Nuclear plants on Earth are way more than 140 times the mass of the fuel, but that's mostly concrete for the radiation shielding. If you're only worrying about

Nuclear plants on Earth are way more than 140 times the mass of the fuel, but that's mostly concrete for the radiation shielding. If you're only worrying about the core and cooling infrastructure it's not nearly as bad. But of course as everyone has mentioned cooling things in space is hard and you'll want to minimize the number of moving parts because maintenance is impossible and you can't use convection to move heat around which makes it even more difficult. Remember that on Earth nuclear reactors create electricity by boiling water to turn turbines. Such a system will be far more difficult to design for space.