Ukraine failed to live up to the agreement with Russia. They never returned their nuclear weapons. Of course that was a very long time ago and I suppose that by now the nukes Ukraine had at the time are no longer functional. I do not believe Ukraine has the technologies necessary to make their own fissionable material.
Keep in mind that rockets/missiles and the fuels required to launch them expire after a period of time. So does the core of the nuclear warhead. So I don't believe Ukraine itself poses a genuine threat where nuclear weapons are concerned.

@iThink

Do you have any evidence that Ukraine kept its nuclear weapons?
You said this before and provided no evidence.
Nuclear weapons are very well documented and it is not plausible that Russia would have missed even one, what to say all of the weapons, and then just signed treaties.

Then, no, Pu-239 has a half-life of 24,000 years. All warheads made irrespective when they were made are all still good to go.
It will be thousands of years before they will no longer be viable.

Ukraine has the capacity to maintain ballistic missiles. They use the same conventional fuels as any other long range missile.

So if Ukraine kept any of its warheads, if would still have the capability to use them and would have threatened Russia with them.

So I do not find the story Ukraine kept its warheads as believable.

@Hanno Just do some searching around the web for stories about how Ukraine failed to return the nuclear weapons to Russia. It's a very well covered story.
The thing about the thermonuclear core of a weapon/bomb is that it degrades over time. Yes it will remain radioactive for many many decades - centuries even. But relative to its "half-life" it becomes incapable of being triggered to critical mass and causing a thermonuclear mushroom cloud explosion. It might be used as a so called "dirty bomb" I suppose but I doubt the core is large enough to spread radioactive fallout over a very wide area.
Just do a web search on the subject of maintaining the nuclear arsenal. You will see that periodic upgrades of the nuclear cores are necessary.
I always thought of it sort of like a battery - wet cell, dry cell any kind of battery. A battery sitting without any load put upon it will nevertheless discharge over time. This is why a "new car battery" always needs to hooked up to a battery charger and fully charged before you put it in your car.
Uranium and plutonium are not in a fissionable state when it is mined. It requires a very particular and long process of refinement in order to make it capable of reaching critical mass.
As I said before the info is available on the web. Go do some reading on it. Let me know what you find.

@iThink

I am a nuclear engineer

Your “sources” on the web are nonsense.

Plutonium does not exist in nature. It is manufactured from uranium during nuclear fission in reactors.

Uranium remains radioactive for billions of years, not centuries. All uranium on earth are there since the supernova that created the material that is earth today, which was billions of years ago.
We can easily calculate this from the half lives of U-235 and U-238 and their respective concentrations in natural occurring uranium.

A nuclear warhead is nothing like a battery. An U-235 warhead will survive millions of years due to its 700 million year half life.
A Pu-239 warhead thousands of years due to its 24 000 year half life.
Once the content of U-235 or Pu-239 falls below the criticality level, you cannot “recharge” it.

You need to know at least what half life means before you make statements like that.

What gets damaged is the electronics surrounding the warhead that controls its implosion to reach critical mass.
This needs regular maintenance, however this is conventional electronics and explosives (HMX and RDX).
Ukraine would have no problem maintaining that.

The difficulty in manufacturing nuclear weapons is not the bomb mechanism. That is relatively simple.
It is the enrichment (the refinement you refer to) of uranium to increase the U-225 content to ~90% from 0,72%. We use the weight difference between 235 and 238 in centrifuges. A very energy expensive method.
Or for plutonium warheads the production of pure plutonium with less than 7% Pu-240. This is not a simple task when you understand the neutron capture process and how Pu-239 and Pu-240 behaves.

As the web articles telling you that nuclear warheads are like batteries and lasts only last a few years are nonsense, so is likely the articles telling you that Russia would sign agreements not to attack Ukraine without receiving the warheads first.

What part of Russia makes you think they would just leave warheads that could destroy Moscow behind?

Just because someone wrote it on the web, does not mean it is true.
Web stories does not replace actual understanding.

@iThink

Then you also need to understand the difference between “thermonuclear” and “nuclear” warheads.

