Complex sentence: Most nuclear fuel is uranium dioxide, this is a cubic solid which has a structure similar to that of calcium fluoride, in used fuel the solid state structure of most of the solid remains the same as that of pure cubic uranium dioxide.
Split into simpler sentences: Most nuclear fuel is uranium dioxide, which is a cubic solid with a structure similar to that of calcium fluoride. In used fuel the solid state structure of most of the solid remains the same as that of pure cubic uranium dioxide.