"Proven Technology" != "Many are currently in commercial use". Had I meant the latter, that's what I would have said. The initial theory behind MSRs was well-defined prior to the 1960s, and experimental reactors (notably the ARE and MSRE) demonstrated the viability of the concept 50 years ago. THE LFTR is one of the most interesting modern reactor concepts and is being explored by several countries (I'm personally more of a fan of the LMBR--if only because it's closer to full commercial viability (and would already be there had Clinton and Gore not murdered the project)---but that's another story). The failure of Gen IV reactors to gain widespread commercial usage (especially in the age of global warming) is about fear, politics and economic interests, not the viability of the technology.
