Listen the HEU is a smoke screen propaganda story.
Because for all this focus on 400-600KG of HEU there is another 9000kg of enriched uranium of lower levels.
A hidden cascade or two of IR-9 centrifuges (20+ SWU) can enrich a 3.5% uranium to 90% in a week, maybe a bit more. IR-6 can do it under a month.
so you cannot seriously believe that the U.S. would take the 600kg HEU and then just decide to leave Iran 9000kg of enriched uranium and call it a day. That’s just propaganda they tell the masses who don’t know about the 9000kg or the nuclear fuel cycle.
You either take it all or you don’t take it at all.
I actually tried to run some calculations on your statement to get a ballpark of what can be achieved. Let's assume that each IR-9 has a SWU of 30 per year.
So, based on what I remembered and double-checked, each cascade of Iran's IR-1, IR-2 and IR-6 centrifuges included 164 machines, typically arranged in 30-40 stages and 2-8 machines in parallel.
Since SWU is approximately additive, for IR-9, the total SWU of such a hypothetical configuration would approximately be 4,920 SWU/year. You have two cascades, therefore you get 9,840 SWU/year.
Now you need to compute the total SWU required for enriching that uranium from 3.5% to 90%, assuming a typical tail assay of 0.25%. I cheated here and used Microsoft's copilot for this. The answer is 24,450 SWU. Hence, the time needed for enriching 3.5% LEU into 90% HEU is about 2.47 years. That's about 2 years, 5 months, 2 weeks and 5 days!
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That aside, even if we assume that Iran has the capability to enrich its LEU stockpile to HEU in less than a month, we'll still have the issue of converting our stockpile from UF6 gas into the uranium metal.