system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How can the Iranian nuclear program be "limited" in such a way that reactor grade uranium can be produced but weapons grade cannot?

The current strategy for dealing with Iran is finding a compromise that lets them continue their research programs but limiting it to nuclear energy production(not weapons). If nuclear enrichment is just increasing the concentration of fissile U235, then I would expect the way they do it is by running a centrifuge to separate the isotopes. But it seems to me that if you can get to 85% or whatever the reactor grade is by that method, you could also get to 99% or whatever weapons grade is by simply doing the same thing, but longer/faster. What exactly can be done that would allow the production of nuclear fuel but stop production of WMDs?
gpt
Commercial Reactor grade is 5%, weapons grade is 80% plus, though pretty much every warhead rests in the 90% plus range.

Plausible suggestions to allow Iran to continue the production of nuclear fuel without limiting the program are : 

a) Continual IAEA observation to ensure they don't enrich further

b) Limiting the total amount of centrifuges they can have.

The problem for option 1 is that most of the enrichment works like a pyramid. Ie, the higher the enrichment is, the less material you need to process and the faster the process goes. Allowing them to enrich as much as they want to a lower enrichment means that they can already lay down the base, where most work needs to be done.

For example, processing 1 ton of feed natural uranium to reactor grade costs 800-900 Swu*, while going to reactor grade only costs 1200. (20%, used for research reactors, lies at 1100). Thus, allowing them to have a stockpile of lightly enriched uranium is a significant asset if they want to break out.

Limiting the total amount of centrifuges also isn't going to work, because a simple 1 GW reactor requires about 140 000 Swu per year. A bomb meanwhile, requires about 4000.

Thus, there are no inherent technical solutions. Any effort and solution will need to be diplomatic. 

*Separative Work unit : Basically a unit of how much effort your centrifuges do. Iran's first Gen centrifuges do 1 SWU/year in optimal conditions, Second generation is about ten times as efficient. 

A modern, Western Centrifuge ranges somewhere between 40-60 IIRC.

Addendum : 

I forgot to list 2 additional options. One being to limit the amount of nuclear material they can have, which wouldn't really work because nuclear reactors use so much more material than bombs.

The other is to force them to immediately convert material to fuel elements, which can't be further enriched. However, this relies on them not secretly building a deconversion plant.

All in all, all the restrictions are only going to buy you time, perhaps 2 years at most, till they have enough nuclear material to build a bomb. Whether they have the knowledge to do so, and what they're going to do with 1 bomb is the question.