Watching Oppenheimer got me wandering: Is there any homogeneous substance which, if it replaced the earth's entire atmosphere, would have made the trinity test start a worldwide nuclear chain reaction? Not counting substances that would have spontaneously ignited a chain reaction without the test. Perhaps some alloy of Uranium, Lithium, Deuterium, and Cadmium (thermal neutron absorber)
Any hollow sphere 50km thick with the inner diameter of the earth would count. It can be solid, liquid, or gas. It has to work with only earth's gravity, so a slightly-subcritical brown dwarf or white dwarf doesn't count.
1,000As evidenced by the existence of the sun self-sustaining nuclear fusion is possible. There should exist some helium density between 1 atm and sun where the reaction wouldn't start by itself but would with some energy added. Also, any secondary explosive should work, the rules don't specify that the chain reaction needs to be nuclear.
If the atmosphere was all Uranium-235 this would almost work, because although it would have the capability to sustain a fission chain reaction, it would need a neutron source to kickstart it, which would be provided by the bomb. This doesn’t quite work, though, I think, because there is a small “neutron background” on Earth that comes from cosmic rays/decay of elements in the crust/whatever. Also, 0.0000002% of the time that U-235 decays, it spontaneously fissions, producing neutrons, which would cause the uranium “atmosphere” to spontaneously ignite. But it would work if you ignored these very small sources of neutrons.