Why nuclear weapons degrade over time

 "A man who lies to himself, and believes his own lies, becomes unable to recognize truth, either in himself or anyone else, and he ends up losing respect for himself and for others.

When he has no respect for anyone, he can no longer love, and, in order to divert himself, having no love in him, he yields to his impulses, indulges in the lowest forms of pleasure, and behaves in the end like an animal. 

And it all comes from lying - lying to others and to yourself." - Fyodor Dostoevsky

The quotes have been heavy on Dostoevsky lately.  I'll try to remedy that, but they are great, and I've not seen them often quoted. 

Short nerdy nuke post ahead.  There's been a lot of digital ink spilled recently about the US need to upgrade the good old cold-war nuclear arsenal.  Probably a lot of this has to do with keeping the military contractors happy, but some of it has to do with physics, and the way we create the Plutonium necessary for a nuclear weapon.

Below:  A Plutonium "pit" partially surrounded by explosive lenses

I did a detailed post about Plutonium production about 10 years ago, and it's probably worth reading prior to continuing here.

Plutonium is a man-made element - except for extremely minute (parts per trillion) quantities naturally occurring in Uranium ore.   The half-life of Plutonium-239 is 24,400 years, and it would seem unlikely that just a few decades after manufacture that the potency of the weapon would degrade.  However, as mentioned in the other post, Plutonium that has been produced in a nuclear reactor also contains Plutonium-240.

Plutonium-240 is an extremely problematic isotope, but its creation in a reactor is unavoidable.  Occasionally the Plutonium-239 you are producing in the reactor absorbs an additional neutron, and does not fission, creating Plutonium-240.  

Plutonium-240 decays by spontaneous fission, which is the reason that making a nuclear weapon from Plutonium-239 is so difficult and requires a rapid implosion-type initiation.  However, continuous fission inside the Plutonium pit over time will give rise to fission products that will cause the weapon to degrade.  

In a reactor, the process is called "reactor slagging" - the build-up of neutron poisons causes the reactor to need refueling despite only burning up 4-5% of the fissile Uranium-235.  One of the worst "slag" fission products is Samarium-149, due to two factors:  The large fraction of Samarium-149 produced as a fission product, and its very large appetite for neutrons.  

Samarium-149 has a thermal neutron absorption cross section of 41,000 barns, and a wildly higher cross-section at nigher incident neutron energies - like those inside of a highly supercritical exploding weapon.  This contaminant (and some others) will tend to make the yield less and less reliable over time, as the weapon ages. 

Below:  Incident neutron energy vs absorption cross-section for Samarium-149 (courtesy ReasearchGate-net)

Now you know why the US might be melting down all those old Plutonium pits, chemically extracting the neutron poisons ("slag"), and then re-machining the same Plutonium into new pits.  Just like nuclear fuel reprocessing facilities do for reactors... except this has a far less desirable end-use.

EDIT:  There is also degradation due to decay of the Tritium boosting gas.  Tritium only has a half-life of 12.3 years, so this is something that would be routinely replaced on these weapons.