He's survived a small nuclear blast before... So, no. He can't be.
To be honest, it depends on the writer. Sometimes he takes a bullet to the head and he's down for a few hours, other times he doesn't miss a step. Sometimes he's killed just as easily as the other x-men, which as the third X-men movie shows us is easier than getting between Paris Hilton thighs, other times like I mentioned before, he can withstand a nuclear blast. Really, he's powers of regeneration and strength are entirely dependant on his current popularity. The stronger his selling power is the stronger he is. One day he could have difficulty fighting Sabertooth, the next day he's kicking the crap out of the Hulk. So, the answer to your question fluctuates day to day.