Who? I mean I hear people on the news state it, but never any concerns at a certain 3 letter agencies that is not the department of education. I haven't heard a serious nuclear physicist be concerned with the Iran Deal. And when Iran broke the deal (not long ago) those same people knew it was a bluff, because Iran didn't even have the ability to produce enough enriched material to create bombs. Even running their enrichment centers at full capacity. Which btw, these enrichment centers don't scale linearly, which is why it is easy to vet through satellite photography. Weapon enrichment centers are typically kinda obvious. This may be a key part in what you aren't understanding. I also linked in another comment the Megatons to Megawatts program. You'll notice there the conversion rate of weapons material to fuel was about 30x (remember, reversing that process is not linear).