The first are also called “hydrogen” bombs, and old ones used tritium which has a relatively short half-life. This needs to be replaced after a few decades. However this is not a difficult task to “re-gas” and all modern hydrogen bombs use lithium deuteride, which does not decay, in tritiums place.
Where you may have read that warheads decay, it may have referred to thermonuclear ones and only to their tritium content.

Nuclear warheads does not have this issue.

Every thermonuclear warhead has inside it a nuclear warhead that initiate the secondary part.

Even if the the tritium is removed from a thermonuclear warhead, the conventional nuclear initiator is still several kilotons (similar to the Hiroshima bomb).

@Hanno Found this article published in "Popular Mechanics" - since you are a nuclear engineer I'm sure already know this - but here it is.
[popularmechanics.com]

Its not so much that these politicians are stupid and incompetent but they (by their actions) want War.

@iThink Most treehugger types (includes anti-nukes) decry the idea of any type of Pu running about 'cause it scares them. Why? 'Cause it demands too much attention to keep track of what is going on.
A U fueled core burns U235 and creates a couple of isotopes of Pu. 239 is the bang type, 240 will accumulate. If the fuel element is only in there for a few months it will be rich in 239. As time goes on it will be swamped by the 240. The Canadian Candu reactors were ideal for making weapons grade Pu. Which is why they passed on the idea of the horrendously expensive U enrichment process that the US had to go to to make the WW2 bombs.
What scares the treehugger types is that a just out of the oven fuel element can have its bomb grade Pu separated from all the rest in a chemical factory. Dangerous as 'Ell since practically everything is a heavy metal poison and if you have large bottles collecting the stuff you can generate a critical mass -- boom. But, it is a lot less expensive than enrichment.
This unreasonable fear of Pu is why domestic recycling of fuel cannot happen. Used in many other countries, but not in the US. The treehuggers won and our electricity is much more expensive -- oh, and makes more CO2. Imagine the irony!

@iThink

Thanks for the article. We calculated these “lifetimes” back as students because it also applies for MOX fuel (that is fuel made with recovered plutonium).
The margins are larger for fuel than for nuclear weapons and I had to go and do some calcs again… so I learned some today again.

However experts all agree current warheads last “at least 100 years”, and none of them are near that yet.
So the original argument about “what if” Ukraine has nukes still stands.

The issue is tritium and there you are correct that Ukraine does not have tritium facilities I know of…. However tritium is a lot easier to obtain than plutonium.

@Hanno I believe you - just wanted to show you a source on the topic...which is why people like myself come to the conclusions that we do.

@Hanno The "100 year" thing puzzles me. Yeah, the radioactives (including tritium) might be good for a long time. But, for an implosion device the timers for the regular explosives that compress the material might not have that much of a shelf life. I used to work with a guy who did the routine testing of bombs while in the army. Uhh, testing as in diagnostics, not boom.
Their concern was the equipment wrapped around the boom stuff. If any of that failed, all you got was a pfft. Nice dirty bomb but no crater. Certainly no dead tank crews -- which was why the army had nukes.
As to a delivery system in a land-based war, no issue. Front lines are porous. Just put one in the path of a tank brigade. That region of the world looks kinda like tank attack heaven. But, if wet that soil turns to gumbo. So, heavy vehicles have to use heavy roads -- boom.

@bobbo666

The “at least 100 years” only applies to the Pu - material.

The electronics do not last very long (depends on the amount of Pu-240) in the core. Anything above 7% and it is fried in few years.
Below 2% and it will last some decades.
However making Pu-239 with less than 2% Pu-240 very expensive with very short fuel changes required.

So the electronics need to be replaced every decade and the explosives every other decade.
The exact schedule depends on the design and Pu-240 content.

The “at least 100 years” was being nice to Trump as it should be much longer than that.

It depends on the excess Pu in the core to start with. If the design required 93%, then they use 95% and it will be fine till it dropped to 93%. However if they used 94%, that period of time is much shorter.
So for the older designs it may be that it will drop to minimum level in just 100 years. However Pu-239 decay to U-235, which is also fissile..: so “modern” warheads, those post 1965, should last a long time.