In a post-post-apocalyptic world with adequately schizophrenic tech, a tinkerer decides to build a submarine engine around a lump of radium, or some other reliable source of heat. But instead of using some complicated setup to drive a propeller, the engine simply evaporate water in a chamber, then uses the pressure to push water and/or vapour at the rear in order to create thrust.
Assuming there is no problem of evaporation residues or material strength, what would be the efficiency of such an engine? And assuming problems with evaporation residues, how long would the engine be expected to run before needing to scrape the chambre?
edit: For this question, assume stealth is not a problem. The goal is not to build a SSBN!