I envision the tank being between the engine and the capsule, not wrapped around the engine or something like that. As another commenter points out, you only n

I envision the tank being between the engine and the capsule, not wrapped around the engine or something like that. As another commenter points out, you only need a shadow shield. I don't think there would be much more heat transferred from the engine to the fuel tank than in traditional engines; the neutrons would warm up the fuel some but perhaps naively I assume that effect would be fairly small. Now that I think about it more, I'm not sure. Is it the neutrons that heat the water in a nuclear reactor?