Nuclear designs are like quarterbacks, the best one is always the one "in development". The problems with all of these next-generation designs are not going to

Nuclear designs are like quarterbacks, the best one is always the one "in development". The problems with all of these next-generation designs are not going to be with the physics, but the engineering. These reactors will be operating for 40+ years, and are complex collections of machinery that have wear-out and failure mechanisms that may not be discovered until years after full operation commences - when this machinery is now highly radioactive. Remember the pebble-bed reactor? Sound idea in principle, the Germans found out that its main problem was inherent in the engineering of the fuel - it turned out to produce large amounts of radioactive dust in the reactor. The description of this reactor, which involves robotic fuel handling equipment operating within the containment constantly moving around fuel elements immersed in molten sodium metal seems doomed to create difficult repair problems as these mechanisms will fail and now need to be replaced/repaired within the hot zone of the reactor. It's quite easy to build a system which technically works, but ends up being uneconomic. To my knowledge there has yet to be a successful long-term operating power generation design of any kind using sodium coolant; the Monju plant in Japan was a failure largely because of problems with the coolant loop (including a leak and serious fire). Until these basic engineering issues are proven to be solved, the fuel physics seem kind of beside the point.