Tanks are big machines, and they swallow a lot of gaz/fuel for sure...
But they arent big enough to have nuclear reactors like an aircraft carrier or a sub would...
And tanks are in the frontline, they take damage all the time. Israel lost Merkava tanks in a fight against a militia and the US army lost Abram tanks in the invasion of Irak.
If they had to contain a whole area because of radiation leeks everytime they lose a tank... It would cost way too much.
If tomorrow they make some stable nuclear energy sources, then maybe they would use them on tanks. But right now, fuel is the most effective way to go.