With Gregtech 5 installed, I'm pretty sure the only way to get U-235 is to centrifuge uranium-238 Dust. You get a 20% chance to get a tiny pile of U-235. It costs ~230,000 energy though in a centrifuge. (Note centrifuge, not thermal centrifuge). There may be U-235 ore, but I haven't found it yet. I haven't done much quarrying though