I'm not sure how much documentation is available on the subject. But, would it be possible to include multi-stage nuclear warheads in the game, as these are the predominant type of modern nuclear weapon.
It appears that the game only recognizes the core and fusion booster as fissionable when calculating yield. Some modern devices use fissionable materials elsewhere in core design, such as the tamper, to boost the overall yield. Would it be possible reflect this in the way yield is calculated?
Also for the implosion nukes that are ingame, the explosive lens equations are probably classified too (according to 'ultimate simulation limitations' in the infolinks), so they're already a mockup. This could mean one of two things depending on your standards: either it wouldn't be too much of a departure from the current simulation fidelity to include approximations of thermonuclear devices; or that the current nukes are 'bad enough already' and adding more types is likely to lead to cumulative errors.