining.
The uncertainty there wasn’t in how much energy would be released per reaction, but in how much of the fuel would react before the rest was blown all to smithereens. A fairly common failure mode for nuclear bombs is that it starts off by producing just enough of a bang that it blows apart the critical mass, and doesn’t react further.
100.0%
Not really. They were simply unaware of a specific reaction pathway for Lithium-7. They didn’t know it was fuel at all.
It was assumed that the lithium-7 would absorb one neutron, producing lithium-8, which decays (through beta decay into beryllium-8) to a pair of alpha particles on a timescale of nearly a second, vastly longer than the timescale of nuclear detonation.[31] However, when lithium-7 is bombarded with energetic neutrons with an energy greater than 2.47 MeV, rather than simply absorbing a neutron, it undergoes nuclear fission into an alpha particle, a tritium nucleus, and another neutron.[31] As a result, much more tritium was produced than expected…
The test used lithium with a high percentage of lithium-7 only because lithium-6 was then scarce and expensive; the later Castle Union test used almost pure lithium-6. Had sufficient lithium-6 been available, the usability of the common lithium-7 might not have been discovered.
IIRC the Hiroshima/Nagasaki bombs fissioned something like 1-2% of their mass. As @Chronos noted, the issue is the bombs blow themselves apart which limits how much of them get used up in the reaction.
Too bad; that was an amusing scene in the Christopher Nolan movie.
It’s sounds like science fiction, but if you take a bit of deuterium and tritium, pack it in a gold box, and shoot hundreds of lasers at the box at the same time from all directions, the concentration of energy will cause the deuterium and tritium to fuse.
IIRC vaguely a nuclear physics discussion from decades ago - the reason iron is relatively common is that iron is near the peak of nuclear inert. Larger nuclei if hit sufficiently hard with a neutron split and also release energy, but splitting smaller atoms absorbs energy. (the smaller they get, the more energy, roughly)
See the graph, here…
Fusion is slamming atoms (nuclei) together with sufficient energy to cause them to bind, at which point they release extra energy.
So scientists knew the earth, being composed of almost all elements iron or (much) smaller, was very unlikely to carry on a chain reaction where the nuclear explosion would carry on to the surrounding environment. The energy falls off from the nuclear explosion almost right away to the point where it cannot fuse common elements.
The problem is, at the first nuclear test the scientists didn’t know for sure that there was no possibility there was something they didn’t know.
Reading that does not make it seem an urban legend. The movie may have overstated it but it was something they considered.
From your link:
“Did the actual exchange happen at that moment? No, I don’t think so,” said Alex Wellerstein, an associate professor at Stevens Institute of Technology in Hoboken, N.J., and author of the 2021 book, “Restricted Data: The History of Nuclear Secrecy in the United States.”
“But were there discussions like that? I believe so,” he added.
At a conference in the summer of 1942, almost a full year before Los Alamos opened, physicist Edward Teller raised the possibility of atomic bombs igniting Earth’s oceans or atmosphere. According to Rhodes’s account, Hans Bethe, who headed the theoretical division at Los Alamos, “didn’t believe it from the first minute” but nonetheless performed the calculations convincing the other physicists that such a disaster was not a reasonable possibility.
I mean, if we’re going to get into that, maybe setting off the first bomb when they did destroyed a cloaked alien ship that they didn’t know about and thereby saved the planet. Man, it’s a good thing that they did it even though they didn’t know!
The issue was raised, but the way it was portrayed in the movie was that there was considerable uncertainty. In fact, after they did the calculations the theorists were sure that it was impossible.
“There was never any possibility of causing a thermonuclear chain reaction in the atmosphere. There was never “a probability of slightly less than three parts in a million,” as Dudley claimed. Ignition is not a matter of probabilities; it is simply impossible.”
The second link is to a more extensive transcript of an interview with the leading theorist Bethe.
Bethe : It is such absolute nonsense [laughter], and the public has been interested in it… And possibly it would be good to kill it once more… Teller at Los Alamos put a very good calculator on this problem, [Emil] Konopinski, who was an expert on weak interactors, and Konopinski together with [inaudible] showed that it was incredibly impossible to set the hydrogen, to set the atmosphere on fire. They wrote one or two very good papers on it, and that put the question really at rest. They showed in great detail why it is impossible. But, of course, it spooked [Compton]. Well, let me first say one other thing: Fermi, of course, didn’t believe that this was possible, but just to relieve the tension at the Los Alamos [Trinity] test [on July 16, 1945], he said, “Now, let’s make a bet whether the atmosphere will be set on fire by this test.” [laughter] And I think maybe a few people took that bet. But, for instance, in Compton’s mind it was not set to rest. He didn’t see my calculations. He even less saw Konopinski’s much better calculations, so it was still spooking in his mind when he gave an interview at some point, and so it got into the open literature, and people are still excited about it.