Also we have to assume that the Tsar Bomba detonates away from the fuel storage, and if my memory serves correctly, the reactors in Star Wars use the variant of unobtanium called 'hypermatter' for their fuel, which reacts with regular matter for a 100% efficient mass-to-energy conversion. Now, imagine the effect of rupturing the fuel storage with a big-ass nuclear bomb. All that hypermatter now flying all over the place in a radially outward direction. Look close enough and you might be able to see a hole appear in the side of the Death Star before it completely vaporizes.