Well IIRC Accelerator's shields automatically block light, sound, heat, electricity, gravity, radiation, etc.. unless he allows it to affect him (so he could breathe, or hear other people for example). It's why he looks like an albino and doesn't have much muscle mass.
However the way the shield works is that when an attack or wave or particle or whatever gets within a certain range of him, its direction and speed is reversed reflecting the vector of movement 180 degrees. It's pretty much an automatic reflex and works even if he's sleeping or unconscious. Though at the same time the way his ESP was trained by Kihara, this technique was designed with an intentional flaw against attacks that can redirect themselves in the event they needed to subdue Accelerator. Accelerator got the crap beaten out of him by Kihara in season 2, because he knew how his powers worked and their effective range, and pulled his punches at the last possible second so they would be reflected right into Accelerator's face. Not sure if he tried to fix this weakness later on though.
And I don't think his shields would do any good against anything that would manipulate his matter directly. >_>
Another problem is that he's still human and still needs to breathe. I remember it was said in the manga that he could theoretically survive a direct hit with a nuclear weapon with no ill effects. But he'd still be at risk of suffocating to death standing around inside a huge explosion. At least, it came up when Misaka was trying to break down the oxygen in the air to get him to pass out, and he countered by causing a dust explosion.
I can't really say anything about him and his powers past what the anime covers since I haven't read the novels. Though I don't think there's really anything he could do in this situation.
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The only certain death in the universe is Mother Hitton's Littul Kittons.
Not changing this line until I beat Seven Heroes (2/24/07)