There's an infinite spectrum of possible E-M frequencies, so I have no idea what it would mean to say that 'most' frequencies reflect or not. It is true that rather high-frequency radiation can be reflected by high-Z materials. From your question it almost seems as if you're asking about the reflection of the high-frequency radiation from a fission bomb by the U-238 layers on the outside of a fusion bomb, redirecting the energy toward the fusion material. (No, that's not classified.)
Mike W.
(published on 05/16/2013)
