Please excuse the poorly phrased question, I have a vague memory of reading about this topic quite a long time ago, during the last years of the cold-war, and I was hoping to get a little more up-to-date.
What I recall reading somewhere was that, at the time of the early CNTB negotiations... the US had secretly developed extremely powerful super-computers that could simulate nuclear tests without the need for actual real-life detonations, and that the CNTB treaty was an effort to make that advantage permanent and to keep the Soviets and others from advancing further.
I'm feeling like my source of information must have had a bit of bias, and was painting the US as acting in bad faith. Is there any truth to that allegation? More generally.... what role did super-computer simulations of nuclear detonations play, if any, in non-proliferation treaty negotiations?
US interest in the CTBT was/is definitely predicated on the idea that they would be able to maintain their "credible deterrent" indefinitely without future testing. Supercomputer simulations (importantly coupled with a very intense amount of underground nuclear testing that calibrated those simulations) are part of that; so are technologies like subcritical testing (nuclear testing that results in no nuclear yield, but still provides hydrodynamic information), and several costly facilities (the Dual-Axis Radiographic Hydrodynamic Testing facility at Los Alamos and the National Ignition Facility at Livermore). The idea is that between those three major approaches, the US could live without further nuclear testing. It is not "bad faith" per se — but the US never claimed (and nobody really assumed) that it would cease all nuclear weapons work if it signed the CTBT. The consequence, however, is that it is unlikely that the US would develop any significantly new warhead designs, since without actual testing it is unlikely they would field anything radical, and it is unlikely that they would rely entirely on these non-nuclear alternatives to validate the warheads.