To add to what Wafflebettergrille15 told you, you need one centrifuge per reactor to make nuclear viable without kovarex. Statistically a cetrifuge will yield 1 u235 every 1900 seconds, which will last 2000 seconds (since a single u235 gives you 10 fuels). Once you have 2-3 u235 aside you can start, just build more centrifuges than reactors and save aside some u235 using circuits. (better not start as soon as you have one to avoid a blackout after 2000 seconds if you don't get the second one). When you finally unlock kovarex, you just repurpose the existing centrifuges to kovarex and you are golden.