Recently there was an interesting thread on physics forum about possible uses of spent nuclear fuel. It seems to me that one of the problems with using decay heath from spent nuclear fuel is the low power density. This got me thinking that maybe with enough insulation the low quality heat being produced could be made into high quality heat with high efficiency. What I was picturing was a primary loop surrounded by an insulating void. Also, something to reflect photons back would probably help. Maybe even have it float using magnetic fields in order to minimize the heat transfer away from the loop except at a single point where the thermal energy is used to make steam. I drew a picture although it probably wasn't necessary. Is this at all feasible?