It's never been stated they can survive nuclear bomb's. Also Spartan fours in Gen 2 armor were said to be equal to Spartan twos in Mark six armor. Of whom were almost killed by a fuel rod canon. "When she turned to let off a few bursts in the hunter's direction, the emerald discharge from an assault cannon slammed right into her chest with a deafening roar of static, lifting her off her feet and slamming her onto the ground. Getting knocked over saved her life, for as she thudded onto her back a second green ray of incendiary plasma blasted directly overhead. With her shields completely knocked out and the HUD shield alert honking a furious warning at her, the second blast would have cut her in half." Halo: Evolutions Vol. II "Blunt Instruments", page 28.