So I was reading this article about NASA’s Plutonium 238 problem and it got me thinking about possible substitutes. This is what I came up with. You make thin sheets of beryllium mixed with some kind of alpha particle emitter. You also make thin sheets of some substance that is good for capillary action which doesn’t absorb a lot of neutrons. I think carbon nanotubes might work. You then layer these sheets together in even proportions, and maybe roll the result up into a cylinder. When this gets to space you can expose it to a solution containing uranium enriched to whatever degree makes sense. It will get sucked up into the nanotube sheets through capillary action. This way you would have three sources of thermal energy. The alpha emitter (maybe Americium-241 would work) will release energy, When some of those alpha particles fuse with beryllium it will release more energy plus neutrons and some of those neutrons will encounter uranium-245 causing it to fission releasing even more energy. This way you can get fission energy without needing a chain reaction.