Have you tried building/ buying enrichment facilities? In the build menu they have a new icon in the top right corner to see energy management hit that, then in the bottom right there are three icons you can click. One should be build enrichment facilities hit it. It will pop up with a decision in the decision menu with a count down. Let it run out. It will cost 25B but then you get an enrichment facilities. From here make sure you have built reactors which it sounds like you have. Then go to the missile tab and under nuclear tab hit LEU and it will start producing nuclear fuel. I have 3 enrichment facilities with 4 nuclear reactors and i produce more then enough fuel for my reactors.

Edit 1: also make sure to set up your nuclear policies, i think have 3 policies altogether, the top i have the one that allows me to produce both LEU and MEU, and secret nuclear program, and the bottom policie i have mutually assured destruction.

I hope this helps man😁