Engineered to cool by convection and gravity, yes. Somewhat safer than reactors that melt without continuous active cooling. But that comes at a cost: the goo

Engineered to cool by convection and gravity, yes. Somewhat safer than reactors that melt without continuous active cooling. But that comes at a cost: the good parts of low power density AND the bad part of low power density, such as being "huge" compared to a tiny little submarine reactor of similar power level. Really any reactor "could" be engineered this way but it does make them big. And "big" competes against the natural desire of engineers to run them at high temps thus high pressures to keep efficiency high. But what if lower efficiency results in net cheaper and safer electricity; its not like they're paying silicon valley prices for the land and nuclear fuel is stunningly cheap so burning twice as much is still cheaper than coal, LOL. A lot of the "old school" reactor design was based around the nuclear navy where both weight and volume are NOT cheap, not cheap at all. I don't think you could ever have a pebble bed reactor in an aircraft carrier.