Have we actually built one that gets this kind of efficiency? I've heard people saying that pebble beds could combine high core temperatures with gas turbines t

Have we actually built one that gets this kind of efficiency? I've heard people saying that pebble beds could combine high core temperatures with gas turbines to get around 50% efficiency, but the only commercial-scale pebble beds I know of are the Chinese HTR-PM, which uses steam turbines to get around 40% efficiency. Still damn impressive, of course.