America is building new nukes.
https://www.revealnews.org/article/new-mexico-thrives-on-nuc...
https://www.revealnews.org/article/new-mexico-thrives-on-nuc...
This is scary, as the higher-precision guidance and adjustable lower yields make it more likely to be used; and sad, that President Obama and the various contractors and New Mexico politicians are pushing this.
The socio-political aspects are a much broader topic. Personally I don't like them, but I realize that once Pandora's Box Of Technology has been opened, there's no going back.
Not only that, but, because of the secrecy and security, many of the processes for building these exotic materials have been lost.
(I think my knowledge comes from one of Freeman Dyson's accounts.)
I have not heard a clear explanation of why those earlier weapons designers were wrong, only concerns about how they might have been wrong. That more nebulous caution sounds identical to the sorts of statements someone might make as a justification for designing new weapons - for some people designing nuclear weapons and watching them go BOOM is a lot of fun.
In my very limited understanding of the topic, the largest unknown is the changes to the plutonium-gallium alloy of the core. Some worry that it has degraded. Others say "most plutonium pits have a credible lifetime of at least 100 years". Even if it's a problem, one solution is to re-smelt the cores, though currently we do not have the facilities for that. However, we could make those facilities without testing new weapons.
See https://en.wikipedia.org/wiki/Reliable_Replacement_Warhead and http://depletedcranium.com/do-our-nuclear-weapons-work/ for some of the discussion. Also bear in mind the annoying calculus of game theory. A higher inability to use a nuclear weapon may make it less likely to be used.
Nuclear weapons with much, much smaller yields have been in the US arsenal for a long time (see https://en.wikipedia.org/wiki/W54 for an example of one with a yield of 10 tons of TNT, as compared to the 300 ton minimum claimed in the article for the B61-12) and we haven't used them in war, so I don't see how this one is any different in that respect.
Furthermore, the "high-precision" argument is also not convincing. You can make an argument -- dubious, but it can be made -- that existing precision conventional weapons are more likely to be used because it's possible to destroy an enemy target even if it's located right next door to noncombatants. But even at its lowest setting, the B61-12 would completely destroy dozens of square blocks and cause additional damage dozens farther out; there's no way that wouldn't lead to massive civilian casualties if it was used in a city.