Yes, Obsidian is nerfed by IC². It seriously made reinforced Stone useless... why to produce expensive shielding if you can just mine loads of obsidian with a diamond drill.
Reinforced Stone IS the best material to absorb a Nuclear Explosion.
However, depending on the amount of Uranium Cells in a Reactor, you would need up to 5-6 layers of Reinforced Stone for the worst case.
Assuming you are not designing a reactor meant to explode, merely containing a handfull of cells, 1-2 layers are sufficient, though.