It’s pretty crazy that they are still working on improving nuclear bombs. If there is one area where the current state is “good enough” it should be that area. I can see the need for maintaining what we have already but why try to advance the technology? A pure fusion bomb would be cleaner as far as I know but do we really want to make nuclear war more feasible?
> I can see the need for maintaining what we have already [...]
The officially-stated goal of these labs (pulsed power, fusion, and hydrodynamic test facilities [0]) is indeed for maintaining existing nuclear weapons, not to design new ones (and also for doing basic research during free time). This was called the Science Based Stockpile Stewardship program [1] - ensure that existing nuclear weapons would remain functional in the foreseeable future. (Interestingly, the lesser-known hydrodynamic test facilities such as the Dual-Axis Radiographic Hydrodynamic Test Facility are more useful for weapon designs than fusion facilities).
The idea is to test materials under extreme lab conditions to help computer modeling, so that it would still be possible to do minor design changes to replace obsolete or end-of-life parts (the FOGBANK incident [1] came to mind). Understanding long-term aging is also a stated goal.
In their defense, some of the questions and that they're asking are about how our stockpile degrades over time - which hopefully means keeping weapons on ice longer instead of having to reprocess and build as many new ones.
Pure fusion bombs are very destabilizing, yes. The current nuclear arms control regime depends on uranium enrichment/plutonium production being heavy industry, requiring large facilities with unusual equipment.
A research program for EPFCG pure fusion weapons could be quite small, only requiring a few hundred people and little equipment that couldn't be manufactured indigenously. Test explosions could be done at the kilogram scale, producing no radiation detectable from orbit or seismic effects, then easily scaled to kiloton yields.
It depends on your answer to the question, is nuclear war inevitable?
If it is, making weapons that are as clean as possible is a reasonable goal. If you get some knowledge that's applicable elsewhere, that's a nice bonus.
If you don't think it's inevitable, you can likely justify making them cleaner because it will likely have applications elsewhere by the logic of fusion weapons being the only place we've been able to harvest usable energy thusfar.
The difficulty with "clean fusion bombs" is that bomb makers can always increase the yield of a fusion bomb by making it dirty. Fusion releases neutrons, and these neutrons have enough energy to fission the common 238 isotope of uranium, which releases roughly 100 times more energy than than the neutron started with.
We already know everything that’s to know about nuclear weapons if used for defense and deterrence. A pure fusion bomb would be relatively clean so it would be much more tempting to use as a regular weapon. What if Putin had such bombs right now?
They are not very useful as attack weapons because they contaminate an area and produce fallout. If you can get the explosive power of a nuke without the fallout it's much more tempting to use one.
That fallout doesn't seem as huge a problem as it is often made out to be. Hiroshima and Nagasaki are not wastelands, they are relatively large cities, and they basically never stopped being ones, even immediately after the bombs fell. The pure destruction of the explosion was a worse problem than the fallout overall - and with today's much higher yields and much more densely populated cities, that seems likely to continue to be the case.
Except this isn't true: the fallout of a nuclear detonation is extremely limited - their physically isn't enough radiological material in the warhead for it to be greater. Troops with chemical suits and respirators could safely operate in a zone which was recently nuked - in fact ground zero is likely to be less contaminated then surrounding areas, since the effect of the blast is to disperse material.
The problem, like all WMDs (weapons of mass destruction), is that they're of limited value against military forces. The distances a military force fights a war over are generally large, and they're mobile - or dug in. The effect of bombarding someone's lines with nuclear weapons though is that you might produce a few holes, but if you tried to advance through them you'd be immediately surrounded since you didn't completely obliterate them.
But they're of devastating effect against civilians, and civilian assets like cities which can be military objectives. Leveling a city rather then taking it is certainly an option, but if that was your plan all along then you could also just drop an ICBM on it - and at that point we're back to "the primary effect of nuclear weapons is making ICBMs useful weapons".
The casualties from a full-scale nuclear war between the United States and Russia were always estimated as "only" being in the hundreds of millions at the top end. But the subsequent famines from the destruction transport and distribution infrastructure, would be in the billions within 6-12 months.
Basically the issue is "tactical" and "strategic" nuclear weapons don't make much a distinction: if it's worth hitting someone's frontlines with a tactical nuke, then why not hit the military base supplying those lines with it? And that base is probably in a city with industry, so why not hit that instead etc. etc. etc.
I suppose another way to put it would be, there's a "hidden" escalation threshold we don't really talk about because no one's been stupid enough to do it: destruction of arable land. You would find that international opinion and weaponry would turn on any country very quickly if they were found to be deliberately targeting and destroying arable land as a policy of invading and denying resources to a neighbor (think literally "salting the earth") on a large, deliberate scale.
Which is all a way of saying, the issue with WMDs is that they're WMDs and WMDs all have broadly similar issues - namely that they will do far more damage to civilians then military targets, even without special preparations, and that they don't allow taking and holding territory. "Clean" nuclear weapons wouldn't change that.
