> We have a couple new nuclear weapon designs, but not really going for bigger or stronger. Just packaging.
There's many other considerations. Like the type and amount of fissile material. To name one that became well known: any plutonium needs to be refreshed (re-breeded I believe is the term) every few years.
Also, look at first designs: https://www.bbc.com/news/newsbeat-35242069 The weight is secret, but I think you can easily see it's going to be deep into "extremely impractical" territory.
Surely you can see why someone (especially aircraft designers) might ask for better versions. Ideally you'd like a version that fits on the hypersonic missiles and those things ... are just not going to work. The size. The shape. The weight. None of them will work.
Then a quick theoretical exploration will tell you that the minimum theoretical size of such a device is tiny. The scare was about "suitcase sized", but if you actually do the calculation looking for the minimum ... to do it however you need to create an explosion of the correct shape to get anywhere near those minimum sizes. And explosion simulations are a problem that utterly sucks ... Oh and these are secret military projects, these simulations, not very optimal. The people doing them are best described as loyal, and not as great physicists. Not saying they're terrible, but in the movie Oppenheimer you can clearly see why the best and brightest are not available for these things.
> There's many other considerations. Like the type and amount of fissile material. To name one that became well known: any plutonium needs to be refreshed (re-breeded I believe is the term) every few years.
Already mentioned in the comment you replied to:
> > Now, stockpile stewardship -- being sure that stuff will keep working without ongoing testing -- is a bit expensive in compute. You need early 2010s supercomputer power.
> Surely you can see why someone (especially aircraft designers) might ask for better versions. Ideally you'd like a version that fits on the hypersonic missiles and those things ... are just not going to work. The size. The shape. The weight. None of them will work.
The "new" US nuclear weapons are direct derivatives of older designs produced with less computing. B61-13 a direct child and similar footprint to 1967's B61. W87-1 direct descendent of the 80's W87. W93 is closest to a "clean sheet' design but hits similar size targets.
> Then a quick theoretical exploration will tell you that the minimum theoretical size of such a device is tiny. The scare was about "suitcase sized", but if you actually do the calculation looking for the minimum ... to do it however you need to create an explosion of the correct shape to get anywhere near those minimum sizes. And explosion simulations are a problem that utterly sucks ...
The backpack W54 was developed in 1959. Tinier devices were developed in the 60s and 70s but ultimately abandoned for proliferation rather than technical concerns. The total computing power used for this was miniscule.
Nuclear weapons are just not pushing forward supercomputing anymore. They did, for awhile, for stockpile stewardship, but we've become relatively confident that our stockpile will still work.