Do you have any insight/thoughts on why the US made this push to prevent reprocessing, if it's not actually possible to use that process to build a warhead?

"Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium"

https://www.osti.gov/opennet/forms?formurl=document/press/pc...

A successful test was conducted in 1962, which used reactor-grade plutonium in the nuclear explosive in place of weapon-grade plutonium. The yield was less than 20 kilotons.

This test was conducted to obtain nuclear design information concerning the feasibility of using reactor-grade plutonium as the nuclear explosive material. The test confirmed that reactor-grade plutonium could be used to make a nuclear explosive. This fact was declassified in July 1977. The release of additional information was deemed important to enhance public awareness of nuclear proliferation issues associated with reactor-grade plutonium that can be separated during reprocessing of spent commercial reactor fuel.

The Carter reprocessing ban was enacted in April 1977, a few months before this information was released to the public.

There is no evidence that any existing nuclear weapons state has started with plutonium reprocessed from civilian power reactors. Building "production" reactors that produce plutonium without generating electricity is easier and the plutonium quality is higher. However, reprocessing commercial fuel is a potential loophole for nations that want to maintain a latent nuclear weapons capability as a plausibly deniable part of a civilian nuclear power program. There are indications that Japan values its reprocessed plutonium from civilian reactors in this light.

More pointedly, would Saudi Arabia, Israel, or the US trust an ostensibly civilian Iranian nuclear power program that included plutonium separation and reprocessing? Would Iran trust a Saudi program of the same type?

The high Pu-240 content of spent nuclear fuel from commercial reactors is not a strong enough technical barrier against weapons use to be reassuring in circumstances of low initial trust between parties. That's why it makes at least some sense that the United States did not want reprocessing to be a routine feature of civilian nuclear power programs.

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EDIT: everything following this line ^ is very speculative, so take with a shaker of salt. But I think it's an interesting idea that I haven't seen anyone else write about.

There are some exotic chemical reactions that give rise to mass-independent isotope fractionation [1]. For isotopes subject to this effect, enrichment can be more efficient via these mass-independent effects than would be indicated by conventional mass-dependent enrichment mechanisms like gas diffusion or centrifuge enrichment. There is evidence that these effects apply in uranium. The behavior is to separate even and odd isotopes rather than heavy and light isotopes [2].

If a similar mass-independent enrichment process were discovered for plutonium, it would mean a couple of things:

1) It might be industrialized secretly, in the context of weapons, long before it becomes a publicly known process with corresponding anti-proliferation safeguards and targeted inspections.

2) It would mean that aged spent nuclear fuel from commercial reactors could become an excellent raw material for making weapons plutonium. The other major contaminant isotope, Pu-241, has a half life of only 14 years. A few decades of cooling largely eliminates it. Then the Pu-240 would be separated from Pu-239 by the mass-independent fractionation process.

[1] https://en.wikipedia.org/wiki/Mass-independent_fractionation

[2] https://link.springer.com/article/10.1007/s11631-016-0109-3